R. Pitt

July 21, 2002

Temporary Sediment Ponds for Construction Site Erosion Control

Introduction

Safety of Wet Detention Ponds

Maintenance Requirements of Wet Detention Ponds

Guidelines To Enhance Pond Performance

Pond Surface Area and Shape

Pond Water Depth

Pond Side Slopes

Outlet Structures

Emergency Spillways

Detention Pond Design Fundamentals

Upflow Velocity

Effects of Short-Circuiting on Particulate Removals in Wet Detention Ponds

Residence Time and Extended Detention Ponds

Runoff Particle Size Distributions

Particle Settling Velocities

Design Based on NURP Detention Pond Monitoring Results

Introduction To Storage-Indication Method

Design of Wet Detention Ponds for the Control of Construction Site Sediment

Specific Sediment Ponds and Filter Fence Standards for Construction Sites

Sediment Barrier/Fence (SF)

Definition

Purpose

Conditions Where Practice Applies

Planning Considerations

Design Criteria

Maintenance

Construction Specifications for Sediment Barriers/Fences

Sediment Basin (SB)

Definition

Purpose

Conditions Where Practice Applies

Basin Design

Maintenance

Construction Specifications for Sediment Basin

The Use of the WinDETPOND Program to Statistically Evaluate Wet Pond Performance

Example Pond Design for Construction Site Sediment Control

Example Design using Filter Fences for Construction Site Sediment Control

Sediment Capture Behind Filter Fences

Example Calculation of Sediment Capture Behind Filter Fence

Filter Fences to Slow Water Flowing Down Critical Slopes

Pressure Force on Filter Fences

Conclusions

References

Appendix 6-A: User Guide for WinDETPOND

Example Design Calculations and Evaluation Using WinDETPOND

Steps in Entering Data for Evaluation in WinDETPOND

 

 

Introduction

The use of temporary sediment ponds for erosion control is a common practice at many construction sites. In some cases, these ponds are re-built after the construction period and used as permanent ponds for stormwater control. However, in many cases, they are filled in and their area used as part of the land development. Because sediment ponds for erosion control have relatively short lives, their design criteria and construction methods differ from more permanent stormwater control ponds. The particle trapping mechanisms are the same for both types of ponds, but the influent hydrology and particle size distributions can be substantially different. The following discussion therefore stresses the special features of temporary sediment control ponds for construction sites. Also discussed are filter fences, for two reasons: 1) small drainage areas are usually controlled using filter fences, while large areas require sediment ponds, they are therefore complementary practices with similar objectives, and 2) filter fences remove sediment from the flowing water in much the same way as sediment ponds, by sedimentation (not “filtration”).

Problems caused by erosion are recognized as the major source of water quality impairment for many locations throughout the US. Siltation is the second largest single cause of impaired water quality in the nations rivers and lakes, with large amounts of sediment reducing stream flow capacity and destroying important aquatic life habitat. Compared to other sediment sources, erosion rates from construction sites can be ten times the erosion rates from row crops and one hundred times the erosion rates from forests or pastures. Nationally, typical construction site erosion yields are 10 to 150 tons/acre/year. Unfortunately, these problems are exacerbated in Alabama for the following reasons:

· Extremely High Rainfall Energy (Alabama has highest in the nation)

· Highly Erodable Soils (northern part of state has fine grained, highly erosive soils)

· Steep Site Topography (northeastern part of state has steep hills undergoing development)

Rain energy is the major driving force causing erosion and is directly related to the rainfall intensity. Because of our commonly occurring high intensity rains in Alabama, the rainfall erosion index varies from 250 to 550+ for Alabama (most of the state is about 350), which includes the highest values in the U.S. Months having the greatest erosion potential are February and March, while September through November have the lowest erosion potential, although highly erosive conditions can occur in any month.

Temporary construction site sediment ponds have sediment loads that are very large and the particulates can be very small. Sizeable accumulations of sediment can therefore occur in short periods of time. Due to the lack of protection from scour, dry detention ponds have much smaller removal benefits than wet ponds (having at lest 3 ft. of standing water). If well designed and properly maintained, suspended solids removals of 70 to 90% can be obtained in wet ponds, while dry ponds seldom provide more than 30% suspended solids reductions.

There are a number of basic design guidelines needed to maximize sediment removal and to minimize potential problems in ponds, including:

                · Need at least three, and preferably six feet, of permanent standing water over most of the pond to protect sediments from scouring.

                · Ideally, the pond length should be about three to five times the width for maximum detention efficiency and the inlets and outlets need to be widely spaced to minimize short-circuiting.

                · Correct pond side slopes are very important to improve safety and to minimize mosquito problems. An underwater shelf near the pond edge needs to be planted with rooted aquatic plants to hinder access to deep water, if the pond will be in place for several years. Short-term temporary ponds commonly used at construction sites will not enable vegetation to become established.

                · Outlet structures should be designed for low outflows during low pond depths to maximize particulate retention. Place underwater dams or deeper sediment trapping forebays near pond inlets to decrease required dredging areas.

                · Protect the inlet and outlet areas from scour erosion and cover the inlets and outlets with appropriate safety gratings. Provide an adequate emergency spillway.

Basic pond design guidelines must also be followed to provide the expected level of sediment removal. The following list is a typical example of these guidelines:

                · Engineering design guidelines (covering such things as foundations, fill materials, embankments, gratings, anti-seep collars, and emergency spillway construction), such as published by the U.S. Natural Resources Conservation Service and the Corps of Engineers must be followed.

                · Pond size is dictated mostly by desired particle control and water outflow rate. For construction sites, the pond water surface should be about 1.5% of the watershed area draining to the pond for approximately 5 mm control (this design will remove all particles greater than about 5 mm from the runoff water and corresponds to about 90% suspended solids reductions). If the pond area is only about 0.5% of the drainage area, the smallest particle sizes controlled would be about 20 mm, resulting in about 65%, or less, suspended solids reductions. The use of chemicals can increase the removal of sediment in ponds. In an early example, Colston (1974) used alum to increase suspended solids and turbidity removals up to about 85 to 97 percent. More recent examples show similar removal benefits when using chemical-assisted sedimentation.

Safety of Wet Detention Ponds

The most important wet detention pond design guidelines are to maintain public safety. The following discussion briefly summarizes common suggestions to maintain and improve safety at wet detention facilities. Death by drowning is the most common safety concern associated with wet detention ponds. Marcy and Flack (1981) state that drownings in general most often occur because of slips and falls into water, unexpected depths, cold water temperatures, and fast currents. Four methods to minimize these problems include: eliminate or minimize the hazard, keep people away, make the onset of the hazard gradual, and provide escape routes. Many of the design suggestions and specifications contained in this discussion are intended to accomplish these methods.

Jones and Jones (1982) consider safety and landscaping together because landscaping can be an effective safety element. They feel that appropriate slope grading and landscaping can provide a more desirable approach than wide-spread fencing around a wet detention pond. Unfortunately, landscaping is not very effective for temporary pond installations, so pond side slopes are most critical. Fences are expensive to install and maintain and usually produce unsightly pond edges. They collect trash and litter, challenge some individuals who like to defy barriers, and impede emergency access if needed. Marcy and Flack (1981) state that limited fencing may be appropriate in special areas. When the pond side slopes cannot be made gradual (such as when against a railroad right-of-way or close to a roadway), steep sides having submerged retaining walls may be needed. A chain link fence located directly on the top of the retaining wall very close to the water’s edge would be needed (to prevent human occupancy of the narrow ledge on the water side of the fence). Another area where fencing may be needed is at the inlet or outlet structures. However, fencing usually gives a false sense of security, as most can be easily crossed (Eccher 1991).

Gradual slopes near the water edge and a submerged ledge close to shore are usually the best solution to maximize safety. Aquatic plants on the ledge would decrease the chance of continued movement to deeper water and thick vegetation on shore near the water edge would discourage access to the water edge and decrease the possibility of falling into the water accidentally. Pathways should not be located close to the water’s edge, or turn abruptly near the water.

Marcy and Flack (1981) also encourage the placement of escape routes in the water whenever possible. These could be floats on cables, ladders, hand-holds, safety nets, or ramps. They should not be placed to encourage entrance into the water.

The use of inlet and outlet trash racks and antivortex baffles is also needed to prevent access to locations having dangerous water velocities. Several types are recommended by the NRCS (SCS 1982), as shown on Figure 6-1. Racks need to have openings smaller than about 6 inches to prevent people from passing through them and need to be placed where water velocities are less than three feet per second to allow people to escape (Marcy and Flack 1981). Besides maintaining safe conditions, racks also help keep trash from interfering with the outlet structures operation.

Figure 6-1. Various trash racks and baffles used by the SCS (NRCS). (SCS 1982).

Eccher (1991) lists the following pond attributes to ensure maximum safety:

1) There should be no major abrupt changes in water depth in areas of uncontrolled access,

2) slopes should be controlled to insure good footing,

3) all slope areas should be designed and constructed to prevent or restrict weed and insect growth (generally requiring some form of hardened surface on the slopes), and

4) shoreline erosion needs to be controlled.

Maintenance Requirements of Wet Detention Ponds

The most important maintenance for temporary construction site erosion ponds is to conduct periodic inspections and to make sure that the sediment accumulation is not excessive and prematurely filling the pond.

Temporary sediment ponds need to be inspected after each major storm. The inspection should include checking the pond embankments for subsidence and erosion. The conditions of the emergency spillway and inlets and outlets also need to be determined during the inspection. The adequacy of any channel erosion protection measures near the pond should also be investigated. Sediment accumulation in the pond (especially near, and in, the inlets and outlets) also needs to be examined.

Large sediment accumulations in detention ponds can have significantly adverse affects on pond performance. Bedner and Fluke (1980) reported on the long term effects of detention ponds that received little maintenance. Lack of dredging actually caused the silted-in ponds to become a major sediment source to downstream areas. Poorly maintained ponds only delayed the eventual delivery of the sediment downstream, they did not prevent it.

During major storms, construction site erosion ponds can literally fill up during a single storm. Most of the sedimentation would occur near the inlet and the resulting sediment accumulation would be very uneven throughout the pond. Normally, sediment removal in a permanent wet pond may be needed about every five to ten years, but may be needed every few months at construction sites. It is therefore necessary to plan for required maintenance during the design and construction of sediment ponds. Ease of access of heavy equipment and the possible paving of a sediment trap near the inlet would ease maintenance problems. Dredged sediment is usually placed directly onto trucks, or is placed on the pond banks for dewatering before hauling to the disposal location. One common practice is to keep an area adjacent to the detention pond available for on-site sediment disposal. Small mounds can be created of the dried sediment and covered with top soil and planted.

Poertner (1974) reviewed various sediment removal procedures. An underwater scoop can be pulled across the pond bottom and returned to the opposite side with guiding cables. If drains and underwater roads were built during the initial pond construction, the pond can be drained and front-end-loaders, draglines, and trucks can directly enter the pond area. Small hydraulic dredges can also be towed on trailers to ponds. The dredge pumps sediment to the shore through a floating line where the sediment is then dewatered and loaded into trucks or piled. A sediment trap (forebay) can also be constructed near the inlet of the pond. The entrances into the pond are widened and submerged dams are used to retain the heavier materials in a restricted area near the inlets. This smaller area can then be cleaned much easier and with less expense than the complete pond.

Guidelines To Enhance Pond Performance

The Natural Resources Conservation Service (NRCS, renamed from SCS, undated) has prepared a design manual that addresses specific requirements for such things as anti-seep collars around outlet pipes, embankment widths, type of fill required, foundations, emergency spillways, etc., for a variety of wet detention pond sizes and locations. That manual must be followed for detailed engineering requirements. The Alabama Soil and Water Conservation Committee, Natural Resources Conservation Service, Montgomery, AL, also has prepared the Alabama Handbook for Erosion Control (1993; currently being updated) that describes the construction and maintenance of sediment basins, and many other practices.

Pond Surface Area and Shape

Surface area is one of the most important design considerations for particle removal. Hittman (1976) reports that pond length to width ratios of about five have produced maximum pond efficiencies (decreased short-circuiting) during dye tests. If a long and narrow pond cannot be constructed, Schueler (1986) suggests that baffles or gabions be placed within the pond to lengthen the flow path between the inlets and outlets. Bondurat, et al. (1975) has also suggested that the idealized pond shape would be triangular: narrow near the inlet and wider near the outlet. This triangular configuration would allow more efficient particle settling by having a continually decreasing forward velocity. Very irregular pond shapes may decrease circulation and cause localized nuisance problems. The pond shape should be irregular for aesthetic considerations, but with minimal opportunities for water stagnation.  Short-circuiting in adequately sized ponds has little detrimental effect on pond performance. However, it can be serious in under-sized ponds.

Pond Water Depth

A storage volume above the permanent pool elevation of the pond affects the pond’s ability to absorb excess flows for flood control. Harrington (1986) found that increasing the wet pool depth increases sedimentation efficiency (due to flocculation), but that surface area increases were much more effective in enhancing the water quality performance of wet ponds. A minimum wet pool depth is very critical in wet ponds to decrease scour losses of previously settled material. Without an adequate permanent pool depth, very little water quality benefits can be expected from wet ponds.

Extra pond depth needs to be considered for sediment storage between removal operations (Schimmenti 1980). Wiegand, et al. (1986) state that it costs about five times as much to removal sediment during pond dredging operations (about $14 per cubic yard) as it does to provide extra sediment storage capacity (sacrificial volume) during initial pond construction (about $3 per cubic yard). This sacrificial storage should be provided as deeper forebays near the pond inlets (Driscoll 1986). These forebays, or the use of underwater dams, need to be designed as pre-sedimentation traps to encourage the deposition of sediment in a relatively restricted area. This would result in more frequent sediment removal operations, but at a much lower cost.

Sufficient water depth (at least three feet over the maximum deposited sediment thickness) is also needed to decrease the potential of sediment scour caused by increased flows during large storms (EPA 1983). Hey and Schaefer (1983) found that a depth of five feet was sufficient to protect the unconsolidated sediment from resuspension in Lake Ellyn.

Pond Side Slopes

Reported recommended side slopes of detention ponds have ranged from 1:4 (one vertical unit to four horizontal units) to 1:10. Steeper slopes will cause problems with grass cutting and may erode. Steep slopes are not as aesthetically pleasing and are more dangerous than gentle slopes (Chambers and Tottle 1980). Sclueler (1986) also recommends a minimum slope of 1:20 for land near the pond to provide for adequate drainage.

The slope near the waterline, and for about one foot below, should be relatively steep (1:4) to provide relatively fast pond drawdown after common storms. However, a flat underwater shelf several feet wide and about one foot below the normal pond surface is needed as a safety measure to make it easier for anyone who happens to fall into the pond to regain their footing and climb out. This shelf should also be planted with native rooted aquatic plants (macrophytes) to create a barrier making unwanted access to deep water difficult for permanent ponds.

Outlet Structures

Most of the effort given to alternative outlet structure designs has been for dry detention ponds. Wet ponds usually only have a surface weir, outlet pipe, or other simple overflow device to allow the passage of displaced pond water during rains. With the use of a more sophisticated outlet device (such as a floating wier), located at the normal wet pond surface elevation, more efficient particulate removals and flood control benefits may occur.

Hittman (1976) recommends that wide outflow (and inflow) channels be used to decrease erosion. If wide flow channels are not possible, then energy dissipaters to reduce the water velocity should be used. The Natural Resources Conservation Service (was SCS 1982) has prepared design guidelines for outlet structures for wet detention ponds. These guidelines include a turf covered embankment having a trapezoidal cross section, a pipe passing through the embankment as the major outlet with a metal riser and upstream trash rack, and an emergency spillway.

Controlled emptying of a detention pond at low outlet flow rates is desirable for effective sediment removal and flood control. A small diameter outlet pipe, or a small orifice on a plate, is usually used to achieve low outflows. The rate of discharge varies for these outlets because of varying overlying water levels. High flow rates occur with higher water levels and the outlet flows decrease with falling water levels. Selecting an appropriate outlet structure has significant effects on pond performance. To have a constant pond performance for all events (if desired), the shape of the outlet must allow a constant upflow velocity (pond outflow rate divided by pond surface area).

Emergency Spillways

All detention ponds must also be equipped with emergency spillways. Mason (1982) states that the preferred location of an emergency spillway is on undisturbed ground rather than over a prepared embankment to reduce the erosion potential. Detention ponds treating runoff from small contributing areas can safely handle overflows as sheetflows through well designed swales.

The Natural Resources Conservation Service guidelines for designing runoff control measures must be followed when designing emergency spillways for wet detention ponds. In addition, if the detention pond is large, special regulations of the state and the Army Corps of Engineers must be followed.

Detention Pond Design Fundamentals

The basic design approaches for wet detention ponds consider either slug flow or completely mixed flow. Martin (1989) reviewed these flow regimes and conducted five tracer studies in a wet detention pond/wetland in Orlando, FL, to determine the actual flow patterns under several storm conditions. Completely mixed flow conditions assumes that the influent is completely and instantaneously mixed with the contents of the pond. The concentrations are therefore uniform throughout the pond. Under plug flow conditions, the flow proceeds through the pond in an orderly manner, following streamlines and with equal velocity. The concentrations vary in the direction of flow and are uniform in cross section. The steady state resident time for both flow conditions is the same for both flow patterns, namely the pond volume divided by the discharge rate. Historically, wet detention ponds have been designed using the plug flow concept, probably because it had been used in conventional clarifier designs for water and wastewater treatment. In reality, detention ponds exhibit a combination flow pattern that Martin terms moderately mixed flow. He found that the type of mixing that actually occurs is dependent on the ratio of the storm volume to the pond storage volume (the flushing ratio). If the ratio is less than one, plug flow likely predominates. If the ratio is greater than one, the flow type is not as obvious. With faster moving water in the pond, short-circuiting effectively reduces the available pond storage volume (and therefore the resident time), with less effective treatment.

Upflow Velocity

Linsley and Franzini (1964) stated that in order to get a fairly high percentage removal of particulates, it is necessary that a sedimentation pond be properly designed. In an ideal system, particles that do not settle below the bottom of the outlet will pass through the sedimentation pond, while particles that do settle below/before the outlet will be retained. The path of any particle is the vector sum of the water velocity (V) passing through the pond and the particle settling velocity (v). Therefore, if the water velocity is slow, slowly falling particles can be retained. If the water velocity is fast, then only the heaviest (fastest falling) particles are likely to be retained. The critical ratio of water velocity to particle settling velocity must therefore be equal to the ratio of the sedimentation pond length (L) to depth to the bottom of the outlet (D):

     

as shown on Figure 6-2.


Figure 6-2. Critical Velocity and Pond Dimensions

The water velocity is equal to the water volume rate (Q, such as measured by cubic feet per second) divided by the pond cross-sectional area (a, or depth times width: DW):


 

or


 

The pond outflow rate equals the pond inflow rate under steady state conditions. The critical time period for steady state conditions is the time of travel from the inlet to the outlet. During critical portions of a storm, the inflow rate (Qin) will be greater than the outflow rate (Qout) due to freeboard storage. Therefore, the outflow rate controls the water velocity through the pond. By substituting this definition of water velocity into the critical ratio:


 

The water depth to the outlet bottom (D) cancels out, leaving:


 

Or


 

However, pond length (L) times pond width (W) equals pond surface area (A). Substituting leaves:


 

and the definition of upflow velocity:


 

where                     Qout = pond outflow rate (cubic feet per second),

                                A = pond surface area (square feet: pond length times pond width), and

                                v = upflow velocity, or critical particle settling velocity (feet per second).

Therefore, for an ideal sedimentation pond, particles having settling velocities less than this upflow velocity will be removed. Only increasing the surface area, or decreasing the pond outflow rate, will increase pond settling efficiency. Increasing the pond depth does lessen the possibility of bottom scour, decreases the amount of attached aquatic plants, and decreases the chance of winter kill of fish. Deeper ponds may also be needed to provide sacrificial storage volumes for sediment between dredging operations. For construction site sediment ponds, it should be assumed that inlet zones are restricted to the pond surface and that the outlet zones are full depth, providing a worst-case situation (as verified during field tests).


For continuous flow conditions (such as for water or wastewater treatment), the following relationships can be shown:

and 


 

                where t = detention (residence) time. With


 

                and substituting:


 

but


 

therefore,

leaving:

               

It is seen that the surface overflow rate (Q/A) is equivalent to the ratio of depth to detention time. It is therefore not possible to predict pond performance by only specifying detention time. If pond depth was also specified (or kept within a typical and narrow range), then detention time could be used as a performance specification for a continuous or slug flow condition. However, it is not possible to hold all of the water in a detention pond for the specified detention time. Outlet devices typically release water at a high rate of flow when the pond stage is increased (resulting in minimal detention times during peak flow conditions) and lower flow rates at lower stages, after most of the detained water has already been released. The average detention time is therefore difficult to determine and is likely very short for most of the water during a moderate to large storm. It is much easier to design and predict pond performance using the surface overflow rate relationships for variable flow stormwater conditions.

The surface overflow rate (the ratio of outflow rate to pond surface area) can be kept constant (or less than a critical value) for all pond stages. This results in a much more direct method in designing or evaluating pond performance. Pond performance curves can therefore be easily prepared relating surface overflow rate (and therefore critical particle control) for all stages at a pond site.

Effects of Short-Circuiting on Particulate Removals in Wet Detention Ponds


Under dynamic conditions, particle trapping can be predicted using the basic Hazen theory presented by Fair and Geyer (1954) that considers short-circuiting effects:

where     yo = initial quantity of solids having settling velocity of vo

y = quantity of these particles removed

y/yo = proportion of particles removed having this settling velocity

                Q = wet pond discharge

                A = wet pond surface area

                n = short-circuiting factor (number of hypothetical basins in series)

This equation is closely related to the basic upflow velocity equation (or surface overflow rate) developed previously and is also included in DEPTOND. The short-circuiting factor is typically given a value of 1 for very poor conditions, 3 for good conditions, and 8 for very good conditions. Short-circuiting allows some large particles to be discharged that theoretically would be completely trapped in the pond. However, field monitoring of particle size distributions of detention pond effluent shows that this has a very small detrimental effect on the suspended solids (and pollutant) removal rate of a pond. Figure 6-3 shows the effects of different n values on the removal of particles having different settling rates (v) compared to the critical settling rate (Q/A). For a particle having a settling rate equal to the critical values (v = Q/A), the ideal settling indicates 100% removal, while for “best performance” (n = ¥), the actual removal would be only about 65%. If the pond had an n of 1 (very poor performance), the removal of this critical particle would be only 50%.

Figure 6-3. Performance curves for settling basins of varying effectiveness (AWWA 1971).

The degradation of performance is much worse for particles having settling rates much larger than the critical rate. However, most wet detention ponds are greatly over-sized according to their ability to remove large particles, so this degraded performance has minimal effect on the overall suspended solids removal. The suggested detention pond design presented in this discussion only operates at the “design” stage (where the critical particle size is being

removed) a few times a year. At all other times, the smallest particles being removed in the ponds are much smaller than the critical size used in the pond design. Most larger particles are effectively trapped because they are much larger than the design particle size (the pond is over-sized for these large particles), even if they are not being removed at their highest possible rate. In most cases, a few relatively large particles (much larger than the critical design particle size) will be observed in the pond effluent, but they have little effect on the overall SS removal.

Figure 6-4 shows example particle settling distributions for a pond, comparing effluent conditions using the short-circuiting effects of Hazen’s theory. The most common particle size (the mode) changes very little for the different effluent conditions. However, there are more larger-sized particles present in the effluent using Hazen’s theory compared to the ideal theory, and the median size obviously increases as the value for n decreases.

Figure 6-4. Influent and effluent particle settling rate distributions for settling basins of varying effectiveness (AWWA 1971).

Very little degraded performance was observed at a pond monitored during NURP (EPA 1983) in Lansing, MI, that was expected to have significant short-circuiting. A golf course pond located across the street from a commercial strip was converted into a stormwater pond, but the inlets and outlets were adjacent to each other in order to reduce construction costs. It was assumed that severe short circuiting would occur because of the close proximity of the inlet and outlet, but the pond produced suspended solids removals close to what was theoretically predicted, and similar to other ponds having much similar pond area to watershed area ratios. Actually, the close inlet and outlet may have resulted in less short-circuiting because the momentum of the inflowing waters may have forced the water to travel in a general circular pattern around the pond, instead of directly flowing across the pond (and “missing” some edge area) if the outlet was located at the opposite side of the pond.

Seven events were studied at the Madison, WI, Monroe St, wet detention pond to find the short-circuiting “n” factors using observed and predicted particle size distributions in effluent water. Particle size distributions were measured using the Sedigraph method at the USGS Denver laboratory. This technique measures settling rates of different size suspended solid particulates down to 2 mm. The value of n is calculated using the concentrations of large particles that are found in the effluent. In ideal settling, no particles greater than the theoretical critical size (about 5 mm for Monroe St.) should appear in the effluent. However, there is always a small number of these larger particles. It is generally assumed that short-circuiting is responsible for these large particles. The measured values for n were one, or less, indicating a high degree of short‑circuiting in the pond. However, these observations were possibly affected by scour of bottom deposits near the subsurface effluent pipes. The maximum effect of short-circuiting on pond performance is shown in the following table, showing the average reduction in suspended solids removals for different n values, compared to the best performance (n value equal to 8):

n value                  % SS removal                       reduction in % SS

                                (average)                               removal compared to n=8

8                             85

3                             84                                            1

1                             80.7                                         4.3

0.5                          78.5                                         6.5

0.2                          59                                            26

The calculated values of n (based on matching measured effluent particle size distributions with distributions calculated using different values of n) ranged from about 0.2 to 1, indicating “very poor performance”, or worse. The median value of n observed was about 0.35, indicating a degradation in annual average suspended solids capture efficiency of no more than about 10 percent. The effects of this short‑circuiting, even with the extremely low values of n for Monroe St., only has a minimal effect on the suspended solids percentage removals. The Monroe St. pond provided an average suspended solids reduction of 87%, compared to the design goal of 90%. These values are quite close and the short-circuiting has a negligible effect on actual performance, as the pond surface is relatively large (0.6% of the drainage area) and the outlets were efficiently modified during the retrofitting activities. 

Although the pond is producing very good suspended solids removals as designed, the particle size distributions of the effluent indicate some short circuiting (some large particles are escaping from the pond). The short circuiting has not significantly reduced the effectiveness of the pond (measured as the percentage of suspended solids captured). Therefore, care should be taken in locating and shaping ponds to minimize short circuiting problems, but not at the

expense of other more important factors (especially size, or constructing the pond at all). Poor pond shapes probably cause greater problems by producing stagnant areas where severe aesthetic and nuisance problems originate.

Residence Time and Extended Detention Ponds

Residence time is defined as the ratio of volume to average flow rate, resulting in a time dimension. It can be assumed to be the average length of time any parcel of water remains in the pond. As in any pond performance measure or design criteria, residence time values are very dependent on good pond configurations. Harrington (1986) stresses the need to subtract pond “dead zones” from pond volume when calculating residence times. Dead zones (and associated short-circuiting) can significantly reduce pond effectiveness.

Designing a wet pond for the treatment of runoff based on residence time is usually not recommended. Barfield (1986) states that residence (detention) time is not a good criteria for pond performance, but the ratio of peak discharge rate to pond surface area (the peak upflow velocity) is a good criteria of performance. The state of Maryland uses a residence time standard as part of their design criteria for “extended detention” ponds. These ponds are normally dry between events, or have a small and shallow wet pond area near the outlet, and greatly extend in surface area during storms. For these types of ponds, Harrington (1986) found, through computer modeling studies, that a residence time of about nine days is needed to achieve a 70 percent reduction of particulate residue. Nine days is longer than the inter-event period for most rains in the midwest and the southeast, which is about three to five days. These types of ponds are therefore not expected to be very useful for locations where the interevent periods of rains is short, or the drain-down time of the pond is rapid.

Unfortunately, dry ponds usually do not allow permanent retention of the settled particles. Subsequent storms usually scour the fine particles previously settled to the pond bottom. As stated previously, dry detention ponds have not been shown to be consistently effective water quality control devices. The use of a small permanently wet detention pond or wetland at the downstream end of a dry detention pond could help recapture some of these scoured particles. As noted above, a wet detention pond above a dry pond is usually a much better solution, as the wet pond would then act as a pre-treatment pond, keeping particles and debris out of the dry pond which should be designed for peak flow rate reductions.

 

The discussion on upflow velocity as a design criteria showed the relationship between particle settling rates and upflow velocity, while this discussion showed the relationship between particle settling rates and residence times. There must therefore be a relationship between residence time and upflow velocity. Residence time is dependent on pond volume and outlet rate, while upflow velocity is dependent on pond surface area and outflow rate. The relationship between residence time and upflow velocity is therefore equal to the relationship between pond volume and pond surface area, or the pond depth. When a pond depth of five feet is used, the residence times of ponds

designed using the upflow velocity method are generally the same residence times needed for similar control levels using the residence time criteria. Even though the two procedures result in the same basic design, it is still recommended that the upflow procedure be used for wet detention ponds during storm events. The depth and configuration design criteria are very critical for the other pond uses (aquatic life, aesthetics, and safety, besides scour prevention) and they should not be varied as part of the major design elements.

Runoff Particle Size Distributions

Knowing the settling velocity characteristics associated with stormwater particulates is necessary when designing wet detention ponds. Particle size is directly related to settling velocity (using Stokes law, for example, and using appropriate shape factors, specific gravity and viscosity values) and is usually used in the design of detention facilities. Particle size can also be much more rapidly measured in the laboratory than settling velocities. Settling tests for stormwater particulates need to be conducted for about three days in order to quantify the smallest particles that are of interest in the design of wet detention ponds. Probably the earliest description of conventional particle settling tests for stormwater samples was made by Whipple and Hunter (1981).

Whipple and Hunter (1981) contradict the assumption sometimes used in modeling detention pond performance that pollutants generally settle out in proportion to their concentrations. However, Grizzard and Randall (1986) have shown a relationship between particulate concentrations and particle size distributions. High particulate concentrations were found to be associated with particle size distributions that had relatively high quantities of larger particulates, in contrast to waters having low particulate concentrations. The high particulate concentration water would therefore have increased particulate removals in detention ponds. This relationship is expected to be applicable for pollutants found mostly in particulate forms (such as suspended solids and most heavy metals), but the relationship between concentration and settling would be much poorer for pollutants that are mostly in soluble forms (such as filterable residue, chlorides and most nutrients). Therefore, the partitioning of specific pollutants between the “particulate” and “dissolved” forms, and eventually for different particulate size fractions, is needed.

Smith (1982) also states that settleability characteristics of the pollutants, especially their particle size distribution, is needed before detention pond analyses can be made. Kamedulski and McCuen (1979) report that as the fraction of larger particles increase, the fraction of the pollutant load that settles also increases. Randall, et al. (1982), in settleability tests of urban runoff, found that non-filterable residue (suspended solids) behaves liked a mixture of discrete and flocculant particles. The discrete particles settled out rapidly, while the flocculant particles were very slow to settle out. Therefore, simple particle size information may not be sufficient when flocculant particles are also present. Particle size analyses should include identification of the particle by microscopic examination to predict the extent of potential flocculation.

Figure 6-5 shows approximate stormwater particle size distributions derived from several upper Midwest and Ontario analyses, from all of the NURP data (Driscoll 1986), and for several eastern sites that reflect various residue concentrations (Grizzard and Randall 1986). Pitt and McLean (1986) microscopically measured the particles in selected stormwater samples collected during the Humber River Pilot Watershed Study in Toronto. The upper Midwest data sources were two NURP projects: Terstriep, et al. (1982), in Champaign/Urbana Ill. and Akeley (1980) in Washtenaw County, Michigan.

Figure 6-5. Particle size distributions for various stormwater sample groups.

Tests have also been conducted to examine the routing of particles through the Monroe St. detention pond in Madison, Wisconsin (Roger Bannerman, Wisconsin Department of Natural Resources, personal communication). This detention pond serves an area that is mostly comprised of medium residential, with some strip commercial areas. This joint project of the Wisconsin Department of Natural Resources and the U.S. Geological Survey has obtained a number of inlet and outlet particle size distributions for a wide variety of storms. The observed median particle sizes ranged from about 2 to 26 mm, with an average of 9 mm. The following list shows the average particle sizes corresponding to various distribution percentages for the Monroe St. outfall:

                                Percent larger                                                        Particle Size

                                   than size                                                             (mm)

                                                10 %                                                    450

                                                25                                                            97

                                                50                                                              9.1

                                                75                                                              2.3

                                                90                                                             0.8

These distributions included bedload material that was also sampled and analyzed during these tests.

Figure 6-6 shows the particle size distribution for the inflow events, including bedload, for a series of about 50 runoff events at the Monroe St. detention pond in Madison, WI. The median size is about 8 mm, but it ranges from about 2 to 30 mm. About 10% of the particles may be larger than 400 mm. The largest particle size observed was larger than 2 mm. The bedload added about 10% of the mass of these particulates and was associated with the largest sizes. The settling velocities of discrete particles can be predicted using Stoke’s and Newton’s settling equations. Probably more than 90% of all stormwater particulates (by volume and mass) are in the 1 to 100 mm range, corresponding to Laminar flow conditions. In most cases, stormwater particulates have specific gravities in the range of 1.5 to 2.5 (determined by conducting settling column, sieving, and microscopic evaluations of the samples, in addition to particle counting), corresponding to a relatively narrow range of settling rates for a specific particle size.

Figure 6-6. Inlet particle size distributions observed at the Monroe St. wet detention pond.

Limited data are also available concerning the particle size distribution of erosion runoff from construction sites. Hittman (1976) reported erosion runoff having about 70 percent of the particles (by weight) in the clay fraction (less than four mm), while the exposed soil being eroded only had about 15 to 25 percent of the particles (by weight) in the clay fraction. When the available data is examined, it is apparent that many factors affect runoff particle sizes. Rain characteristics, soil type, and on-site erosion controls are all important. This distribution is generally comparable to the “all NURP” particle size distribution presented previously. The critical particle sizes corresponding to the 50 and 90 percent control values are as follows for the different data groups:

                                                                                90 %                       50%

                Monroe St.                                            0.8                           9.1 mm

                All NURP                                               1                              8

                Midwest                                                                3.2                           34

                Low solids conc.                                  1.4                           4.4

                Medium solids conc.                           3.1                           21

                High solids conc.                                 8                              66

In addition to high rain energy, many Alabama soils are also highly erosive and result in construction site runoff that is very difficult to control. Based on about 70 construction site erosion samples collected in the Birmingham area (Nelson 1996; Pitt 1998), the characteristics of this runoff include:

·  Measured suspended solids concentrations ranged from 100 to more than 25,000 mg/L (overall median about 4,000 mg/L).

· Turbidity ranged from about 300 to >50,000 NTU, with an average of about 4,000 NTU

· Particle sizes: 90% were smaller than about 20 mm (0.02 mm) in diameter and median size was about 5 mm (0.005 mm).

· Measured Birmingham construction site erosion discharges range from about 100 to 300 tons/acre/year

There were obvious relationships between rain conditions and the observed runoff quality during these local Birmingham studies:

Measured conditions:

Low intensity rains (<0.25  in/hr)

Moderate intensity

rains (about 0.25 in/hr)

High intensity rains

(>1 in/hr)

Suspended solids, mg/L

400

2,000

25,000

Particle size (median), mm

3.5

5

8.5

Nelson 1996 and Pitt 1998

These construction site data would therefore correspond to the “low,” or “all NURP” particle size distributions. The particle size distribution of material leaving construction sites is therefore quite small and hard to control. Small particle sizes are much more difficult to remove by most erosion control strategies commonly employed that usually employ sedimentation (sediment ponds and “filter” fences). Settling velocities (or particle sizes) are used with the outflow rate to determine the required surface area for a sediment pond.

These stormwater data show that construction site runoff likely has smaller particle size distributions than most stormwater; construction site runoff has median sizes generally in the range of 3 to 8 mm, while stormwater at many locations has larger particles, with median sizes from about 8 to 65 mm.

Particle Settling Velocities

The settling velocities of discrete particles are shown in Figure 6-7, based on Stoke’s and Newton’s settling relationships. Probably more than 90% of all runoff particulates are in the 1 to 100 mm range, corresponding to laminar flow conditions, and appropriate for using Stoke’s law. This figure also illustrates the effects of different specific gravities on the settling rates. In most cases, stormwater particulates have specific gravities in the range of 1.5 to 2.5, while construction site runoff particles would be closer to 2.5. This corresponds to a relatively narrow range of settling rates for a specific particle size. Particle size is much easier to measure than settling rates and it is generally recommended to measure particle sizes using automated particle sizing equipment (such as a Coulter Counter Multi-Sizer III) and to conduct periodic settling column tests to determine the corresponding specific gravities. If the particle counting equipment is not available, then small scale settling column tests (using 50 cm diameter Teflonä columns about 0.7 m long) can be used.

Figure 6-7. Type 1 (discrete) settling of spheres in water at 10° C (Reynolds 1982).

Particle settling observations in actual detention ponds have generally confirmed the ability of well designed and operated detention ponds to capture the “design” particles. Gietz (1983) found that particles smaller than 20 mm were predominate (comprised between 50 to 70 percent of the sediment) at the outlet end of a “long” monitored pond, while they only made up about ten to 15 percent of the sediment at the inlet end. Particles between 20 and 40 mm were generally uniformly distributed throughout the pond length, and particles greater than 40 mm were only found in the upper (inlet) areas of the pond. The smaller particles were also found to be resuspended during certain events.

Design Based on NURP Detention Pond Monitoring Results

The EPA (1983) determined that long-term detention pond performance could be estimated based on geographical location and the ratio of the pond surface area to contributing source area. Driscoll (1989; and EPA 1986) presented  a basic methodology for the design and analysis of wet detention ponds. A pond operates under dynamic conditions when the storage of the pond is increasing with runoff entering the pond and with the stage rising, and when the storage is decreasing when the pond stage is lowering. Quiescent settling occurs during the dry period between storms when storage is constant and when the previous flows are trapped in the pond, before they will be partially or completely displaced by the next storm. The relative importance of the two settling periods depends on the size of the pond, the volume of each runoff event, and the inter-event time between the rains.

Driscoll (1989) produced a summary curve, shown as Figure 6-8, that relates wet pond performance to the ratio of the surface area of the pond to the drainage area, based on the numerous NURP wet detention pond observations. The NURP ponds were in predominately residential areas and were drained with conventional curb and gutters. This figure indicates that wet ponds from about 0.3 to 0.8 percent of the drainage area should produce about 90% reductions in suspended solids. Southeastern ponds need to be larger than ponds in the Rocky Mountain region because of the much greater amounts of rain and the increased size of the individual events in the southeast. Also, wet ponds intending to remove 90% of the suspended solids need to be about twice as large as ponds with only a 75% suspended solids removal objective.

Figure 6-8. Regional differences in detention pond performance (EPA 1983).

Introduction To Storage-Indication Method

The discharged water from a detention pond is simply displaced pond water. In some cases, observed outlet water characteristics during a specific storm cannot be related to the inlet water characteristics. If the storm is small, the volume of water coming into the pond can be substantially less than the resident water in the pond. In these cases, the outlet water is mostly “left-over” water from a previous event or from relatively low volume (but long duration) baseflows that had previously entered the pond since the last storm. However, if the storm is large, then the water being discharged from the pond is mostly related to the specific event. Therefore, analyses of detention pond behavior must consider the relative displacement of pond water. Long-term continuous analyses comparing many adjacent storms resulting in seasonal inlet and outlet discharges of pollutants may be more appropriate than monitoring simple paired samples.

The following discussion on routing includes a fairly simple procedure to examine these pond water displacement considerations and their effects on particulate trapping. The Source Loading and Management Model (WinSLAMM) and the Detention Pond Analysis model (WinDETPOND) include a computerized version of the storage-indication method. The pond routing calculation procedure presented in the remainder of this section is based on the Natural Resources Conservation Service Technical Release-20 (TR-20) procedures (SCS 1982), as presented by McCuen (1982). The reservoir routing subroutine in TR-20 (RESVOR) is based on the storage equation:

               

where I is the pond inflow and O is the pond outflow. The difference between the inflow and outflow must be equal to DS/DT, the change in pond storage per unit of time. McCuen presents a series of equations and their solutions that require the preparation of a “storage-indication” curve to produce the pond outflow hydrograph. The storage-indication curve is a plot of pond outflow (O) against the corresponding pond storage at that outflow (S) plus 1/2 of the outflow times the time increment. When the pond outflow hydrograph is developed, the upflow velocity procedure described earlier can be used to estimate pond pollutant removal and peak flow rate reduction performance.

The relationship between the pond stage and the surface area for the pond under study is also needed in order to calculate the storage volume available for specific pond stages. Figure 6-9 is an example stage-area curve developed from topographic maps of the Monroe Street detention pond in Madison, Wisconsin. The normal pond wet surface is at 13 feet (arbitrary datum) and the emergency spillway is located at 16 feet, for a resultant useable stage range of three feet.

 
 
Figure 6-9. Pond-stage surface area relationship for example problem.

Table 6-1 shows the calculations used to produce the storage-indication figure (Figure 6-10) for the Monroe St. pond. This example assumes some pond modifications: two 90o V-notch weirs, with a maximum stage range increased to 3.5 feet available before the emergency spillway is activated. The storage calculations assume an initial storage value of zero at the bottom of the V-notch weirs (13.0 feet). The time increment used in these calculations is ten minutes, or 600 seconds. The storage-indication curve shown as Figure 10 is therefore a plot of pond outflow (cfs) verses pond storage plus 300 (1/2 of 600 seconds) times the outflow rate. The storage-indication figure must also include the stage verses outflow and storage verses outflow curves (also from Table 6-1).

Table 6-1. Calculation of Storage-Indication Relationships for Example Pond and 1.5-Inch, 3-Hour Rain.

Datum Stage (H)

(ft)

Discharge Rate1 (O)

(ft3/sec)

Surface Area

(ft2)

Storage (S)

(ft2)

S + ½ ODt

(see footnote 2)

0

    0

59,100

           0

           0

0.1

    0.016

59,800

    5,980

    5,985

0.2

    0.09

60,500

  12,100

  12,130

0.3

    0.25

61,250

  18,375

  18,450

0.4

    0.51

61,850

  24,740

  24,890

0.5

    0.88

62,520

  31,260

  31,520

0.6

    1.4

63,300

  37,980

  38,400

0.7

    2.1

64,200

  44,940

  45,570

0.8

    2.9

65,000

  52,000

  52,870

0.9

    3.8

65,800

  59,200

  60,340

1.0

    5.0

66,767

  66,770

  68,270

1.2

    7.9

68,300

  82,000

  84,370

1.5

  14

71,000

107,000

111,200

1.8

  22

73,500

130,000

136,600

2.0

  28

75,148

150,300

158,700

2.5

  49

79,400

200,000

214,700

3.0

  78

83,928

251,800

275,200

3.5

115

87,500

306,300

340,800

1 Using two 90° V-notch weirs:

                Q = 2(2.5H2.5)

2 S+ ½ O Dt = S + O (½ D t) = S + 300 (O)

                D t = 600 seconds

 

Figure 6-10. Pond-stage/storage indication curve for example problem.

Design of Wet Detention Ponds for the Control of Construction Site Sediment

A wet detention pond performance specification for water quality control needs to result in a consistent level of protection for a variety of conditions, and to allow a developer a large range of options to best fit the needs of the site. It must also be easily evaluated by the reviewing agency and be capable of being integrated into the complete stormwater management program for the watershed. It should have minimal effects on the hydraulic routing of stormwater flows, unless a watershed‑wide hydraulic analyses is available that specifies the specific hydraulic effects needed at the specific location.

The following suggested specifications should meet these objectives under most conditions. However, the specific pond sizes should be confirmed through continuous long-term simulations using many years of actual rainfall records for the area of interest (such as possible by using WinDETPOND). These guidelines should therefore be considered as a starting point and modified for specific local conditions. As an example, it may be desirable to provide less treatment than suggested by the following guidelines (Vignoles and Herremans 1996). The following guidelines were developed by Pitt (1993a and 1993b), based on literature information and on his personal experience.

1) The wet pond should have a minimum water surface area corresponding to land use, and desired pollutant control. The following values were extrapolated from extensive wet detention pond monitoring, mainly the EPA’s NURP (EPA 1983) studies and other research. For construction sites, these required pond areas are 1.5% of the drainage area for 5 mm control and 0.5% for 20 mm control. For most locations, these would correspond to annual suspended solids controls of about 90 percent for the 5 mm particle size, and about 65 percent for the 20 mm particle size objectives. If any undeveloped areas are in the pond drainage, the pond area would have to be increased in area by about 0.6% of those areas. Similarly, if any paved areas were in the drainage, the increase in pond area would need to be 3% of the paved area. Obviously, to be most efficient, any extra drainage areas should be kept to a minimum.

The following table shows how the pond area can be estimated based on drainage area characteristics:

 

Land area

Pond size factor

Resulting pond area

Paved area

0.6 acres

3%

0.018 acres

Undeveloped area

3.8 acres

0.6%

0.023 acres

Construction area

27.6 acres

1.5%

0.414 acres

Total:

32.0 acres

 

0.455 acres

As illustrated in the example appendix, the total land area needed for the pond will be substantially larger than this value, as this area is the pond surface area during dry weather. The pond freeboard volume (for water quality control), plus the emergency spillway area, will increase the needed area dedicated for the pond.

2) The pond freeboard storage should be equal to the runoff associated with a 1.25 inch rain for the land use and development type. It should be noted that this storage volume is associated with the runoff volume from a specific type of rain and not for a set runoff volume. This has the benefit of providing the same level of control for all land uses. As an example, many ordinances require capture and treatment of the first 0.5 inch, or 1 inch, of runoff for an area. Unfortunately, this has the effect of providing very uneven levels of control because of different rainfall-runoff characteristics for different land uses. As an example, a residential area may require a rain of about 1.50 inches to produce 0.5 inches of runoff. However, a commercial area, such as a strip commercial development, would only require a rain of about 0.6 inches to produce 0.5 inches of runoff. It is obvious that the residential area is providing treatment for a much more severe rain, with a correspondingly  greater level of annual control, compared to the commercial area. By requiring a set amount of control associated with a rain having the same re-occurrence interval, a more consistent effort and benefit is obtained throughout the community. About 0.5 inches of runoff would occur at construction sites for sandy soil areas and about 0.6 inches of runoff for clayey soil areas for this rain depth. Again, if other land areas are also in the drainage in addition to the construction area, the pond treatment volume would have to be increased. For any paved areas, the 1.25 inch rain would produce about 1.1 inches of runoff, and for undeveloped areas, the 1.25 inch rain would produce about 0.1 (for sandy soils) to 0.3 (for clayey soils) inches of runoff.

The following table shows how the pond storage volume can be estimated based on drainage area characteristics (assuming clayey soil conditions):

 

Land area

Pond WQ volume factor

Resulting pond WQ volume

Paved area

0.6 acres

1.1 inches

0.66 acre-inches

Undeveloped area (clayey)

3.8 acres

0.3 inches

1.14 acre-inches

Construction area (clayey)

27.6 acres

0.6 inches

16.56 acre-inches

Total:

32.0 acres

 

18.36 acre-inches

(1.53 acre-ft)

Figure 6-12 is a schematic showing a cross section of the pond. The area below the invert of the major control device is the dead storage and is provided to minimize scour of the retained particulates. The water quality storage volume in the detention pond is the volume associated with the runoff associated with a 1.25 inch rain. The topmost layer in the detention pond is additional storage that is provided for drainage benefits. This storage would be provided (with the appropriate additional outlet structure) only if a basin-wide hydraulic analyses has been conducted to insure that inappropriate interferences of the different flood hydrographs would not occur. Also, it is important to note that an emergency spillway must also be provided above the water quality storage area. Therefore, the additional storage for drainage benefits as shown in this figure would at least be provided to cover the range of stage of the emergency spillway. In addition, the dead storage area must be provided to minimize scour and to provide sediment storage. At least 3 ft of water must be over the maximum stored sediment.


Figure 6-12. Cross-section of pond showing water quality storage portion

3) The selection of the outlet devices for the wet detention pond (primary water quality device plus emergency spillway). This outlet device must be selected based upon the desired pollutant control at every specific pond stage in the wet detention pond. This specification regulates the detention time periods and the “draining” period to produce consistent removals for all rains. The ratio of outlet flow rate to pond surface area for each stage value needs to be at the most 0.00013 ft3 /sec/ft2 for 5 mm (about 90 percent annual) control and 0.002 (ft3/sec/ft2) for 20 mm (about 65 percent annual) control. In practice, the desired pond surface area to stage relationship (simply the “shape” of the hole) is compared to the minimum surface areas needed at each stage for various candidate outlet structures. As an example, the following list summarizes the minimum surface areas needed for 5 mm particle control for different stage values. Also shown are the freeboard storage values below each elevation:

                                                45° V‑notch                                           90° V‑notch                                   24” pipe

stage                      storage                   surface                   storage                   surface                   storage                   surface  

feet                         acre‑ft                     acres                       acre‑ft                     acres                       acre‑ft                     acres

0.5                           <0. 01                      0.032                       0.02                         0.08                         0.07                         0.28

1.0                           0.05                         0.18                         0.15                         0.44                         0.39                         0.98

1.5                           0.22                         0.5                           0.56                         1.2                           1.1                           1.8

2.0                           0.60                         1.0                           1.5                           2.5                           2.1                           2.4

3.0                           1.6                           2.8                           6.2                           6.8                           4.5                           2.4

4.0                           5.9                           5.8                           17                            14                            6.9                           2.4

5.0                           14                            10                            36                            25                            9.3                           2.4

6.0                           27                            16                            67                            39                            12                            2.4

The large stages above the normal wet pond depth may result in unsafe conditions for most wet detention ponds. A maximum depth of about 3 feet above the normal wet pond depth is recommended.

Tables 6-2 through 6-5 provide a quick method of selecting appropriate outfall devices for a potential pond location. These tables indicate the minimum amount of pond surface area needed at each stage to provide a five mm critical control level for a variety of conventional outfall devices. Table 6-5 presents multipliers to adjust the minimum areas for other critical particle sizes. In order to improve the pond performance by selecting a two mm critical particle size instead of five mm, the pond surface area would have to be increased by about 6.7 times. If the critical particle size was increased to ten mm, then the required pond surface would be reduced by about 0.27 compared to the pond surface areas needed for five mm control.

Table 6-2. Surface Area Requirements for 5-mm Particle Size Control for Various V-notch Weirs.

Head (ft)

Flow

(cfs)

22.5°

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

30°

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

45°

Storage (ac-ft)

Reqd. area (acres)

0.5

0.1

<0.01

0.01

0.1

<0.01

0.02

0.2

<0.01

0.03

1

0.5

0.03

0.1

0.7

0.05

0.1

1.0

0.05

0.2

1.5

1.4

0.1

0.2

1.9

0.2

0.3

2.9

0.2

0.5

2

2.8

0.3

0.5

3.8

0.3

0.7

5.9

0.6

1.0

3

7.8

1.2

1.4

11

1.6

1.8

16

1.6

2.8

4

16

3.3

2.8

22

4.4

3.8

33

5.9

5.8

5

28

7.2

4.9

38

9.6

6.6

58

14

10

6

44

14

7.7

60

18

10

91

27

16

 

Flow

(cfs)

60°

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

90°

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

120°

Storage (ac-ft)

Reqd. area (acres)

0.5

0.3

<0.01

0.05

0.4

0.02

0.08

0.8

0.04

0.1

1

1.4

0.07

0.3

2.5

0.2

0.4

4.4

0.3

0.8

1.5

4.0

0.3

0.7

6.9

0.6

1.2

12

1.7

2.1

2

8.2

0.8

1.4

14

1.5

2.5

25

3.3

4.4

3

28

3.5

3.9

39

6.2

6.8

69

12

12

4

46

9.5

8.1

80

17

14

140

30

25

5

81

21

14

140

36

25

250

69

43

6

130

39

22

220

67

39

390

120

68


Table 6-3. Surface Area Requirements for 5-mm Particle Size Control for Various Rectangular Weirs.

Head (ft)

Flow

(cfs)

2 ft.

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

5 ft.

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

10 ft.

Storage (ac-ft)

Reqd. area (acres)

0.5

2.1

0.10

0.4

5.7

0.3

1.0

12

0.5

2.0

1

6

0.5

1.1

16

1.2

2.8

33

2.4

5.7

1.5

10

1.2

1.8

29

3.2

5.0

59

6.3

10

2

15

2.3

2.6

43

6.4

7.6

90

13

16

3

24

5.7

4.2

80

17

14

160

35

29

4

32

11

5.6

110

34

20

250

71

43

5

37

17

6.5

150

47

26

340

120

59

6

39

23

6.9

190

77

33

430

190

75

 

Flow

(cfs)

15 ft.

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

20 ft.

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

30 ft.

Storage (ac-ft)

Reqd. area (acres)

0.5

17

0.8

3.0

23

1.0

4.1

35

1.5

6.1

1

49

3.7

8.6

66

5.1

12

99

7.3

17

1.5

90

9.9

16

120

13

21

180

20

32

2

140

20

24

190

27

32

280

40

49

3

250

54

44

340

72

59

510

110

89

4

380

110

66

510

150

89

780

220

140

5

520

190

91

710

250

120

1100

390

190

6

680

290

120

920

390

160

1400

610

250

Table 6-4. Surface Area Requirements for 5-mm Particle Size Control for Various Drop-tube Structures.

Head (ft)

Flow

(cfs)

8”

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

12”

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

18”

Storage (ac-ft)

Reqd. area (acres)

0.5

0.5

0.02

0.09

0.9

0.04

0.2

1.6

0.07

0.3

1

0.7

0.07

0.1

2.2

0.2

0.4

4.4

0.3

0.8

1.5

0.7

0.1

0.1

2.2

0.4

0.4

6.5

0.8

1.1

2

0.7

0.2

0.1

2.2

0.6

0.4

6.5

1.4

1.1

3

0.7

0.3

0.1

2.2

0.9

0.4

6.5

2.5

1.1

4

0.7

0.4

0.1

2.2

1.3

0.4

6.5

3.6

1.1

5

0.7

0.6

0.1

2.2

1.7

0.4

6.5

4.7

1.1

6

0.7

0.7

0.1

2.2

2.1

0.4

6.5

5.8

1.1

 

Flow

(cfs)

24”

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

30”

Storage (ac-ft)

Reqd. area (acres)

Flow

(cfs)

36”

Storage (ac-ft)

Reqd. area (acres)

0.5

1.6

0.07

0.3

1.9

0.08

0.3

2.0

0.09

0.4

1

5.6

0.4

1.0

6.3

0.4

1.1

7.2

0.5

1.3

1.5

11

1.1

1.8

13

1.3

2.3

16

1.5

2.8

2

14

2.1

2.4

21

2.8

3.7

27

3.4

4.7

3

14

4.5

2.4

25

6.9

4.4

42

9.4

7.3

4

14

6.9

2.4

25

11

4.4

42

17

7.3

5

14

9.3

2.4

25

16

4.4

42

24

7.3

6

14

12

2.4

25

20

4.4

42

31

7.3


Table 6-5. Corrections for Needed Surface Areas for Particle Size Controls other than 5 mm.

Particle size for control (mm)

Typical percentage of particles larger than indicated size

Particle settling rate (cm/sec)

Required area multiplier, compared to 5 mm

1

100

1.5 x 10-4

27

2

94

6 x 10-4

6.7

5

88

4 x 10-3

1.0

10

78

1.5 x 10-2

0.27

20

62

6 x 10-2

0.067

40

47

2 x 10-1

0.02

100

28

8 x 10-1

0.005

As an example, if a site had a surface area of 3 acres at two feet above the lowest invert level, a number of outlet devices could be used to provide at least five mm critical control:

                · all V-notch weirs from 22.5o through 90o (but not 120o)             

                · only a 2 foot long rectangular weir

                · all pipes from 8” to 24”

Obviously, all stage levels have to be examined and the most critical device selected that provides the desired level of control. In a similar manner, it would be possible to specify the shape of a pond (area versus stage) to closely match the natural topography with minimal required grading by selecting an outfall structure that provides close to the required outfall rates.

These procedures will result in the largest storms that do not enter the secondary spillway to have treatment levels equal to the critical particle size specified. As an example, the above calculations focus on the 5 mm particle, at least, being controlled at all stage depths of the primary outfall structures in order to provide an approximate 90 percent annual control of suspended solids. The outfall device is selected to provide an outfall rate no greater than a critical value, that when divided by the pond surface area at that stage, will be no larger than the settling rate of the critical particle size. In almost all cases, the critical stage will be at the top of the primary outfall device, and all stages below that will more than meet the critical objective, and will therefore be controlling particles much smaller than the critical size specified in the objective. It may seem that the pond is therefore over-designed and that the pond is larger than needed. However, the 5 mm critical particle size is typically substantially larger than the 90th percentile particle size, and the added control provided at the lower stages in the pond is generally needed to provide this level of control on an annual basis. As indicated previously, the 90th percentile particle size is typically only 3 mm, or smaller.

An emergency spillway is always needed, even for temporary detention ponds at construction sites. Most local regulatory agencies will require an emergency spillway that is capable of discharging a specific design storm, typically in the range of 25 to 100-yr events, depending on the size of the pond. The typical procedure is to use the SCS (now NRCS) (1986) version of TR-55. The graphical peak discharge method (chapter 4) is commonly used to estimate the peak flow associated with the design storm, and the chapter 6 methods are then used to estimate the emergency spillway design. This spillway design should consider the outlet device selected for water quality benefits also. TR-55 is attached to module 4 and these procedures are well described in that material. As an example, Figure 6-11 shows that for type II and III rains, the storage volume would have to be about 0.55 of the runoff volume, if the peak runoff rate is to be reduced to 0.1 of its influent peak flow rate.

Figure 6-11. SCS TR-55 plot used to size additional freeboard needed for emergency spillway.

The SCS methods can be used indirectly to size an emergency spillway. The pond is sized to provide the water quality benefits, and this storage volume is taken as Vs in Figure 6-11. The design storm volume that must safely be accommodated by the emergency spillway is taken as Vr. The ratio of these values can be used with this figure to estimate the peak flow attenuation that the pond will provide. The peak inflow discharge rate, qi, can be estimated using the SCS graphical peak discharge method (or the tabular hydrograph method, or WinTR-55). The peak outfall discharge, qo, is then calculated based on the measured attenuation factor. As an example, consider:

                Vs = 1.53 acre-ft

                Vr = 7.5 acre-ft     

                And Vs/Vr = 0.20

               

Therefore, for type II or III rain categories:

                qo/qi = 0.72

if the calculated peak discharge rate entering the pond (qi) = 8.7 cfs, the resulting peak discharge rate leaving the pond, qo, (through the water quality primary outlet plus the emergency spillway) is therefore: 0.72 (8.7) = 6.3 cfs. TR-55 shows how to calculate the needed emergency spillway for a specific discharge goal, considering multiple outlet structures. This method will help determine the size of the spillway, plus the additional freeboard that must be added to the pond design to accommodate the emergency spillway.

4) The ponds must also be constructed according to specific design guidelines to insure the expected performance and adequate safety. The guidelines need to specify such things as pond depth, side slopes, and shape.

Specific Sediment Ponds and Filter Fence Standards for Construction Sites

The Alabama Handbook (USDA 1993), along with other erosion control manuals, contain descriptions of many “structural” practices that can be used on construction sites to capture sediment that has already eroded. The following are modified excerpts from the Alabama Handbook are for sediment fences and sediment ponds. These sections contain recommendations for the use of these controls for Alabama conditions. This handbook is currently being revised, with the new edition expected to be available in 2003, and local USDA extension offices should be consulted for updated recommendations.

Sediment Barrier/Fence (SF)

Definition

A sediment barrier is a temporary structure constructed of a silt fence or hay bales along a contour, property line or development line. Other barrier materials may include sand bags, brush piles or other filtering material.

Purpose

To retard sheet and rill erosion and to intercept and detain small amounts of sediment from disturbed areas during site development in order to prevent sediment from leaving the construction site.

Conditions Where Practice Applies

1. On or along the down slope side of disturbed areas where sheet and rill erosion would occur.

2. At storm drain inlets, along right-of-way lines, property lines or along the edge of developing areas.

3. Where the size of the drainage area is no more than one-fourth acre per 100 feet of sediment barrier length; where the total maximum length of slope behind the barrier does not exceed 100 feet; and where the maximum gradient behind the barrier is 25% (4:1). The length of slope behind the barrier should not exceed 50 feet where the slope is steeper than 50% (2:1). (Note: these criteria do not imply that rows of filter fabric barriers can be located along a long slope, with each fence spaced at these distances.)

4. In small waterways, minor swales or ditch lines where the maximum contributing drainage area is less than two acres and where maximum flow is less than one cubic feet per second.

5. Sediment barriers should not be used on high sediment producing areas.

Planning Considerations

Silt fences are usually preferable to hay bales because silt fences can trap a much higher percentage of suspended solids. The success of silt fences depends on a proper installation so as to develop maximum efficiency of trapping. Silt fences as well as hay bales should be carefully installed to meet the intended purpose.

Sediment barriers may be used on developing sites. They should be installed on the contour so that flow will not concentrate and cause bypassing, overtopping and/or failure. Barriers may be placed on a slight grade (less than 0.5%) when the flow can be safely released at the end of the barrier and the length does not exceed 500 feet.

The primary sediment barrier is a silt fence. A silt fence is specifically designed to allow water to pass through while retaining sediment on the site. Silt fences shall be installed to be stable under the flows expected from the site. Silt fences are composed of woven geotextile supported between steel or wooden posts. Silt fences are commercially available with geotextile attached to the post and can be rolled out and installed by driving the post into the ground. This type of silt fence is simple to install, but more expensive than some other installations. Silt fences must be trenched in at the bottom to prevent rills from developing under the fence.

Hay bale barriers are the next most common sediment barrier. Hay bales are laid end to end along the contour and anchored in place by driving wooden stakes through the bales into the soil. Some embedment is needed to prevent water from going under the barrier. The bales should be embedded three or four inches on the upslope side of the bales on steep sites. Sediment barriers shall be of sufficient length to eliminate end flow whenever it is constructed across a swale or ditch line. The plan configuration shall resemble an arc or horseshoe with ends oriented upstream.

Design Criteria

1. No formal design is required.

2. Silt fences are normally limited to situations in which only sheet or overland flow is expected. They normally cannot filter the volumes of water generated by channel flow. Silt fences are normally constructed of synthetic fabric (woven geotextile) and the life is expected to be the duration of most construction projects.

3. Hay bales have an expected life of six to twelve months and replacement should be anticipated on projects lasting a longer time. They are applicable to ditch lines, around drop inlets and at other temporary locations.

4. Silt fences shall be a minimum of 30” high above ground and shall not exceed 36”.

5. Posts shall extend at least 18” below ground.

Maintenance

The contractor shall maintain sediment barriers until the upslope area is satisfactorily vegetated or the job is accepted.

Sediment barriers are a high maintenance measure. They shall be inspected immediately after each rainfall and at least weekly during normal construction activities, and daily during prolonged rainfall. Any needed repairs shall be made immediately. (The proposed EPA NPDES Phase II construction site regulations propose site inspections and needed repairs after at least every 0.5 inches of rainfall).

Sediment deposits shall be removed when the deposits reach one-half the height of the barrier. Any sediment deposits remaining after the sediment barrier is no longer required shall be smoothed to conform to the natural topography, and seeded and mulched in accordance with permanent vegetation specifications given in the Alabama Handbook.

Construction Specifications for Sediment Barriers/Fences

1. Silt fence may be premanufactured equivalent to Mirofi Envirofence or Armco Propex Silt Stop. Silt fence may also be built on the site with post, wire and fabric of approved materials.

2. When silt fence is built on the site, the fabric shall be Filter X, Poly-filter X, Mirafi 100X, Laurel Erosion Control Cloth, Bidim or other similar fabrics which have been developed for sediment control applications. Wooden posts shall be sound and have a minimum actual diameter of 3 inches. Steel posts of Standard T or U section weighing not less than 1.33 pounds per linear feet may be used; woven wire fencing shall have a minimum 14 gauge with a maximum 6 inches mesh opening.

3. The silt fence shall be installed at locations shown on the drawings or as staked by the engineer.

4. The filter fabric shall be purchased in a continuous roll cut to the length of the barrier to avoid the use of joints. When joints are necessary, the fabric shall be spliced together only at a support post with a minimum 36 inch overlap, and securely sealed. The overlap shall be pointed downstream if the barrier has any grade along its length.

5. A trench shall be constructed along the bottom of the silt fence.

6. The posts for the silt fence shall be spaced a maximum of 10 feet apart. The posts shall extend into the ground at least 18 inches and more depth is recommended when the ground is soft or concentrated flow is expected to cross the sediment barrier.

7. The woven wire fencing shall extend into the trench a minimum of 4 inches and securely fastened to the uphill side of the post with 1 inch staples or other appropriate fasteners.

8. The filter fabric shall be securely fastened to the fencing with staples or hog rings or other fasteners made for this purpose. The filter fabric shall be installed in the trench correctly. The trench shall be backfilled and the soil compacted over the filter fabric.

9. Premanufactured silt fence shall be installed in accordance with manufacturers directions and details, but must be embedded in the soil.

10. Hay bales shall be tame hay, or straw from wheat, oats, barley or rye. Bales containing noxious weeds or seeds will not be acceptable. The bales shall be dense and securely tied with string.

11. Stakes for hay bale barriers shall be nominal 1 inch by 2 inch wood. The wood shall be sound with a minimum cross section of 1.25 square inches. The minimum actual dimension shall be not less than one-half inch. The minimum length shall be 3 feet. The stakes shall be driven into the ground at least 12 inches. Equivalent metal rods or steel bars may be used, but wood is preferred.

12. Hay bale barriers shall be placed at locations shown on the drawings and as selected by the engineer.

13. Hay bales shall be placed end to end with two stakes in each bale to anchor it in place. Bales shall be placed in full contact with the ground, not spanning rills or depressions which would allow water to flow under the bales. The bale string must not contact the ground, with the bale rotated so the string is parallel to the ground surface.

14. When hay bale barriers are installed on steep sites (steeper than 25% or 4:1), the upstream side of the hay bale shall be embedded 3 inches. Embedment soil should be placed on upslope site of the bale and used to seal the barrier.

15. When hay bale barriers are installed across swales or concentrated flow areas, the entire hay bale should be embedded 4 inches.

Sediment Basin (SB)

Definition

A basin created by the construction of a barrier or dam across a natural drainage path or by excavating a basin or by a combination of both. Basins usually consist of a dam, a pipe outlet (principal spillway), and an emergency spillway. The size of the structure will depend upon the location, size of drainage area, soil type, rainfall pattern, available storage and selected outflow releases.

Purpose

To detain rainfall runoff water, reduce and/or maintain peak discharges, and trap sediment from highly erodible areas in order to protect properties and drainage ways located downstream from damage by sedimentation and debris. The water is temporarily stored and a portion of the sediment carried by the water drops out and is retained in the basin while the water is automatically released. The basin is for temporary use and may be removed after the disturbed area in the drainage area has been stabilized.

Conditions Where Practice Applies

This practice applies to critical areas where physical site conditions, construction schedules, or other restrictions preclude the installation or establishment of erosion control practices to satisfactorily reduce runoff volumes, erosion, and sedimentation. The structure may be used in combination with other practices and should remain in place until the sediment-producing area is permanently stabilized.

This standard applies to the installation of temporary sediment basins on sites where: (1) failure of the structure would not result in loss of life or interruption of use or service of public utilities, (2) the peak rate of runoff from the drainage area into the basin, produced by the 10­year/24-hour storm, does not exceed 200 cfs, (3) the peak flowrate through the principal spillway does not exceed 50 cfs, (4) the maximum height of the embankment measured from the low point of the original centerline cross-section does not exceed 15 feet, and (5) the total storage below the emergency spillway crest does not exceed 50 ac-ft.

Design Criteria

Compliance With Laws and Regulations. Design and construction shall comply with State and local laws, ordinances, rules and regulations. The design criteria as stated in this standard may be exceeded by a professional engineer with experience in hydrology and hydraulics.

Location. The sediment basin should be located to obtain the maximum storage benefit from the terrain and for ease of cleanout of the trapped sediment. It should be located to minimize interference with construction activities and construction of utilities.

Volume of the Basin. The volume of the sediment basin, as measured from the bottom of the basin to the elevation of the principal spillway crest shall be at least 67 cubic yards per acre of the total drainage area of the basin. This storage is equivalent to 1/2 inch of runoff per acre of total drainage area and provides for sediment control and stormwater management of the site.

Sediment basins shall be cleaned out when 50% of the basin volume has filled with sediment. Cleanout shall be performed to restore the original design volume in the sediment basin. The elevation corresponding to the sediment cleanout level shall be determined and shall be stated in the design data as a distance below the top of the riser. In no case shall the sediment cleanout level be higher than 1 foot below the top of the riser and this cleanout elevation should be clearly marked on the riser.

As a minimum, provisions should be made to dewater the basin down to the sediment cleanout elevation. Dewatering of the basin may be accomplished by using perforations in the riser, or by providing a maximum 4 inch diameter hole at the sediment cleanout elevation or other acceptable methods.

Shape of the Basin. To improve trapping efficiency of the basin, the effective flow length must be twice the effective flow width. This basin shape may be accomplished by properly selecting the site of the basin, by excavation or by using baffles. The dimensions necessary to obtain the required basin volume and shape shall be clearly shown on the plans to facilitate plan review, construction and inspection.

Basin Design

Runoff shall be computed by the SCS method or other acceptable methods. Runoff computations for the principal spillway and emergency spillway design storms shall be based upon the worse soil-cover conditions expected to prevail in the contributing drainage area during the anticipated effective life of the structure.

The combined capacities of the principal and emergency spillway shall be sufficient to pass the peak rate of runoff from a 25-year/24-hour storm.

1. Principal spillway - A spillway consisting of a vertical pipe or box type inlet (riser) joined (watertight connection) to a pipe (barrel) which shall extend through the embankment and outlet beyond the downstream toe of the fill. The maximum capacity of the principal spillway shall not exceed the peak runoff rate produced by the 10-year/24-hour pre-development peak discharge assuming a meadow type of land use. The barrel and riser size shall be determined through routing procedures of a 10-year/24-hour storm (land use conditions during construction) taking into account the storage characteristics of the basin between the top of the riser and the crest of the emergency spillway. if during the design process the routed peak discharge through the principal spillway exceeds the pre­development discharge, the pipe sizes will need to be reduced, the storage increased and then redesigned. The minimum size of the barrel shall be 8 inches in inside diameter. The principal spillway should be adequately sized to remove 50% of the runoff volume of the 10­ year/24-hour storm within a 3 day period.

a. Crest elevation - The crest elevation of the riser shall be a minimum of one foot below the elevation of the control section of the emergency spillway.

b. Watertight riser and barrel assembly - The riser and all pipe connections shall be completely watertight except for the inlet opening at the top and dewatering openings, and shall not have any other holes, leaks, rips or perforations.

c. Dewaterinq the basin - As previously mentioned, the basin shall be dewatered to the sediment cleanout elevation. A dewatering device shall be included in the sediment basin plans submitted for approval and shall be installed during construction of the basin. Dewaterinq shall be done in such a manner as to remove the relatively clean water without removing any of the sediment that has settled out and without removing any appreciable quantities of floating debris. Dewaterinq may be accomplished using perforations in the riser, or by a maximum 4 inch hole at the sediment cleanout elevation or other acceptable methods.

d. Anti-vortex device and trash rack - An antivortex and trash rack shall be securely installed on top of the riser and shall be the concentric type.

e. Base - The riser shall have a base attached to the bottom of the pipe with a watertight connection and shall have sufficient counter weight to prevent flotation of the riser. Two approved bases for risers ten feet or less in height are (1) a concrete base 18” thick with the riser embedded 9” in the base, and (2) a ¼” minimum thickness steel plate attached at the base of the riser by a continuous weld around the circumference of the riser to form a watertight connection. The plate shall have at least 2.5 feet of stone, gravel or compacted earth placed on top of it to prevent flotation. For both cases, each side of the square base shall have dimensions twice the riser diameter. For risers greater than ten feet high, computations shall be made to design a base which will prevent flotation. The minimum factor of safety shall be 1.20 (downward forces = 1.20 x upward forces).

f. Anti-Seep Collars - anti-seep collars shall be installed around all conduits through earth fills of impoundment structures according to the following criteria:

1. Collars shall be placed to increase the seepage length along the pipe (barrel) by a minimum of 15 percent of the pipe length located within the normal saturation zone.

2. Collar spacing shall be between 5 and 14 times the vertical projection of each collar.

3. All collars shall be placed within the normal saturation zone.

4. The assumed normal saturation zone (phreatic line) shall be determined by projecting a line at a slope of 4 horizontal to 1 vertical from the point where the normal water (riser crest) elevation touches the upstream slope of the fill to a point where this line intersects the invert of the pipe (barrel) outlet. All fill located within this line may be assumed as saturated.

g. Outlet - An outlet shall be provided, including a means of conveying the discharge in an erosion­ free manner to an existing stable channel. Where discharge occurs at the property line, drainage easements will be obtained in accordance with local ordinances. Adequate notes and references will be shown on the erosion and sediment control plan.

Protection against scour at the discharge end of the principal spillway shall be provided. Measures may include an impact basin, riprap, revetment, excavated plunge pools, or other acceptable methods.

2. Emergency Spillways - The entire flow area of the emergency spillway shall be constructed in undisturbed soil material (not fill). The emergency spillway cross ­section shall be trapezoidal with a minimum bottom width of eight feet. This spillway channel shall have a straight control section of at least 20 feet in length and a straight outlet section for a minimum distance equal to 25 feet or 1/2 the base width of the embankment fill.

a. Elevation - The elevation of the emergency spillway control section will be determined through routing procedures of the 10-year/24-hour storm taking into account the principal spillway and the storage characteristics above the top of the riser.

b. Capacity - The minimum capacity of the emergency spillway shall be that required to pass the difference between the peak rate of runoff from the 25-year/24-hour storm and the 10-year/24-hour storm. Emergency spillway dimensions may be determined by using the method described in this standard.

c. Velocities - The velocity of flow in the exit channel shall not exceed 6 feet per second for vegetated channels. For channels with erosion protection other than vegetation, velocities shall be within the non-erosive range for the type of protection used.

d. Erosion Protection - Vegetation, riprap, asphalt or concrete shall be provided to prevent erosion.

e. Freeboard - Freeboard is the difference in elevation between the design high water elevation in the emergency spillway and the top of the settled embankment. The freeboard shall be a minimum of one foot. The minimum elevation between the control section of the emergency spillway and the top of the dam shall be 2 feet.

Entrance of Runoff into Basin. Points of entrance of surface runoff into excavated sediment basins shall be protected to prevent erosion and sediment generation. Dikes, swales or other water control devices shall be installed as necessary to direct runoff into the basin. Points of runoff entry should be located as far away from the riser as possible, to maximize travel time.

Maintenance

Repair all damages caused by soil erosion or construction operations at or before the end of each working day. Sediment shall be removed from the basin when it reaches the specified distance below the top of the riser. This sediment shall be placed in such a manner that it will not erode from the site. The sediment shall not be deposited downstream from the embankment, adjacent to a stream or in a floodplain.

Final Disposal. When temporary structures have served their intended purpose and the contributing drainage area has been property stabilized, the embankment and resulting sediment deposits shall be removed and the site stabilized in accordance with the approved sediment control plan.

Construction Specifications for Sediment Basin

Site Preparation. Areas under the embankment and under structural works shall be cleared, grubbed, and stripped of topsoil. All trees, vegetation, roots and other objectionable material shall be removed and disposed of by acceptable methods. In order to facilitate cleanout or restoration, the pool area (measured at the top of the riser pipe) will be cleared of all brush and trees.

Keyway Trench. A keyway trench will be excavated along the center-line of earth fill embankments. The minimum depth shall be 2 feet and the actual depth determined by the engineer during excavation. The keyway trench shall extend up both abutments to the riser crest elevation. The minimum bottom width shall be 8 feet, but wide enough to permit operation of compaction equipment. The side slopes shall be no steeper than1:1. Compaction requirements shall be the same as those for the embankment. The trench shall be drained during the backfilling and compacting operations.

Embankment. The embankment shall have a minimum 8 ft. top width and 3:1 side slopes. Flatter side slopes may be needed for stability purposes on certain types of soils. The fill material shall be taken from approved areas shown on the plans. It shall be clean mineral soil free of roots, woody vegetation, oversized stones, rocks or other objectionable material. Relatively pervious materials such as sand or gravel (Unified Soil Classes GW, GP, SW & SP) shall not be placed in the embankment. Areas on which fills are to be placed shall be scarified prior to placement of fill. Fill material shall be placed in six inch to eight inch thick continuous layers over the entire length of the fill. Compaction shall be achieved by traversing the entire surface of each lift with two tracks of earth hauling equipment or with the use of a roller type compactor. Earth fill shall be considered too wet for placement if free moisture can be seen on the surface of a hand molded sample and too dry for placement if the material can not be easily molded by hand into a firm ball. The core of the embankment should be at least 8 feet wide and consist of the most impermeable material available at the site. The core should extend from the bottom of the keyway trench to the crest of the emergency spillway. The embankment shall be constructed to an elevation 10 percent higher than the design height to allow for settlement.

Principal Spillway. The riser shall be securely attached to the barrel or barrel stub making a watertight structural connection. The barrel stub must be attached to the riser at the same angle or grade as the outlet conduit to ensure the riser is vertical. The connection between the riser and the riser base shall be watertight. All connections between barrel sections must be achieved by approved watertight band assemblies or other approved methods. The barrel and riser shall be placed on a firm, smooth foundation of impervious soil as the embankment is constructed. Breaching the embankment to install the barrel is unacceptable. Pervious materials such as sand, gravel, or crushed stone shall not be used as backfill around the pipe or anti-seep collars. The fill material within 2 feet of the pipe spillway shall be placed in four inch layers and compacted by hand equipment under and around the pipe to at least the same density as the adjacent embankment. A minimum depth of two feet of hand compacted backfill shall be placed over the pipe spillway before crossing it with construction equipment. Steel base plates used to prevent flotation of risers shall have at least 2­-1/2 feet of compacted earth, stone or gravel placed over it.

Emergency Spillway. The emergency spillway shall be installed in undisturbed ground. The achievement of planned elevations, grades, design width, entrance and exit channel slopes are critical to the successful operation of the emergency spillway and must be constructed within a tolerance of ± 0.2 feet. Side slopes of the emergency spillway should be a minimum of 3:1.

Vegetative Treatment. The embankment, borrow areas, emergency spillway and other disturbed areas shall be stabilized in accordance with the appropriate vegetative standards immediately following construction of the basin. Vegetative treatment shall be applied to these areas within 7 days after the site has been completed.

Erosion and Pollution Control. Construction operations will be carried out in such a manner that erosion and water pollution will be minimized. State and local laws concerning pollution abatement shall be complied with.

Safety. State and local requirements shall be met concerning fencing and signs warning the public of hazards of soft sediment and floodwater.

The Use of the WinDETPOND Program to Statistically Evaluate Wet Pond Performance

WinDETPOND was developed by Bob Pitt and John Voorhees to enable a continuous simulation of wet stormwater detention ponds (www.winslamm.com). This continuous simulation is important to understand the storm to storm variation and long-term performance for typical rain conditions. The basic analysis procedures in WinDETPOND are similar to the detention pond analysis procedures provided in WinSLAMM, the Source Loading and Management Model, but offers some additional model output choices to enable more detailed evaluations of individual detention facilities. Appendix 6-A is a user’s guide for WinDETPOND which also includes a simple design example. Additional assistance is provided in the Help components of the model.

WinDETPOND uses conventional procedures to predict hydraulic conditions (pond storage-indication routing) and the behavior of particulates in stormwater as it passes through a detention pond (surface overflow rates described by the Hazen equation and quiescent settling using Stoke’s and Newton’s laws), as described in previous discussions. WinDETPOND was specifically designed for continuous long-term evaluations, using lengthy rain series. In its current Windows configuration, it is limited only by computer resources (and available time) in the number of rains that it can evaluate. It is also currently quite fast, requiring only a few minutes on most computers to complete a single run using several decades of rainfall data. Whereas most computer-based pond models require time increment direction from the user and frequently crash due to unstable algorithms, DETPOND predicts reasonable calculation increments based on the duration of each rain and interevent period. If the calculation appears to approach unstable conditions, it automatically starts over with a reduced calculation increment. In addition, if the pond design is too small or if the outfall is inadequate, causing catastrophic overflow conditions, the program doesn’t crash, but continues using the last known outfall or surface area value, and notes that the pond overflowed. The tabular output of the model can also be easily imported into spreadsheets and graphing programs to produce statistical summaries of the pond performance.

WinDETPOND can be easily used to evaluate an existing design or pond under a wide variety of rain conditions. It can be used with a single event (most commonly used when observed influent hydrograph data is available) or with a lengthy rain series (when the program predicts runoff and hydrograph characteristics).

Example Pond Design for Construction Site Sediment Control

Table 6-6 shows the conditions for an area on a construction site that needs a sediment pond. The drainage area, 53 acres, is mostly active construction site, but some undeveloped land and paved areas also drain to the pond location. The pond therefore needs to be enlarged to accommodate the additional runoff from these areas. The table shows the percentage of the drainage area needed to be used a pond, along with the pond volume to obtain approximately 90% suspended solids reduction.

               

Table 6-6. Size of Pond for Construction Area

 

Area (acres)

% of area needed for pond surface

Pond surface area (acres)

Water quality volume (inches of runoff)

Pond volume (acre-inches)

Construction area

37

1.5%

0.56

0.6

22.2

Undeveloped area

14

0.5

0.07

0.3

4.2

Paved area

2

3.0

0.06

1.1

2.2

 

53

 

0.69

 

28.6

The total water quality volume (“live storage”) of the pond is 28.6 acre-inches, or 2.38 acre-ft. The surface of the pond between events (during dry weather) is 0.7 acres, or about 1.3% of this drainage area. The top area of the pond and associated side slopes are calculated based on various assumed pond depths, as shown in Appendix 6-A. In this example, a pond depth of 3 ft, and approximate side slopes of 12% and a top area of 0.9 acres are used. An additional 1 ft of storage to accommodate an emergency spillway is also provided, with a maximum top area needed of about 1 acre. The selection of the main discharge device is based on the water surface at the top of this water quality prism. A 12 inch vertical stand pipe, having its opening at the normal pond water surface level, seems to be a good choice, based on Table 6-4 data.

Three feet of standing water is needed above the maximum sediment depth in order to minimize scour. In addition, sacrificial sediment storage must also be provided in the pond. Using RUSLE, the total construction period sediment load to the pond can be estimated. For this example, it is assumed that the construction period is a full year, and the following conditions apply:

                R = 350

                LS = 4.95 (based on typical slope lengths of 600 ft at 10% slope)

                k = 0.28

                C = 0.25 (assuming that ¼ of the construction site area is being actively being worked, and the rest of the

                       area is effectively protected)

The calculated unit area erosion loss for this construction period is therefore about 243 tons per acre per year. Since the construction period is one year and the area is 37 acres, the total sediment loss is estimated to be about 4490 tons. For a loam soil, the sediment volume is about 4600 yd3.  The pond area at the bottom of the 3 ft of standing water is assumed to be about ½ acre, requiring about 2 ft of sediment storage. Therefore, the following lists the pond areas for each depth increment:

Pond depth (ft)

Pond area (acres)

0

0

1

0.35

2

0.50

3

0.57

4

0.63

5

0.70

6

0.77

7

0.73

8

0.90

9

0.97

This design was then entered into WinDETPOND and evaluated. Table 6-7 shows some of the program results for this pond. A series of rains ranging from 0.01 to 4.0 inches was also used. The maximum pond stage is estimated to be about 7.4 ft for the 4 inch rain, more than a half foot below the broad-crested weir emergency spillway. The peak reduction factor (the reduction of the influent peak flow rate at the outfall) is very large for the small events, as expected, and still remains about 0.5 for the largest event. This will help reduce erosive flows to the receiving waters. The “event flushing ratio” indicates the volume of runoff compared to the water volume in the pond before the event. Again, this value is very small for the small events and increases to greater than 1 for rains larger than about 3 inches. The last 2 columns indicate sedimentation performance of the pond. The flow-weighted particle size in the effluent is greater than 4 mm after 3 inches of rain. However, the expected percentage suspended solids control (assuming the “low” particle size distribution) remains greater than 80% for all rains less than about 2 inches. The worst case shown, for the 4 inch rain, drops down to less than 40% control.

Table 6-7. Summarized Results from WinDETPOND to Evaluate Detention Pond at Construction Site

DETPOND for Windows Version 8.4.1

(c) Copyright Robert Pitt and John Voorhees 1996

All Rights Reserved

Pond file name:  C:\PROGRAM FILES\WINDETPOND\EROSION CONTROL POND EXAMPLE.PND

Pond file description:  This is an example of an erosion control pond

Rain file name:  C:\Program Files\WinDetpond\BHAMSRCE.RAN

Date of run:  07-18-2002    Time of run:  22:59:47

Detention Pond Water Quality Performance Summary, by Event

Rain    Rain   Rain      Rain       Maximum    Event     Peak      Event     Flow-       % Part

Number  Depth  Duration  Intensity  Pond       Inflow    Reduction Flushing  weighted    Solids   

        (in)  (hrs)     (in/hr)     Stage      Volume    Factor    Ratio     Particle    Removed

                                    (ft)       (ac-ft)     (%)               Size(Ideal) (Ideal) 

  1    0.01    3.00      0.00       5.00       0.000      1.00      0.000     0.0         100.0 

  2    0.05    7.00      0.01       5.00       0.002      0.99      0.001     0.0         100.0 

  3    0.10    8.00      0.01       5.01       0.007      0.99      0.003     0.1          99.8 

  4    0.25   10.00      0.02       5.07       0.052      0.99      0.022     0.1          99.5 

  5    0.50   12.00      0.04       5.19       0.137      0.97      0.059     0.3          98.9  

  6    0.75   14.00      0.05       5.30       0.230      0.94      0.099     0.5          98.2  

  7    1.00   14.00      0.07       5.42       0.342      0.90      0.147     0.7          96.7   

  8    1.50   14.00      0.11       5.64       0.610      0.85      0.262     1.2          88.5  

  9    2.00   14.00      0.14       5.87       0.939      0.78      0.403     1.8          80.2 

 10    2.50   14.00      0.18       6.26       1.528      0.67      0.656     2.9          68.1 

 11    3.00   14.00      0.21       6.64       2.266      0.57      0.973     4.0          57.2  

 12    4.00   14.00      0.29       7.37       4.014      0.50      1.724     6.5          39.1  

As noted earlier in Modules 3 and 4, most of the erosion potential is associated with the numerous moderate (greater than 1 inch) and the few large rains (up to 4 inches) that likely occur during the year. This pond will likely provide 65 to 95+% control for the moderate rains, but will drop off significantly for the largest rains. It is possible to improve the performance of the pond by changing the outlet weir to a smaller capacity device which would provide additional retention for the larger events. Table 6-8 illustrates how this temporary pond would affect the annual particulate solids losses from this construction site. The overall pond performance is expected to be about 75% effective, much less than the initial goal of 90% control. The performance of this pond could be improved if the design was better optimized for the larger, more erosive events. This could be done by choosing a more restrictive outlet device at higher pond stages, for example.

Table 6-8.  Performance of Temporary Sediment Pond at Construction Site (Birmingham rains)

Rain range (inches)

Mid Point Rain (inches)

% of annual R in category

% particulate solids removed for pond

Weighted total annual particulate solids removal (%)

0.01 to 0.05

0.03

0.0

100

0

0.06 to 0.10

0.08

0.1

100

0.1

0.11 to 0.25

0.18

0.7

99.8

0.7

0.26 to 0.50

0.38

3.5

99.5

3.5

0.51 to 0.75

0.63

4.8

98.9

4.7

0.76 to 1.00

0.88

8.2

98.2

8.1

1.01 to 1.50

1.26

16.1

96.7

15.6

1.51 to 2.00

1.76

15.4

88.5

13.6

2.01 to 2.50

2.26

10.9

80.2

8.7

2.51 to 3.00

2.76

7.5

68.1

5.1

3.01 to 4.00

3.5

16.3

57.2

9.3

over 4.01

5.67

16.5

39.1

6.5

4583 events

41.5 years

100.0

 

75.9 % annual particulate solids removal

Example Design using Filter Fences for Construction Site Sediment Control

There are three aspects of filter fences that can be evaluated, as demonstrated in the following examples: 1) sediment capture behind the fence, 2) slowing water flowing down a slope, and 3) pressure forces on the fence from the water and resisting forces from the soil on the fence stakes. The first two aspects determine the erosion and sediment control benefits of filter fences, while the third aspect determines how filter fences may fail structurally.

Sediment Capture Behind Filter Fences

Relatively few field investigations have been conducted to examine the effectiveness of filter fences, and other controls, at construction sites. Important tests have been performed by Barrett, et al. (1995), Horner, et al. (1990),  Schueler and Lugbill (1990), and Smoot, et al. (1992). Caltrans is also currently conducting comprehensive tests of construction erosion controls and their results should become available soon.

Perhaps the most comprehensive study for filter fences was conducted by Barrett, et al. (1995) at Austin, TX, area highway construction sites, supplemented with controlled laboratory tests. Silt fences at six active highway construction sites were evaluated in terms of suspended solids and turbidity reduction. Two installations used non-woven fabrics, and four installations used woven fabrics. Manual grab sampling was used to obtain representative sediment samples of all size distributions during 10 rains. Uncontrolled discharges due to obvious filter fence failures (mostly undercutting flows or tears in the fabric) were excluded from sampling; only locations where the flows passed through the fabric were sampled. Samples were collected upslope of the pooled water behind the filter fence, in the pool backed up by the filter fence, and downstream of the filter fence. This sampling strategy was used to differentiate sedimentation from filtration effects, and to obtain an overall control efficiency. Because of highly variable concentrations above the pool, most of their data analysis relied on comparisons between the samples collected from the pool and the effluent from the filter fabric, reflecting filtering removal and not sedimentation.

The observed suspended solids removal rates were highly variable, ranging from -61 to 54%, with a median of 0%. Typical effluent suspended solids concentrations after the filter fence were 500 mg/L. Similar poor results were obtained for turbidity removals (-32 to 49% range, with a median removal of 2%). As indicated by the negative removal rates, the effluent from the fabric sometimes had greater suspended solids concentrations than were found in the pool. The removal of suspended solids due to sedimentation, however, was estimated to be about 50%, based on partial field observations. At one location where the lower portion of the fabric was clogged, a shallow upstream pool lasted for an extended period and removals of about 65% were measured.

The poor removal efficiency due to filtration was explained by comparing the particle sizes of the suspended solids and the apparent opening sizes of the fabrics (typically from 100 to 1,000 mm). Silt and clay-sized particles comprised the majority of the solids collected (68 to 100%, with a median of 96%) from the pond and below the filter fences. Any large particles present in the flowing waters were thought to have been settled in the pool before the fence. The diameters of the remaining particles passing through the fence were therefore smaller than the openings in the fabric and were able to pass through unhindered. Earlier work by Schueler and Lugbill (1990) in Maryland substantiated the small particles observed in Texas. During settling column studies on construction site runoff, Schueler and Lugbill found that 90% of the incoming sediment was smaller than 15 mm, with the largest particles observed being only 50 mm. During their sediment pond evaluation tests, however, they did observe sediment deltas forming near the influent location, indicating that sand-sized particles were transported to the sediment ponds and represented a minor portion of the total load. These larger particles were apparently not included in the grab samples as they form part of the bed load.

Barrett, et al. (1995) found that filter fence installations are not designed as hydraulic structures, with frequent failures caused by excessive runoff. Runoff around the ends of fences, and even over-topping of the fences was observed several times during their monitoring project. However, other downstream controls were in place to mitigate these failures. Besides failures caused by lack of design, they also observed deficiencies in performance that were caused by improper installation and maintenance, including:

                · inadequate filter fabric splicing

                · fence failure due to sustained over-topping

                · unrepaired holes in fabric

                · flow beneath fabric due to inadequate trenching of the bottoms of the fabric fences into the ground

Laboratory flume tests were also conducted on filter fabrics, enabling flow rates and suspended solids concentrations to be controlled at specific conditions. Austin silty clay, after passing through a 3 mm sieve, was used to make a test slurry. The median particle size in this mixture was 20 mm, and 30% was finer than 3 mm. The apparent openings in the filter fabrics tested ranged from 600 to 850 mm for 3 woven fabrics and 150 mm for the one non-woven fabric tested. During testing, the woven fabrics had median suspended solids removal rates of 68 to 87% (ranges of 46 to 97%), while the non-woven fabric had a median removal rate of 93% (range of 73 to 99%). The non-woven fabric also had the longest detention times during the tests due to its lowest flow rate. In comparison, a rock berm was also tested (having the highest flow rate and therefore shortest detention time) and had a median SS removal efficiency of 42% (36 to 49% range). The SS reductions in the testing flume was 34% without any controls in place due to sedimentation of the larger test particles while flowing over the rough bed. This high background reduction level therefore significantly reduces these reported flume test measurements. The corrected berm removal rate was only 7%, for example, after taking into consideration the background reductions. Similar reductions would have to be made for the filter fabric test results.

An interesting observation during the flume tests was that while the detention times increased with time since the start of the tests, due to partial clogging of the fabrics, the woven fabrics all had decreased detention times after being exposed to large rains. Apparently, the rains helped wash some of the caked-on mud from the fabrics. This was not observed for the non-woven fabrics where clogging was internal and more permanent. During recent tests on stormwater filtration, several filter types were tested by Clark and Pitt (1999). They found that all of the fabrics examined totally clogged after accumulating a layer of about 3 mm of clay. This clogging layer preferentially forms near the bottom of the fabric, usually indicating the depth of the ponding. This clogging significantly decreases the flow rates through the fabric, allowing extended detention and therefore increased performance.

Barrett, et al. (1995) concluded that the poor filtering performance of the filter fences in good condition was due to the small particles in comparison to the large fabric openings. Previously reported high filtration control efficiencies conducted during laboratory experiments were faulty due to the use of unrealistically large test particles. Median particles during field tests at construction sites indicate that almost all of the particles are as silts and clays. The relatively minor sand fractions are easily deposited during sheetflows, or in ponded areas. Sedimentation effectiveness was found to be highly dependent on the detention time in the ponded areas behind the filter fabrics. The detention time is controlled by the geometry of the upstream pond, hydraulic properties of the fabric, and maintenance of the filter fence. Holes in the fabric, under-cutting due to inadequate trenching of the bottom of the fabric, and overtopping or bypassing around the ends of filter fabric fences, all effectively decrease the detention time in the pond behind the fabrics and contribute to very low observed field performance of filter fabrics.

Example Calculation of Sediment Capture Behind Filter Fence

It is possible to calculate the expected level of control for a filter fence at a specific site using the upflow velocity concept presented earlier:


               

The performance of a filter fence can therefore be calculated by knowing the ratio of the discharge through the fence divided by the surface area of the ponded area. Both of these values are directly related to the depth of water detained behind the filter fence. This value can be easily calculated assuming an even slope uphill from the fence and using the manufacture’s value for unit area flow capacity. The ponded surface area increases directly with the water depth, depending on the slope. The total outfall rate also increases directly with the water depth. Therefore, the critical particles being trapped in the pond behind the filter fence is only dependent on the slope and fabric. Figure 6-12 is a plot of the particle size controlled, in mm, for different ground slopes (%) and filter fabric flow rates (ft/sec), using Stokes’ law for calculating the critical particle sizes associated with the upflow velocity:

               

                where:

                v= settling rate of particle, cm/sec   

                g = 981 cm/sec2

                k = kinematic viscosity = 0.01 cm2/sec

                spgr = specific gravity of particulate = 2.65

                d = particle diameter, cm

Particle sizes greater than about 500 mm would actually be larger than shown due to transition and turbulent settling for larger particle sizes. Figure 6-5 can be used to estimate the approximate suspended solids control corresponding to the critical particle size. For example, if the calculated critical particle size is 10 mm (such as for a 2% slope and a 0.02 ft/sec filter fabric flow rate), the expected SS control would be about 25 to 45% for the size distributions likely appropriate for construction site runoff. A 5% slope and 0.25 ft/sec flow rate would result in about a 60 mm critical particle size, and the SS control would only be about 5 to 15%. It is also possible to use WinDETPOND to calculate filter fence performance and to consider variable flow rates for different water depth, as shown in laboratory tests, and to consider the partial clogging near the bottom of the fabric. WinDETPOND can also be used to determine the likelihood of the filter fence being over-topped for different storm and site conditions.

                         

Figure 6-12. Filter fabric conditions and critical particle size controlled.

Filter Fences to Slow Water Flowing Down Critical Slopes

Filter fences intercepting sheetflows may also slow the water flowing down critical slopes. The upslope length of the ponded area will be obviously protected from rain impaction and by flowing water. This length can be estimated for different water depths impounded behind a filter fence. As an example, for a 5% slope and for a 1 ft water depth, the would extend uphill 20 ft. In addition, some of the downslope area beneath of filter fence (if not installed on the toe of the slope, as generally recommended), will also have reduced flow velocities, compared to the same slope without the filter fence. WinDETPOND can be used to calculate the reduction in flow rates for the flows entering the ponded area compared to the discharge water through a filter fence. Generally, non-woven filter fabrics have much lower flow rates compared to woven filter fabrics. The sheetflow calculation information in Module 4 can also be used to estimate the flow rates on slopes of different roughness and slopes. As an example, Figure 7-13 is a repeat of Figure 4-12 and indicates the sheetflow travel times for different slopes having a roughness value of 0.15, corresponding to relatively short grass. A slope of 10% that is 100 ft long would have a travel time of about 5 minutes, or a velocity of about 0.33 ft/sec. Of course, if a bare filter fabric without grass is on the slope, the sheetflow velocity would be much larger. There are non-woven fabrics that have flow rates appreciably less than this amount, so a filter fabric could result in critical slopes being exposed to reduced periods of high flows.

Figure 7-13. Sheetflow travel times for different slopes.

Pressure Force on Filter Fences

The pressure equation can be used to calculate the forces acting on filter fences. The  following calculation shows the resisting force needed for a 10 ft span of filter fence with 2 ft of standing water:

               

               

The momentum equation can be used when the flow rates should be considered:

               

               

However, the exit flow rate (V2) is usually assumed to be zero (the water seepage exiting the fence) while the flow rate through the fence (V1) is a very small value (0.0007 ft/sec for a typical filter fabric flow rate of 0.3 gal/ft2-min). These small velocities have little effect on the forces acting on the fence.

The forces acting on a filter fence can therefore be very large and the filter fence stake systems must be selected to withstand this force. In addition, the resisting forces of the soil also act on the fence stake to hold it upright, and also needs to be considered. Wet clayey soils may need long stakes driven deeply in the ground to resist this pressure.

Conclusions

This discussion has shown that the use of relatively simple design criteria can be used to provide excellent water quality benefits over a wide range of storm conditions. WinDETPOND can be used to evaluate a wide variety of pond designs and can be used to develop appropriate design guidelines for different climatic conditions.

Detention ponds are probably the most commonly used runoff quality control devices and have substantial literature documenting their performance and problems. Wet detention ponds have been shown to be very effective, if their surface area is large enough in comparison to the drainage area and expected runoff volume. Small wet ponds and all dry ponds have been shown to be much less effective. Care must also be taken to minimize safety and environmental hazards associated with ponds.

Wet detention ponds have been shown to be an extremely robust stormwater control practice. Even though their cost may be high, their level of pollutant reduction is also high, resulting in very cost-effective pollutant removals. Physical sedimentation is the main removal process occurring in wet ponds, resulting in much better removals of particulate bound pollutants than “filterable” forms of pollutants. Temporary sediment ponds at construction sites are most suitable where the area to be controlled is larger than about 10 acres (the typical upper limit for filter fencing). They have been found to be generally the most effective erosion control (after prevention).

 

References

Akeley, R.P. “Retention ponds for control of urban stormwater quality.” In Proceedings National Conference on Urban Erosion and Sediment Control: Institutions and Technology, EPA-905/9-80-002, Chicago, January 1980.

Barfield, B. “Analysis of the effects of existing and alternate design criteria on the performance of sediment detention ponds.” 1986 Sediment and Stormwater Conference, sponsored by the Maryland Water Resources Administration, Sediment and Stormwater Division, Salisbury State College, Maryland, July-August 1986.

Barrett, M.E., J.E. Kearney, J.F. Malina, R.J. Charbeneau, and G.H. Ward. An Evaluation of the Use and Effectiveness of Temporary Sediment Controls. Center for Research in Water Resources. Technical Report CRWR 261. The University of Texas at Austin. August 1995.

Bedner, R.E. and D.J. Fluke. Demonstration of Debris Basin Effectiveness in Sediment Control. Environmental Protection Agency, Industrial Environmental Research Laboratory - Cincinnati, EPA-600/7-80-154. Cincinnati, Ohio, 1980.

Bondurant, J.A., C.E. Brockway, and M.J. Brown. “Some aspects of sedimentation pond design.” Proceedings National Symposium on Urban Hydrology and Sediment Control, University of Kentucky, Lexington, 1975.

Chambers, G.M. and C.H. Tottle. Evaluation of Stormwater Impoundments in Winnipeg, Report SCAT-1, Environment Canada, Ottawa, April 1980.

Clark, S. and R. Pitt. Stormwater Treatment at Critical Areas, Vol. 3:  Evaluation of Filtration Media for Stormwater Treatment. U.S. Environmental Protection Agency, Water Supply and Water Resources Division, National Risk Management Research Laboratory. EPA/600/R-00/016, Cincinnati, Ohio. 442 pgs. October 1999.

Colston, N.V., Jr. Characterization and Treatment of Urban Land Runoff. EPA-670/2-74-096, U.S. Environmental Protection Agency, 1974.

Driscoll, E. D. “Detention and retention controls for urban stormwater.” Engineering Foundation Conference: Urban Runoff Quality - Impact and Quality Enhancement Technology, Henniker, New Hampshire, edited by B. Urbonas and L. A. Roesner, published by the American Society of Civil Engineers, New York, June 1986.

Eccher. C.J. “Thoughtful design is prime factor in water safety.” Lake Line. pp. 4-8. May 1991.

EPA (USA Environmental Protection Agency). Final Report for the Nationwide Urban Runoff Program. Water Planning Division, PB 84-185552, Washington, D.C., December 1983.

Gietz, R. J. Urban Runoff Treatment in the Kennedy-Burnett Settling Pond. For the Rideau River Stormwater Management Study, Pollution Control Division, Works Department, Regional Municipality of Ottawa-Carleton, Ottawa, Ontario, March 1983.

Grizzard, T. J., C. W. Randall, B. L. Weand, and K. L. Ellis. “Effectiveness of extended detention ponds.” Engineering Foundation Conference: Urban Runoff Quality - Impact and Quality Enhancement Technology, Henniker, New Hampshire, edited by B. Urbonas and L. A. Roesner, published by the American Society of Civil Engineers, New York, June 1986.

Harrington, B. W. Feasibility and Design of Wet Ponds to Achieve Water Quality Control. Sediment and Stormwater Division, Maryland Water Resources Administration, July 1986.

Hittman Assoc. Methods to Control Fine-Grained Sediments Resulting from Construction Activity. U.S. Environmental Protection Agency, Pb-279 092, Washington, D.C., December 1976.

Horner, R.R., J. Guedry, and M.H. Kortenhof. Improving the Cost Effectiveness of Highway Construction Site Erosion and Pollution Control. Washington State Transportation Center, Washington State Dept. of Transportation, Seattle, WA. 1990.

Jones, J. E. and D. E. Jones, “Interfacing considerations in urban detention ponding.” Proceedings of the Conference on Stormwater Detention Facilities, Planning, Design, Operation, and Maintenance, Henniker, New Hampshire, Edited by W. DeGroot, published by the American Society of Civil Engineers, New York, August 1982.                    

Kamedulski, G. E. and R. H. McCuen. “The effect of maintenance on storm water detention basin efficiency.” Water Resources Bulletin, Vol. 15, No. 4, pg. 1146, August 1979.

Linsley, R. K., and J. B. Franzini. Water Resources Engineering. McGraw-Hill, New York, 1964.

Marcy, S. J. and J. E. Flack. “Safety considerations in urban storm drainage design.” Second International Conference on Urban Storm Drainage, Urbana, Illinois, June 1981.

Martin, E.H. “Mixing and residence times of stormwater runoff in a detention system.” Engineering Foundation Conference: Design of Urban Runoff Quality Controls. Potosi, Missouri. July 10 - 15, 1988. Edited by L. A. Roesner, B. Urbonas and M.B. Sommen. pp. 164 - 179. Published by the American Society of Civil Engineers, New York, June 1989.

Mason, J.M.,Jr. “On-site stormwater detention: An overview.” Public Works, February 1982.

McCuen, R. H. A Guide to Hydrologic Analysis Using SCS Methods. Prentice-Hall, Englewood Cliffs, New Jersey, 1982.

Nelson, John. MSCE published thesis, Department of Civil and Environmental Engineering, UAB. Characterizing Erosion Processes and Sediment Yields on Construction Sites. 1996.

Pitt, R. “Runoff controls in Wisconsin’s Priority Watersheds.” Advanced Topics in Urban Runoff Research conference. Proceedings edited by B. Urbonas and L.A. Roesner. Engineering Foundation and ASCE, New York. pp. 290-313. 1986.

Pitt, R., and J. McLean. Toronto Area Watershed Management Strategy Study: Humber River Pilot Watershed Project. Ontario Ministry of the Environment, Toronto, Ontario, 1986.

Pitt, R. “Sediment Control in Alabama.” 41st Annual Transportation Conference. Montgomery, AL. February 1998.

Pitt, R. “Detention pond design for water quality improvement.” National ASCE Hydraulic Conference. San Francisco, California. July 1993a.

Pitt, R. “The stormwater quality detention pond model (DETPOND).” 26th Annual Water Resources Conference. University of Minnesota. October 1993b.

Poertner, H.G. Practices in Detention of Urban Stormwater Runoff. American Public Works Association, OWRR Contract No. 14-31-0001-3722, Chicago, 1974.

Randall, C. W. “Stormwater detention ponds for water quality control.” Proceedings of the Conference on Stormwater Detention Facilities, Planning, Design, Operation, and Maintenance, Henniker, New Hampshire, Edited by W. DeGroot, published by the American Society of Civil Engineers, New York, August 1982.                              

Schimmenti, F.G. “Stormwater detention basins must control more than runoff.” American City and County, December 1980.

Schueler, T. R. New Guidebook Review Paper No. 2, Wet Pond BMP Design. Metropolitan Washington Council of Governments, Washington, D.C., 1986.

Schueler, T.R., and J. Lugbill. Performance of Current Sediment Control Measures at Construction Sites. Metropolitan Washington Council of Governments, Sediment and Stormwater Administration of the Maryland Dept. of the Environment. 19990.

SCS (now NRCS) (U.S. Soil Conservation Service). Computer Program for Project Formulation, Hydrology. Technical Release Number 20 (TR-20). U.S. Dept. of Agriculture, 1982.

Smith, W. G. “Water quality enhancement through stormwater detention.” Proceedings of the Conference on Stormwater Detention Facilities, Planning, Design, Operation, and Maintenance, Henniker, New Hampshire, Edited by W. DeGroot, published by the American Society of Civil Engineers, New York, August 1982.                             

Smoot, J.T., T.D. Moore, J.H. Deatherage, and B.A. Tschantz. Reducing Nonpoint Source Water Pollution by Preventing Soil Erosion and Controlling Sediment on Construction Sites. Transportation Center of Tennessee. 1992/

Terstriep, M.L., G.M. Bender, D.C. Noel. Nationwide Urban Runoff Project, Champaign, Illinois: Evaluation of the Effectiveness of Municipal Street Sweeping in the Control of Urban Storm Runoff Pollution. Contract No. 1-5-39600, U.S. Environmental Protection Agency, Illinois Environmental Protection Agency, and the State Water Survey Division, University of Illinois, December 1982.

Vignoles M., and L. Herremans. “Metal pollution of sediments contained in runoff water in the Toulouse city.” (in French). Novatech 95, 2nd International Conference on Innovative Technologies in Urban Storm Drainage. May 30 - June 1, 1995. Lyon, France. pp. 611-614. Organized by Eurydice 92 and Graie. 1995.

Whipple, W. and J. V. Hunter. “Settleability of urban runoff pollution.” Journal WPCF, Vol. 53, No. 12, pg. 1726, 1981.

Wiegand, C., T. Schueler, W. Chittenden, and D. Jellick. “Comparative costs and cost effectiveness of urban best management practices.” Engineering Foundation Conference: Urban Runoff Quality - Impact and Quality Enhancement Technology, Henniker, New Hampshire, edited by B. Urbonas and L. A. Roesner, published by the American Society of Civil Engineers, New York, June 1986.                         


Appendix 6-A: User Guide for WinDETPOND

The following example shows the initial steps in designing a wet detention pond and the development of a WinDETPOND file for that pond in order to enable water quality evaluations. The pond sizing criteria can be examined in relation to site constraints and the pond design modified, if needed, based on these evaluations.

Example Design Calculations and Evaluation Using WinDETPOND

The following discussion presents a calculation example using the design criteria presented earlier:

                · Assume a medium density residential area of 150 acres with a goal of approximately 90% suspended solids control (corresponding to 5mm critical particle size).

                · The wet pond surface would therefore be: 0.008(150 acres) = 1.2 acres

                · The runoff volume for 1.25” rain => 0.5" runoff (based on typical development conditions and small storm hydrology; CN= 90 and Rv= 0.4).

                · Therefore, wet storage volume: 0.5"(150 acres) => 6.3 acre-feet

                · The depth associated with the wet storage volume can be estimated assuming a prismatic cross-section (simplified, compared to a conical section):


 

Approximately:  [1.2 + x(1.2)]y/2 = 6.3 acre-ft.

                               

                                                re-arranging gives:   x =[(10.5)/y] - 1

The following table can be used to give simultaneous depths for different x multipliers and top of pond areas for the “live-storage” area of the pond (the section affected by the primary water quality outlet device and located on top of the permanent pool depth, and below the invert of the emergency spillway and additional storage needed for flood control):

                                                y (depth, ft)           x (multiplier)                          top area

                                                                2                              4.3                           4.3 (1.2 acres) = 5.2 acres

                                                                3                              2.5                           3.0  acres

                                                                4                              1.6                           1.9 acres

                                                                5                              1.1                           1.3 acres

Depths less than 2 feet are too shallow and could require very large pond top surface areas for this example. “Live depths” greater than 5 feet may be too deep for most locations and obviously result in very steep side slopes for this example.

The following table summarizes the calculations for the side slopes of the pond (assuming a simple circular shaped pond, as shown below):


 


                r = (A/p)1/2   =  [1.2acres(43,560 ft2 per acre)/p)]1/2 = 130 ft

                Depth                     Top Area               Top Radius                           Slope Length                        Side Slope

                (ft)                           (acres)                    (ft)                                           (ft)                                          

                2                              5.2                           270                                          270 - 130 = 140                      2/140 = 1.4%

                3                              3.0                           200                                          200 - 130 = 70                        3/70 = 4.3%

                4                              1.9                           160                                          160 - 130 = 30                        4/30 = 13%

                5                              1.3                           135                                          135 - 130 = 5                          5/5 = 100%

                · The preliminary pond cross-section is therefore:

 

                · The outfall device is selected by comparing the maximum allowable discharge rate for the surface area of the pond at several pond depth increments. These maximum allowable discharges are compared with weir ratings (as tabulated in the text, for example) to select the permissible weirs that can be used:

                                                Qout = vA

                                                v = 1.3 X 10-4 ft/sec for 5 mm particle

                                Stage                                                      Pond Area                             Maximum

                                (above normal                                       (acres)                                    Allowable Discharge (cfs)

                                water surface, ft)

                                                0                                              1.2                                           6.8

                                                0.5                                           1.5                                           8.5

                                                1                                              1.8                                           10

                                                1.5                                           2.1                                           12

                                                2                                              2.4                                           14

                                                3                                              3.0                                           17 (usually most critical)

                Therefore, use a single 45o V-notch weir, or two 22-1/2o V-notch weirs.

                · Select emergency spillway (mandatory) and additional flood control storage volume (if necessary) using NRCS TR-55 (SCS 1986) procedures.

                · Figure A-1 is an example program check sheet for a WinDETPOND model evaluation, while the next section shows how this information is entered into a data file for analysis.


Figure A-1a. WinDETPOND model check sheet for example calculation.

Figure A-1b. WinDETPOND model check sheet for example calculation.


Steps in Entering Data for Evaluation in WinDETPOND

Enter the main WinDETPOND program by double-clicking on the WinDetpond.exe file located in the directory where the program was installed, or select the file from the “start, programs, WinDetpond” list. The following window will open:

 Select the “continue” button to open the following window:

Notice that the status for each of the four main categories are listed as “incomplete.” The next steps in creating the file include entering this data. The first step for this window is to select the file name “edit” box and entering a file name, as shown below:

After the file name is typed in, click on the save button, after ensuring that the correct directory is listed. The next step under “file name information” is to enter a site description. Any short statement can be entered that will enable tracking the files or the site test conditions. The last part of this element is selecting the particle size file, as shown below:

All available particle size files are listed. If the desired file is not listed, check the directory to ensure that the correct directory is shown. When the desired file is selected, click “OK.” 

The next major category of information is the stage-area values. When that “edit” box is selected, the following window is displayed:

The first information to be entered is the initial stage elevation. This is the water depth in the pond at the beginning of the study period. It is generally the normal water elevation (above the pond bottom datum). However, it can be different reflecting actual conditions (such as being lower than the lowest invert because of evaporation that may have occurred during an extended dry period, or higher because the pond has not completely drained since the preceding rain). When that number is entered, the program automatically starts requesting stage and surface area data. The bottom-most stage (at depth zero) is already entered (required to have a surface area of zero acres). When all of the stage-area data is entered, select continue, or change the user defined pond efficiency factor first. The sequence is displayed in the following window:

The “User Defined Pond Efficiency Factor, n” is given as 5, but can be changed by over-typing. This is the n factor used in the Hazen equation and is equivalent to the number of pond cells. Large numbers imply very little short-circuiting, while small numbers imply that substantial numbers of large particles may be leaving the pond.

The next major data requirement group is the outlet information. Select “edit” to bring up the following window (this one has the rectangular weir already listed, normally, this would be empty and the user would select the desired outlet):

When the rectangular weir is selected, the following window is brought up to enable the user to describe the weir dimensions and location:

The user needs to refer to the diagram (on Figure A-1) to ensure that the weir heights are correct. The program also checks to make sure that the sum of the “height of bottom of weir opening to top of weir” plus the “height from datum to bottom of weir opening” adds up to equal the total depth of the pond entered previously. After entering the data and clicking on “continue”, the user selects the V-notch weir for this example, bringing up the following window:

The user selects the v-notch weir angle and the height data, and then clicks “continue.”

The next data requirement set relates to the rain file. A rainfall series is selected from the available list, and the starting and ending dates contained in the file are automatically listed. If these dates are not correct, they can be edited by selecting the “edit” button near each date, as shown in the following window, and typing in the desired dates:

If a user-defined hydrograph is to be evaluated (such as for entering a single design storm calculated using TR-55, for example, or to enter actual observed inflow rates), then the “single event” type of rainfall data is selected and the program prompts for that information.

The last series of data requirements is the drainage basin information, as shown in the following window:

 

In our example, the “combined surface characteristics” is selected, which uses the correct runoff characteristics associated with small and intermediate-sized events. The area associated with each surface category is entered, and then the “continue” button is clicked. The “SCS Curve Number Procedure” simply uses a constant curve number for each event, but still uses the basic triangular hydrograph (and not the TR-55 tabular hydrograph, which is not accurate for these smaller rains). The WinSLAMM data file option allows more resolution in describing the surface areas, and is especially helpful if the same file is being used for a SLAMM analysis, but the greater detail in WinDETPOND is desired for an outfall wet detention pond. When these data are entered, the main screen shows that the status of each data requirement category is “complete.” The file needs to be saved again, as shown in the following window:

The file name is verified by clicking on “OK” in the following dialog box:

Finally, the large “calculate” button is clicked and after a few seconds, the program is completed. The file viewer is then clicked and the output file is selected. The following window then appears:

This example shows the default file output format, or one line per event. The “file, output” drop down menu offers several other options. The file is automatically saved as a comma separated value (CSV) file that can be directly opened with a spreadsheet program. In addition, the input file can also be saved to a file that can be opened in a spreadsheet for examination. The input file for this example is shown as Table A-1, while the output file (after adding some column statistics in Excel) is shown in Table A-2. It is also possible to plot these data from within the spreadsheet, or in any graphing program.


Table A-1. Input File Associated with Example Problem

Pond file name:  G:\WDP71\CLASSEXP.PND

Pond file description:  This is an example of the design procedure

Particle Size file name:  G:\WDP71\MEDIUM.CPZ

Output Format Option:  Water Quality Summary:  One Line per Event

Output device:  Print Output to File (extension .DPO)

Date:  02-17-2000

Drainage Basin Runoff Procedure:

         Combined Surface Characteristics

         1.  All directly connected impervious areas (acres):   45

         2.  All pervious areas (acres):   75

         3.  All impervious areas draining to pervious areas (acres):   30

Outlet Characteristics:

    Outlet number 1

      Outlet type:  V - Notch Weir

           1.  Weir angle (degrees):  45

           2.  Weir height from invert:   4

           3.  Invert elevation above datum (ft):   3

Outlet Characteristics:

    Outlet number 2

      Outlet type:  Rectangular Weir

           1.  Weir length (ft):   20

           2.  Weir height from invert:   1

           3.  Invert elevation above datum (ft):   6

Initial stage elevation (ft):   3

User defined pond efficiency factor (n):   5

Pond Stage, Surface Area, and Stage-related Outfall Devices (if applicable)

Entry    Stage     Pond Area     Natural Seepage    Other Outflow

Number    (ft)      (acres)          (in/hr)             (cfs)

   0      0.00       0.0000             0.00               0.00

   1      0.50       0.1000             0.00               0.00

   2      1.00       0.1300             0.00               0.00

   3      1.50       0.1700             0.00               0.00

   4      2.00       0.2000             0.00               0.00

   5      2.50       0.9000             0.00               0.00

   6      3.00       1.2000             0.00               0.00

   7      3.50       1.5000             0.00               0.00

   8      4.00       1.8000             0.00               0.00

   9      4.50       2.1000             0.00               0.00

  10      5.00       2.4000             0.00               0.00

  11      5.50       2.7000             0.00               0.00

  12      6.00       3.0000             0.00               0.00

  13      6.50       3.3000             0.00               0.00

  14      7.00       3.6000             0.00               0.00

Rain Information

Rain file name:  G:\wdp71\BHAM5290.RAN

         Rain starting date : 01/01/76

         Rain ending date : 12/31/76

Table A-2. Output Data for Example Analysis (one-line per event)

DETPOND for Windows Version 7.1.6

                           

© Copyright Robert Pitt and John Voorhees 1996

                     

All Rights Reserved

                             
                                   

Pond file name: G:\wdp71\classexp.pnd

                       

Pond file description: this is an example of the design procedure

                     

Rain file name: G:\wdp71\bham5290.ran

                       

Model Run Start Date: 01/01/76    Model Run End Date: 12/31/76

                   

Date of run: 02-17-2000    Time of run: 18:43:18

                       
                                   

Detention Pond Water Quality Performance Summary, by Event

                   

Rain Number

Rain Date

Rain Depth (in)

Time (Julian days)

Rain Dur. (hrs)

Intrevnt Dur. (days)

Rain Intensity (in/hr)

Maximum Pond Stage (ft)

Minimum Pond Stage (ft)

Event Inflow Volume (ac-ft)

Event Hydr Outflow (ac-ft)

Event Infil Outflow (ac-ft)

Event Evap Outflow (ac-ft)

Event Total Outflow (ac-ft)

Flow-weighted Particle Size

Approx. Part Res Control (%)

Peak Reduction Factor

Event Flushing Ratio

2,641

½/76

0.46

8765.8

9

3.03

0.05

4.14

3.00

2.24

1.948

0

0

1.948

1.1

97.7

0.72

2.074

2,642

1/7/76

0.58

8770.2

9

2.73

0.06

4.42

3.23

2.931

2.906

0

0

2.906

1.5

96.2

0.63

2.714

2,643

1/11/76

0.25

8774.3

5

0.88

0.05

3.85

3.25

1.089

0.879

0

0

0.879

0.6

99.3

0.84

1.008

2,644

1/13/76

0.03

8775.9

2

0.07

0.01

3.39

3.36

0.017

0.068

0

0

0.068

0.2

99.8

0.39

0.015

2,645

1/13/76

0.01

8776.3

1

0.22

0.01

3.36

3.32

0.002

0.052

0

0

0.052

0.1

99.9

N/A

0.002

2,646

1/13/76

0.38

8776.7

2

6.24

0.19

4.34

3.17

1.939

2.122

0

0

2.122

1.7

95.3

0.89

1.795

2,647

1/20/76

0.05

8783.2

5

3.33

0.01

3.2

3.13

0.046

0.09

0

0

0.09

0

100

0.91

0.043

2,648

1/24/76

0.03

8787.3

2

0.78

0.01

3.14

3.13

0.017

0.016

0

0

0.016

0

100

0.95

0.015

2,649

1/25/76

2.33

8788.4

20

8.33

0.12

5.64

3.12

14.977

14.99

0

0

14.99

3.3

88.8

0.22

13.868

2,650

2/5/76

0.51

8799.7

9

4.23

0.06

4.27

3.12

2.523

2.427

0

0

2.427

1.3

96.9

0.68

2.336

2,651

2/11/76

0.01

8805.3

1

6.6

0.01

3.19

3.11

0.002

0.112

0

0

0.112

0

100

0.54

0.002

2,652

2/18/76

0.67

8812

8

2.22

0.08

4.61

3.11

3.678

3.444

0

0

3.444

1.7

95.3

0.63

3.405

2,653

2/21/76

0.61

8815.5

3

12.59

0.2

4.79

3.10

3.318

3.511

0

0

3.511

2.2

93.1

0.8

3.072

2,654

3/5/76

0.85

8828.5

23

0

0.04

4.47

3.10

4.801

4.465

0

0

4.465

1.7

95.2

0.36

4.445

2,655

3/8/76

1.11

8831.7

17

0.91

0.07

4.85

3.31

6.224

6.283

0

0

6.283

2.2

93.3

0.36

5.763

2,656

3/12/76

0.3

8835.1

5

0

0.06

4.01

3.31

1.366

0.642

0

0

0.642

1.2

97.6

0.81

1.265

2,657

3/12/76

1.18

8835.6

4

1.82

0.29

5.77

3.37

6.892

7.52

0

0

7.52

3.2

89.3

0.62

6.382

2,658

3/15/76

3.64

8838

27

1.24

0.13

6.02

3.24

25.13

25.319

0

0

25.319

3.8

86.7

0.12

23.268

2,659

3/20/76

0.04

8843.3

2

0.2

0.02

3.26

3.24

0.029

0.031

0

0

0.031

0.1

99.9

0.88

0.027

2,660

3/20/76

1.14

8843.8

6

2.93

0.19

5.4

3.24

6.616

6.576

0

0

6.576

2.8

90.8

0.58

6.126

2,661

3/24/76

0.04

8847.7

6

0.81

0.01

3.27

3.21

0.029

0.102

0

0

0.102

0.1

99.9

0.6

0.027

2,662

3/26/76

1.56

8849.4

17

0.62

0.09

5.22

3.21

9.111

8.928

0

0

8.928

2.7

91.1

0.31

8.436

2,663

3/29/76

2.2

8852.5

12

0

0.18

5.93

3.35

13.098

11.551

0

0

11.551

3.8

86.6

0.33

12.128

2,664

3/30/76

2.09

8853.4

22

8.99

0.09

5.44

3.11

12.864

14.718

0

0

14.718

3

89.8

0.2

11.911

2,665

4/11/76

0.21

8865.7

5

1.42

0.04

3.67

3.11

0.878

0.618

0

0

0.618

0.4

99.6

0.89

0.813

2,666

4/13/76

0.05

8867.9

7

9.74

0.01

3.32

3.1

0.046

0.32

0

0

0.32

0.1

99.9

0.56

0.043

2,667

4/24/76

0.84

8878.7

9

3.9

0.09

4.78

3.11

4.528

4.388

0

0

4.388

2

94.1

0.58

4.192

2,668

4/30/76

0.09

8883.9

8

0

0.01

3.31

3.21

0.165

0.055

0

0

0.055

0.1

99.9

0.87

0.153

2,669

4/30/76

0.94

8884.6

11

4.31

0.09

4.88

3.19

5.245

5.374

0

0

5.374

2.2

93.3

0.48

4.856

Table A-2. Output Data for Example Analysis (one-line per event) (cont.)

Rain Number

Rain Date

Rain Depth (in)

Time (Julian days)

Rain Dur. (hrs)

Intrevnt Dur. (days)

Rain Intensity (in/hr)

Maximum Pond Stage (ft)

Minimum Pond Stage (ft)

Event Inflow Volume (ac-ft)

Event Hydr Outflow (ac-ft)

Event Infil Outflow (ac-ft)

Event Evap Outflow (ac-ft)

Event Total Outflow (ac-ft)

Flow-weighted Particle Size

Approx. Part Res Control (%)

Peak Reduction Factor

Event Flushing Ratio

2,670

5/6/76

1.71

8890.5

15

0

0.11

5.44

3.19

10.482

8.863

0

0

8.863

3.3

88.8

0.32

9.705

2,671

5/7/76

0.03

8891.5

2

0.07

0.01

4.19

3.8

0.017

0.723

0

0

0.723

1.5

96.3

N/A

0.015

2,672

5/8/76

0.3

8891.9

8

1.34

0.04

4.17

3.34

1.386

2.109

0

0

2.109

1.1

97.6

0.56

1.283

2,673

5/10/76

0.06

8894.5

2

0.03

0.03

3.37

3.33

0.067

0.052

0

0

0.052

0.2

99.8

0.87

0.062

2,674

5/10/76

0.2

8894.8

6

1.68

0.03

3.78

3.29

0.832

0.905

0

0

0.905

0.5

99.5

0.8

0.77

2,675

5/13/76

3.83

8897.4

34

0

0.11

5.86

3.3

26.954

25.826

0

0

25.826

3.8

86.8

0.11

24.958

2,676

5/15/76

0.01

8899.4

1

0.68

0.01

4

3.54

0.002

0.784

0

0

0.784

1.2

97.8

N/A

0.002

2,677

5/16/76

0.07

8900.2

2

6.24

0.04

3.57

3.15

0.092

0.633

0

0

0.633

0.3

99.7

0.73

0.085

2,678

5/22/76

2.33

8906.8

25

0.21

0.09

5.47

3.15

15.033

14.822

0

0

14.822

3.1

89.5

0.19

13.919

2,679

5/26/76

0.02

8910.7

4

0.15

0

3.31

3.26

0.007

0.068

0

0

0.068

0.1

99.9

N/A

0.007

2,680

5/27/76

0.02

8911.5

1

0.43

0.02

3.27

3.24

0.007

0.039

0

0

0.039

0.1

99.9

0.74

0.007

2,681

5/28/76

0.23

8912

8

0

0.03

3.77

3.24

0.994

0.522

0

0

0.522

0.7

99.3

0.79

0.92

2,682

5/28/76

0.05

8912.9

3

3.05

0.02

3.56

3.22

0.046

0.548

0

0

0.548

0.3

99.7

0.2

0.043

2,683

6/1/76

0.48

8916.4

10

15.5

0.05

4.26

3.08

2.488

2.655

0

0

2.655

1.3

96.8

0.63

2.304

2,684

6/18/76

0.03

8933.4

1

0.6

0.03

3.1

3.08

0.017

0.005

0

0

0.005

0

100

0.99

0.016

2,685

6/19/76

1.78

8934.1

24

7.4

0.07

5.15

3.1

10.778

10.74

0

0

10.74

2.7

91.1

0.23

9.98

2,686

6/30/76

0.46

8945.1

3

3.63

0.15

4.4

3.13

2.414

2.256

0

0

2.256

1.6

95.5

0.85

2.235

2,687

7/4/76

1.17

8949.2

14

7.19

0.08

5

3.14

6.626

6.751

0

0

6.751

2.4

92.4

0.4

6.136

2,688

7/13/76

0.26

8958.5

1

2.89

0.26

3.88

3.14

1.163

0.99

0

0

0.99

0.9

98.9

0.97

1.077

2,689

7/16/76

0.03

8961.5

1

4.81

0.03

3.27

3.14

0.017

0.175

0

0

0.175

0.1

99.9

0.88

0.016

2,690

7/21/76

0.09

8966.5

1

1.89

0.09

3.26

3.14

0.164

0.1

0

0

0.1

0.1

99.9

0.99

0.152

2,691

7/23/76

0.26

8968.5

1

3.81

0.26

3.92

3.19

1.163

1.109

0

0

1.109

1

98.5

0.96

1.077

2,692

7/27/76

0.91

8972.5

2

0.07

0.46

5.43

3.23

5.207

3.302

0

0

3.302

3.2

89.1

0.82

4.821

2,693

7/27/76

0.1

8972.9

1

0.31

0.1

4.37

3.83

0.216

1.182

0

0

1.182

1.9

94.4

0.48

0.2

2,694

7/28/76

1.63

8973.3

6

0.35

0.27

6.06

3.69

9.856

10.094

0

0

10.094

3.6

87.5

0.46

9.126

2,695

7/29/76

0.17

8974.6

3

0.18

0.06

3.94

3.63

0.615

0.702

0

0

0.702

1

98.5

0.78

0.569

2,696

7/30/76

0.23

8975.2

3

0.76

0.08

4.06

3.49

0.947

1.173

0

0

1.173

1.1

97.8

0.81

0.877

2,697

7/31/76

0.07

8976.4

1

6.02

0.07

3.54

3.15

0.091

0.556

0

0

0.556

0.3

99.7

0.88

0.085

2,698

8/6/76

0.3

8982.6

2

0.57

0.15

3.99

3.16

1.392

0.826

0

0

0.826

1.1

98.1

0.93

1.289

2,699

8/7/76

0.54

8983.5

1

7.93

0.54

4.89

3.14

2.849

3.31

0

0

3.31

2.6

91.6

0.91

2.638

2,700

8/15/76

0.06

8991.5

3

0.47

0.02

3.19

3.14

0.066

0.027

0

0

0.027

0

100

0.96

0.061

2,701

8/16/76

0.93

8992.5

3

7.63

0.31

5.34

3.15

5.323

5.297

0

0

5.297

2.9

90.2

0.76

4.929

2,702

8/24/76

0.86

9000.5

11

1.23

0.08

4.76

3.15

4.763

4.502

0

0

4.502

1.9

94.3

0.52

4.41

2,703

8/27/76

0.34

9003.4

6

0

0.06

4.11

3.34

1.621

0.891

0

0

0.891

1.4

96.8

0.76

1.5

2,704

8/28/76

0.11

9004

4

0

0.03

3.84

3.69

0.28

0.471

0

0

0.471

0.9

99

0.52

0.259

2,705

8/28/76

0.17

9004.4

2

0.87

0.09

3.97

3.47

0.599

0.947

0

0

0.947

1

98.4

0.84

0.554

2,706

8/29/76

0.03

9005.6

1

2.47

0.03

3.47

3.24

0.017

0.351

0

0

0.351

0.2

99.8

0.53

0.016

2,707

9/1/76

1.41

9008.2

10

0.71

0.14

5.44

3.24

8.393

8.109

0

0

8.109

2.9

90.5

0.43

7.771

2,708

9/3/76

0.25

9010.4

7

0

0.04

3.92

3.44

1.097

0.763

0

0

0.763

1

98.6

0.73

1.016

2,709

9/4/76

0.05

9011.2

7

0

0.01

3.65

3.43

0.046

0.383

0

0

0.383

0.4

99.6

N/A

0.043

2,710

9/5/76

0.44

9012

14

0

0.03

4.16

3.43

2.195

2.054

0

0

2.054

1.3

97

0.53

2.032

2,711

9/6/76

0.04

9013.6

1

0.64

0.04

3.54

3.39

0.03

0.235

0

0

0.235

0.3

99.7

0.64

0.028

2,712

9/7/76

0.11

9014.4

2

2.2

0.05

3.55

3.26

0.278

0.463

0

0

0.463

0.3

99.7

0.92

0.257

Table A-2. Output Data for Example Analysis (one-line per event) (cont.)

Rain Number

Rain Date

Rain Depth (in)

Time (Julian days)

Rain Dur. (hrs)

Intrevnt Dur. (days)

Rain Intensity (in/hr)

Maximum Pond Stage (ft)

Minimum Pond Stage (ft)

Event Inflow Volume (ac-ft)

Event Hydr Outflow (ac-ft)

Event Infil Outflow (ac-ft)

Event Evap Outflow (ac-ft)

Event Total Outflow (ac-ft)

Flow-weighted Particle Size

Approx. Part Res Control (%)

Peak Reduction Factor

Event Flushing Ratio

2,713

9/10/76

0.01

9016.9

1

10.89

0.01

3.26

3.09

0.002

0.217

0

0

0.217

0.1

99.9

0.05

0.002

2,714

9/21/76

0.06

9028

2

5.16

0.03

3.15

3.09

0.067

0.06

0

0

0.06

0

100

0.99

0.062

2,715

9/26/76

0.12

9033.4

2

0.45

0.06

3.35

3.1

0.345

0.085

0

0

0.085

0.1

99.9

0.98

0.319

2,716

9/27/76

0.03

9034.2

1

1.43

0.03

3.3

3.22

0.017

0.115

0

0

0.115

0.1

99.9

0.84

0.016

2,717

9/28/76

2.39

9035.8

16

4.93

0.15

5.85

3.17

15.04

15.111

0

0

15.111

3.5

88

0.26

13.926

2,718

10/6/76

0.04

9043.1

2

0.16

0.02

3.19

3.17

0.029

0.014

0

0

0.014

0

100

0.95

0.027

2,719

10/6/76

0.01

9043.5

1

1.35

0.01

3.18

3.15

0.002

0.036

0

0

0.036

0

100

0.6

0.002

2,720

10/8/76

0.01

9045

1

0.39

0.01

3.15

3.15

0.002

0.01

0

0

0.01

0

100

0.73

0.002

2,721

10/8/76

0.15

9045.6

5

7.33

0.03

3.48

3.13

0.506

0.526

0

0

0.526

0.2

99.8

0.92

0.469

2,722

10/16/76

0.05

9053.7

6

2.47

0.01

3.16

3.12

0.046

0.054

0

0

0.054

0

100

0.93

0.043

2,723

10/20/76

0.15

9057

2

4.66

0.08

3.47

3.12

0.491

0.428

0

0

0.428

0.2

99.8

0.97

0.455

2,724

10/25/76

0.64

9062

14

2.86

0.05

4.39

3.17

3.35

3.286

0

0

3.286

1.5

96.1

0.52

3.102

2,725

10/30/76

0.54

9067

11

10.77

0.05

4.32

3.11

2.762

2.901

0

0

2.901

1.4

96.4

0.6

2.557

2,726

11/11/76

0.23

9079.4

13

0.8

0.02

3.66

3.11

0.996

0.751

0

0

0.751

0.4

99.6

0.76

0.922

2,727

11/14/76

0.91

9082.1

19

3.28

0.05

4.62

3.19

5.072

5.205

0

0

5.205

1.8

94.6

0.37

4.696

2,728

11/20/76

0.22

9088.3

7

4.95

0.03

3.73

3.17

0.938

0.965

0

0

0.965

0.5

99.5

0.83

0.868

2,729

11/26/76

0.12

9094.3

9

0

0.01

3.38

3.17

0.332

0.145

0

0

0.145

0.1

99.9

0.88

0.307

2,730

11/27/76

0.02

9095.4

2

0.24

0.01

3.31

3.28

0.007

0.052

0

0

0.052

0.1

99.9

0.27

0.007

2,731

11/28/76

0.73

9096

22

5.12

0.03

4.37

3.15

3.941

4.109

0

0

4.109

1.5

95.9

0.38

3.649

2,732

12/6/76

0.59

9104.4

19

1.86

0.03

4.23

3.15

3.089

2.979

0

0

2.979

1.3

96.9

0.47

2.86

2,733

12/11/76

1.09

9109.1

38

0

0.03

4.45

3.23

6.291

6.124

0

0

6.124

1.8

95

0.22

5.825

2,734

12/14/76

0.25

9112.8

5

4.33

0.05

3.91

3.19

1.089

1.304

0

0

1.304

0.8

99

0.81

1.008

2,735

12/20/76

0.87

9117.9

9

3.94

0.1

4.84

3.2

4.703

4.685

0

0

4.685

2.1

93.7

0.56

4.354

2,736

12/25/76

1.35

9123.2

13

3.3

0.1

5.22

3.21

7.948

7.934

0

0

7.934

2.7

91.3

0.39

7.359

2,737

12/30/76

0.01

9128.5

1

0.18

0.01

3.21

3.21

0.002

0.014

0

0

0.014

0

100

0.39

0.002

2,738

12/30/76

0.19

9128.8

7

1.99

0.03

3.65

3.21

0.765

0.696

0

0

0.696

0.3

99.7

0.84

0.708

                                   
                                   
   

Rain Depth (in)

Rain Dur. (hrs)

Intrevnt Dur. (days)

Rain Intensity (in/hr)

Maximum Pond Stage (ft)

Minimum Pond Stage (ft)

Event Inflow Volume (ac-ft)

Event Hydr Outflow (ac-ft)

Event Infil Outflow (ac-ft)

Event Evap Outflow (ac-ft)

Event Total Outflow (ac-ft)

Flow-weighted Particle Size

Approx. Part Res Control (%)

Peak Reduction Factor

Event Flushing Ratio

 

minimum:

3.83

 

38.00

15.50

0.54

6.06

3.83

26.95

25.83

0.00

0.00

25.83

3.80

100.00

0.99

24.96

 

maximum:

0.01

 

1.00

0.00

0.00

3.10

3.00

0.00

0.01

0.00

0.00

0.01

0.00

86.60

0.05

0.00

 

st dev:

0.76

 

7.71

3.22

0.09

0.83

0.15

5.03

4.95

0.00

0.00

4.95

1.15

4.04

0.25

4.66

 

average:

0.56

 

7.57

2.70

0.08

4.13

3.23

3.21

3.20

0.00

0.00

3.20

1.22

96.55

0.64

2.97

 

COV

1.35

 

1.02

1.19

1.18

0.20

0.05

1.57

1.55

na

na

1.55

0.94

0.04

0.40

1.57

 

median:

0.24

 

5.00

1.39

0.05

3.96

3.19

1.04

0.90

0.00

0.00

0.90

1.00

98.45

0.68

0.97

 

total:

55.15

 

742

265

     

314

314

0.00

0.00

314

     

291

 

number:

98