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 (Q_in) will be greater than the outflow rate (Q_out) 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                     Q_out = 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).

and 

                where t = detention (residence) time. With

                and substituting:

but

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

where     y_o = initial quantity of solids having settling velocity of v_o

y = quantity of these particles removed

y/y_o = 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:

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 90^o 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.

¹ Using two 90° V-notch weirs:

                Q = 2(2.5H^2.5)

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:

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):

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.