The use of temporary ponds for sediment 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 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”).

Temporary construction site sediment ponds have sediment loads that are very large while 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 feet of permanent standing water over most of the pond to protect sediments from scouring. Additional depth is also needed for sediment storage between cleanout operations.

· 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 (SCS 1982) 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 90% suspended solids reductions. If the pond area is only about 0.5% of the drainage area, the resulting removal would be about 65%, or less, of suspended solids. 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 to 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 sloped 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 (SCS 1982) has prepared a design manual that addresses specific requirements for such things as anti-seep collars around outlet pipes, embankment widths, types 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. Stagnation is a much more serious problem degrading pond water quality.

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 (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 foe all pond stages).

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 assume 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 may reduce the available pond storage volume (and therefore the resident time), with less effective treatment.

Sediment pond at landfill

Permanent pond acting as sediment trap during final construction

Series of small sediment pond at complex construction site (Atlanta, GA)

Temporary pond at highway construction site in area where hauling trucks are washed prior to re-entering roads (WI)

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):

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:

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).

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 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 settling velocity of v_o

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. 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