<IP_ADDRESS>.3 TEMPORARY STORM DRAIN DIVERSION General Description A temporary storm drain diversion is a pipe that reroutes an existing drainage system to discharge flow into a sediment trap or basin. This practice reduces the amount of sedimentladen runoff from construction sites that enters waterbodies without treatment. Temporary storm drain diversions can be used when a permanent storm water drainage system has not yet been installed. It should be recognized that diversion channels can also be installed but are not considered in the following discussion.

Applicability A temporary storm drain diversion should be used to temporarily redirect discharge to a permanent outfall and should remain in place until the area draining to the storm sewer is no longer disturbed. Temporary storm drain diversions can also be combined with other structures and used as a sediment trapping device when the completion of a permanent outfall has been delayed; alternatively, a sediment trap can be placed below a permanent outfall to remove sediment before the final flow discharge.

Design and Installation Criteria Since the diversion is only temporary, the layout of piping and the overall impact of the diversion's installation on post construction drainage patterns must be considered. Once construction is completed, the temporary diversion should be moved to restore the original system. The following activities should be done at this time: Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 52 ° The storm drain should be flushed before the sediment trap is removed.

° The outfall should be stabilized.

° Graded areas should be restored.

° State or local specifications should be checked for more detailed requirements and an appropriate professional should certify that the design meets local hydrologic and hydraulic requirements.

Effectiveness If installed properly to capture the bulk of runoff from a construction site, temporary storm drain diversions can be effective in reducing the discharge of sediment laden, untreated water to waterbodies. When used in combination with other erosion and sediment control practices such as minimized clearing or vegetative and chemical stabilization, the level of pollution from a construction site can be substantially reduced or eliminated.

Limitations Installation of a temporary storm drain diversion may result in the disturbance of existing storm drainage patterns. Care must be taken to ensure that the original system is properly restored once the temporary system is removed. The most common source of problems is excessive velocity at the outlet. Installation of an outlet stabilization structure is typically required and may be constructed of riprap, reinforced concrete, geotextile linings, or a combination.

Maintenance Once installed, temporary storm drain diversions require very little maintenance. Frequent inspection and maintenance of temporary storm drain systems, especially after large storms, should ensure that pipe clogging does not occur and that runoff from the site is being successfully diverted. After removal of the temporary diversion, the permanent storm drain system should be carefully inspected to ensure that drainage patterns have not been altered by the temporary system.

Cost Depending on the size of the construction site, a temporary storm drain diversion can be costly. Costs include those associated with materials needed to construct the diversion and sediment trap or basin (mainly piping, concrete, and gravel), and also labor costs for installation and removal of the system, all of which may involve excavation, regrading, and inspections. Based on the variety of conditions that can affect storm drain diversion designs, typical costs per installation are not presented here. However, site specific cost estimates can be produced using unit cost Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 53 values along with site specific quantity estimates. R. S. Means (2000) indicates a range of pipe costs for surface placement, between $5.00 per linear foot for 4" diameter PVC piping, and $9.20 per linear foot for 10" diameter PVC piping. On construction sites, temporary inlets and outlets are usually formed by small rock lined depressions. Assuming 4 cubic yards of crushed rock (1.5" mean diameter) per opening, an inlet and outlet combine to add approximately $200 per pipe installation, based on $25 per cubic yard of stone (R. S. Means, 2000).

<IP_ADDRESS>.4 PIPE SLOPE DRAIN General Description Pipe slope drains are used to reduce the risk of erosion on slopes by discharging runoff to stabilized areas. Consisting of a metal or plastic flexible pipe if temporary, or pipes or paved chutes if permanent, these drains are placed from the top to the bottom of a slope to carry surface runoff from the top to the bottom of a slope that has already been damaged by erosion or is at high risk for erosion. These drains are also used to drain saturated slopes that have the potential for soil slides.

Applicability Temporary slope drains can be used on most disturbed slopes to eliminate gully erosion problems resulting from concentrated flows discharged at a diversion outlet. Slope drains should be used as a temporary measure for as long as the drainage area remains disturbed. They will need to be moved once construction is complete and a permanent storm drainage system is established. Appropriate restoration measures will then need to be taken, such as adjusting grades and flushing sediment from the pipe before it is removed (UNEP, 1994).

Design and Installation Criteria Pipe slope drains can be placed directly on the ground or buried under the surface. The inlet should be located at the top of the slope and should be fitted with an apron, attached with a water tight connection. Filter cloth should be placed under the inlet to prevent erosion. Flexible pipes, which are positioned on top of the ground, should be securely anchored with grommets placed 10 feet on center. The outlet at the bottom of the slope should also be stabilized with riprap. The riprap should be placed along the bottom of a swale that leads to a sediment trapping structure or another stabilized structure.

Slope drain pipe sizes are based on drainage area and the size of the design storm. Pipes should be connected to a diversion ridge at the top of the slope by covering with compacted fill material where it passes through the ridge. Discharge from a slope drain should be to a sediment trap, sediment basin, or other stabilized outlet (UNEP, 1994). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 54 Pipe slope drains should be installed perpendicular to the contour down the slope, and the design should be able to handle the peak runoff for the 10 year storm. Recommendations of slope drain diameter are summarized in Table 5 11 (NAHB, n. d).

Table 5 11. Recommended Pipe/ Tubing Sizes for Slope Drains Maximum Drainage Area (acres) Pipe/ Tubing Diameter a (inches) Pipe/ Tubing Diameter b (inches) Pipe/ Tubing Diameter c (inches) 0 0.5 0.5 12 12 8 0.75 10 1.0 12 1.5 18 18 Individually designed 2.5 21 3.5 24 24 5.0 30 a UNEP, 1994. b USDOT, 1995. c IDNR, 1992.

Recently graded slopes that do not have permanent drainage measures installed should have a temporary slope drain and a temporary diversion installed. A temporary slope drain used in conjunction with a diversion conveys storm water flows and reduces erosion until permanent drainage structures are installed.

The following are design recommendations for temporary slope drains:

° The drain should consist of heavy duty material manufactured for the purpose and have grommets for anchoring at a spacing of 10 feet or less.

° Minimum slope drain diameters should be observed for varying drainage areas.

° The entrance to the pipe should consist of a standard flare end section of corrugated metal. The corrugated metal pipe should have watertight joints at the ends. The rest of the pipe is typically corrugated plastic or flexible tubing, although for flatter, shorter slopes, a polyethylene lined channel is sometimes used.

° The height of the diversion at the pipe should be the diameter of the pipe plus 0.5 foot.

° The outlet should be located at a reinforced or erosion resistant location. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 55 ° Temporary slope drains should be designed to adequately convey runoff for a desired frequency storm, typically either 2 years or 10 years depending on local regulations. Both the size and the spacing can be determined based on the contributing drainage area. Drains are spaced at intervals corresponding to the specified drainage areas. For larger drainage areas and critical locations, the drains should be sized on an individual basis (USDOT, 1995).

° Slope drains should be constructed in conjunction with diversion berms such that the berms are not overtopped. At the pipe inlet, the top of the berm should be a minimum of 300 millimeters (11.81 inches) higher than the top of the pipe. The entrance should be constructed of a standard flared end section or a Tee section if designed properly. The entrance should be placed in a 150 millimeters (5.90 inches) minimum depressed sump (USDOT, 1995).

° The outlet of the slope drain must be protected with a riprap apron. If the slope drain is draining a disturbed area and sufficient right of way is available, the drain may empty into a sediment trap (USDOT, 1995). Table 5 12 summarizes slope drain characteristics.

Table 5 12. Slope Drain Characteristics Capacity 2 yr frequency, 24 hr duration storm event Material Strong, flexible pipe, such as heavy duty, nonperforated, corrugated plastic Inlet section Standard "T" or "L" flared end section with metal toe plate Connection to ridge at top of slope Compacted fill over pipe with minimum dimensions, 1.5 ft depth, 4 ft top width, and 6 in higher than ridge Outlet Pipe extends beyond toe of slope and discharges into a sediment trap or basin unless contributing drainage area is stable Source: IDNR, 1992.

Effectiveness There is currently no information on the effectiveness of pipe slope drains.

Limitations The area drained by a temporary slope drain should not exceed 5 acres. Physical obstructions substantially reduce the effectiveness of the drain. A common slope drain problem is overtopping of the inlet due to an undersized or blocked pipe, or erosion at the outlet point due to insufficient protection (UNEP, 1994). Other concerns are failures from overtopping because of inadequate pipe inlet capacity and reduced diversion channel capacity and ridge height. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 56 Solutions to common problems include the following (IDNR, 1992):

° Washout  A washout along a pipe due to seepage and piping may be caused by inadequate compaction, insufficient fill, or installation that may be too close to the edge of the slope.

° Overtopping caused by undersized or blocked pipe  The drainage area may be too large.

° Overtopping caused by improper grade of channel and ridge  A positive grade should be maintained.

° Overtopping caused by poor entrance conditions and trash buildup at the pipe inlet  Deepen and widen the channel at the pipe entrance and frequently inspect and clear the inlet.

° Erosion at outlet  The pipe should be extended to a stable grade or an outlet stabilization structure is needed.

° Displacement or separation of pipe  The pipe should be tied down and the joints secured.

Maintenance Pipe slope drains must be inspected after each significant runoff event for evidence of erosion and uncontrolled runoff. Any repairs to the drain should be made immediately. Significant amounts of sediment trapped at the outfall should also be removed in a timely manner and disposed of properly (NAHB, n. d.).

The following actions should be taken to properly maintain a pipe slope drain (IDNR, 1992):

° Inspect slope drains and supporting diversions once a week and after every storm event.

° Check the inlet for sediment or trash accumulation; clear and restore to proper entrance condition.

° Check the fill over the pipe for settlement, cracking, or piping holes; repair immediately.

° Check for holes where the pipe emerges from the dike; repair immediately.

° Check the conduit for evidence of leaks or inadequate anchoring; repair immediately.

° Check the outlet for erosion or sedimentation; clean and repair, or extend if necessary.

° Once slopes have been stabilized, remove the temporary diversions and slope drains, and stabilize all disturbed areas. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 57 Cost The cost of pipe slope drains and their installation varies with the design and materials used. Site specific cost estimates can be produced using unit cost values with site specific quantity estimates. R. S. Means (2000) indicates a range of pipe costs for surface placement between $5.00 per linear foot for 4" diameter PVC piping, and $9.20 per linear foot for 10" diameter PVC piping. On construction sites, temporary inlets and outlets are usually formed by small rock lined depressions. Assuming 4 cubic yards of crushed rock (1.5" mean diameter) per opening, an inlet and outlet combine to add approximately $200 per pipe installation, based on $25 per cubic yard of stone (R. S. Means, 2000).

<IP_ADDRESS>.5 STONE CHECK DAM General Description A check dam is a small temporary barrier or dam constructed across a drainage channel or swale to reduce the velocity of the flow. By reducing the flow velocity, the erosion potential is reduced, detention times are lengthened, and more sediments are able to drop out of the water column. Check dams can be constructed of stone, gabions, treated lumber, or logs (NAHB, n. d.).

Check dams are inexpensive and easy to install. They may be used permanently if designed properly to allow a high proportion of sediment in the runoff to settle out and reduce velocity and may provide aeration of the water (NAHB, n. d.). However, the use of check dams in a channel should not be a substitute for the use of other sediment trapping and erosion control measures. As with most other temporary structures, check dams are most effective when used in combination with other storm water and erosion and sediment control measures.

Applicability Check dams are commonly used (1) in channels that are degrading but where permanent stabilization is impractical because of their short period of usefulness and (2) in eroding channels where construction delays or weather conditions prevent timely installation of erosion resistant linings (IDNR, 1992).

Check dams are also useful in steeply sloped swales, in small channels, in swales where adequate vegetative protection cannot be established, or in swales or channels that will be used for a short period of time where it is not practical to line the channel or implement other flow control practices (USEPA, 1993). In addition, check dams are appropriate where temporary seeding has been recently implemented but has not had time to fully develop and take root. The contributing drainage area should range from 2 to 10 acres. Check dams should be used only in small open channels that will not be overtopped by flow once the dams are built and should not be built in stream channels, either intermittent or perennial (UNEP, 1994). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 58 Design and Installation Criteria Check dams can be constructed from a number of different materials. Most commonly, they are made of rock, logs, sandbags, or straw bales. Rock or stone is often preferred because of its cost effectiveness and longevity. Logs and straw bales will decay with time and are not recommended as they may cause waterway blockage if they fail. When using rock or stone, the material diameter should be 2 to 15 inches. The stones should be extended 18 inches beyond the banks, and the side slopes should be 2: 1 or flatter. Lining the upstream side of the dam with a foot of 1 to 2 inch gravel may improve the efficiency of the dam (NAHB, n. d.). Logs should have a diameter of 6 to 8 inches. Regardless of the material used, careful construction of a check dam is necessary to ensure its effectiveness.

The distance between rock check dams will vary depending on the slope of the ditch, with closer spacing when the slope is steeper. The size of stone used in the check dam should also vary with the expected design velocity and discharge. As velocity and discharge increase, the rock size should also increase. For most rock check dams, 3 inches to 12 inches is a suitable stone size. To improve the sediment trapping efficiency of check dams, a filter stone can be applied to the upstream face. A well graded coarse aggregate that is less than 1 inch in size can be used as a filter stone.

All check dams should have a maximum height of 3 feet. The center of the dam should be at least 6 inches lower than the edges. This design creates a weir effect that helps to channel flows away from the banks and prevent further erosion. Additional stability can be achieved by implanting the dam material approximately 6 inches into the sides and bottom of the channel (VDCR, 1995).

When installing more than one check dam in a channel, outlet stabilization measures should be installed below the final dam in the series. Because this area is likely to be vulnerable to further erosion, riprap or some other stabilization measure is highly recommended.

Effectiveness Field experience has shown that rock check dams are more effective than silt fences or straw bales to stabilize wet weather ditches (VDCR, 1995). Straw bales have been shown to have very low trapping efficiencies and should not be used for check dams. For long channels, check dams are most effective when used in a series, creating multiple barriers to sediment laden runoff. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 59 Limitations Check dams should not be used in perennial streams unless approved by an appropriate regulatory agency (USEPA, 1992; VDCR, 1995). Because the primary function of check dams is to slow runoff in a channel, they should not be used as a stand alone substitute for other sediment trapping devices. Also, leaves have been shown to be a significant problem as they clog check dams; therefore, increased inspection and maintenance might be necessary in the fall.

Common problems with check dams include channel bypass and severe erosion when overtopped and ineffectiveness due to accumulated sediment and debris. When designing check dams, the fact that they will reduce the capacity of a channel to transmit storm water runoff and thus will need to be sized appropriately should be taken into account (UNEP, 1994). The check dam may also kill grass linings in the channel if the water level remains high after it rains or if there is significant sedimentation. In addition, a check dam may reduce the hydraulic capacity of the channel and create turbulence, which erodes the channel banks (NAHB, n. d.).

Maintenance Check dams should be inspected periodically to ensure that they have not been repositioned as a result of storm water flow. In addition, the center of a check dam should always be lower than its edges. Additional stone may have to be added to maintain the correct height. Sediment should not be allowed to accumulate to more than half the original dam height. Any required maintenance should be performed immediately. When check dams are removed, care must be taken to remove all dam materials to ensure proper flow within the channel. The channel should subsequently be seeded for stabilization (NAHB, n. d.).

Cost The cost of check dams varies based on the material used for construction and the width of the channel to be dammed. In general, it is estimated that check dams constructed of rock cost about $100 per dam (USEPA, 1992). Brown (1997) estimated rock check dam would cost approximately $62 per installation, including the cost for filter fabric bedding. Other materials, such as logs and sandbags, may be a less expensive alternative, but they might require higher maintenance costs.

<IP_ADDRESS>.6 LINED WATERWAYS General Description Lined channels convey storm water runoff through a stable conduit. Vegetation lining the channel reduces the flow velocity of concentrated runoff. Lined channels usually are not designed to control peak runoff loads by themselves and are often used in combination with other BMPs such as subsurface drains and riprap stabilization. Where moderately steep slopes Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 60 require drainage, lined channels can include excavated depressions or check dams to enhance runoff storage, decrease flow rates, and enhance pollutant removal. Peak discharges can be reduced through temporary detention in the channel. Pollutants can be removed from storm water by filtration through vegetation, by deposition, or in some cases by infiltration of soluble nutrients into the soil. The degree of pollutant removal in a channel depends on the residence time of the water in the channel and the amount of contact with vegetation and the soil surface, but pollutant removal is not generally the major design criterion.

Often construction increases the velocity and volume of runoff, which causes erosion in newly constructed or existing urban runoff conveyance channels. If the runoff during or after construction will cause erosion in a channel, the channel should be lined or flow control practices instituted. The first choice of lining should be grass or sod since this reduces runoff velocities and provides water quality benefits through filtration and infiltration. If the velocity in the channel would erode the grass or sod, riprap, concrete, or gabions can be used (USEPA, 2000). Geotextile materials can be used in conjunction with either grass or riprap linings to provide additional protection at the soil lining interface.

Applicability Lined channels typically are used in residential developments, along highway medians, or as an alternative to curb and gutter systems. Grass lined channels should be used to convey runoff only where slopes are 5 percent or less. These channels require periodic mowing, occasional spot seeding, and weed control to ensure adequate grass cover (UNEP, 1994).

Lined channels should be used in areas where erosion resistant conveyances are needed, such as in areas with highly erodible soils and slopes of less than 5 percent. They should be installed only where space is available for a relatively large cross section. Grassed channels have a limited ability to control runoff from large storms and should be used with the recommended allowable velocities for the specific soil types and vegetative cover.

Design and Installation Criteria The design of a lined waterway requires proper determination of the channel dimensions. It must ensure that (1) the velocity of the flowing water will not wash out the waterway and that (2) the capacity of the waterway is sufficient to carry the surface flow from the watershed without overtopping.

Vegetative Lined Channels. Grass lined channels have been previously discussed in detail and are only summarized in this section. The allowable velocity of water in the waterway depends upon the type, condition, and density of the vegetation, as well as the erosive characteristics of the soil. Uniformity of vegetative cover is important because the stability of the most sparsely covered area determines the stability of the channel. Grasses are a better vegetative cover than legumes because grasses resist water velocity more effectively. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 61 Vegetative lined channels may have triangular, parabolic, or trapezoidal cross sections. Side slopes should not exceed 3: 1 to facilitate the establishment, maintenance, and mowing of vegetation. A dense cover of hardy, erosion resistant grass should be established as soon as possible following grading. This may necessitate the use of straw mulch and the installation of protective netting until the grass becomes established. If the intent is to create opportunities for runoff to infiltrate into the soil, the channel gradient should be kept near zero, the channel bottom must be well above the seasonal water table, and the underlying soils should be relatively permeable (generally, with an infiltration rate greater than 2 centimeters [0.78 inches] per hour).

Rock Lined Channels. Riprap lined channels may be installed on somewhat steeper slopes than grass lined channels. They require a foundation of filter fabric or gravel under the riprap. Generally, side slopes should not exceed 2: 1, and riprap thickness should be 1.5 times the maximum stone diameter. Riprap should form a dense, uniform, well graded mass (UNEP, 1994).

Lined channels should be sited in accordance with the natural drainage system and should not cross ridges. The channel design should not have sharp curves or significant changes in slope. Channels should not receive direct sedimentation from disturbed areas and should be established only on the perimeter of a construction site to convey relatively clean storm water runoff and separated from disturbed areas by a vegetated buffer or other BMP to reduce sediment loads.

Basic design recommendations for lined channels include the following:

° Construction and vegetation of the channel should occur before grading and paving activities begin.

° Design velocities should be below 5 feet per second.

° Geotextiles can be used to stabilize vegetation until it is fully established.

° Covering the bare soil with sod or geotextiles can provide reinforced storm water conveyance immediately.

° Triangular shaped channels should be used with low velocities and small quantities of runoff; parabolic grass channels are used for larger flows and where space is available; trapezoidal channels are used with large flows of low velocity (low slope).

° Outlet stabilization structures might be needed if the runoff volume or velocity has the potential to exceed the capacity of the receiving area.

° Channels should be designed to convey runoff from a 10 year storm without erosion. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 62 ° The sides of the channel should be sloped less than 3: 1, with V shaped channels along roads sloped 6: 1 or less for safety.

° All trees, bushes, stumps, and other debris should be removed during construction.

Effectiveness Lined channels can effectively transport storm water from construction areas if they are designed for expected flow volumes and velocities and if they do not receive sediment directly from disturbed areas.

Limitations Lined channels, if improperly installed, can alter the natural flow of surface water and have adverse impacts on downstream waters. Additionally, if the design capacity is exceeded by a large storm event, the vegetation might not be sufficient to prevent erosion and the channel might be destroyed. Clogging with sediment and debris reduces the effectiveness of grass lined channels for storm water conveyance.

Common problems in lined channels include erosion of the channel before vegetation is fully established and gullying or head cutting in the channel if the grade is too steep. In addition, trees and brush tend to invade lined channels, causing maintenance problems.

Riprap lined channels can be designed to safely convey greater runoff volumes on steeper slopes. However, they should generally be avoided on slopes exceeding 10 percent because stone displacement, erosion of the foundation, or channel overflow and erosion resulting from a channel that is too small can occur. Thus, channels established on slopes greater than 10 percent will usually require protection with rock gabions, concrete, or other highly stable and protective surfaces (UNEP, 1994).

Maintenance Maintenance requirements for lined channels are relatively minimal. During the vegetation establishment period, the channels should be inspected after every rainfall. Other maintenance activities that should be carried out after vegetation is established are mowing, litter removal, and spot vegetation repair. The most important objective in the maintenance of lined channels is maintaining a dense and vigorous growth of turf. Periodic cleaning of vegetation and soil buildup in curb cuts is required so that water flow into the channel is unobstructed. During the growing season, channel grass should be cut no shorter than the level of design flow, and the cuttings should be removed promptly. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 63 Cost Costs of grassed channels range according to depth, with a 1.5 foot deep, 10 foot wide grassed channel estimated at $6,395 to $17,075 per trench, while a 3.0 foot deep, 21 foot wide grassed channel is estimated at $12,909 to $33,404 per trench (SWRPC, 1991).

Readers are also referred to the discussion of costs for grass lined channels, which contains many of the design and cost elements required for installing lined waterways. Designers have a range of options for lining new channels. Geosynthetic turf reinforcement mattings (TRMs) can be used for immediate erosion protection in channels exposed to runoff flows. The Erosion Control Technology Council (a geotextile industry support association) suggests TRMs cost approximately $7.00 per square yard (installed) for channel protection (ECTC, 2002a). R. S. Means indicates machine placed riprap costs of approximately $40 per cubic yard. The riprap maximum size is typically between 6 and 12 inches, depending on the channel design velocity. A cubic yard of riprap will cover between 36 and 18 square feet of channel bed for these riprap sizes (assuming depth of riprap is 1.5 times the maximum size). These estimates suggests that riprap lining will be between $10 and $20 per square foot of channel (Costs include materials, labor, and equipment, with overhead and profit).

<IP_ADDRESS> SEDIMENT TRAPPING DEVICES The devices listed under this group of BMPs trap sediment primarily through impounding water and allowing for settling to occur (Haan et al., 1994). Silt fence, super silt fence, straw bale dikes, sediment traps, and sediment basins all control flow through a porous flow control system such as filter fabric or straw bales or they use a dam to impound water with a pipe, open channel, or rock fill outlet. The filtering capacity of silt fence (filter fabric) contributes only a small amount of trapping, but serves to make the fence less porous and hence increases ponding . For steady state flows, the trapping that occurs behind the flow control device can be shown to be directly proportional to the surface area and indirectly proportional to flow through the system (Haan et al., 1994). The ratio of the surface area to flow is known as the overflow rate, and trapping in such systems is predicted by the ratio of overflow rate to particle settling velocity. Although flows in nature are inherently non steady state and more complex than steady state systems, studies have shown that the best predictor of trapping in such systems is still the ratio of settling velocity to overflow rate (Hayes et al., 1984). In the case of non steady state, the overflow rate is best defined by the ratio of peak discharge from the system to a surface area (Hayes et al., 1984; McBurnie et al., 1990).

The amount of trapping in these structures depends on the size of the structure, flow rates into the system, hydraulics of the flow control system, the size distribution of the sediment flowing into the structure, and the chemistry of the sediment water system (Haan et al., 1994). Trapping can be enhanced by chemical treatment of flows into the structure, but the impacts have not been widely defined for varying mineralogy and chemistry of the sediment water system (Haan et al., 1994; Tapp and Barfield, 1986). Recent studies have been conducted on the application of Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 64 polyacrilamides (PACs) to disturbed areas for enhancing settling (Benik et al., 1998; Masters et al., 2000; Roa Espinosa et al., 2000), but results have not been definitive. No known studies have evaluated the impacts of PAC application to disturbed areas on settling in sediment trapping devices.

Sediment flowing into sediment trapping devices is composed of primary particles and aggregated particles. Aggregates are formed when clays, silts, and sands are cemented together to form larger particles that have settling velocities far greater than those of any individual particles alone although the degree of aggregation depends on the amount of cementing material present (typically clays and organic matter). Since the aggregates have higher settling velocities than primary particles, the degree of aggregation that is present has a large impact on the trapping that occurs. Procedures are available to measure the combined size distribution of aggregate and primary particle size distribution (Barfield et al., 1979; Haan et al., 1994). Procedures are also available to predict particle size distributions of aggregates and primary particles (Foster et al., 1985) but have not been found to be very accurate for subsoils exposed during construction in at least one study (Barfield et al., 1983).

In the absence of chemical treatment, the sediment that can be captured in sediment trapping devices is typically the settleable solids. To trap the smaller size clay particles, structures with surface areas larger than the construction site itself would have to built in many cases (Barfield, 2000). Chemical treatment can be used to reduce the size, but it has not been adopted on a wide scale because of the cost and complexity of the operation (Tapp et al., 1981).

Sediment trapping devices also provide some storm water detention by virtue of detaining flows long enough to allow sediment to settle out and be deposited. However, to operate as a storm water detention structure, the design should include storm water detention as well.

Virtually all of the available information on sediment trapping structures, both theoretical and experimental, is on impacts to receiving waters and not downstream effects. In a very limited analysis, Barfield (2000) combined the SEDIMOT II computer model together with the FLUVIAL model to theoretically evaluate the impact of sediment trapping structures on downstream geomorphology in a Puerto Rican watershed.

<IP_ADDRESS>.1 SILT FENCE General Description Silt fences are used as temporary sediment barriers consisting of filter fabric anchored across and supported by posts. Their purpose is to retain sediment from small disturbed areas by reducing the velocity of sediment laden runoff and promoting sediment deposition (Smolen et al., 1998). Silt fences capture sediment by ponding water and allowing for deposition, not by filtration. Silt fence fabric first screens silt and sand from runoff, resulting in clogging of the lower part of the Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 65 fence. The pooling water allows sediments to settle out of the runoff. Silt fences work best in conjunction with temporary basins, traps, or diversions.

Applicability Silt fences are generally placed at the toe of fills, along the edge of waterways, and along the site perimeter. The fences should not be used in drainage areas with concentrated and high flows, in large areas, or in ditches and swales where concentrated flow is present.

The drainage area for the fence should be selected based on design storms and local hydrologic conditions so that the silt fence is not expected to overtop. A typical design calls for no greater than ¼ acre per 100 feet of fence, but this is highly variable depending on climate. The fence should be stable enough to withstand runoff from a 10 year peak storm. Table 5 13 lists the maximum slope length specified by the USDOT. These slope lengths should be based on sediment load and flow rates. This would mean that the values given below should be adjusted for climatic conditions instead of "one size fits all" for a silt fence to ensure maximum effectiveness.

Table 5 13. Maximum Slope Lengths for Silt Fences Slope (%) 18 inch (460 mm) Fence 30 inch (760 mm) Fence

2 250 ft (75 m) 500 ft (150 m) 5 100 ft (30m) 250 ft (75 m) 10 50 ft (15 m) 150 ft (45 m) 20 25 ft (8 m) 70 ft (21 m) 25 6 m (20 ft) 55 ft (17 m) 30 15 ft (5 m) 45 ft (14 m) 35 15 ft (5 m) 40 ft (12 m) 40 15 ft (5 m) 35 ft (10 m) 45 10 ft (3 m) 30 ft (9 m) 50 10 ft (3m) 25 ft (8m) Source: USDOT, 1995.

Typical standards and specifications call for the silt fence to be located on fairly level ground and follow the land contour. However, field evaluations by Barfield and Hayes (1992, 1999) in South Carolina and Kentucky indicate that installations on the contour as well as along a slope have problems with undercutting. In either case, the installations are such that a slight slope may occur along the fence in spite of the best installation practices. Runoff can move down the contour until a weak spot occurs in the buried toe and undercuts the fence. Alternatively, flow may move to a low spot where it accumulates and causes an overtopping. In either case, trapping by the silt fence is essentially zero, and flows have then been concentrated at a point causing downslope channel erosion. Development Document for Construction and Development Proposed Effluent Guidelines 1 d15 :15 percent by weight of suspended solids are smaller than those that are trapped by this device; Similarly d50 indicates that 50 percent by weight of suspended solids are smaller than those trapped.

June 2002 5 66 Design and Installation Criteria Design criteria are of two types:

Hydrologic design for a required trapping of sediment and flow rate to pass the design storm. Selection of appropriate installation criteria such that the silt fence will perform as designed.

Hydrologic Design Hydrologic design should result in a design that passes the design storm without causing damage while trapping the required amount of sediment. It is necessary to use either a database or some type of model to develop the appropriate hydrologic design. Efforts to model the sediment trapping that occurs through the use of a silt fence have resulted in models that predict the settling in the ponded area upstream from the fence (Barfield et al., 1996; Lindley et al., 1998). The results from model simulations show that trapping depends primarily on the surface area of the impounded water and the flow rate through the filter. The models utilize a clear water slurry flow rate, typically specified by the manufacturer, to predict discharge. However, numerous studies have shown that sediment laden flows cause clogging of the geotextiles used to construct the fence, dependent on the opening size and size of the sediment (Britton et al., 2001; Wyant, 1980; Barrett et al., 1995; Fisher and Jarret, 1984). Thus, results from model studies to date are suspect and need to be modified to account for the impacts of clogging on flow rate. Barfield et al., (2000) developed a model of flow rate using conditional probability concepts, but the results have not been experimentally verified.

Design aids have been developed for silt fence, using simulations from the SEDIMOT III model (Hayes and Barfield, 1995). In the model, predictions are made about trapping efficiency using the ratio of settling velocity for the d151 of the eroded sediment, divided by the ratio of discharge to ponded surface area. The design aids yield conservative estimates as compared to the SEDIMOT III model, but the database used for generating the design aid is based on the assumption that clogging does not impact flow rates. The discussion above shows that assumption to be erroneous.

The bottom line on the discussion above is that it is not possible to predict with any expected accuracy the trapping efficiency of silt fence under a given set of conditions.

Installation Criteria General installation criteria for the silt fence should incorporate the following factors: Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 67 ° The fabric must have sufficient strength to counter forces created by contained water and sediment (Sprague, 1999).

° The posts must have sufficient strength to counter the forces transferred to them by the fabric (Sprague, 1999).

° The fabric must be installed to ensure that the loads are all adequately transferred through the fabric to the posts or the ground without overstressing (Sprague, 1999).

° The fence must be designed based on site specific hydrologic and soil conditions such that it will not overtop during design events.

° The fence must be installed (anchored) with a buried toe of sufficient depth so that it does not become detached from the soil surface.

° In general, the fence requires a metal wire backing to provide sufficient strength to prevent failure from the weight of trapped sediment and to prevent the toe of the fabric from being removed from the ground.

° Maximum drainage area behind the fence should be determined based on the local rainfall and the infiltration characteristics of the soil and cover.

Silt fence material is typically synthetic filter fabric or a pervious sheet of polypropylene, nylon, polyester, or polyethylene yarn. The fabric should have ultraviolet ray inhibitors and stabilizers to provide for a minimum useful construction life of 6 months or the duration of construction, whichever is greater. The height of the fence fabric should not exceed 3 feet. If standard strength filter fabric is used, it should be reinforced with a wire fence, extending down into the trench that buries the toe. The wire should be of sufficient strength to support the weight of the deposited sediment and water. In general, a minimum 14 gauge and a maximum mesh spacing of 6 inches is called for (Smolen et al., 1988). Typical requirements for the silt fence physical properties, as specified in selected local BMP standards and specifications, are included in Table 5 14. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 68 Table 5 14. Typical Requirements for Silt Fence Fabric Physical Property Requirements Woven Fabric Non Woven Fabric Filtering Efficiency 85% 85%

Tensile Strength at 20% (maximum) Elongation Standard Strength —30 pound/ linear inch Extra Strength —50 pound/ linear inch Standard Strength —50 pound/ linear inch Extra Strength —70 pound/ linear inch Slurry Flow Rate 0. 3 gallon/ square feet/ minute 4. 5 gallon/ square feet/ minute Water Flow Rate 15 gallon/ square feet/ minute 220 gallon/ square feet/ minute UV Resistance 70% 85%

Source: NCDNR, 1988; IDNR 1992.

It should be pointed out that these numbers, particularly the flow rates, could vary widely depending on the local soil condition due to possible clogging of the filter material.

Material for the posts used to anchor the filter fabric can be constructed of either wood or steel. Wooden stakes should be buried at a depth sufficient to keep the fence, when loaded with sediment and water, from falling over. The depth of burial should depend on soil strength characteristics when saturated and post diameter. Many standards and specifications set a minimum length of the post of 5 feet long, and a diameter of 4 inches for posts composed of softwood (e. g, pine), and 2 inches for posts composed of hardwood (e. g., oak)( Smolen et al., 1988). Steel posts should also be designed based on local soil strength characteristics when wet. Some standards and specifications for these posts set a minimum weight of 1.33 pound/ linear feet with a minimum length of 4 feet. Steel posts should also have projections to adhere filter fabric to the post (Smolen et al., 1988).

A silt fence should be erected in a continuous fashion from a single roll of fabric so as to eliminate unwanted gaps in the fence. If a continuous roll of fabric is not available, the fabric should overlap from both directions only at posts with a minimum overlap of 6 inches and be rolled together with a special flexible rod to keep the ends from separating. Fence posts should be spaced at a distance based on wet soil strength characteristics and post size and strength; generally, the posts are spaced approximately 4 to 6 feet apart. If standard strength fabric is used in combination with wire mesh, the spacing can be larger. Typically, the standards and specifications call for the posts to be no more than 10 feet apart. If extra strength fabric is used without wire mesh reinforcement, some standards call for the support posts to be spaced no more than 6 feet apart (VDCR, 1995). Again, this spacing should depend on wet soil strength characteristics and post size.

A silt fence must provide sufficient storage capacity or be stabilized over flow outlets such that the storage volume of water will not overtop the fence. The return period event (size of the rainfall event managed) used for design is typically a prerogative of the regulatory agency. For temporary fences, a 2 year storm event is typically used as a design standard. Fences that will be in place for 6 months or longer are commonly designed based on a 10 year storm event Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 69 (Sprague, 1999). The space behind the fence used for impoundment volume must be sufficient to adequately contain the sediment that will be deposited. Each storm will deposit sediment behind the fence, and after a period of time the amount of sediment accumulated will render the fence useless. Frequency of fence management is a function of its sizing (i. e. whether the fence was installed for a 2 year or a 10 year storm event) (Sprague, 1999) and the amount of erosion that occurs in the area draining to the fence.

Effectiveness The performance of silt fences has not been well defined. Laboratory studies using carefully controlled conditions have shown trapping efficiencies in the range of 40 to 100 percent, depending on the type of fabric, overflow rate, and detention time (Barrett et al., 1995; Wyant, 1980; Wishowski et al., 1998). Field studies have been limited and quite inadequate; however, the results show that field trapping efficiencies are very low. In fact, Barrett et al. (1995) obtained a value of zero percent trapping averaged over several samples with a standard error of 26 percent. Barrett et al. (1995) cite the following reasons for the field tests not showing the expected results:

° Inadequate fabric splices ° Sustained failure to correct fence damage resulting from overtopping ° Large holes in the fabric ° Under runs due to inadequate "toe ins"

° Silt fence damaged and partially covered by the temporary placement of stockpiles of materials Field inspections conducted by Barfield and Hayes (1992) were made in which more than 50 construction sites in South Carolina and Kentucky were visited. Inspections found that silt fence was seldom installed and, when installed, was rarely set up according to specifications. In areas where installations did meet standards, it was obvious that flows sought the weakest spot on the fence and either flowed through cuts in the fabric, or undercut or overtopped the fence. This flow was thus changed from the overland flow coming into the site to concentrated flow, causing significant erosion.

Silt fences are effective at removing large particle sediment, primarily aggregates, sands, and larger silts. Sediment is removed through impounding of water to slow velocity. It is argued that the silt fence will not contribute to a reduction in small particle sediment and is not effective against other pollutants (WYDEQ, 1999). EPA (1993) reports the following effectiveness ranges for silt fences constructed of filter fabric: average total suspended solids removal of 70 percent, sand removal of 80 to 90 percent, silt loam removal of 50 to 80 percent, and Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 70 silt clay loam removal of 0 to 20 percent. However, the EPA numbers from the Nationwide Urban Runoff Program should not be considered to apply to every location. The actual trapping will vary widely for a given design because of differences in hydrologic regimes and soil types.

The advantages of using silt fences include: minimal labor requirement for installation, low cost, high efficiency in removing sediment, durability, and sometimes reuse (Sprague, 1999). Silt fences are the most readily available and cost effective control options where options like diversion are not possible. Silt fences are also a popular choice; because contractors have used them extensively, the familiarity makes silt fence use more likely for future construction activities. The visibility of a silt fence is also an advantage, for the fence is "advertising" the use of erosion and sediment control structures. In addition, the silt fence visibility makes site inspection easier for contractors and government inspectors (CWP, 1996).

Limitations Silt fences should not be installed along areas where rocks or other hard surfaces will prevent uniform anchoring of fence posts and entrenching of the filter fabric because an insufficient anchor will greatly reduce the effectiveness of silt fencing and may create runoff channels leading off site. In addition, open areas where wind velocity is high may present a maintenance challenge, as high winds may accelerate deterioration of the filter fabric (Smolen et al., 1988). When the pores of the silt fence fabric become clogged with sediment, pools of water are likely to form on the uphill side of fence. Siting and design of the silt fence should account for this problem and care should be taken to avoid unnecessary diversion of storm water from these pools which might cause further erosion damage. Silt fences can act as a diversion if placed slightly off contour and can control shallow, uniform flows from small, disturbed areas and deliver sediment laden water to deposition areas.

Silt fences will sag or collapse if a site is too large, if too much sediment accumulates, if the approach slope is too steep, or if the fence was not adequately supported. If the fence bottom is not properly installed or the flow velocity is too fast, fence undercuts or blowouts can occur because of excess runoff. Erosion around the end of the fence can occur if the fence ends do not extend upslope to prevent flow around the fence (IDNR, 1992).

Maintenance Site operators should inspect silt fences after each rainfall event to ensure they are intact and that there are no gaps at the fence ground interface or tears along the length of the fence. If gaps or tears are found, they should be repaired or the fabric should be replaced immediately. Accumulated sediments should be removed from the fence base when the sediment reaches one third to halfway up the height of the fence. Sediment removal should occur more frequently if accumulated sediment is creating a noticeable strain on the fabric and there is the possibility that the fence might fail from a sudden storm event. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 71 Cost There is a wide range of data on installation costs for silt fences. EPA estimates these costs at approximately $6.00 per linear foot (USEPA, 1992) while SWRPC estimates unit costs between $2.30 and $4.50 per linear foot (SWRPC, 1991). Silt fences have an annual maintenance cost that is 100 percent of installation cost (Brown et al., 1997). These values are significantly greater than that reported by R. S. Means (2000), which indicates a 3 foot tall silt fence installation cost between $0.68 and $0.92 per linear foot (for favorable and challenging installations). It should be noted that the R. S. Means value covers just a single installation, without the expected costs of maintenance (e. g., removal of collected sediment). In addition, the type of silt fence fabric employed will also affect the total installation costs.

<IP_ADDRESS>.2 SUPER SILT FENCE General Description Super silt fence is a modification of a standard silt fence. The two central differences between the standard silt fence and the super silt fence is that the super silt fence has toe that is buried more deeply and the backing material is chain link fence held in place by steel posts– a concept that originated in Maryland. The Maryland super silt fence requires a Geotextile Class F fabric over a chain link fence to intercept sediment laden runoff from small drainage areas. The super silt fence provides a barrier that can collect and hold debris and soil more effectively than a standard silt fence, preventing material from entering critical areas. It is best used where the installation of a dike would destroy sensitive areas, woods, and wetlands.

Applicability Super silt fences can be used in the same conditions as a silt fence. Fences should follow the contour of the land. Table 5 15 lists the distance a super silt fence should be from a slope to ensure maximum effectiveness (MDE, 1994).

Table 5 15. Slope Lengths for Super Silt Fences Slope (%) Slope Length Minimum Maximum 0 10 Unlimited Unlimited 10 20 200 feet 1,500 feet 20 33 100 feet 1,000 feet 33 50 100 feet 500 feet 50+ 50 feet 250 feet Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 72 Design and Installation Criteria As with the standard silt fence, design criteria are of two types, hydrologic design for a required trapping of sediment and flow rate to pass the design storm and selection of appropriate installation criteria such that the silt fence will perform as designed.

Hydrologic Design Hydrologic design criteria are the same as the criteria for the standard silt fence.

Installation Criteria The criteria used for the Maryland super silt fence indicate the following, although they have not been tested with field data:

° The fence should be placed as close to the contour as possible, with no section of the silt fence exceeding a grade of 5 percent for a distance of more than 50 feet.

° Fabric should be no more than 42 inches in height and should be held in place with a 6 foot chain link fence.

° Fabric should be attached to the steel pole using wire ties or staples. Fabric should be securely fastened to the chain link fence with ties spaced every 24 inches at the top and midsection.

° Fabric should be embedded into the ground at a minimum of 8 inches.

° Edges of fabric should overlap by 6 inches.

Table 5 16 describes the physical properties of Geotextile class F fabric (MDE, 1994).

Table 5 16. Minimum Requirements for Super Silt Fence Geotextile Class F Fabric Physical Properties Requirements Tension Strength 50 pound/ inch Tensile Modulus 20 pound/ inch Flow Rate 0.3 gallon/ ft 2 /minute Filtering Efficiency 75% Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 73 Effectiveness Performance data have not been collected for super silt fences. The fences have been proposed for locations within a sensitive watershed, or where site conditions prohibit the use of a standard silt fence. However, until performance data are collected under field conditions, effectiveness is speculative.

Limitations Super silt fences are not as likely to fail structurally as are standard silt fences, but they are more expensive than standard silt fences.

Maintenance Maintenance requirements for super silt fences are generally the same as for standard silt fences.

Cost The cost of the super silt fence is more than the standard silt fence because of deeper burial at the toe and the cost of chain linked fencing. R. S. Means (2000) indicates a rental price of $10 to $11 per linear foot of chain linked fence for periods up to 1 year. Overall, rental is expected for most construction site installation because rental rates are approximately half the price of permanent chain link fencing.

<IP_ADDRESS>.3 STRAW BALE DIKE General Description The straw bale dike is a temporary measure used to trap sediment from small, sloping disturbed areas. It is constructed of straw bales (not hay bales) wedged tightly together and placed along the contour downslope of disturbed areas. The bales are placed in a shallow excavation, and the upslope side is sealed with soil. Stakes are driven through the bales into the soil to help hold the bales in place. The dike works by impounding water, which allows sediment to settle out in the upslope area (Haan et al., 1994). Straw bale dikes are recommended for short duration application and are usually effective for less than 3 months because of rapid decomposition (USDOT, 1995).

Applicability Straw bale dikes are generally placed at the toe of fills to provide for a broad shallow sediment pool. The dikes should not be used in drainage areas with concentrated and high flows, in large areas, or in ditches and swales. The location of the straw bale dike should be fairly level, at least Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 74 10 feet from the toe, and should follow the land contour. Table 5 17 lists the distance a straw bale dike should be placed from a slope to ensure maximum effectiveness.

Table 5 17. Maximum Land Slope and Distances Above a Straw Bale Dike Land Slope (%) Maximum Distance Above Dam (ft) Less than 2 100 2 5 75 5 10 50 10 20 25 More than 20 15 Source: USDOT, 1995.

Design and Implementation Criteria Hydrologic Design Hydrologic design dictates the structure necessary to withstand a storm without causing damage while trapping the required amount of sediment. Either a database or some type of model are needed to find the appropriate design. Efforts to model the sediment trapping that occurs in straw bale dikes have resulted in models that predict the settling in the ponded area upstream from the fence (Barfield et al., 1996; Lindley et al., 1998). The results from model simulations show that trapping depends primarily on the surface area of the impounded water and flow rate through the filter. The models utilize a clear water slurry flow rate to predict discharge. It is anticipated, based on visual observations, that sediment will clog the straw bale barrier, reducing the slurry flow rate. Thus, results from model studies to date are suspect and need to be modified to account for the impact of clogging on flow rate.

Installation Criteria The USDOT's BMP Manual and the Indiana BMP Manual (IN Manual) calls for bales to be:

° Anchored by driving two 36 inch long (minimum) steel rebars or 2 x 2 inch hardwood stakes through each bale;

° Sized according to the standard bale size of 14 inches x 18 inches x 35 inches;

° Placed in an excavated trench at least 4 inches deep, a bale's width, and long enough that the end bales are somewhat upslope of the sediment pool;

° Abutted tightly against each other; and,

° Sized such that impounded water depth should not exceed 1.5 feet. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 75 The USDOT BMP Manual does not require that straw bale dikes be designed; however, the Indiana Manual limits the drainage area to ¼ acre per 100 feet of dam and the total drainage area draining to a straw bale dike to 2 acres.

Effectiveness The information on performance of straw bale dikes is very limited. In laboratory studies of bales at varying orientations, Kouwen (1990) found that trapping efficiencies ranged from 60 to 100 percent. Field data on trapping have not been collected; however, visual inspection of sites indicate that straw bales are not properly installed to prevent flows from undercutting or flowing between bales (Barfield and Hayes, 1992, 1999). In addition, bales deteriorate rapidly and need to be replaced frequently. Because of these problems, the use of straw bale dikes as a perimeter control is not recommended, except in special circumstances. Only 27 percent of Erosion and Sediment Control (ESC) experts rated the straw bale dike as an effective ESC practice, although its use was still allowed in half of the communities surveyed (Brown and Caraco, 1997).

Limitations Straw bale dikes should not be used as a diversion, in streams, in channels, or in areas with concentrated flow. The bales are not recommended for paved areas because of the inability to anchor the bales (IDNR, 1992).

Care must be taken to ensure that the bales are not installed in an area where there is a concentrated flow of runoff, in a drainage area that is too large, or on an excessive slope (IDNR, 1992). Under these conditions, erosion around the end of the bales, overtopping and undercutting of the bales, and bale collapsing and dislodging are likely to occur. Overtopping will also occur if the storage capacity is underestimated and where provisions are not made for safe bypass of storm flow (IDNR, 1992). Undercutting will occur if the bales are not entrenched at least 4 inches and backfilled with compacted soil or were not abutted or chinked properly. Straw bale dikes are likely to collapse or dislodge if the bales are not adequately staked, or if too much sediment is allowed to accumulate before cleanout (IDNR, 1992).

Maintenance For the straw bale dike to be most effective, it is important to replace deteriorated bales when appropriate.

Cost The cost of straw bale dikes are relatively low, making their use relatively attractive. R. S. Means (2000) indicates a staked straw bale unit cost of $2.61 per linear foot (Costs include materials, labor, and equipment, with profit and overhead). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 76 <IP_ADDRESS>.4 SEDIMENT TRAP General Description A sediment trap is a temporary control device used to intercept sediment laden runoff and to trap sediment to prevent or reduce off site sedimentation. It is normally a more temporary type of structure than a sediment pond and is constructed to control sediment on the construction area during a selected phase of the construction operation. A sediment trap can be formed by excavation and/ or embankments constructed at designated locations accessible for cleanout. The outlet for a sediment trap is typically a porous rock fill structure, which serves to detain the flow, but a pipe structure can also be used. A temporary sediment trap may be located in a drainageway, at a storm drain inlet, or at other points of discharge from a disturbed area. They may be constructed independently or in conjunction with diversions and may be used in most drainage situations to prevent excessive siltation of pipe structures (USEPA, 1992).

Applicability Sediment traps can simplify the storm water control plan design process by trapping sediment at specific spots at a construction site (USEPA, 1992). They should be installed as early in the construction process as possible and are primarily effective as a short term solution to trapping sediment from construction sites (WYDEQ 1999). Natural drainage patterns should be noted, and sites where runoff from potential erosion can be directed into the traps should be selected. Traps are most effective when capturing runoff from areas where 2 to 5 acres drain to one location. Sediment traps should not be located in areas where their failure resulting from excess storm water runoff can lead to further erosive damage of the landscape. Alternative diversion pathways should be designed to accommodate these potential overflows. Traps should be accessible for clean out and located so that they do not interfere with construction activity. In addition, the traps are easily adaptable to most conditions.

Design and Implementation Criteria Hydrologic Design A sediment trap should be designed to maximize surface area and sediment settling. This will increase the effectiveness of the trap and decrease the likeliness of backup during and after periods of high runoff intensity. The design of a trap includes determining the storage volume, surface area, dimensions of spillway or outlet, and elevations of embankment (USDOT, 1995). Sediment traps should be designed to meet a 2 year, 24 hour duration storm event, but the selection of a return period varies among regulatory agencies (IDNR, 1992).

Storage volume is created by a combination of excavation of land and construction of an embankment to detain runoff (USDOT, 1995). Trap storage volume and length of spillway are determined as a function of the runoff volume and rate for the design storm. These parameters Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 77 will vary depending on return period rainfall and watershed hydrologic characteristics. Some standards specify a storage volume per acre disturbed. For example, Smolen et al. (1998) specified that approximate storage capacity of each trap should be at least 67 cubic yards per acre disturbed draining into the trap, but more recent guidelines suggest 134 cubic yards per acre of drainage area (VDCR, 2001). Any national standard, however, should be based on runoff volume and peak discharge in order to be generally applicable. Local regulations can translate this into applicable volume and area standards.

A more important criterion than storage volume relates to sediment trapping. If a trapping efficiency is specified, as in the case of South Carolina (SCDHEC, 1995), it is necessary to design for trapping efficiency. If a TSS or settleable solids effluent criterion is adopted (SCDHEC, 1995), settleable solids must be estimated. In both cases, a national standard should address how to estimate trapping efficiency or settleable solids. Efforts to model the sediment trapping that occurs in sediment traps have resulted in models that predict the settling in the ponded area (Barfield et al., 1996; Lindley et al., 1998). The results from model simulations show that trapping depends primarily on surface area of the impounded water and flow rate through the rock fill outlet. In fact, the ratio of peak outflow rate to surface area is the best simple predictor of trapping. The models utilize a modification of the Herrera and Felton (1991) relationship developed by Haan et al. (1994) to predict discharge rates. The predicted flow rates do not take into account clogging that can occur in rock fill. No models or procedures are available to estimate this clogging or its impact on flow criteria.

Design aids have also been developed for sediment traps, using simulations from the SEDIMOT III (Barfield et al., 2001; Hayes et al., 2001). In the model, predictions are made of trapping efficiency using the ratio of settling velocity for the d15 of the eroded sediment, divided by the ratio of discharge to ponded surface area. The design aid yields conservative estimates, but the database used for generating the design aid is based on the assumption that flow rates are not impacted by clogging. This latter assumption is not likely to be a critical issue, but should be addressed in future research.

Installation Specifications USDOT standards call for the embankment to be constructed of compacted earth, at a maximum height of 5 feet (1.5 meters), a width of 4 to 5 feet (1.2 meters), and side slopes of 2: 1or flatter. These values may change as a result of local criteria and with changing soil characteristics. Temporary vegetation should be applied to the embankment (USDOT). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 78 Two types of outlet structures are typically used for sediment traps, a rock outlet and a pipe outlet. Spillways of large stones or aggregate are the most common type of outlet designed for sediment traps. The crest of the spillway should be constructed 1 foot below the top of the embankment and the spillway depth 1.5 feet below the top of the embankment. Weir length of the spillway is determined based on the contributing drainage area (Table 5 18) (USDOT, 1995). The outlet apron should be a minimum of 5 feet long, and situated on level ground with a filter fabric foundation to ensure exit velocity of drainage to receiving stream is nonerosive (IDNR, 1992).

The length of the rock outlet should be determined based on peak discharge required and rock characteristics, typically rock diameter. Flow rate calculations can be made with the relationship of Herrera and Felton (1991) as modified by Haan et al. (1994). Alternatively, the USDOT has specified the weir length for a given drainage area as shown in Table 5 18. However, the values should be adjusted for each climatologic area to account for local hydrologic and return period rainfall.

Table 5 18. Weir Length for Sediment Traps Contributing Drainage Area Weir Length (ft) 1 4 2 5 3 6 4 10 5 12 Source: USDOT, 1995.

The pipe outlet, constructed of corrugated metal or PVC pipe riser, is an alternative to the rock outlet. Pipe diameter is based on the peak discharge rate required. To obtain appropriate freeboard, the top of pipe should be placed 1.5 feet below embankment elevation. Perforated pipe is sometimes used. USDOT suggests perforations of 1 inch (25 mm) diameter holes or 0.5 x 6 inch (13 x 15 mm) slits in the upper two thirds of the pipe; however, the discharge should be calculated for this pipe specification to ensure that it matches the required peak discharge.

The pipe should be placed vertically and horizontally above wet storage elevation (USDOT, 1995). Riprap should be used as an outlet protection and placed at the outlet of the barrel to prevent scour from occurring (USDOT, 1995). A stable channel should be provided to convey discharge to the receiving channel (USDOT, 1995).

Effectiveness If it is assumed that the flow can be accurately controlled by the rock fill outlet, sediment traps should operate as effectively as sediment basins, with trapping efficiencies reduced as a result of smaller surface areas. The NURP study (USEPA, 1993), Stahre and Urbonas (1990), and Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 79 Haan, et al., (1994), report that sediment basins effectively trapped sediment and chemical as shown in Table 5 19.

Table 5 19. Range of Measured Long Term Pollutant Removal for Sediment Detention Basins Item Removable Percentage Total suspended solids (TSS) 50 70 Total phosphorus (TP) 10 20 Nitrogen 10 20 Organic matter 20 40 Lead 75 90 Zinc 30 60 Hydrocarbons 50 70 Bacteria 50 90 Source: Stahre and Urbonas, 1990.

Information on the actual effectiveness of sediment trapsis limited. The discussion should start first with the flow hydraulics of the rock fill outlet typically employed as a principal spillway for sediment traps. Procedures for estimating flow through rock fill have been developed by Herra and Felton (1991) to estimate flow as a function of average rock diameter, standard deviation of rock size, and flow length. If these parameters could be controlled in an actual situation, the flow could be accurately predicted. However, given that standard construction practices consist of end dumping the rock fill in place, one would expect little correlation between design and construction and the actual discharge and trapping efficiency would be expected to be dramatically different from the design. This analysis does not mean that sediment traps are ineffective, but that a given design could not be guaranteed to meet the effluent criteria, even though the predictions indicate compliance. Sediment trapping efficiency is a function of surface area and inflow rate (Smolen, 1988). Those traps that provide pools with large length towidth ratios have a greater chance of success.

Sediment traps remove larger size sediment, primarily sized from silt to sands, by slowing water velocity and allowing for sediment settling in ponded water (Haan et al., 1994). Although sediment traps allow for settling of eroded soils, because of their short detention periods for storm water they typically do not remove fine particles such as silts and clays without chemical treatment. Sediment settling ability is related to the square of the particle size; halving particle sizes quadruples the time needed to achieve settlement (WYDEQ 1999). To increase overall effectiveness, traps should be constructed in smaller areas with low slopes.

Sediment traps are typically designed to remove only sediment from surface water, but some non sediment pollutants are trapped as well (Haan et al., 1994). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 80 Limitations Common concerns associated with sediment traps are included in Table 5 20.

Table 5 20. Common Concerns Associated with Sediment Traps Common Concern Result Inadequate spillway size Results in overtopping of the dam and possible failure of the structure Omission or improper installation of geotextile fabric Results in piping under the sides or bottom of the stone and outlet section Low point in embankment caused by inadequate compaction and settling Results in overtopping and possible failure Stone outlet apron does not extend to stable grade Results in erosion below the dam Stone size too small or backslope too steep Results in stone displacement Inadequate vegetative protection Results in erosion of embankment Inadequate storage capacity Caused by sediment not being removed from the basin enough Contact slope between stone spillway and earth embankment too steep Results in piping failure Outlet pipe installed in vertical side of trench Results in piping failure of embankment Corrugated tubing used as outlet pipe Results in crushed pipe and inadequate outlet capacity Source: IDMR, 1992.

Maintenance The primary maintenance consideration for temporary sediment traps is the removal of accumulated sediment from the basin, which must be done periodically to ensure the continued effectiveness of the sediment trap. Sediments should be removed when the basin reaches approximately 50 percent sediment capacity.

A sediment trap should be inspected after each rainfall event to ensure the trap is draining properly. Inspectors should also check the structure for damage from erosion or piping. The depth of the spillway should be checked and maintained at a minimum of 1.5 feet below the low point of the trap embankment.

Cost The cost of installing temporary sediment traps ranges from $0.20 to $2.00 per cubic foot of storage (about $1,100 per acre of drainage). For a recent national assessment, USEPA (1999) estimated the following costs for sediment traps, which vary as a function of the volume of storage: $513 for 1,800 cubic yards, $1,670 for 3,600 cubic yards, and $2,660 for 5,400 cubic yards. In addition, it has been reported that a sediment trap has an annual maintenance cost of 20 percent of installation cost (Brown et al., 1997). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 81 <IP_ADDRESS>.5 SEDIMENT BASINS General Description A sediment basin is a storm water detention structure formed by constructing a dam across a drainageway or excavating a storage volume at other suitable locations and using it to intercept sediment laden runoff. Sediment basins are generally larger and more effective in retaining sediment than temporary sediment traps and typically remain active throughout the construction period. Jurisdictions that require postdevelopment flow to be less than or equal to predevelopment flow during construction may employ the designed detention facilities as a temporary sediment basin during construction.

When sediment basins are designed properly, they can control sediment pollution through the following functions (Faircloth, 1999):

° Sediment laden runoff is caught to form an impoundment of water and create conditions where sediment will settle to the bottom of the basin.

° Treated runoff is released with less sediment concentration than when it entered the basin.

° Storage is provided for accumulated sediment, and resuspension by subsequent storms is limited.

Applicability Sediment basins should be located at a convenient concentration point for sediment laden flows (NCDNR, 1988). Ideal sites are areas where natural topography allows a pond to be formed by constructing a dam across a natural swale; such sites are preferred to those that require excavation (Smolen et al., 1998).

Sediment basins are also applicable in drainage areas where it is anticipated that other erosion controls, such as sediment traps, will not be sufficient to prevent off site transport of sediment. Choosing to construct a sediment basin with either an earthen embankment or a stone/ rock dam will depend on the materials available, location of the basin, and desired capacity for storm water runoff and settling of sediments.

Rock dams are suitable where earthen embankments would be difficult to construct or where riprap is readily available. Rock structures are also desirable where the top of the dam structure is to be used as an emergency overflow outlet. These riprap dams are best for drainage areas of less than 50 acres. Earthen damming structures are appropriate where failure of the dam will not result in substantial damage or loss of property or life. If properly constructed, sediment basins with earthen dams can handle storm water runoff from drainage basins as large as 100 acres. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 82 Design and Implementation Criteria Hydrologic Design A sediment basin can be constructed by excavation or by erecting an earthen embankment across a low area or drainage swale. Sediment basins can be designed to drain completely during dry periods, or they can be constructed so that a shallow, permanent pool of water remains between storm events. Depending on the size of the basin constructed, the basin may be subject to additional regulation, particularly state and federal regulations related to dam safety.

Sediment basins can be used for any size watershed, but the U. S. Department of Transportation recommends a drainage area range of 5 to 100 acres (USDOT, 1995). Components of a sediment basin that must be considered in the hydrologic design include the following (Haan et al., 1994):

° A sediment storage volume sized to contain the sediment trapped during the life of the structure or between cleanouts.

° A permanent pool volume (if included) above the sediment storage to protect trapped sediment and prevent resuspension as well as providing a first flush of discharge that has been subjected to an extended detention period.

° A detention volume that contains storm runoff for a period sufficient to trap the necessary quantity of suspended solids.

° A principal spillway that can be a drop inlet pipe and barrel, a trickle tube, or other type of controlled release structure.

° An emergency spillway that is designed to handle excessive runoff from the rarer events and prevent overtopping.

The following recommended procedures for conducting the hydrologic design are summarized from Haan et al. (1994).

Sediment Storage Volume. This volume should be sufficient to store the sediment trapped during the life of the structure or between cleanouts. Sediment storage volume can be calculated based on sediment yield using relationships such as the Revised Universal Soil Loss Equation with an appropriate delivery ratio (Renard et al., 1994) or a computer model such as SEDIMOT III (Barfield et al., 1996). Many design specifications, however, base the sediment storage volume on a volume per acre disturbed. This volume is highly site specific, depending on rainfall distributions, soil types, and construction techniques. It is recommended that care be exercised in developing appropriate values to be sure that existing variations in rainfall throughout a state or region are incorporated in the statutory requirements. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 83 Permanent Pool Volume. Providing a first flush of discharge that has been subjected to an extended detention period can help to minimize degradation of water quality and justify some permanent pool. The recommended capacity of the permanent pool varies with the regulatory agency. The U. S. Department of Transportation, for example, recommends 67 cubic yards per acre (126 m 3 /ha) (USDOT, 1995). If an effluent criterion such as allowable peak TSS or peak settleable solids is used, the final design of both permanent pool and detention volume should be selected only after using a computer model to predict the expected peak effluent concentrations.

Detention Volume. Storm runoff must be contained for a period of time sufficient to trap the necessary quantity of suspended solids. Since inflow is occurring simultaneously with outflow, the detention time for each plug of flow is different and should be considered individually. The size of the detention volume, as stated above, should also be developed in concert with determining the size of the permanent pool volume as well as the size of the principal spillway. When effluent TSS and settleable solids criteria are used, the size of the detention volume and permanent pool volume should be determined through on a computer model calculation of expected effluent concentrations for a given design. The return period used to size the detention volume depends on the regulatory agency, but a return period of 10 years is typical.

Principal Spillway. The principal spillway is a hydraulic outlet structure sized to provide the appropriate outflow rate to meet the effluent or trapping efficiency criteria. The principal spillway should have a dewatering device that slowly releases water contained in the detention storage over an extended period of time and at a rate determined to trap the required amount of sediment and/ or provide for the appropriate effluent concentration in the design storm. The more common outlet structures are the drop inlet structure and the trickle tube. Sizing of the principal spillway should follow standard hydrologic and sedimentology design procedures but sizing the structure to simply pass the design storm is inappropriate and will not result in meeting an effluent or trapping efficiency standard. The size to be used in a given structure should be determined based on the effluent or trapping efficiency standard being targeted and site specific hydrologic and soil conditions. Appropriate design will require the use of a computer model such as SEDIMOT III (Barfield et al., 1996) or design aids such as those developed for South Carolina (Hayes and Barfield, 1995). In general, the design is developed to maximize surface area, which will minimize peak discharge. Since failure of the dam could result in downstream damage, the design should be done and certified by a licensed engineer with expertise in hydrologic computation.

It has been proposed that a surface skimmer made of PVC, aluminum, or stainless steel and designed to prevent trash from clogging and can also be used to replace conventional principal spillways. The skimmer puts the basin drain just below the water surface, allowing for a constant head rather than variable head from the bottom. It is proposed that the skimmer allows water to be released from the top of the basin, which would be the cleanest water, and that the skimmer properly regulates the fill and draining of the basin (Fairchild, 1999). The skimmer floats on the surface of the basin and rises as water in the basin rises during the storm. After the storm the skimmer slowly releases water from the basin. As the basin drains, the skimmer Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 84 settles to the bottom, draining the entire pool except for a pool directly under the skimmer. The skimmer can be attached directly to an outlet pipe that drains through the dam or can be attached to an outlet pipe through a riser. It is important to point out that use of the skimmer is controversial and not universally recognized as a good concept. Conventional hydraulic flow theory would not concur with the statement that the flow would come only from the surface, unless the pond had significant thermal gradients preventing flow from deeper levels. A single hole placed just above the sediment cleanout level can also dewater the basin slowly.

Emergency Spillway. Since overtopping of the dam can cause failure and downstream damage, an emergency spillway is necessary to handle excessive runoff from the rarer events and prevent overtopping. The design storm for the emergency spillway will depend on the hazard classification of the sediment basin. Typical return periods vary between 25 and 100 years, with 25 years recommended by the USDOT. Sizing of the emergency spillway is typically accomplished to simply transmit the rare event without eroding the base of the spillway. Procedures for making the hydrologic and hydraulic computations are summarized in Haan et al. (1994). Again, since failure of the dam could result in downstream damage, the design should be done and certified by a licensed engineer with expertise in hydrologic computation.

Installation Criteria The embankment for permanent sediment basins should use standard geotechnical construction techniques. The fill is typically constructed of earthen fill material placed and compacted in continuous layers over the entire length of the fill. USDOT recommends 6 to 8 inch layers (USDOT, 1995). The embankment should be stabilized with vegetation after construction of the basin. A cutoff trench should be excavated along the centerline of the dam to prevent excessive seepage beneath the dam, and sized using standard geotechnical computations. USDOT recommends that a minimum depth of the cutoff trench should be about 2 feet (600 mm), the height should be to the riser crest elevation, the minimum bottom width should be 4 feet (1.2 m) or wide enough for compaction equipment, and slopes should be no steeper than 1: 1.

Sediment basins can also be constructed with rock dams in a design that is similar to a sediment basin with an earthen embankment. It is important to remember that rock fill is highly heterogeneous and that flow rates calculated with any available procedure are not likely to match those that will actually occur. Since sediment trapping is inversely proportional to flow rate, the trapping efficiency will be impacted significantly. No data are available to determine the variability of rock fill in actual installations so that confidence intervals can be placed on predicted flow rates. Such data should be collected and the confidence intervals calculated prior to recommending the use of rock dams as outlets on any structures other than sediment traps.

Effectiveness The effectiveness of a sediment basin depends primarily on the sediment particle size and the ratio of basin surface area to inflow rate (Smolen et al., 1998; Haan et al., 1994). Basins with a Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 85 large surface area to volume ratio will be most effective. Studies by Barfield and Clar (1985) showed that a surface area to peak discharge ratio of 0.01 acres per cubic square foot would trap more than 75 percent of the sediment coming from the Coastal Plain and Piedmont regions in Maryland. This efficiency might vary for other regions of the country and should not be used as a national standard. Studies by Hayes et al. (1984) and Stevens et al. (2001), however, show that similar relationships can be developed for other locations.

Laboratory data collected on pilot scale facilities are available on the trapping efficiency of sediment basins, on effluent concentrations, on dead storage and flow patterns, and on the impacts of chemical flocculants on sediment trapping (Tapp et al., 1981; Wilson et al., 1984; Griffin et al., 1985; Jarrett et al., 1999; Ward et al., 1977, 1979). In general, the laboratory studies show that pilot scale ponds can be expected to trap from 70 to 90 percent of sediment, depending on the sediment characteristics, pond volume, and flow rate. The trapping efficiency and effluent concentration are, in general, related to the overflow rate and can be reasonably well predicted using a plug flow model (Ward et al., 1977, 1979) and a Continuously Stirred Tank Reactor (CSTR) model (Wilson et al., 1982; Wilson et al., 1984). Extensive field scale data are available on long term trapping efficiency in storm water detention basins (Brune, 1953) in which the annual trapping efficiency is related to the annual capacity inflow ratio of the basin. These structures are not representative of those used for sediment ponds, but would be representative of those used for regional detention. A more limited database is available on single storm sediment trapping in the larger structures (Ward, et al., 1979) and on a field laboratory structure at Pennsylvania State University (Jarret et al., 1999).

For maximum trap efficiency, Smolen et al. (1988) recommend the following:

° Allow the largest surface area possible, maximize the length to width ratio of the basin to prevent short circuiting, and ensure use of the entire design settling area;

° Locate inlets for the basin at the maximum distance from the principal spillway outlet;

° Allow the maximum reasonable time to detain water before dewatering the basin; and,

° Reduce the inflow rate into the basin and divert all sediment free runoff.

Jarett (1999) has shown that the smaller the depth of the basin, the more sediment is discharged. A 0.15 m (0.49 ft) deep basin lost twice as much sediment as a 0.46 m (1.50 ft) deep basin. Jarrett also found that the performance of a sediment basin will increase with the use of a skimmer in the principal spillway. The sediment discharged was 1.8 times greater with just a perforated riser than with a skimmer in the principal spillway. In addition, increasing the dewatering time, which will allow for more sediment deposition, decreases the sediment loss from the basin (Jarett, 1999). Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 86 Limitations Neither a sediment basin with an earthen embankment nor a rock dam should be used in areas of continuously running water (live streams). The use of sediment basins is not intended for areas where failure of the earthen or rock dam will result in loss of life, or damage to homes or other buildings. In addition, sediment basins should not be used in areas where failure will interfere with the use of public roads or utilities.

Because sediment basins are usually temporary structures, they are often designed poorly and rarely receive the adequate attention and maintenance. As a result, these basins will not achieve the function for which they were designed, especially when conventional outlets cannot properly meter outflow to create an impoundment, thus allowing rapid release of sediment laden water from the bottom of the basin to escape (Faircloth, 1999).

Common concerns associated with sediment basins are included in Table 5 21.

Table 5 21. Common Concerns Associated with Sediment Basins Common Concern Result Piping failure along conduit Caused by improper compaction, omission of anti seep collar, leaking pipe joints, or use of unsuitable soil Erosion of spillway or embankment slopes Caused by inadequate vegetation or improper grading and sloping Slumping or settling of embankment Caused by inadequate compaction or use of unsuitable soil Bank failure due to slumping Caused by steep side slopes Erosion and caving below principal spillway Caused by inadequate outlet protection Basin not located properly for access Results in difficult, ineffective, and costly maintenance Sediment not properly removed Results in inadequate storage capacity and potential resuspension Lack of anti flotation Results in the riser and barrel being blocked with debris Principal and emergency spillway on design plans Results in improper disposal of accumulated sediment Safety or health hazard from pond water Caused by gravel clogging the dewatering system Principal spillway too small Results in frequent operation of emergency spillway and increased erosion potential Source: IDNR, 1992.

Maintenance Routine inspection and maintenance of sediment basins is essential to their continued effectiveness. Basins should be inspected after each storm event to ensure proper drainage from the collection pool and determine the need for structural repairs. Erosion from the earthen embankment or stones moved from rock dams should be replaced immediately.

Sediment basins must be located in an area that is easily accessible to maintenance crews for removal of accumulated sediment. Sediment should be removed from the basin when its storage Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 87 capacity has reached approximately 50 percent. Trash and debris from around dewatering devices should be removed promptly after rainfall events.

Cost The sediment basin has a 25 percent annual maintenance cost as a percentage of installation (Brown et al., 1997).

If constructing a sediment basin with less than 50,000 cubic feet of storage space, the cost of installing the basin ranges from $0.20 to $1.30 per cubic foot of storage (about $1,100 per acre of drainage). The average cost for basins with less than 50,000 cubic feet of storage is approximately $0.60 per cubic foot of storage (USEPA, 1993).

If constructing a sediment basin with more than 50,000 cubic feet of storage space, the cost of installing the basin ranges from $0.10 to $0.40 per cubic foot of storage (about $550 per acre of drainage). The average cost for basins with greater than 50,000 cubic feet of storage is approximately $0.30 per cubic foot of storage (USEPA, 1993).

As an alternative costing method, designers can use cost curves developed for permanent basins used to manage storm water from urban areas. However, since permanent storm water basins typically include design features that would not be included in temporary sediment basins, this approach is expected to greatly overestimate the actual costs to construct sediment basins. For many sites, sedimentation basins installed for erosion and sediment control during the construction phase are retained/ modified to meet other runoff management requirements. For example, site flood prevention requirements for the 10 year rainfall event can be met with a pond made from a converted sedimentation basin. As a result, sedimentation basins installation costs are partially offset by a later cost reduction or savings. Work by the Center for Watershed Protection (CWP, 1996), provides capital cost equations for different types of sediment basins for permanent installations. For example,

dry extended duration ponds CC = 8.16 (Vs) ^ 0.78 and for all ponds regardless of type (including wet ponds)

CC = 20.18 (Vs) ^ 0.70 Where: CC = base construction cost, not including design, engineering, and contingencies Vs = Storage volume below the crest of the emergency spillway, in cubic feet Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 88 Design, engineering, and contingency costs are given as approximately 32 percent of the base construction costs. Base construction costs for permanent ponds are composed of approximately 48 percent excavation/ grading cost, 36 percent control structure cost, and 16 percent appurtenances cost. R. S. Means (2000) suggests the cost to remove the eroded sediment collected in a small basin during construction is approximately $4 per cubic yard (value includes a 100 percent surcharge for wet excavation). Disposal of material on site will be an additional cost that can only be computed from site specific conditions. The cheapest management of dredge material is application to land areas adjacent to the basin, followed with application of a vegetative cover.

<IP_ADDRESS> OTHER CONTROL PRACTICES <IP_ADDRESS>.1 STONE OUTLET STRUCTURE Description A stone outlet structure is a temporary stone dike installed in conjunction with and as a part of an earth dike. The purpose of the stone outlet structure is to impound sediment laden runoff, provide a protected outlet for an earth dike, provide for diffusion of concentrated flow, and allow the area behind the dike to dewater slowly. The stone outlet structure can extend across the end of the channel behind the dike or be placed in the dike itself. In some cases, more than one stone outlet structure can be placed in a dike.

Applicability Stone outlet structures apply to any point of discharge where there is a need to discharge runoff at a protected outlet or to diffuse concentrated flow for the duration of the period of construction. The drainage area to this practice is typically limited to one half acre or less to prevent excessive flow rates. The stone outlet structure should be located so as to discharge onto an already stabilized area or into a stable watercourse. Stabilization should consist of complete vegetative cover and paving, sufficiently established to be erosion resistant.

Design and Installation Criteria Design criteria are of two types, hydrologic design for a required trapping of sediment and/ or flow rate to pass the design storm; and selection of appropriate installation criteria such that the stone outlet will perform as designed Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 89 Hydrologic Design The hydrologic design should be based on the design storm and standard hydraulic calculations and should include the following considerations:

° Design Rainfall and Design Storm. The design storm should be specified by the regulatory authority. Typically a return period of 2 to 5 years is used. Runoff rates should be calculated with standard hydrologic procedures, as allowed by the regulatory authority.

° Drainage Area. The drainage area to this structure is typically limited to less than half an acre to ensure that the flow rates are not excessive.

° Length of Crest and Height of Stone Fill. The crest length and height of stone fill should be of sufficient size to transmit the design storm without overtopping. The volume of water stored behind the dike can be estimated, but would require a routing of the storm flow in the design storm. Flow through the stone outlet can be calculated using the relationships of Herrera and Felton (1991) as modified by Haan et al. (1994). The height of the fill should be small enough to prevent excessive flow velocities through the stone fill and prevent undercutting.

° Outlet Stabilization. The discharge from the stone outlet should be stabilized with vegetated waterways or riprap until the flow reaches a stable channel. Design of the stabilized outlet should follow procedures presented earlier.

Installation Criteria Specifications A stone outlet structure should conform to the following specifications:

° The outlet should be composed of 2 to 3 inch stone or recycled concrete equivalent is preferred, but clean gravel may be used if stone is not available.

° The crest of the stone dike should be at least 6 inches lower than the lowest elevation of the top of the earth dike and should be level.

° The stone outlet structure should be embedded into the soil a minimum of 4 inches.

° The minimum length of the crest of the stone outlet structure should be 6 feet.

° The baffle board should extend 1 foot into the dike and 4 inches into the ground and be staked in place.

° The drainage area to this structure should be less than half an acre. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 90 <IP_ADDRESS>.2 ROCK OUTLET PROTECTION Description Rock outlet structures are rocks that are placed at the outfall of channels or culverts to reduce the velocity of flow in the receiving channel to nonerosive rates.

Applicability This practice applies where discharge velocities and energies at the outlets of culverts are sufficient to erode the next downstream reach and it applicable to outlets of all types such as sediment basins, storm water management ponds, and road culverts.

Design and Installation Criteria Hydrologic Design Hydrologic design consists primarily of selecting the design runoff rate and sizing the outlet protection. Standard hydrologic calculations should be used to make the calculation, using an appropriate return period storm for the outlet being protected. Typical return periods range from 2 to 10 years.

Sizing the outlet protection consists of:

° Selecting the Type of Outlet Protection. The outlet protection may consist of a plunge pool (scour hole), an apron type arrangement, or an energy dissipation basin (Haan et al., 1994). The design of each differs. Plunge pools are typically used for outlet pipes that are elevated above the water surface. Aprons are used for other types of outlets.

° Selecting the Geometry of the Outlet. Plunge pool geometry is based on the flow rate, pipe size and slope, tailwater depth, and size of the riprap lining (Haan et al., 1994). Apron dimensions are determined by the ratio of the tailwater depth to pipe diameter (Haan et al., 1994). Energy dissipation basins are used as an alternative to the plunge pool. Dimensions are a function of the brink depth in the pipe at the design flow, pipe diameter, and size of riprap (Haan et al., 1994).

° Size of Rock Lining. The size of the rock lining is a function of the discharge, pipe size, tailwater depth, and geometry selected. Details on sizing the rock are given in Haan et al. (1994).

The design method presented here applies to the sizing of rock riprap and gabions to protect a downstream area. It does not apply to rock lining of channels or streams. The design of rock outlet protection depends entirely on the location. Pipe outlets at the top of cuts or on slopes Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 91 steeper than 10 percent cannot be protected by rock aprons or riprap sections due to reconcentration of flows and high velocities encountered after the flow leaves the apron.

Installation Criteria The following criteria should be considered.

° Bottom Grade: The outlet protection apron should be constructed with no slope along its length. There should be no obstruction at the end of the apron. The elevation of the downstream end of the apron should be equal to the elevation of the receiving channel or adjacent ground.

° Alignment: The outer protection apron should be located so that there are no beds in the horizontal alignment.

° Materials: The outlet protection may be done using rock riprap, or gabions. Riprap should be composed of a well graded mixture of stone sized so that 50 percent of the pieces, by weight, should be larger than the size determined by using the charts. The minimum d50 size to be used should be 9 inches. A well graded mixture is defined as a mixture composed primarily of larger stone sizes but with a sufficient mixture of other sizes to fill the smaller voids between the stones. The diameter of the largest stone in such a mixture should be 2.0 times the size selected in Table 5 22 (MDE, 1994).

° Thickness: The SHA riprap specification values are summarized in Table 5 22.

Table 5 22. Riprap Sizes and Thicknesses (SHA Specifications) D50 (inches) D100 (inches) Thickness (inches) Class I 9.5 15 19 Class II 16 24 32 Class III 23 34 46 ° Stone Quality: Stone for riprap should consist of field stone or rough and hewn quarry stone. The stone should be hard and angular and of a quality that will not disintegrate on exposure to water or weathering. The specific gravity of the individual stones should be at least 2.5. Recycled concrete equivalent may be used provided it has a density of at least 150 pounds per cubic foot and does not have any exposed steel or reinforcing bars.

° Filters: A filter is a layer of material placed between the riprap and the underlying soil surface to prevent soil movement into and through the riprap to prevent piping, reduce uplift pressure, and collect water. Riprap should have a filter placed under it in all cases. A filter can be of two general forms: a gravel layer or a geotextile. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 92 ° Gabions: Gabion baskets may be used as rock outlet protection, provided they are made of hexagonal triple twist mesh with heavily galvanized steel wire. The maximum lined dimension of the mesh opening should not exceed 4.5 inches. The area of the mesh opening should not exceed 10 square inches. Gabions should be fabricated in such a manner that the sides, ends, and lid can be assembled at the construction site into a rectangular basket of the specified sizes. Gabions should be of a single unit construction and should be installed according to the manufacturer's specifications. The area on which the gabion is to be installed should be graded as shown on the drawings. Foundation conditions should be the same as for placing rock riprap. Geotextiles should be placed under all gabions. Gabions must be keyed in to prevent undermining of the main gabion structure.

° The subgrade for the filter, riprap, or gabion should be prepared to the required lines and grades. Any fill required in the subgrade shall be compacted to a density of approximately that of the surrounding undisturbed material.

° The rock or gravel should conform to the specified grading limits when installed in the riprap or filter, respectively.

° Geotextiles should be protected from punching, cutting, or tearing. Any damage other than occasional small holes should be repaired by placing another piece of geotextile fabric over the damaged part or by completely replacing the geotextile fabric. All overlaps, whether for repairs or for joining two pieces of geotextile fabric, should be a minimum of 1 foot in length.

° Stone for the riprap or gabion outlets may be placed by equipment. They should be constructed to the full course thickness in one operation and in such a manner as to avoid displacement of underlying materials. Care should be taken to ensure that the stone is not placed so that rolling will cause segregation of stone by size, i. e., the stone for riprap or gabion outlets should be delivered and placed in a manner that will ensure that it is reasonably homogeneous with the smaller stones filling the voids between the larger stones. Riprap must be placed in a manner to prevent damage to the filter blanket or geotextile fabric. Hand placement will be required to the extent necessary to prevent damage to the permanent works.

° Stone should be placed so that it blends in with the existing ground and the depth to the stone surface is sufficient to transmit the flow without spilling over onto the unprotected surface.

Effectiveness There is currently no information on the effectiveness of rock outlet structures. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 93 Limitations Common problems with rock outlet structures include the following:

° Foundation not excavated deep enough or wide enough— restricts the flow cross section, resulting in erosion around the apron and sour holes at the outlet.

° Riprap apron should be placed on a suitable foundation to prevent downstream erosion.

° Riprap installed smaller than specified— results in rock displacement; selectively grouting over the rock materials may stabilize the situation.

° Riprap not extended enough to reach a stable section of the channel— results in downstream erosion.

° No filter installed under the riprap— results in stone displacement and erosion of the foundation.

Maintenance Once a riprap outlet has been installed, the maintenance needs are very low. It should be inspected after high flows to see if scour has occurred beneath the riprap, if flows have occurred outside the boundaries of the riprap and caused scour, or if any stones have been dislodged. Repairs should be made immediately.

Cost R. S. Means indicates machine placed riprap costs of approximately $40 per cubic yard. For a riprap maximum size between 15 and 24 inches, a cubic yard of riprap will cover between 13.5 and 17 square feet for channel bed (assuming depth of riprap as given in Table 5 22). This suggests that riprap lining will be between $21 and $27 per square foot of outlet (includes materials, labor, and equipment, with overhead and profit). R. S. Means (2000) provides a cost range for gabions ($ 2.80 to $9 per square foot of coverage) for stone fill depths of 6" to 36", respectively. These costs include all costs of materials, labor, and installation.

<IP_ADDRESS>.3 SUMP PIT Description A sump pit is a temporary pit from which pumping is conducted to remove excess water while minimizing sedimentation. The purpose of the sump pit is to filter water being pumped to reduce sedimentation to receiving streams. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 94 Applicability Sump pits are constructed when water collects and must be pumped away during excavating, cofferdam dewatering, maintenance or removal of sediment traps and basins, or other uses as applicable, such as for concrete wash out.

Design and Installation Criteria Hydrologic Design The only hydrologic calculation is determining the expected flow rate and volume to be handled. This should follow standard hydrologic computational procedures based on design rainfall, surface and soil conditions, and the size of the pump.

Installation Criteria and Specifications The number of sump pits and their locations should be determined by the designer and included on the plans. Contractors may relocate sump pits to optimize use, but discharge location changes should be coordinated with inspectors.

A perforated vertical sandpipe is wrapped with ½ inch hardware cloth and geotextiles and then placed in the center of an excavated pit which is then backfilled with filter material consisting of anything from clean gravel to stone. Water is then pumped from the center of the sandpipe to a suitable discharge area such as into a sediment trap, sediment basin, or stabilized area.

A sump pit should conform to the following specifications:

° Pit dimensions are variable, with the minimum diameter being twice the diameter of the sandpipe.

° The sandpipe should be constructed by perforating a 12 to 36 inch diameter pipe, then wrapping it with ½ inch hardware cloth and geotextiles. The perforations should be ½ x 6 inch slits or 1 inch diameter holes 6 inches on center.

° The sandpipe should extend 12 to 18 inches above the lip of the pit or riser crest elevation (basin dewatering), and filter material should extend 3 inches minimum above the anticipated standing water level.

Effectiveness There is currently no information on the effectiveness of the sump pit. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 95 Limitations The sump pit must be properly maintained and pumped regularly to avoid clogging.

Maintenance To maintain, sump pits must be removed and reconstructed when water can no longer be pumped out of the sandpipe.

Cost R. S. Means (2000) provides information appropriate for assessment of a wide range of dewatering scenarios (i. e., different sump sizes, dewatering durations, and discharge conditions). In general, installation of earthen sump pits are listed as costing approximately $1.50 per cubic foot of sump volume. Piping to and away from the sump ranges from $30 to $60 per linear foot. Pump rentals and operation range between $150 and $500 per day of pumping, depending on the rate of dewatering. All costs include material, labor, and equipment, with overhead and profit.

<IP_ADDRESS>.4 SEDIMENT TANK Description A sediment tank is a compartmented tank container through which sediment laden water is pumped to trap and retain the sediment. The purpose of a sediment tank is to trap and retain sediment prior to pumping the water to drainageways, adjoining properties, and rights of way below the sediment tank site.

Applicability A sediment tank should be used on sites where excavations are deep and space is limited, such as urban construction, where direct discharge of sediment laden water to streams and storm drainage systems should be avoided.

Design and Installation Criteria The location of sediment tanks should facilitate easy cleanout and disposal of the trapped sediment to minimize interference with construction activities and pedestrian traffic. The tank size should be determined according to the storage volume of the sediment tank, 1 cubic foot of storage for each gallon per minute of pump discharge capacity. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 96 Effectiveness There is currently no information on the effectiveness of sediment tanks.

Limitations The sediment tank does not provide any natural infiltration; thus, the trapped sediment and storm water must be disposed of properly.

Maintenance To properly maintain the sediment tank, it needs to be in a location that is easy to access.

Cost There is currently no information on the cost of sediment tanks.

<IP_ADDRESS>.5 STABILIZED CONSTRUCTION ENTRANCE Description The purpose of stabilizing entrances to a construction site is to minimize the amount of sediment leaving the area as mud attached to motorized vehicles. Installing a pad of gravel over filter cloth where construction traffic leaves a site can help stabilize a construction entrance. As a vehicle drives over the gravel pad, mud and other sediments are removed from the vehicle's wheels (sometimes by washing) and offsite transport of soil is reduced. The gravel pad also reduces erosion and rutting on the soil beneath the stabilization structure. The fabric reduces the amount of rutting caused by vehicle tires by spreading the vehicle's weight over a larger soil area than just the tire width. The filter fabric also separates the gravel from the soil below, preventing the gravel from being ground into the soil.

Applicability Typically, stabilized construction entrances are installed at locations where construction traffic leaves or enters an existing paved road. However, the applicability of site entrance stabilization should be extended to any roadway or entrance where vehicles will access or leave the site.

From a public relations point of view, stabilizing construction site entrances can be a worthwhile exercise. If the site entrance is the most publicly noticeable part of a construction site, stabilized entrances can improve the appearance to passersby and improve public perception of the construction project by reducing the amount of mud tracked onto adjacent streets. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 97 Design and Installation Considerations Hydrologic Design Not applicable.

Installation Criteria and Specifications All entrances to a site should be stabilized before construction begins and further disturbance of the site area occurs. The stabilized site entrances should be long enough and wide enough so that the largest construction vehicle that will enter the site will fit in the entrance with room to spare. If many vehicles are expected to use an entrance in any one day, the site entrance should be wide enough for the passage of two vehicles at the same time with room on either side of each vehicle. For optimum effectiveness, a rock construction entrance should be at least 50 feet long and at least 10 to 12 feet wide (USEPA, 1992). If a site entrance leads to a paved road, the end of entrance should be "flared" (made wider as in the shape of a funnel) so that long vehicles do not go off the stabilized area when turning onto or off of the paved roadway.

If a construction site entrance crosses a stream, swale, roadside channel, or other depression, a bridge or culvert should be provided to prevent erosion from unprotected banks.

Stone and gravel used to stabilize the construction site entrance should be large enough so that they are not carried off site with vehicle traffic. In addition, sharp edged stone should be avoided to reduce the possibility of puncturing vehicle tires. Stone or gravel should be installed at a depth of at least 6 inches for the entire length and width of the stabilized construction entrance.

Effectiveness Stabilizing construction entrances to prevent sediment transport off site is effective only if all entrances to the site are stabilized and maintained. Also, stabilization of construction site entrances may not be very effective unless a wash rack is installed and routinely used (Corish, 1995) but this can be problematic for sites with multiple entrances with high vehicle traffic.

Limitations Although stabilizing a construction entrance is a good way to help reduce the amount of sediment leaving a site, some soil may still be deposited from vehicle tires onto paved surfaces. To further reduce the chance that these sediments will pollute storm water runoff, sweeping of the paved area adjacent to the stabilized site entrance is recommended.

For sites using wash stations, a reliable water source to wash vehicles before leaving the site might not be initially available. In this case, water may have to be trucked to the site at Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 98 additional cost. Discharge from the wash station should be directed into an appropriate sediment control structure.

Maintenance Stabilization of site entrances should be maintained until the remainder of the construction site has been fully stabilized. Stone and gravel might need to be periodically added to each stabilized construction site entrance to keep the entrance effective. Soil that is tracked offsite should be swept up immediately for proper disposal.

For sites with wash racks at each site entrance, sediment traps will have to be constructed and maintained for the life of the project. Maintenance will entail the periodic removal of sediment from the traps to ensure their continued effectiveness.

Cost Without a wash rack, construction site entrance stabilization costs range from $1,000 to $4,000. On average, the initial construction cost is around $2,000 per entrance. When maintenance costs are included, the average total annual cost for a 2 year period, is approximately $1,500.

If a wash rack is included in the construction site entrance stabilization, the initial construction costs range from $1,000 to $5,000, with an average initial cost of $3,000 per entrance. Total annual cost, including maintenance for an estimated 2 year life span, is approximately $2,200 per year (USEPA, 1993).

<IP_ADDRESS>.6 LAND GRADING Description Land grading involves reshaping the ground surface to planned grades as determined by an engineering survey, evaluation, and layout. Land grading provides more suitable topography for buildings, facilities, and other land uses and helps to control surface runoff, soil erosion, and sedimentation both during and after construction.

Applicability Land grading is applicable to sites with steep topography or easily erodible soils because it stabilizes slopes and decreases runoff velocity. Grading activities should maintain existing drainage patterns as much as possible. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 99 Design and Installation Criteria Before grading activities begin, decisions should be made regarding the steepness of cut and fill slopes and how the slopes will be protected from runoff, stabilized, and maintained. A grading plan that establishes which areas of the site will be graded, how drainage patterns will be directed, and how runoff velocities will affect receiving waters should be prepared. The grading plan also includes information regarding when earthwork will start and stop, establishes the degree and length of finished slopes, and dictates where and how excess material will be disposed of (or where borrow materials will be obtained if needed). Berms, diversions, and other storm water practices that require excavation and filling should also be incorporated into the grading plan.

A low impact development BMP that can be incorporated into a grading plan is site fingerprinting, which involves clearing and grading only those areas necessary for building activities and equipment traffic. Adhering to strict limits of clearing and grading helps to maintain undisturbed temporary or permanent buffer zones in the grading operation and provides a low cost sediment control measure that will help reduce runoff and off site sedimentation. The lowest elevation of the site should remain undisturbed to provide a protected storm water outlet before storm drains or other construction outlets are installed.

Effectiveness Land grading is an effective means of reducing steep slopes and stabilizing highly erodible soils when implemented with storm water management and erosion and sediment control practices in mind. Land grading is not effective when drainage patterns are altered or when vegetated areas on the perimeter of the site are destroyed.

Limitations Construction sites are routinely graded to prepare a site for buildings and other structures. Improper grading practices that disrupt natural storm water patterns might lead to poor drainage, high runoff velocities, and increased peak flows during storm events. Clearing and grading of the entire site without vegetated buffers promotes off site transport of sediments and other pollutants. Grading plans should be designed with erosion and sediment control and storm water management goals in mind; grading crews should be carefully supervised to ensure that the plan is implemented as intended. Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 100 Maintenance All graded areas and supporting erosion and sediment control practices should be periodically checked, especially after heavy rainfalls. All sediment should be promptly removed from diversions or other storm water conveyances. If washouts or breaks occur, they should be repaired immediately. Prompt maintenance of small scale eroded areas is essential to prevent these areas from becoming significant gullies.

Cost Land grading is practiced at virtually all construction sites— additional site planning to incorporate storm water and erosion and sediment controls in grading plans can require several hours of planning by a certified engineer or landscape architect. Extra time might be required to excavate diversions and construct berms, and fill materials might be needed to build up lowlying areas or fill depressions.

Where grading is performed to manage on site storm water, R. S. Means (2000) suggests the cost of fine grading, soil treatment, and grassing to be approximately $2 per square yard of earth surface area. Shallow excavation/ trenching (1 to 4 feet deep) with a backhoe in areas not requiring dewatering can be performed for $4 to $5 per cubic yard of removed material. Larger scale grading requires a site specific assessment of an alternative grading apparatus and a detailed fill/ excavation material balance to retain as much soil on site as possible.

<IP_ADDRESS>.7 TEMPORARY ACCESS WATERWAY CROSSING Description A temporary stream crossing is a structure erected to provide a safe and stable way for construction vehicle traffic to cross a running watercourse. The primary purpose of such a structure is to provide streambank stabilization, to reduce the risk of damaging the streambed or channel, and to reduce the risk of sediment loading from construction traffic. A temporary stream crossing may be a bridge, culvert, or ford.

Applicability Temporary stream crossings are applicable wherever heavy construction equipment must be moved from one side of a stream channel to the other or where lighter construction vehicles will cross the stream a number of times during the construction period. In either case, an appropriate method for ensuring the stability of the streambanks and preventing large scale erosion is necessary.

A bridge or culvert is the best choice for most temporary stream crossings. If properly designed, each can support heavy loads, and materials used to construct most bridges and culverts can be Development Document for Construction and Development Proposed Effluent Guidelines June 2002 5 101 salvaged after they are removed. Fords are appropriate in steep areas subject to flash flooding, where normal flow is shallow or intermittent across a wide channel. Fords should be used only where stream crossings are expected to be infrequent.

Design and Installation Criteria Because of the potential for stream degradation, flooding, and safety hazards, stream crossings should be avoided on a construction site whenever possible. Consideration should be given to alternative site access routes before arrangements are made to erect a temporary stream crossing. If it is determined that a stream crossing is necessary, an area where the potential for erosion is low should be selected. The stream crossing structure should be selected during a dry period if possible to reduce sediment transport into the stream.

If needed, over stream bridges are generally the preferred temporary stream crossing structure. The expected load and frequency of the stream crossing, however, will govern the selection of a bridge as the correct choice for a temporary stream crossing. These types of temporary bridges usually cause minimal disturbance to a stream's banks and cause the least obstruction to stream flow and fish migration. They should be constructed only under the supervision and approval of a qualified engineer.