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Successful control of erosion and sedimentation from construction activities should involve a system of BMPs which targets each stage of the erosion process. The most efficient approach involves minimizing the potential sources of sediment from the outset. This means limiting the extent and duration of land disturbance to the minimum needed, and protecting surfaces once they are exposed. The second stage of the BMP system involves controlling the amount of runoff and its ability to carry sediment by diverting incoming flows and impeding internally generated flows. The third stage involves retaining sediment that is picked up on the project site through the use of sediment-capturing devices. On most sites successful erosion and sedimentation control requires a combination of structural and vegetative practices. All of these stages are better performed using advance planning and good scheduling. The following is a list of BMPs for erosion and sediment control during construction activities in development settings, divided by function and identifying the stages of the BMP system to which they apply. Much of the material in this section was taken from Erosion and Sediment Control Planning and Design Manual by Smolen et al. (1988). For a discussion of permanent BMPs to treat stormwater following completion of construction for the life of the development, please refer to the Urban Stormwater BMPs.
The timing of land disturbing activities and installation of erosion and sedimentation control measures must be coordinated to minimize water quality impacts. In terms of major activities, the BMP system is typically installed in reverse order, starting with sediment capturing devices, followed by key runoff control measures and runoff conveyances, and finally involving major land clearing activities after the minimization and capture elements are in place. Often, construction operations which generate significant off-site sediment have failed to sequence activities in the proper order.
| Mulching: A protective blanket of straw or other plant residue, gravel, or synthetic material applied to the soil surface to minimize raindrop impact energy and runoff, foster vegetative establishment, reduce evaporation, insulate the soil, and suppress weed growth. Mulch provides immediate protection, and straw mulch is also typically used as a matrix for spreading plant seed. Organic mulches such as straw, wood chips, and shredded bark have been found to be the most effective. Straw typically requires some kind of tacking, such as liquid emulsions or netting. Netting may also be needed to hold mulch in place on slopes. Mats made from a wide variety of organic and synthetic materials are useful in establishing grass in channels and waterways, and they promote seedling growth. Mulching assists in the first, source reduction, and second, conveyance, stages of a BMP system. |
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| Permanent Seeding: Establishment of perennial vegetative cover with seed to minimize runoff, erosion, and sediment yield on disturbed areas. Disturbed soils typically require amendment with lime, fertilizer, and roughening. Seeding should be done together with mulching. Mixtures are typically most effective, and species vary with preferences, site conditions, climate, and season. Permanent seeding assists in the first, source reduction stage of a BMP system. |
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Riprap: A layer of stone designed to protect and stabilize areas subject to erosion, slopes subject to seepage, or areas with poor soil structure. Riprap is used on slopes where vegetation cannot be established, channel slopes and bottoms, stormwater structure inlets and outlets, slope drains, streambanks (see STREAMBANK BMPs, click HERE ), and shorelines. It should be a well-graded mixture of stone sizes, and should be underlain by a filter blanket of gravel, sand and gravel, or synthetic material to prevent soil movement into or through the riprap. Riprap can assist in all stages of a BMP system.
Sodding: Permanent stabilization of exposed areas by laying a continuous cover of grass sod. Sod is useful for providing immediate cover in steep critical areas and in areas unsuitable for seed, such as flowways and around inlets. Sod must be rolled over after placement to ensure contact, and then watered. Sodded waterways and steep slopes may require netting and pegging or stapling. Sodding assists in the first, source reduction, and second, conveyance, stages of a BMP system.
Surface Roughening: Roughening a bare, sloped soil surface with horizontal grooves or benches running across the slope. Grooves can be large-scale, such as stair-step grading with small benches or terraces, or small-scale, such as grooving with disks, tillers, or other machinery, or with heavy tracked machinery which should be reserved for sandy, noncompressible soils. Roughening aids the establishment of vegetative cover, improves water infiltration, and decreases runoff velocity, assisting in the first, source reduction, and second, pollutant transport, stages of a BMP system.
Temporary Gravel Construction Access: A graveled area or pad located at points where vehicles enter and leave a construction site, this BMP provides a buffer area where vehicles can drop their mud and sediment to avoid transporting it onto public roads, to control erosion from surface runoff, and to help control dust. This measure assists in the third, pollutant capture stage of a BMP system.
| Temporary Seeding: Planting rapid-growing annual grasses, small grains, or legumes to provide initial, temporary stabilization for erosion control on disturbed soils that will not be brought to final grade for more than approximately one month. Seeding is facilitated by fertilizing and surface roughening. Broadcast seeds must be covered by raking or chain dragging, while hydroseed mixtures are spread in a mulch matrix. Temporary seeding assists in the first, source reduction stage of a BMP system. |
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Topsoiling: Preserving and subsequently re-using the upper, biologically active layer of soil to enhance final site stabilization with vegetation. Topsoiling should not be conducted on steep slopes. Stockpiled soil should be contained with sediment barriers, and temporarily seeded for stability. Surfaces which will receive topsoil should be roughened just prior to spreading the soil to improve bonding. Spread topsoil should be lightly compacted to ensure good contact with the subsoil. Topsoil can act as a mulch, promoting final vegetation establishment, increasing water infiltration, and anchoring more erosive subsoils, assisting in the first, source reduction, and second, pollutant transport, stages of a BMP system.
Runoff Diversion: A structure that channels upslope runoff away from erosion source areas, diverts sediment-laden runoff to appropriate traps or stable outlets, or captures runoff before it leaves the site, diverting it to locations where it can be used or released without erosion or flood damage. Diversions include graded surfaces to redirect sheetflow, diversion dikes or berms which force sheetflow around a protected area, and stormwater conveyances (swales, channels, gutters, drains, sewers) which intercept, collect and redirect runoff (USEPA, 1992). Diversions can be either temporary or permanent in nature. Temporary diversions include excavation of a channel along with placement of the spoil in a dike on the downgradient side of the channel, and placement of gravel in a ridge below an excavated swale. Permanent diversions are used to divide a site into specific drainage areas, should be sized to capture and carry a specific magnitude of design storm, and should be constructed of more permanent materials. A water bar is a specific kind of runoff diversion that is constructed diagonally at intervals across a linear sloping surface such as a road or right-of-way that is subject to erosion. Water bars are meant to interrupt the accumulation of erosive volumes of water through their periodic placement down the slope, and divert the resulting segments of flow into adjacent undisturbed areas for dissipation. Runoff diversions assist in the second, conveyance, stage of a BMP system.
| Grass-Lined Channel: A swale vegetated with grass, which is dry except following storms and serves to convey specified concentrated stormwater runoff volumes, without resulting in erosion, to disposal locations. Typical uses include roadside swales, outlets for runoff diversions, site stormwater routing, and drainage of low areas. Channels should conform to the natural drainage patterns. Channels are not meant to collect sediment, as it will reduce their conveyance capacity. Lining with geotextile or other material is required if design flows are to exceed 2 feet per second. Channel vegetation should be allowed to establish before flows are introduced. Channels assist in the second, conveyance, stage of a BMP system. |
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Hardened Channel: A channel with erosion-resistant linings of riprap, paving, or other structural material designed for the conveyance and safe disposal of excess water without erosion. Hardened channels replace grass-lined channels where conditions are unsuitable for the latter, such as steep slopes, prolonged flows, potential for traffic damage, erodible soils, or design velocity over 5 feet per second. Channels assist in the second, conveyance, stage of a BMP system.
Paved Flume: A small concrete-lined channel to convey water down a relatively steep slope without causing erosion. Flumes serve as stable, permanent elements of a stormwater system receiving drainage from above a relatively steep slope, typically conveyed by diversions, channels, or natural drainageways. Setting the flume well into the ground is important, particularly on fill slopes. Some means of energy dissipation should be provided at the outlet, and an inlet bypass route should be available for extreme flows. Flumes assist in the second, conveyance, stage of a BMP system.
| Temporary Slope Drain: Flexible tubing or conduit extending temporarily from the top to the bottom of a cut or fill slope for the purpose of conveying concentrated runoff down the slope face without causing erosion. These are generally used in conjunction with diversions to convey runoff down a slope until permanent water disposal measures can be installed. Temporary slope drains assist in the second, conveyance, stage of a BMP system. |
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Level Spreader: An outlet designed to
convert concentrated runoff to sheet flow and disperse
it uniformly across a slope without causing erosion.
This structure is particularly well-suited for returning
natural sheet flows to exiting drainage that has been
altered by development, especially for returning sheet
flows to receiving ecosystems such as wetlands where
dispersed flow may be important for maintain
pre-existing hydrologic regimes. The outlet's receiving
area must be uniformly sloped and not susceptible to
erosion. Particular care must be taken to construct the
outlet lip completely level in a stable, undisturbed
soil to avoid formation of an outlet channel and
subsequent erosion. Erosion-resistant matting of some
kind may be necessary across the outlet lip depending on
expected flows. Alternative designs to minimize such
channeling include hardened structures, stiff grass
hedges, and segmenting discharge flows into a number of
smaller, adjacent spreaders. The level spreader is often
used as an outlet for runoff diversions. Level spreaders
assist in the second, conveyance, stage of a BMP system.
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| Outlet Stabilization Structure: A structure designed to control erosion at the outlet of a channel or conduit by reducing flow velocity and dissipating flow energy. This should be used where the discharge velocity of a structure exceeds the tolerances of the receiving channel or area. Designs will vary based on discharge specifics and tailwater conditions. A riprap-lined apron is the most commonly used practice for this purpose because of its relatively low cost and ease of installation. Riprap stilling basins or plunge pools should be considered in lieu of aprons where overfalls exit at the ends of pipes or where high flows would require excessive apron length. Outlet stabilization structures assist in the second, conveyance, stage of a BMP system. |
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Block and Gravel Inlet Protection: A temporary sediment control barrier formed around a storm drain inlet by the use of standard concrete block and gravel, to filter sediment from stormwater entering the inlet prior to stabilization of the contributing area soils, while allowing use of the inlet for stormwater conveyance. The height of the barrier should allow overflow into the inlet and not let overflow bypass the inlet to unprotected lower areas. An alternative design eliminates the blocks and involves only a gravel doughnut around the inlet. This practice can be used in combination with other temporary inlet protection devices, such as excavation and fabric. Inlet protection structures assist in the third, capture, stage of a BMP system.
| Excavated Drop Inlet Protection: A temporary excavated area around a storm drain drop inlet or curb inlet designed to trap sediment prior to discharge into the inlet. This practice allows use of the permanent inlet early in the development prior to stabilization of the contributing area soils. Frequent maintenance is required. This practice can be used in combination with other temporary inlet protection devices, such as fabric and block and gravel. Inlet protection structures assist in the third, capture, stage of a BMP system. |
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Fabric Drop Inlet Protection: A temporary fabric barrier placed around a drop inlet to help prevent sediment from entering storm drains during construction operations, while allowing use of the inlet for stormwater conveyance. The height of the barrier should allow overflow into the drop inlet and not let overflow bypass the inlet to unprotected lower areas. This practice can be used in combination with other temporary inlet protection devices, such as excavation and block and gravel. Inlet protection structures assist in the third, capture, stage of a BMP system.
| Sediment Basin/Rock Dam: An earthen or rock embankment located to capture sediment from runoff and retain it on the construction site, for use where other on-site erosion control measures are not adequate to prevent off-site sedimentation. Sediment basins are more permanent in nature than sediment traps, and can be designed as permanent features of a development. Basins are most commonly used at the outlets of diversions, channels, slope drains, or other runoff conveyances that discharge sediment-laden water. Earthen basins should use barrel and riser discharge structures, while rock dams can be designed to discharge over the top of the embankment, where a crest should be constructed as the low point. Smaller gravel should line the inside face of the rock dam. Sediment basins and rock dams assist in the third, capture, stage of a BMP system. |
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Sediment Fence (Silt Fence)/ Straw Bale Barrier:
A temporary sediment barrier consisting of filter fabric
buried at the bottom, stretched, and supported by posts, or
straw bales staked into the ground, designed to retain
sediment from small disturbed areas by reducing the velocity
of sheet flows. Because silt fences and straw bales can cause
temporary ponding, sufficient storage area and overflow
outlets should be provided. Ends must be well-anchored (USEPA,
1993). Sediment fences and straw bale barriers assist in the
third, capture, stage of a BMP system.
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Sediment Trap: A small, temporary ponding
basin formed by an embankment or excavation to capture
sediment from runoff. Traps are most commonly used at
the outlets of diversions, channels, slope drains, or
other runoff conveyances that discharge sediment-laden
water. It is important to consider provisions to protect
the embankment from failure from runoff events that
exceed the design capacity. Plan for nonerosive
emergency bypass areas. Make traps readily accessible
for periodic maintenance. High length-to-width ratios
minimize the potential for short-circuiting. The pond
outlet should be a stone section designed as the low
point. Sediment traps assist in the third, capture,
stage of a BMP system. |
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Sod Drop Inlet Protection: A permanent grass sod
sediment filter area around a storm drain drop inlet for use
once the contributing area soils are stabilized. This area is
well-suited for lawns adjacent to large buildings. Inlet
protection structures assist in the third, capture, stage of a
BMP system.
| Vegetated Filter Strip (VFS): A low-gradient vegetated area that filters solids from overland sheet flow. VFSs can be natural or planted, should have relatively flat slopes, and should be vegetated with dense-culmed, herbaceous, erosion-resistant plant species. The main factors influencing removal efficiency are the vegetation type and condition, soil infiltration rate, and flow depth and travel time, which are affected by size of contributing area, and slope and length of strip. Channelized flows decrease the effectiveness of VFSs. VFSs are often used as buffers bordering on construction areas. Level spreaders are often used to distribute runoff evenly across the VFS (Dillaha, 1989; USEPA, 1993). |
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Streambank Stabilization: For a discussion of different technologies used to stabilize streambanks from erosion due to excess runoff or artificially increased flows, please click here.
Streambed Stabilization: For a discussion of different technologies used to stabilize streambeds from erosion due to artificially increased flows, headcut erosion or gully erosion, please click here.
Temporary Stream Crossing: A bridge, ford, or temporary structure installed across a stream or water course for short-term use by construction vehicles or heavy equipment, intended to keep sediment out of the stream and avoid damage to the streambed. Stream crossings should be avoided if at all possible, since they are a direct source of water pollution, they can cause flooding, and they are expensive to construct. While bridges are the most expensive method, they are the most preferred, as they cause the least disturbance to streambeds, banks, and surrounding floodplain, they provide the least obstruction to flow, and have the least erosion potential. Culvert crossings are the most common form of crossing, but can cause the most damage to the stream environment, cause the most flow blockage, and therefore can result in the most erosion. Fords involve making cuts in the banks and placing stone over filter cloth in the stream. They are often used in steep areas subject to flash flooding where normal flow is shallow (<3 inches deep) or intermittent. Fords should only be used where crossings are infrequent and banks are low. Temporary crossings may overtop during peak storm events, unlike permanent crossings. Fill in the floodplain should be kept to a minimum to reduce erosion potential and avoid upstream flooding. Choose crossing sites where erosion potential is low. Try to locate temporary crossings where permanent crossings will occur. Where appropriate, install in-stream sediment traps immediately below stream crossings to reduce downstream sedimentation. Temporary stream crossings, and bridge designs in particular, should be undertaken by a qualified engineer.
Dillaha, T.A., J.H. Sherrard, and D. Lee, 1989. Long-Term Effectiveness of Vegetative Filter Strips. Water Environment and Technology, 1:418-421.
Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, A.L. Lanier, W.W. Woodhouse, and S.W. Broome, 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission, NC Dept. of Natural Resources and Community Development, Raleigh, NC.
USEPA, 1992. Storm Water Management For Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. EPA 832-R-92-006. U.S. Environmental Protection Agency, Office Of Water, Washington, DC.
USEPA, 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution In Coastal Waters. EPA-840-B-92-002, January 1993. U.S. Environmental Protection Agency, Office of Water, Washington, DC.
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NCSU Water Quality Group |
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