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[[file:Temporary_Sediment_Basin_with_Forebay.jpg|thumb|300px|alt=Temporary_Sediment_Basin_with_Forebay|<font size=3>Example of a temporary sediment trap with forebay to remove initial inflows prior to final settling and discharge. Note extension of rock forebay berm up side slopes – to prevent bypass – and geotextile along banks to prevent erosion. (Source: Tetra Tech) </font size>]]
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[[File:Technical information page image.png|100px|left|alt=image]]
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[[file:Temporary_Sediment_Basin_with_Forebay.jpg|thumb|300px|alt=Temporary_Sediment_Basin_with_Forebay|<font size=3>Temporary sediment trap with forebay, click image for more information (Source: Tetra Tech) </font size>]]
  
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==Temporary/Permanent Sediment Traps and Basins==
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Sediment traps and basins are settling ponds formed by excavation and/or an <span title="A levee, an artificial bank raised above the immediately surrounding land to redirect or prevent flooding"> '''embankment'''</span> that intercept and retain sediment-laden runoff from a construction site for a sufficient period of time to allow the majority of sediment to settle out prior to being released from the site. They may be constructed as smaller sediment traps – serving disturbed areas of less than five acres – or as larger sediment basins, handling mass grading runoff from subdivisions, commercial/institutional sites, or roadway projects. Proper use of these structures can greatly reduce sediment transport off-site; if properly designed, installed, and maintained, sediment removal efficiency of 80 percent or greater can be achieved, depending on <span title="an index (means of expression) indicating what sizes (particle size) of particles are present in what proportions (relative particle amount as a percentage where the total amount of particles is 100 %) in the sample particle group to be measured"> '''soil particle size'''</span>. Sediment traps are often temporary and usually decommissioned after the disturbed area is stabilized (i.e., with [https://stormwater.pca.state.mn.us/index.php?title=Minnesota_plant_lists vegetation] or other cover). Temporary sediment basins can be converted to <span title="stormwater management practice that will be operational after the land disturbing activities are completed"> '''permanent stormwater management'''</span> basins after construction is complete. Sediment traps and basins are very useful on construction sites with moderate to <span title="the definition of a steep slope is somewhat arbitrary and depends on the context and other site characteristics, such as soil type and vegetative cover. Slopes greater than 12 percent are considered highly erodible, while slopes greater than 18% are often considered severe"> '''steep slopes'''</span>. The selection of traps versus basins primarily depends on the size of the <span title="Contributing area is defined as the total area, including pervious and impervious surfaces, contributing to a BMP. It is assumed that in most cases, with the exception of green roofs and many permeable pavement systems, impervious surfaces will constitute more than 50 percent of the contributing area to the BMP and that most of this impervious is directly connected."> '''contributing drainage area'''</span> and plans for post-construction stormwater management, as discussed below.
Sediment traps and basins are settling ponds formed by excavation and/or an embankment that intercept and retain sediment-laden runoff from a construction site for a sufficient period of time to allow the majority of sediment to settle out prior to being released from the site. They may be constructed as smaller sediment traps – serving disturbed areas of an acre or less – or as larger sediment basins, handling runoff from subdivisions, commercial/institutional sites, or roadway projects. Proper use of these structures can greatly reduce sediment transport off-site; if properly designed, installed, and maintained, sediment removal efficiency of 80% or greater can be achieved. Sediment traps are often temporary, and usually decommissioned after the disturbed area is stabilized (i.e., with vegetation or other cover). Temporary sediment basins can be converted to permanent stormwater management basins after construction is complete. Sediment traps and basins are very useful on construction sites with moderate to steep slopes, and the selection of traps versus basins primarily depends on the size of the contributing drainage area and plans for post-construction stormwater management, as discussed below.
 
  
 
==Purpose and function==
 
==Purpose and function==
[[file:Small_Sediment_Trap.jpg|300px|thumb|alt=Small Sediment trap|<font size=3>Example of a small sediment trap. Note the rock-lined overflow berm, stabilized banks, and silt fence protection along inflow section. Silt fence in foreground helps prevent excessive sediment flow into the trap, reducing maintenance dredging needs. (Source: Georgia SWCC)</font size>]]
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[[file:Small_Sediment_Trap.jpg|300px|thumb|alt=Small Sediment trap|<font size=3>Small sediment trap, click image for more information (Source: Georgia SWCC) </font size>]]
  
Sediment traps retain runoff using embankments and other barriers and discharge through an overflow or piped outlet to a vegetated swale or other drainage feature. Sediment basins are typically larger than sediment traps and may feature earthen embankments that retain runoff for longer periods of time, releasing runoff via floating, perforated, or slotted risers, or when the water level exceeds the height of the riser pipe in the outlet structure.
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Sediment traps and basins function by intercepting and detaining site runoff, which allows soil particles to settle out prior to discharge. Sediment traps may serve several small catchments on a site, retaining runoff using embankments and other barriers and discharging through an <span title="protection of a surface by incorpoating or covering it with an erosion resistant material, such as rock"> '''armored'''</span> overflow or piped outlet to a vegetated <span title="are configured as shallow, linear channels. They typically have vegetative cover such as turf or native perennial grasses"> [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) '''swale''']</span> or other drainage feature. Sediment basins typically serve larger areas than sediment traps and may feature earthen embankments that retain runoff for longer periods of time, releasing runoff via floating, perforated, or slotted risers or floating <span title="a device to retain or remove floatables (e.g. oil) from water"> '''skimmers'''</span> that draw clarified water from the surface
  
 
==Applicability==
 
==Applicability==
Sediment traps and basins represent one of the most effective and reliable measures for treating sediment-laden runoff from construction sites. These structures are typically placed near the perimeter of the site, where flows concentrate in swales, ditches, or other low areas. Phasing of construction activities should be designed to allow sediment traps and basins to be utilized before upslope areas are disturbed, if possible, and until the contributing drainage area is fully stabilized. Specific considerations related to site applicability and permit applicability are discussed below.
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Sediment traps and basins are very similar in their siting, sizing, and design, with sediment traps serving areas of five acres or less and basins handling larger areas. They represent one of the most effective and reliable measures for treating sediment-laden runoff from construction sites. These structures are typically placed near the perimeter of the site, where flows concentrate in <span title="are configured as shallow, linear channels. They typically have vegetative cover such as turf or native perennial grasses"> [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) '''swales''']</span>, ditches, or other low areas. Sediment traps and basins should be constructed prior to disturbance of upslope areas, if possible, and continue functioning until the contributing drainage area is fully stabilized. Specific considerations related to site applicability and permit applicability are discussed below.
  
 
===Site applicability===
 
===Site applicability===
[[file:Siting_and_Design_Considerations_for_Sediment_Traps.tif.jpg|300px|thumb|alt=Sediment Trap Design Criteria|<font size=3> Siting and design considerations for sediment traps/basins (Source: Created by Tetra Tech for US EPA and State of Kentucky)</font size>]]
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[[file:Siting_and_Design_Considerations_for_Sediment_Traps.tif.jpg|300px|thumb|alt=Sediment Trap Design Criteria|<font size=3> Siting and design considerations for sediment traps/basins, click image for more information (Source: Created by Tetra Tech for US EPA and State of Kentucky) </font size>]]
  
Disturbed soils on a construction site have the potential to leave the site via stormwater runoff and negatively impact bodies of water, roadways, and neighboring property. Therefore, sediment traps and basins should be placed in such a way that they interrupt concentrated or sheet flows of stormwater discharge across a construction site. Sediment traps can be placed near the point of discharge and are often built in series to intercept and treat flow moving down long drainage paths through a site. Stormwater basins should be placed in low lying areas on the outer edge of a construction site where water naturally flows or is directed according to site plans. Sediment traps are extremely useful in perimeter control areas where silt fences will likely fail. Do not site sediment traps in high-velocity flow areas (e.g., culvert outlets, steep ditches) where excessive turbulence and scour erosion may interfere with sediment settling processes. Neither practice should be placed in surface waters (including intermittent streams) or within their required [[Construction stormwater best management practice – buffer zones|buffer zones]].
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Disturbed soils on a construction site have the potential to [https://stormwater.pca.state.mn.us/index.php?title=General_principles_for_erosion_prevention_and_sediment_control_at_construction_sites_in_Minnesota leave the site via stormwater runoff] and negatively impact <span title="A stream, river, lake, ocean, or other surface or groundwaters into which treated or untreated wastewater is discharged">  [https://stormwater.pca.state.mn.us/index.php?title=Protection_and_restoration_of_receiving_waters '''receiving water''']</span>, roadways, and neighboring property. Sediment traps and basins should intercept <span title="Storm runoff, flowing in a confined feature such as a channel, ditch, swale, river, etc. Concentrated flow often occurs after a maximum of 300 feet of sheet flow. concentrated flow can lead to severe down- or side-cutting in the resulting channel."> '''concentrated'''</span> or sheet flows of stormwater discharge from a construction site. Place sediment traps near the point of discharge and build traps in series to intercept and treat flow moving down long drainage paths through a site. Place stormwater basins in low lying areas on the outer edge of a construction site where water naturally flows or is directed according to site plans. Sediment traps are particularly useful in <span title="Temporary structural stormwater BMPs that surround and contain a site of exposed soil to prevent sediments from leaving the site."> [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Perimeter_controls_for_disturbed_areas '''perimeter control''']</span> areas where [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Perimeter_controls_for_disturbed_areas silt fences] will likely fail. Do not site sediment traps in high-velocity flow areas (e.g., culvert outlets, steep ditches) where excessive turbulence and <span title="forceful removal of soil or other material by flowing water"> '''scour'''</span> erosion may interfere with sediment settling processes. Neither practice should be placed in surface waters (including intermittent streams) or within their required <span title="a vegetative setback between development and streams, lakes, and wetlands whose aim is to physically protect and separate the resource from future disturbance or encroachment"> [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_best_management_practice_%E2%80%93_buffer_zones '''buffer zones'''].</span>
  
 
===Permit applicability===
 
===Permit applicability===
Section 14 (Temporary Sediment Basins) of the MPCA Construction Stormwater General Permit (2018) has several requirements regarding sediment basins, including:
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[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins Section 14 (Temporary Sediment Basins)] of the [https://stormwater.pca.state.mn.us/index.php?title=2018_Minnesota_Construction_Stormwater_Permit MPCA Construction Stormwater General Permit (2018)] has several requirements regarding sediment basins, including the following.
  
*(Section 14.2) Sediment basins are required where ten (10) or more acres of disturbed soil drain to a common location. If the acreage of disturbed soil is reduced to less than ten (10) acres due to establishment of permanent cover, the temporary basin is no longer required. Temporary sediment basins may be converted to permanent basins after completion of construction.
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.2 '''Section 14.2''']: Temporary sediment basins are required where ten (10) or more acres of disturbed soil drain to a common location. If the acreage of disturbed soil is reduced to less than ten (10) acres due to establishment of permanent cover, the temporary basin is no longer required. Permittees may convert temporary sediment basins to permanent basins after completion of construction.
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.3 '''Sections 14.3 and 14.4''']: For each acre of land that drains to the basin, the basin must provide sufficient live storage to hold runoff from a [https://stormwater.pca.state.mn.us/index.php?title=Rainfall_frequency_maps 2-year 24-hour storm event] or provide 1,800 cubic feet of <span title="water held temporarily, typically in a constructed pond, above the permanent (dead storage) pool"> '''live storage'''</span>, whichever is greater. Where permittees have not calculated the two (2)-year, 24-hour storm runoff amount, the temporary basin must provide 3,600 cubic feet of live storage per acre of the basins' drainage area.
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.5 '''Sections 14.5, 14.6, and 14.7''']: Outlet structures must prevent <span title="A condition that occurs when water flows along a nearly direct pathway from the inlet to the outlet of a tank or basin, often resulting in shorter contact, reaction, or settling times in comparison with the calculated or presumed detention times."> '''short-circuiting'''</span> and discharge of floating debris, withdraw water from the surface of the sediment pond, and have sufficient <span title="Interception of incoming flow to deflect, scatter, or otherwise neutralize the erosive force of concentrated, moving stormwater."> [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Outlet_energy_dissipation '''energy dissipation''']</span> for the outlet within 24 hours after connecting to a surface water.
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.8 '''Sections 14.8 and 14.9''']: Temporary basins must be located outside of surface waters and applicable <span title="a vegetative setback between development and streams, lakes, and wetlands whose aim is to physically protect and separate the resource from future disturbance or encroachment"> [https://stormwater.pca.state.mn.us/index.phptitle=Construction_stormwater_best_management_practice_%E2%80%93_buffer_zones '''buffer zones.''']</span> Temporary basins must be constructed prior to disturbance of ten (10) or more acres of soil draining to a common location.
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.10 '''Section 14.10'''] The general construction permit allows other effective sediment controls (e.g., a series of smaller sediment basins and/or sediment traps, silt fences, vegetative buffer strips, etc.) if sediment basin requirements are infeasible to meet on a particular site. This determination must be documented in the <span title="Stormwater pollution prevention plan."> '''SWPPP'''</span> ([https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_best_management_practice_%E2%80%93_Stormwater_Pollution_Prevention_Plan Stormwater Pollution Prevention Plan]).
  
*(Sections 14.3 and 14.4) For each acre of land that drains to the basin, the basin must provide sufficient live storage to hold runoff from a 2-year 24-hour storm event or provide 1,800 cubic feet of live storage, whichever is greater. If storm calculations are not conducted, the temporary basin must provide 3,600 cubic feet of storage for every acre drained.
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====Specific permit language====
  
*(Sections 14.5, 14.6, and 14.7) Outlet structures must be designed to prevent short-circuiting and discharge of floating debris, withdraw water from the surface of the sediment pond, and have sufficient energy dissipation for the outlet within 24 hours after connecting to a surface water.  
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''Applicability''<br>
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*Specifically, [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.2 '''section 14.2'''] of the MPCA Construction Stormwater General Permit (2018) states: “Where ten (10) or more acres of disturbed soil drain to a common location, permittees must provide a temporary sediment basin to provide treatment of the runoff before it leaves the construction site or enters surface waters. Permittees may convert a temporary sediment basin to a permanent basin after construction is complete. The temporary basin is no longer required when permanent cover has reduced the acreage of disturbed soil to less than ten (10) acres draining to a common location.
  
*(Sections 14.8 and 14.9) Temporary basins must be located outside of surface waters and applicable buffer zone requirements, and temporary basins must be constructed prior to disturbance of ten (10) or more acres of soil draining to a common location.
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''Sizing''<br>
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.3 '''Section 14.3 states''']: “The temporary basin must provide live storage for a calculated volume of runoff from a two (2)-year, 24-hour storm from each acre drained to the basin or 1,800 cubic feet of live storage per acre drained, whichever is greater.”
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.4 '''Section 14.4 states''']: “Where permittees have not calculated the two (2)-year, 24-hour storm runoff amount, the temporary basin must provide 3,600 cubic feet of live storage per acre of the basins' drainage area”.
  
*(Section 14.10) The general construction permit allows other effective sediment controls (e.g., a series of smaller sediment basins and/or sediment traps, silt fences, vegetative buffer strips, etc.) if sediment basin requirements are infeasible to meet on a particular site. This determination must be documented in the SWPPP.
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''Outlets''<br>
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.5 '''Section 14.5 states''']: “Permittees must design basin outlets to prevent short-circuiting and the discharge of floating debris.”
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.6 '''Section 14.6 states''']: “Permittees must design the outlet structure to withdraw water from the surface to minimize the discharge of pollutants. Permittees may temporarily suspend the use of a surface withdrawal mechanism during frozen conditions. The basin must include a stabilized emergency overflow to prevent failure of pond integrity.”
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.7 '''Section 14.7 states''']: “Permittees must provide <span title="Interception of incoming flow to deflect, scatter, or otherwise neutralize the erosive force of concentrated, moving stormwater."> [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Outlet_energy_dissipation '''energy dissipation''']</span> for the basin outlet within 24 hours after connection to a surface water.
  
'''Specific permit language'''
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''Location and timing''<br>
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.8 '''Section 14.8 states''']:“Permittees must locate temporary basins outside of surface waters and any <span title="a vegetative setback between development and streams, lakes, and wetlands whose aim is to physically protect and separate the resource from future disturbance or encroachment"> [https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_best_management_practice_%E2%80%93_buffer_zones '''buffer zone''']</span> required in item [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_23_Discharges_to_Special_(Prohibited,_Restricted,_Other)_and_Impaired_Waters#23.11 23.11 of the general permit].”
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.9 '''Section 14.9 states''']: “Permittees must construct the temporary basins prior to disturbing 10 or more acres of soil draining to a common location.”
  
''Applicability''
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''Alternatives''<br>
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*[https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.10 '''Section 14.10 states''']: “Where a temporary sediment basin meeting the requirements of item 14.3 through 14.9 is infeasible, permittees must install effective sediment controls such as smaller sediment basins and/or sediment traps, silt fences, vegetative buffer strips or any appropriate combination of measures as dictated by individual site conditions. In determining whether installing a sediment basin is infeasible, permittees must consider public safety and may consider factors such as site soils, slope, and available area on-site. Permittees must document this determination of infeasibility in the SWPPP”.
  
Specifically, Section 14.2 of the MPCA Construction Stormwater General Permit (2018) states: “Where ten (10) or more acres of disturbed soil drain to a common location, permittees must provide a temporary sediment basin to provide treatment of the runoff before it leaves the construction site or enters surface waters. Permittees may convert a temporary sediment basin to a permanent basin after construction is complete. The temporary basin is no longer required when permanent cover has reduced the acreage of disturbed soil to less than ten (10) acres draining to a common location.
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Additional information on basin drainage is provided in [[Construction stormwater treatment - dewatering]].
  
''Sizing''
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==Effectiveness==
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When designed, installed, and maintained properly, sediment traps and basins have sediment-trapping efficiencies typically between 60 and 80 percent. They need little maintenance compared to other practices that treat sediment-laden runoff, provided upland areas are brought to final grade and stabilized promptly. Sediment traps and basins provide good control of <span title="there is no specific definition of coarse sediment, but in stormwater management it typically refers to coarse sand, which includes particles greater than 1/2-mm in diameter, using the Wentworth scale."> '''coarse sediment'''</span> and are moderately effective for trapping medium-size sediment particles. However, they have a relatively low trapping efficiency for fine silt and clay particles suspended in runoff. Longer detention times, use of a flocculant (e.g., floc log contact with upstream incoming flows), or additional sediment control measures may be necessary for removal of finer-textured sediment. In general, the larger the storage volume and the longer the detention time of the stormwater, the more efficient sediment basins are at removing finer particles. Oil, grease, and other floatables may also be removed if using an outlet pipe that features perforated openings, a screen, a skimmer, or other means to draw ponded water from just below the surface. Effectiveness in removing floatables is dependent on the outlet design. As a secondary design benefit, sediment traps and basins can also help control the flow rate and potentially the  volume of runoff from a site.
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{{:Expected performance for temporary/permanent sediment traps and basins}}
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==Planning Considerations==
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[[file:Stone_Sediment_Trap.jpg|left|300px|thumb|alt=Stone Sediment Trap|<font size=3>Where to install sediment traps/basins, click image for more information (Source: Tetra Tech) </font size>]]
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[[file:Permanent_Stormwater_Basin.jpg|300px|thumb|alt=Permanent Stormwater Basin|<font size=3>Permanent stormwater basin, click image for more information (Source: Tetra Tech) </font size>]]
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[[file:Sediment_Basin_with_Baffles.jpg|300px|thumb|alt=Sediment Basin with Baffles|<font size=3>Sediment basin with baffles, click image for more information (Source: Tetra Tech) </font size>]]
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'''Sediment Traps'''
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In order to successfully prevent off-site migration of sediment, sediment traps need to be properly functioning and in the correct location on site. They should be installed early on in the project before the site clearing phase begins. The natural drainage of a construction site should be considered prior to planning sediment trap size and location. Locate areas of potential sediment runoff, determine the likely pathway for water draining from those sites, and place sediment traps between the source of sediment and the site perimeter or water body that will receive the runoff. For maximum effectiveness, sediment traps should be placed as close as possible to the source of sediment runoff. If the site configuration prevents the use of a single sediment trap, multiple smaller traps arranged in series can be used to meet the design requirement.
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'''Sediment Basins'''
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If sediment traps are not sufficient for stormwater and sediment control, use sediment basins due to their larger capacity. Basins have a larger footprint than sediment traps and should be sited at a low point on the construction site, where runoff converges. If stormwater does not naturally flow towards the sediment basin and cannot feasibly be directed to it, a series of sediment traps or other BMPs may be more appropriate for sediment control. It is recommended that the slopes that carry runoff to the basin be greater than 1 percent but no more than 25 percent to promote flow towards the basin. Similar to sediment traps, install sediment basins prior to soil disturbing activities wherever possible. When selecting an area to place a sediment basin, look for areas that meet the following recommendations.
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#Capable of storing sediment and stormwater from as much of the planned disturbed area as practical
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#Provide access for maintenance throughout the project
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#Are far enough away (minimum of 20 feet of separation) from existing building foundations to preserve their integrity
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#Where groundwater levels will be lower than the basin bottom
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#Limit treatment to runoff from disturbed areas only
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#Minimally interfere with the construction site
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Temporary sediment basins are often converted to <span title="stormwater management practice that will be operational after the land disturbing activities are completed"> '''permanent stormwater management'''</span> structures once all disturbed contributory drainage areas have been permanently stabilized and approved by a stormwater inspection entity and all storm drains have been flushed. The outlet structure must be installed in accordance with an approved stormwater management design plan. Additional grading may be necessary to achieve the required storage volume of the basin. Prior to transition to a post-construction stormwater basin, the basin must be cleared of accumulated sediment, fully stabilized, and inspected to ensure that side slopes and the volume, containment berm, outlet, and inlets comply with stormwater basin design requirements. [[Design criteria for stormwater ponds|Link here for design information for permanent stormwater ponds]].
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==Design==
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[[file:Sediment_Trap_Design.jpg|300px|thumb|alt=Sediment Trap Design|<font size=3>Typical sediment trap design, click image to enlarge (Source: Iowa SUDAS, 2014) </font size>]]
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Designs for traps and basins should specify a minimum length to width ratio of 2:1 if possible. The surface area of a sediment trap or basin may range from approximately four to ten percent or more of the area draining to them, depending on their shape, depth, incoming soil particle size, use of flocculants, and other factors. In general, longer flow paths through the trap/basin and greater detention times results in higher rates of sediment removal via settling processes.
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===Sediment traps===
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A temporary sediment trap should only be used in a location with a drainage area of five (5) acres or less and where it will be used for two years or less.
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Sediment traps must have an outlet to carry runoff from the structure. The outlet can be a pipe outlet, stabilized rock outlet, or other suitable structure. The outlet must be capable of handling the runoff from a 2-year-frequency, 24-hour-duration storm without failure or significant erosion. Overflow outlets should be stabilized with coarse aggregate and/or <span =title"Riprap is a permanent layer of large, angular stone, cobbles, or boulders that is typically used to armor, stabilize, and protect the soil surface against erosion and scour in areas of concentrated flow or wave energy."> [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices_-_Riprap '''riprap''']</span> and <span title="Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain"> '''geotextile'''</span> fabric. For more information on outlet stabilization, [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Outlet_energy_dissipation link here].
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Sediment trap sizing is dependent on the anticipated drainage area and volume of stormwater to treat. A side slope ratio of 2:1 or flatter is recommended for sediment traps. A minimum length to width ratio of 2:1 should be provided, and the distance between the inlet and outlet should be maximized to increase sediment removal efficiency. Where flow paths are shortened due to tight site conditions, silt fencing can be used as baffles within the trap to slow flows through the trap and increase sediment removal.
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The example design table below provides general guidance on sizing rectangular sediment traps with sloped sides, based on contributing drainage area. Volumes for sediment traps constructed by installing a rock <span title="a level space, shelf, or raised barrier (usually made of compacted soil) separating two areas"> '''berm'''</span> or earthen embankment across a swale can be estimated using the triangular pyramid volume formula. As a rule of thumb, a total storage volume of 3,600 ft<sup>3</sup> should be provided for every acre of contributing drainage. This volume may be equally divided between wet (retention) and dry (drawdown, or dewatered) storage. Wet storage is provided in a permanent pool and dry storage provides extended settling time.
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{{:Sediment trap sizing table}}
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Embankments are located at the lowest point of the sediment trap and typically consist of mixed size rock or a stabilized earthen berm with a rock-armored overflow notch. The bottom of the embankment should be level with the wet storage portion of the trap. The design table below summarizes the recommended embankment top width (also referred to as weir length) for sediment traps as a function of contributing drainage area. <br>
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{{:Recommended embankment widths for sediment traps}}
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===Sediment Basins===
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Per the [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins 2018 MPCA Construction Stormwater General Permit], sediment basins are required when ten (10) or more acres of disturbed land discharges to a common location. If the site is located within one mile of a <span title="waters with qualities that warrant extra protection"> [https://stormwater.pca.state.mn.us/index.php?title=Special_Waters_and_Impaired_Waters '''special''']</span> or <span title="Impaired waters are bodies of water that exceed the limits of one or more parameters for surface water quality> [https://stormwater.pca.state.mn.us/index.php?title=Special_Waters_and_Impaired_Waters '''impaired water''']</span>, [https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_23_Discharges_to_Special_(Prohibited,_Restricted,_Other)_and_Impaired_Waters#23.10  sediment basins are required when five (5) or more acres of disturbed land discharges to a common location]. The maximum drainage area for a single sediment basin should not exceed 100 acres.
 +
 
 +
Sediment basins consist of a principal spillway, dewatering device, and an emergency (overflow) spillway. A principal spillway conveys the design volume of treated stormwater away from the site and discharges it. It consists of a vertical riser that controls basin water levels and an outlet pipe. A dewatering device releases the runoff over time, allowing the sediment to settle out. Dewatering devices can take many forms including modifications to risers and floating skimmers. The MPCA Construction Stormwater General Permit ([https://stormwater.pca.state.mn.us/index.php?title=MN_CSW_Permit_Section_14_Temporary_Sediment_Basins#14.6 Section 14.6]) requires emergency spillways to safely pass stormwater from storms exceeding the basin design storm without compromising the integrity of the basin.
 +
 
 +
The total storage volume provided by sediment basins should be a minimum of 3,600 cubic feet per acre of contributing area. Contributing drainage area should include the entire drainage area, not only the disturbed area. The total storage volume should be divided equally between wet (retention) and dry (drawdown, or dewatered) storage. Storage volume is measured from the bottom of the basin to the elevation of the principal spillway crest.
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[[file:Sediment_Trap_Design_with_Spillway.jpg|300px|thumb|alt=Sediment Trap Design with Spillway|<font size=3>Typical design for a sediment basin with an emergency spillway, click image to enlarge (Source: Iowa SUDAS, 2014) </font size>]]
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[[File:Skimmer image.png|300px|thumb|alt=image of skimmer|<font size=3>Floating skimmer installed in temporary sediment basin to draw and discharge cleaner water from the top of the impoundment. (Source: J. W. Faircloth & Son, Inc.) </font size>]]
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 +
The following design recommendations apply to sediment basins:
 +
 
 +
*The ratio of the surface area (ft<sup>2</sup>) at the designed high water elevation to the discharge rate from a 15-year, 24-hour storm (cfs) is greater than or equal to 0.0035.
 +
*Basin length to width ratio should be 2:1 or greater , in order to maximize settling time.
 +
*Use a <span title="a pond configuration that is narrow at the inlet and wide near the outlet, thus allowing for good circulation."> '''wedge'''</span> design with the inlet located at the narrow end of the basin.
 +
*Locate inflows to the basin as far from the riser as possible to maximize <span title="Travel time is the time it takes for water to move horizontally from one location to another. For stormwater ponds, this is the length of time for water to move from the pond inlet to the pond outlet."> '''travel time'''</span> through the basin and increase effectiveness of treatment. Baffles may be installed to maximize detention time and increase sediment removal efficiency.
 +
*For small temporary sediment traps, the embankment height should not exceed five feet. For temporary sediment basins, the embankment (dam) height should not exceed fifteen (15) feet.
 +
*For embankments up to ten (10) feet in height, the embankment top width should be a minimum of eight (8) feet. For embankments between ten (10) and fifteen (15) feet in height, the minimum top width should be ten (10) feet.
 +
*Soils used for embankments exceeding five feet must be selected and compacted in accordance with NRCS or MnDOT standards.
 +
*For rock sediment trap berms, use mixed size rock to ensure slower drain-down times and maximum sediment settling.
 +
*Design inside embankment side slopes at a ratio of 2:1 or flatter, and outside embankment side slopes 3:1 or flatter. Design all slopes to be stable through the use of erosion controls such as seeding, mulching, and sodding.
 +
*Protect inflows and outlets from erosion by armoring with [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices_-_Riprap riprap] or other appropriate techniques.
 +
*Increase detention time by using rock, filter fabric, rock bags, or similar measures to modify the inlet, and use [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Stabilized_earth/soil_berm berms] or other approaches that slow down exiting flow to allow more sediment to settle.
 +
*Increase sediment removal via flocculation and faster settling through the use of <span title="Polymers are large molecules, or macromolecules, composed of many repeated subunits. For water treatment, they are used as coagulants and may be cationic, anionic, or neutral, depending on the desired treatment."> '''polymers'''</span>, available as powders, flocculant logs/bags, and other products. Follow manufacturer’s directions for applicability and use. For more information see [[Construction stormwater treatment - dewatering, including chemical treatment and sediment filtration]].
 +
*For superior sediment basin discharge water quality, consider use of a floating skimmer. These devices draw clarified water from the surface of the impounded stormwater, where sediment concentrations are the lowest. Several do-it-yourself and <span title="a pre-fabricated stormwater treatment structure utilizing settling, filtration, absorptive/adsorptive materials, vortex separation, vegetative components, and/or other appropriate technology to remove pollutants from storm runoff."> '''proprietary products'''</span> are available to choose from; see [https://www.dot.state.mn.us/research/TS/2014/201418.pdf here for more information].
 +
 
 +
==Construction recommendations and specifications==
 +
While sediment traps differ from basins in design options, sizing, and (typically) permanence, there are some basic construction recommendations common to both BMPs:
  
Section 14.3 states: “The temporary basin must provide live storage for a calculated volume of runoff from a two (2)-year, 24-hour storm from each acre drained to the basin or 1,800 cubic feet of live storage per acre drained, whichever is greater.
+
*Install traps/basins prior to disturbing large portions of the upland construction site, so they are available for sediment removal during the active construction period.
 +
*Assess drainage toward the trap/basin to ensure it conforms to site design. Use berms or ditches to capture flows that may bypass the trap/basin where necessary.
 +
*Clear and <span title="Grubbing or clearing denotes the removal of trees, shrubs, stumps, and rubbish from a site, often from the site on which a transportation or utility corridor"> '''grub'''</span> the area under the embankment, removing any vegetation and the root mat.
 +
*Ensure that earthen containment berms are constructed according to design. Soils used to construct the embankment must be free of roots, woody debris, large rock, organic material, and other non-soil material.
 +
*Compact containment embankment fill soils by traversing with equipment in one-foot lifts during construction..
 +
*Ensure that the berm is capable of handling the lateral stress from ponded water (i.e., apply MnDOT and NRCS soil and compaction specifications for embankments exceeding five feet in height).
 +
*For rock sediment trap berms, use mixed size rock to ensure slower drain-down times and maximum sediment settling.
 +
*Outlets (e.g., overflow, piped) must be stable and able to handle heavy incoming/outgoing flows.
 +
*Ensure there are no large bottom-elevation outlet structure holes, pipe openings, rock voids in the berm, or other features that may cause the trap/basin to drain down less than 12 to 24 hours after a rain.
 +
*Mixed size rock berms or filter fabric can be used, if necessary, to slow flows from permanent stormwater outlets during the construction period.
 +
*Remove limbs, debris, trash, building materials, soil stockpiles, etc. immediately after constructing the trap/basin.
 +
*Stabilize side slopes and berms of traps/basins with [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices_-_temporary_seeding_and_stabilization seed], [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices_-_erosion_control_blankets_and_anchoring_devices erosion control blanket], stone, etc. as soon as they are built, to prevent filling the available volume with sediment from the trap/basin itself. The bottom can remain unstabilized until final sediment removal occurs.
 +
*Ensure that stormwater discharges from the basin will exit onto an armored anti-scour pad or other erosion proof structure/area, and can move downstream via stabilized [https://stormwater.pca.state.mn.us/index.php?title=Dry_swale_(Grass_swale) swales], ditches, channels, or as sheet flow.
 +
*Remove any non-sediment upland materials that may wash into the trap/basin (e.g., limbs, debris, trash, building materials, etc.)
 +
*Stabilize upland areas that may contribute significant amounts of eroded soil to the trap/basin as quickly as possible, to avoid frequent dredging and maintenance.
 +
*Use [https://stormwater.pca.state.mn.us/index.php?title=Sediment_control_practices_-_Perimeter_controls_for_disturbed_areas silt fencing or other sediment controls] where necessary to further reduce sediment inputs to the trap/basin – it is easier to remove sediment from a silt fence than a trap/basin.
  
Section 14.4 states: “Where permittees have not calculated the two (2)-year, 24-hour storm runoff amount, the temporary basin must provide 3,600 cubic feet of live storage per acre of the basins' drainage area”.
+
The [http://www.dot.state.mn.us/pre-letting/spec/ 2018 MnDOT Standard Specifications for Construction] includes several specifications relevant to sediment basins and traps.
  
''Outlets''
+
*[http://www.dot.state.mn.us/pre-letting/spec/2018/2018-spec-book-final.pdf#%5B%7B%22num%22%3A747%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C69%2C420%2C0%5D MnDOT Specification 2573.3] Part A includes guidance for construction of temporary sediment basins and traps. Part A.4 (page 514) states: “Construct temporary sediment basins concurrently with the start of soil disturbing activities. Direct storm water runoff from drainage areas to the basins. Stabilize the exposed side slopes of the basins. Provide an outlet to the basin that discharges water from the surface, separates floatables, and provides scour protection or energy dissipation.” Part A.6 requires the use of a water treatment plan for these devices, stating: “Provide a water treatment plan for turbid or sediment laden water. Submit the water treatment plan to the Engineer before pumping. Do not begin work until the Engineer accepts the water treatment plan including any contractor required permits. Include in the water treatment plan the use of sediment traps, vegetative filter strips, flocculants, or other water treatments per 3875, ‘Water Treatments’.”
 +
*[http://www.dot.state.mn.us/pre-letting/spec/2018/2018-spec-book-final.pdf#%5B%7B%22num%22%3A747%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C69%2C420%2C0%5D MnDOT Specification 2573.3] Part P includes guidance for the maintenance of sediment basins and traps. Part P.2 (page 518) states: “Drain the basin and remove the sediment when the depth of sediment collected in the basin reaches 50 percent of the storage volume determined by the outfall device. Complete drainage and removal within 72 hours or as soon as field conditions allow access. Remove sediment to the original designed or excavated grade or as necessary to restore the function of the device. Restore stabilized condition of side slopes and access road. Clean out and shape temporary sedimentation basins intended for use as permanent water quality management basins as shown on the plans.” Maintenance is discussed in greater detail below.
 +
*[http://www.dot.state.mn.us/pre-letting/spec/2018/2018-spec-book-final.pdf#%5B%7B%22num%22%3A754%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C69%2C642%2C0%5D MnDOT Specification 2573.4] prescribes “method of measurement” related to temporary sediment traps and basins. Part E (Temporary Sediment Traps, page 519) states: “The Engineer will measure sediment trap excavation by volume. The Engineer will measure overflow devices separately.” Further, Part G (Sediment Removal) states: “The Engineer will measure sediment removal from temporary sediment traps and basins by backhoe hours or vac-truck hours. The spreading and hauling of sediment is incidental to the backhoe hour removal.”
 +
*[http://www.dot.state.mn.us/pre-letting/spec/2018/2018-spec-book-final.pdf#%5B%7B%22num%22%3A755%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C69%2C575%2C0%5D MnDOT Specification 2573.5] prescribes “basis of payment” related to temporary sediment traps and basins. Part G (Sediment Traps, page 520) states: “The Department will pay for removal of sediment from sediment traps by backhoe or vac-truck hour. Sediment spreading and disposal are incidental.” Further, Part I (Pay Items, page 521) states that payment by the Department for sediment trap excavation (Item No. 2573.504) shall be calculated on the basis of cubic yards.
 +
*[http://www.dot.state.mn.us/pre-letting/spec/2018/2018-spec-book-final.pdf#%5B%7B%22num%22%3A1134%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C69%2C383%2C0%5D MnDOT Specification 3875] (Water Treatment) outlines requirements for use of passive dewatering treatment methods for sediment traps and basins. Specification 3875.2 Part A (page 694) states: “Use passive dewatering treatment methods, using time and gravity to settle out sediments, if draining basins, traps, ditches, or sumps to prepare the construction site for the next storm event.” Part A.2 provides specifications regarding the design of the riser pipe, stating: “Provide a riser pipe in a pond, basin, or trap outlet structure meeting the following requirements or characteristics.
 +
#Two-thirds the height of the outlet above the floor of the structure
 +
#Made of perforated PVC or metal pipe of the same diameter as the outlet structure
 +
#Surrounded by clean rock from 1 in to 2 in, for the entire height of the riser pipe.
 +
:Install a trash guard on the top overflow.”
  
Section 14.5 states: “Permittees must design basin outlets to prevent short-circuiting and the discharge of floating debris.
+
The relevant specifications noted above in 2573.4 (Method of Measurement) and 2573.5 (Basis of Payment) are important when considering cost implications of these practices, discussed further below.
  
Section 14.6 states: “Permittees must design the outlet structure to withdraw water from the surface to minimize the discharge of pollutants. Permittees may temporarily suspend the use of a surface withdrawal mechanism during frozen conditions. The basin must include a stabilized emergency overflow to prevent failure of pond integrity.”
+
==Inspection and maintenance==
 +
[[file:Full_Sediment_Trap.jpg|300px|thumb|alt=Full Temporary Sediment Trap|<font size=3>Neglected temporary sediment trap, click image for more information (Source: Tetra Tech) </font size>]]
  
Section 14.7 states: “Permittees must provide energy dissipation for the basin outlet within 24 hours after connection to a surface water.
+
===Sediment traps===
 +
The [https://stormwater.pca.state.mn.us/index.php?title=2018_Minnesota_Construction_Stormwater_Permit MPCA Construction Stormwater General Permit] does not include specific requirements for inspection and maintenance of temporary sediment traps. Temporary sediment traps, however, should be inspected at least weekly and after each 0.5-inch or greater rainfall event and repaired immediately. Recommended maintenance includes the following.
 +
*Ensure that trap sidewalls are vegetated or otherwise stabilized to prevent erosion of the structure and filling of the trap volume.
 +
*Check spillway for accumulated debris, erosion, and displacement of rock.
 +
*Immediately address erosion, bulging, or other conditions indicating weakness or potential failure of the containment berm.
 +
*Remove sediment from the trap when sediment accumulates to the clean out level (50 percent of wet storage depth).
 +
Dispose of dredged sediment at fill areas, soil stockpiles, or other locations where it can be stabilized with vegetation or contained via sediment controls (e.g., silt fencing). Dredged sediment containing hazardous wastes (i.e., from demolition debris, waste sites, etc.) must be characterized and disposed of [https://www.pca.state.mn.us/waste/minnesota-rules-hazardous-waste-solid-waste-and-tanks in accordance with MPCA requirements].
 +
*When performing sediment cleanout, inspect the trap for accumulated trash and other debris. Keep inflow, outflow, and interior of trap clear of any debris.
 +
*Prevent erosion at inflow and outflow by installing and maintaining <span =title"Riprap is a permanent layer of large, angular stone, cobbles, or boulders that is typically used to armor, stabilize, and protect the soil surface against erosion and scour in areas of concentrated flow or wave energy."> [https://stormwater.pca.state.mn.us/index.php?title=Erosion_prevention_practices_-_Riprap '''riprap''']</span> or other appropriate measures.
 +
*Stabilize or use <span title="A temporary sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment (loose soil) in stormwater runoff.">  '''silt fencing'''</span> to address upslope areas that are contributing excessive volumes of sediment to the trap and increasing maintenance needs.
 +
*Maintain original dimensions of trap (grade, depth, etc.) throughout life of practice.
  
''Location and timing''
+
===Sediment Basins===
 +
The recommended maintenance activities outlined above for sediment traps also apply to sediment basins. Maintenance of sediment basins is necessary when sediment levels reach half of the design volume of the basin. Once this threshold is reached, 72 hours are allotted for proper maintenance to occur. As noted above, [http://www.dot.state.mn.us/pre-letting/spec/2018/2018-spec-book-final.pdf#%5B%7B%22num%22%3A747%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C69%2C420%2C0%5D MnDOT Specification 2573.3 Part P.2] states: “Drain the basin and remove the sediment when the depth of sediment collected in the basin reaches 50 percent of the storage volume determined by the outfall device. Complete drainage and removal within 72 hours or as soon as field conditions allow access. Remove sediment to the original designed or excavated grade or as necessary to restore the function of the device. Restore stabilized condition of side slopes and access road. Clean out and shape temporary sedimentation basins intended for use as permanent water quality management basins as shown on the plans.”
  
Section 14.8 states: “Permittees must locate temporary basins outside of surface waters and any buffer zone required in item 23.11 [of the general permit].
+
Additionally, during inspections, evaluate the embankment for signs of seepage, settlement, or slumping. Any problems should be repaired immediately. All woody vegetation (e.g., trees, brush, etc.) should be cleared from the embankment and principal spillway. During sediment cleanout, trash and other debris should be removed from the basin, dewatering device, and riser structure.
  
Section 14.9 states: “Permittees must construct the temporary basins prior to disturbing 10 or more acres of soil draining to a common location.”
+
==Costs==
  
''Alternatives''
+
The following table summarizes estimated BMP costs based on [https://www.dot.state.mn.us/bidlet/average-bid-price.html MnDOT data] summarizing average bid prices for awarded projects in 2017.
  
Section 14.10 states: “Where a temporary sediment basin meeting the requirements of item 14.3 through 14.9 is infeasible, permittees must install effective sediment controls such as smaller sediment basins and/or sediment traps, silt fences, vegetative buffer strips or any appropriate combination of measures as dictated by individual site conditions. In determining whether installing a sediment basin is infeasible, permittees must consider public safety and may consider factors such as site soils, slope, and available area on-site. Permittees must document this determination of infeasibility in the SWPPP”.
+
{{:Unit costs related to sediment traps and basins}}
  
Additional information on basin drainage is provided in “Dewatering practices.” <MPCA, this should be linked to the Dewatering Practices wiki page>
+
==Reference materials==
  
==Effectiveness==
+
Except where more stringent recommendations are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include:
When designed, installed, and maintained properly, sediment traps and basins have relatively good sediment-trapping efficiencies (typically between 60 and 80%) and need little maintenance compared to other practices used to treat sediment-laden runoff as long as upland areas are brought to final grade and stabilized promptly. Sediment traps and basins provide good control of coarse sediment and are moderately effective for trapping medium-size sediment particles. However, they have a relatively low trapping efficiency for fine silt and clay particles suspended in runoff. Longer detention times, use of a flocculant, or additional sediment control measures may be necessary for their removal. In general, the larger the storage volume and the longer the detention time of the stormwater, the more efficient sediment basins are at removing finer particles. Oil, grease, and other floatables may also be removed if an outlet pipe is used that features perforated openings, a screen, a skimmer, or other means to draw ponded water from just below the surface. Effectiveness in removing floatables is completely dependent on the outlet design.
+
 
 +
*[https://www.pca.state.mn.us/water/construction-stormwater 2018 Minnesota NPDES/SDS Construction Stormwater General Permit]
 +
*[http://www.dot.state.mn.us/pre-letting/spec/ MnDOT Standard Specifications for Construction]
 +
*[https://metrocouncil.org/Wastewater-Water/Planning/Water-Resources-Management/Water-Quality-Management-Key-Roles.aspx Minnesota Urban Small Sites Best Management Practice Manual (Chapter 3: Runoff Pollution Prevention – Temporary Sedimentation Basins/Traps)]
 +
*[https://www.dot.state.mn.us/environment/erosion/pdf/2006mndotecfieldhandbook.pdf MnDOT Erosion Control Handbook II]
 +
 
 +
The following is a list of additional resources that are not specific to Minnesota:
 +
 
 +
*[https://www.cleanwaterservices.org/media/1464/erosion-prevention-and-sediment-control-manual.pdf Clean Water Services Erosion Prevention and Sediment Control Manual (2008) (4.3.9 Sediment Basin and 4.3.11 Sediment Trap)]
 +
*[https://www.lexingtonky.gov/sites/default/files/2018-12/LFUCG%20Contractor%27s%20Handbook%20Final%20v29Nov2018.pdf Contractor’s Handbook for Erosion, Sediment, and Stormwater Management on Capital Project Construction Sites (Lexington-Fayette Urban County Government) (2018)]
 +
*[https://doee.dc.gov/sites/default/files/dc/sites/ddoe/publication/attachments/2017%20DC%20ESC%20Manual_FINAL.pdf District of Columbia Erosion and Sediment Control Manual (2017) (Chapter 6)]
 +
*[https://iowasudas.org/manuals/design-manual/ Iowa Statewide Urban Design and Specification (SUDAS) Design Manual (2016) (Chapters 7E-12 and 7E-13)]
 +
*[http://dep.ky.gov/formslibrary/Documents/09BMPManual_Final.pdf Kentucky Erosion Prevention and Sediment Control Manual (Revised 2009). (Chapter 4.7)]
 +
*[https://www.dec.ny.gov/docs/water_pdf/2016nysstanec.pdf New York State Standards and Specifications for Erosion and Sediment Control (2016)]
 +
 
 +
<noinclude>
 +
 
 +
==Related pages==
 +
*[https://stormwater.pca.state.mn.us/index.php?title=Construction_stormwater_best_management_practice_%E2%80%93_Site_stabilization Stabilization practices]
 +
 
 +
*[[Erosion prevention practices]]
 +
**[[Erosion prevention practices - temporary seeding and stabilization]]
 +
**[[Erosion prevention practices - natural and synthetic mulches]]
 +
**[[Erosion prevention practices - tackifiers and soil stabilizers]]
 +
**[[Erosion prevention practices - erosion control blankets and anchoring devices]]
 +
**[[Erosion prevention practices - turf reinforcement mats]]
 +
**[[Erosion prevention practices - Riprap]]
 +
*[[Sediment control practices]]
 +
**[[Sediment control practices - Vehicle tracking BMPs]]
 +
**[[Sediment control practices - Perimeter controls for disturbed areas]]
 +
**[[Sediment control practices - Check dams (ditch checks, ditch dikes)]]
 +
**[[Sediment control practices - Diversion barrier controls (cofferdams/temporary dikes)]]
 +
**[[Sediment control practices - Storm drain inlet protection]]
 +
**[[Sediment control practices - Outlet energy dissipation]]
 +
**[[Sediment control practices - Sediment traps and basins]]
 +
**[[Sediment control practices - Stabilized earth/soil berm]]
 +
**[[Construction stormwater treatment - dewatering, including chemical treatment and sediment filtration]]
 +
**[[Construction stormwater best management practice – buffer zones]]
 +
*Other practices
 +
**[[Construction stormwater practices - temporary stream crossing]]
 +
**[[Construction stormwater practices - diversion and working in the dry]]
 +
**[[Construction stormwater treatment - BMP removal after final stabilization]]
  
<font size=3>'''Expected performance for temporary/permanent sediment traps and basins'''.</font size><br>
+
*[[General principles for erosion prevention and sediment control at construction sites in Minnesota]]
 +
*[[Construction stormwater photo gallery]]
  
{{:Expected performance for temporary/permanent sediment traps and basins}}
+
[[Category:Construction stormwater best management practice]]
 +
[[Category:Level 3 - Best management practices/Construction practices/Sediment control practices]]
 +
</noinclude>

Latest revision as of 19:43, 18 July 2022

image
Temporary_Sediment_Basin_with_Forebay
Temporary sediment trap with forebay, click image for more information (Source: Tetra Tech)


Sediment traps and basins are settling ponds formed by excavation and/or an embankment that intercept and retain sediment-laden runoff from a construction site for a sufficient period of time to allow the majority of sediment to settle out prior to being released from the site. They may be constructed as smaller sediment traps – serving disturbed areas of less than five acres – or as larger sediment basins, handling mass grading runoff from subdivisions, commercial/institutional sites, or roadway projects. Proper use of these structures can greatly reduce sediment transport off-site; if properly designed, installed, and maintained, sediment removal efficiency of 80 percent or greater can be achieved, depending on soil particle size. Sediment traps are often temporary and usually decommissioned after the disturbed area is stabilized (i.e., with vegetation or other cover). Temporary sediment basins can be converted to permanent stormwater management basins after construction is complete. Sediment traps and basins are very useful on construction sites with moderate to steep slopes. The selection of traps versus basins primarily depends on the size of the contributing drainage area and plans for post-construction stormwater management, as discussed below.

Purpose and function

Small Sediment trap
Small sediment trap, click image for more information (Source: Georgia SWCC)

Sediment traps and basins function by intercepting and detaining site runoff, which allows soil particles to settle out prior to discharge. Sediment traps may serve several small catchments on a site, retaining runoff using embankments and other barriers and discharging through an armored overflow or piped outlet to a vegetated swale or other drainage feature. Sediment basins typically serve larger areas than sediment traps and may feature earthen embankments that retain runoff for longer periods of time, releasing runoff via floating, perforated, or slotted risers or floating skimmers that draw clarified water from the surface

Applicability

Sediment traps and basins are very similar in their siting, sizing, and design, with sediment traps serving areas of five acres or less and basins handling larger areas. They represent one of the most effective and reliable measures for treating sediment-laden runoff from construction sites. These structures are typically placed near the perimeter of the site, where flows concentrate in swales, ditches, or other low areas. Sediment traps and basins should be constructed prior to disturbance of upslope areas, if possible, and continue functioning until the contributing drainage area is fully stabilized. Specific considerations related to site applicability and permit applicability are discussed below.

Site applicability

Sediment Trap Design Criteria
Siting and design considerations for sediment traps/basins, click image for more information (Source: Created by Tetra Tech for US EPA and State of Kentucky)

Disturbed soils on a construction site have the potential to leave the site via stormwater runoff and negatively impact receiving water, roadways, and neighboring property. Sediment traps and basins should intercept concentrated or sheet flows of stormwater discharge from a construction site. Place sediment traps near the point of discharge and build traps in series to intercept and treat flow moving down long drainage paths through a site. Place stormwater basins in low lying areas on the outer edge of a construction site where water naturally flows or is directed according to site plans. Sediment traps are particularly useful in perimeter control areas where silt fences will likely fail. Do not site sediment traps in high-velocity flow areas (e.g., culvert outlets, steep ditches) where excessive turbulence and scour erosion may interfere with sediment settling processes. Neither practice should be placed in surface waters (including intermittent streams) or within their required buffer zones.

Permit applicability

Section 14 (Temporary Sediment Basins) of the MPCA Construction Stormwater General Permit (2018) has several requirements regarding sediment basins, including the following.

  • Section 14.2: Temporary sediment basins are required where ten (10) or more acres of disturbed soil drain to a common location. If the acreage of disturbed soil is reduced to less than ten (10) acres due to establishment of permanent cover, the temporary basin is no longer required. Permittees may convert temporary sediment basins to permanent basins after completion of construction.
  • Sections 14.3 and 14.4: For each acre of land that drains to the basin, the basin must provide sufficient live storage to hold runoff from a 2-year 24-hour storm event or provide 1,800 cubic feet of live storage, whichever is greater. Where permittees have not calculated the two (2)-year, 24-hour storm runoff amount, the temporary basin must provide 3,600 cubic feet of live storage per acre of the basins' drainage area.
  • Sections 14.5, 14.6, and 14.7: Outlet structures must prevent short-circuiting and discharge of floating debris, withdraw water from the surface of the sediment pond, and have sufficient energy dissipation for the outlet within 24 hours after connecting to a surface water.
  • Sections 14.8 and 14.9: Temporary basins must be located outside of surface waters and applicable buffer zones. Temporary basins must be constructed prior to disturbance of ten (10) or more acres of soil draining to a common location.
  • Section 14.10 The general construction permit allows other effective sediment controls (e.g., a series of smaller sediment basins and/or sediment traps, silt fences, vegetative buffer strips, etc.) if sediment basin requirements are infeasible to meet on a particular site. This determination must be documented in the SWPPP (Stormwater Pollution Prevention Plan).

Specific permit language

Applicability

  • Specifically, section 14.2 of the MPCA Construction Stormwater General Permit (2018) states: “Where ten (10) or more acres of disturbed soil drain to a common location, permittees must provide a temporary sediment basin to provide treatment of the runoff before it leaves the construction site or enters surface waters. Permittees may convert a temporary sediment basin to a permanent basin after construction is complete. The temporary basin is no longer required when permanent cover has reduced the acreage of disturbed soil to less than ten (10) acres draining to a common location. “

Sizing

  • Section 14.3 states: “The temporary basin must provide live storage for a calculated volume of runoff from a two (2)-year, 24-hour storm from each acre drained to the basin or 1,800 cubic feet of live storage per acre drained, whichever is greater.”
  • Section 14.4 states: “Where permittees have not calculated the two (2)-year, 24-hour storm runoff amount, the temporary basin must provide 3,600 cubic feet of live storage per acre of the basins' drainage area”.

Outlets

  • Section 14.5 states: “Permittees must design basin outlets to prevent short-circuiting and the discharge of floating debris.”
  • Section 14.6 states: “Permittees must design the outlet structure to withdraw water from the surface to minimize the discharge of pollutants. Permittees may temporarily suspend the use of a surface withdrawal mechanism during frozen conditions. The basin must include a stabilized emergency overflow to prevent failure of pond integrity.”
  • Section 14.7 states: “Permittees must provide energy dissipation for the basin outlet within 24 hours after connection to a surface water.”

Location and timing

Alternatives

  • Section 14.10 states: “Where a temporary sediment basin meeting the requirements of item 14.3 through 14.9 is infeasible, permittees must install effective sediment controls such as smaller sediment basins and/or sediment traps, silt fences, vegetative buffer strips or any appropriate combination of measures as dictated by individual site conditions. In determining whether installing a sediment basin is infeasible, permittees must consider public safety and may consider factors such as site soils, slope, and available area on-site. Permittees must document this determination of infeasibility in the SWPPP”.

Additional information on basin drainage is provided in Construction stormwater treatment - dewatering.

Effectiveness

When designed, installed, and maintained properly, sediment traps and basins have sediment-trapping efficiencies typically between 60 and 80 percent. They need little maintenance compared to other practices that treat sediment-laden runoff, provided upland areas are brought to final grade and stabilized promptly. Sediment traps and basins provide good control of coarse sediment and are moderately effective for trapping medium-size sediment particles. However, they have a relatively low trapping efficiency for fine silt and clay particles suspended in runoff. Longer detention times, use of a flocculant (e.g., floc log contact with upstream incoming flows), or additional sediment control measures may be necessary for removal of finer-textured sediment. In general, the larger the storage volume and the longer the detention time of the stormwater, the more efficient sediment basins are at removing finer particles. Oil, grease, and other floatables may also be removed if using an outlet pipe that features perforated openings, a screen, a skimmer, or other means to draw ponded water from just below the surface. Effectiveness in removing floatables is dependent on the outlet design. As a secondary design benefit, sediment traps and basins can also help control the flow rate and potentially the volume of runoff from a site.

Expected performance for temporary/permanent sediment traps and basins.
Link to this table

Water quantity
Flow attenuation Secondary design benefit
Runoff volume reduction Secondary design benefit
Water quality
Erosion prevention Little or no design benefit
Sediment control Primary design benefit
Nutrient loading Secondary design benefit
Pollutant removal
Total suspended solids Primary design benefit
Total phosphorus Secondary design benefit
Heavy metals Secondary design benefit
Floatables Secondary design benefit
Oil and grease Secondary design benefit


Planning Considerations

Stone Sediment Trap
Where to install sediment traps/basins, click image for more information (Source: Tetra Tech)
Permanent Stormwater Basin
Permanent stormwater basin, click image for more information (Source: Tetra Tech)
Sediment Basin with Baffles
Sediment basin with baffles, click image for more information (Source: Tetra Tech)

Sediment Traps

In order to successfully prevent off-site migration of sediment, sediment traps need to be properly functioning and in the correct location on site. They should be installed early on in the project before the site clearing phase begins. The natural drainage of a construction site should be considered prior to planning sediment trap size and location. Locate areas of potential sediment runoff, determine the likely pathway for water draining from those sites, and place sediment traps between the source of sediment and the site perimeter or water body that will receive the runoff. For maximum effectiveness, sediment traps should be placed as close as possible to the source of sediment runoff. If the site configuration prevents the use of a single sediment trap, multiple smaller traps arranged in series can be used to meet the design requirement.

Sediment Basins

If sediment traps are not sufficient for stormwater and sediment control, use sediment basins due to their larger capacity. Basins have a larger footprint than sediment traps and should be sited at a low point on the construction site, where runoff converges. If stormwater does not naturally flow towards the sediment basin and cannot feasibly be directed to it, a series of sediment traps or other BMPs may be more appropriate for sediment control. It is recommended that the slopes that carry runoff to the basin be greater than 1 percent but no more than 25 percent to promote flow towards the basin. Similar to sediment traps, install sediment basins prior to soil disturbing activities wherever possible. When selecting an area to place a sediment basin, look for areas that meet the following recommendations.

  1. Capable of storing sediment and stormwater from as much of the planned disturbed area as practical
  2. Provide access for maintenance throughout the project
  3. Are far enough away (minimum of 20 feet of separation) from existing building foundations to preserve their integrity
  4. Where groundwater levels will be lower than the basin bottom
  5. Limit treatment to runoff from disturbed areas only
  6. Minimally interfere with the construction site

Temporary sediment basins are often converted to permanent stormwater management structures once all disturbed contributory drainage areas have been permanently stabilized and approved by a stormwater inspection entity and all storm drains have been flushed. The outlet structure must be installed in accordance with an approved stormwater management design plan. Additional grading may be necessary to achieve the required storage volume of the basin. Prior to transition to a post-construction stormwater basin, the basin must be cleared of accumulated sediment, fully stabilized, and inspected to ensure that side slopes and the volume, containment berm, outlet, and inlets comply with stormwater basin design requirements. Link here for design information for permanent stormwater ponds.

Design

Sediment Trap Design
Typical sediment trap design, click image to enlarge (Source: Iowa SUDAS, 2014)

Designs for traps and basins should specify a minimum length to width ratio of 2:1 if possible. The surface area of a sediment trap or basin may range from approximately four to ten percent or more of the area draining to them, depending on their shape, depth, incoming soil particle size, use of flocculants, and other factors. In general, longer flow paths through the trap/basin and greater detention times results in higher rates of sediment removal via settling processes.

Sediment traps

A temporary sediment trap should only be used in a location with a drainage area of five (5) acres or less and where it will be used for two years or less.

Sediment traps must have an outlet to carry runoff from the structure. The outlet can be a pipe outlet, stabilized rock outlet, or other suitable structure. The outlet must be capable of handling the runoff from a 2-year-frequency, 24-hour-duration storm without failure or significant erosion. Overflow outlets should be stabilized with coarse aggregate and/or riprap and geotextile fabric. For more information on outlet stabilization, link here.

Sediment trap sizing is dependent on the anticipated drainage area and volume of stormwater to treat. A side slope ratio of 2:1 or flatter is recommended for sediment traps. A minimum length to width ratio of 2:1 should be provided, and the distance between the inlet and outlet should be maximized to increase sediment removal efficiency. Where flow paths are shortened due to tight site conditions, silt fencing can be used as baffles within the trap to slow flows through the trap and increase sediment removal.

The example design table below provides general guidance on sizing rectangular sediment traps with sloped sides, based on contributing drainage area. Volumes for sediment traps constructed by installing a rock berm or earthen embankment across a swale can be estimated using the triangular pyramid volume formula. As a rule of thumb, a total storage volume of 3,600 ft3 should be provided for every acre of contributing drainage. This volume may be equally divided between wet (retention) and dry (drawdown, or dewatered) storage. Wet storage is provided in a permanent pool and dry storage provides extended settling time.

Sediment trap sizing table
Link to this table

Drainage area (ac) Total Volume (ft3) Wet Volume (ft3) Dry Volume (ft3) Minimum Depth (ft) Depth of Permanent Pool (ft) Minimum Bottom Length (ft) Minimum Bottom Width
1 3600 1800 1800 2.5 1.5 46 23
2 7200 3600 3600 2.75 1.5 64 32
3 10800 5400 5400 3.0 1.75 76 38

Table assumes 2:1 side slopes; Minimum depth is from trap bottom to weir crest and includes both wet and dry storage


Embankments are located at the lowest point of the sediment trap and typically consist of mixed size rock or a stabilized earthen berm with a rock-armored overflow notch. The bottom of the embankment should be level with the wet storage portion of the trap. The design table below summarizes the recommended embankment top width (also referred to as weir length) for sediment traps as a function of contributing drainage area.

Recommended embankment widths for sediment traps
Link to this table

Contributing Drainage Area (acre) Embankment Width (ft)
1 4
2 6
3 8
4 10
5 12


Sediment Basins

Per the 2018 MPCA Construction Stormwater General Permit, sediment basins are required when ten (10) or more acres of disturbed land discharges to a common location. If the site is located within one mile of a special or impaired water, sediment basins are required when five (5) or more acres of disturbed land discharges to a common location. The maximum drainage area for a single sediment basin should not exceed 100 acres.

Sediment basins consist of a principal spillway, dewatering device, and an emergency (overflow) spillway. A principal spillway conveys the design volume of treated stormwater away from the site and discharges it. It consists of a vertical riser that controls basin water levels and an outlet pipe. A dewatering device releases the runoff over time, allowing the sediment to settle out. Dewatering devices can take many forms including modifications to risers and floating skimmers. The MPCA Construction Stormwater General Permit (Section 14.6) requires emergency spillways to safely pass stormwater from storms exceeding the basin design storm without compromising the integrity of the basin.

The total storage volume provided by sediment basins should be a minimum of 3,600 cubic feet per acre of contributing area. Contributing drainage area should include the entire drainage area, not only the disturbed area. The total storage volume should be divided equally between wet (retention) and dry (drawdown, or dewatered) storage. Storage volume is measured from the bottom of the basin to the elevation of the principal spillway crest.

Sediment Trap Design with Spillway
Typical design for a sediment basin with an emergency spillway, click image to enlarge (Source: Iowa SUDAS, 2014)
image of skimmer
Floating skimmer installed in temporary sediment basin to draw and discharge cleaner water from the top of the impoundment. (Source: J. W. Faircloth & Son, Inc.)

The following design recommendations apply to sediment basins:

  • The ratio of the surface area (ft2) at the designed high water elevation to the discharge rate from a 15-year, 24-hour storm (cfs) is greater than or equal to 0.0035.
  • Basin length to width ratio should be 2:1 or greater , in order to maximize settling time.
  • Use a wedge design with the inlet located at the narrow end of the basin.
  • Locate inflows to the basin as far from the riser as possible to maximize travel time through the basin and increase effectiveness of treatment. Baffles may be installed to maximize detention time and increase sediment removal efficiency.
  • For small temporary sediment traps, the embankment height should not exceed five feet. For temporary sediment basins, the embankment (dam) height should not exceed fifteen (15) feet.
  • For embankments up to ten (10) feet in height, the embankment top width should be a minimum of eight (8) feet. For embankments between ten (10) and fifteen (15) feet in height, the minimum top width should be ten (10) feet.
  • Soils used for embankments exceeding five feet must be selected and compacted in accordance with NRCS or MnDOT standards.
  • For rock sediment trap berms, use mixed size rock to ensure slower drain-down times and maximum sediment settling.
  • Design inside embankment side slopes at a ratio of 2:1 or flatter, and outside embankment side slopes 3:1 or flatter. Design all slopes to be stable through the use of erosion controls such as seeding, mulching, and sodding.
  • Protect inflows and outlets from erosion by armoring with riprap or other appropriate techniques.
  • Increase detention time by using rock, filter fabric, rock bags, or similar measures to modify the inlet, and use berms or other approaches that slow down exiting flow to allow more sediment to settle.
  • Increase sediment removal via flocculation and faster settling through the use of polymers, available as powders, flocculant logs/bags, and other products. Follow manufacturer’s directions for applicability and use. For more information see Construction stormwater treatment - dewatering, including chemical treatment and sediment filtration.
  • For superior sediment basin discharge water quality, consider use of a floating skimmer. These devices draw clarified water from the surface of the impounded stormwater, where sediment concentrations are the lowest. Several do-it-yourself and proprietary products are available to choose from; see here for more information.

Construction recommendations and specifications

While sediment traps differ from basins in design options, sizing, and (typically) permanence, there are some basic construction recommendations common to both BMPs:

  • Install traps/basins prior to disturbing large portions of the upland construction site, so they are available for sediment removal during the active construction period.
  • Assess drainage toward the trap/basin to ensure it conforms to site design. Use berms or ditches to capture flows that may bypass the trap/basin where necessary.
  • Clear and grub the area under the embankment, removing any vegetation and the root mat.
  • Ensure that earthen containment berms are constructed according to design. Soils used to construct the embankment must be free of roots, woody debris, large rock, organic material, and other non-soil material.
  • Compact containment embankment fill soils by traversing with equipment in one-foot lifts during construction..
  • Ensure that the berm is capable of handling the lateral stress from ponded water (i.e., apply MnDOT and NRCS soil and compaction specifications for embankments exceeding five feet in height).
  • For rock sediment trap berms, use mixed size rock to ensure slower drain-down times and maximum sediment settling.
  • Outlets (e.g., overflow, piped) must be stable and able to handle heavy incoming/outgoing flows.
  • Ensure there are no large bottom-elevation outlet structure holes, pipe openings, rock voids in the berm, or other features that may cause the trap/basin to drain down less than 12 to 24 hours after a rain.
  • Mixed size rock berms or filter fabric can be used, if necessary, to slow flows from permanent stormwater outlets during the construction period.
  • Remove limbs, debris, trash, building materials, soil stockpiles, etc. immediately after constructing the trap/basin.
  • Stabilize side slopes and berms of traps/basins with seed, erosion control blanket, stone, etc. as soon as they are built, to prevent filling the available volume with sediment from the trap/basin itself. The bottom can remain unstabilized until final sediment removal occurs.
  • Ensure that stormwater discharges from the basin will exit onto an armored anti-scour pad or other erosion proof structure/area, and can move downstream via stabilized swales, ditches, channels, or as sheet flow.
  • Remove any non-sediment upland materials that may wash into the trap/basin (e.g., limbs, debris, trash, building materials, etc.)
  • Stabilize upland areas that may contribute significant amounts of eroded soil to the trap/basin as quickly as possible, to avoid frequent dredging and maintenance.
  • Use silt fencing or other sediment controls where necessary to further reduce sediment inputs to the trap/basin – it is easier to remove sediment from a silt fence than a trap/basin.

The 2018 MnDOT Standard Specifications for Construction includes several specifications relevant to sediment basins and traps.

  • MnDOT Specification 2573.3 Part A includes guidance for construction of temporary sediment basins and traps. Part A.4 (page 514) states: “Construct temporary sediment basins concurrently with the start of soil disturbing activities. Direct storm water runoff from drainage areas to the basins. Stabilize the exposed side slopes of the basins. Provide an outlet to the basin that discharges water from the surface, separates floatables, and provides scour protection or energy dissipation.” Part A.6 requires the use of a water treatment plan for these devices, stating: “Provide a water treatment plan for turbid or sediment laden water. Submit the water treatment plan to the Engineer before pumping. Do not begin work until the Engineer accepts the water treatment plan including any contractor required permits. Include in the water treatment plan the use of sediment traps, vegetative filter strips, flocculants, or other water treatments per 3875, ‘Water Treatments’.”
  • MnDOT Specification 2573.3 Part P includes guidance for the maintenance of sediment basins and traps. Part P.2 (page 518) states: “Drain the basin and remove the sediment when the depth of sediment collected in the basin reaches 50 percent of the storage volume determined by the outfall device. Complete drainage and removal within 72 hours or as soon as field conditions allow access. Remove sediment to the original designed or excavated grade or as necessary to restore the function of the device. Restore stabilized condition of side slopes and access road. Clean out and shape temporary sedimentation basins intended for use as permanent water quality management basins as shown on the plans.” Maintenance is discussed in greater detail below.
  • MnDOT Specification 2573.4 prescribes “method of measurement” related to temporary sediment traps and basins. Part E (Temporary Sediment Traps, page 519) states: “The Engineer will measure sediment trap excavation by volume. The Engineer will measure overflow devices separately.” Further, Part G (Sediment Removal) states: “The Engineer will measure sediment removal from temporary sediment traps and basins by backhoe hours or vac-truck hours. The spreading and hauling of sediment is incidental to the backhoe hour removal.”
  • MnDOT Specification 2573.5 prescribes “basis of payment” related to temporary sediment traps and basins. Part G (Sediment Traps, page 520) states: “The Department will pay for removal of sediment from sediment traps by backhoe or vac-truck hour. Sediment spreading and disposal are incidental.” Further, Part I (Pay Items, page 521) states that payment by the Department for sediment trap excavation (Item No. 2573.504) shall be calculated on the basis of cubic yards.
  • MnDOT Specification 3875 (Water Treatment) outlines requirements for use of passive dewatering treatment methods for sediment traps and basins. Specification 3875.2 Part A (page 694) states: “Use passive dewatering treatment methods, using time and gravity to settle out sediments, if draining basins, traps, ditches, or sumps to prepare the construction site for the next storm event.” Part A.2 provides specifications regarding the design of the riser pipe, stating: “Provide a riser pipe in a pond, basin, or trap outlet structure meeting the following requirements or characteristics.
  1. Two-thirds the height of the outlet above the floor of the structure
  2. Made of perforated PVC or metal pipe of the same diameter as the outlet structure
  3. Surrounded by clean rock from 1 in to 2 in, for the entire height of the riser pipe.
Install a trash guard on the top overflow.”

The relevant specifications noted above in 2573.4 (Method of Measurement) and 2573.5 (Basis of Payment) are important when considering cost implications of these practices, discussed further below.

Inspection and maintenance

Full Temporary Sediment Trap
Neglected temporary sediment trap, click image for more information (Source: Tetra Tech)

Sediment traps

The MPCA Construction Stormwater General Permit does not include specific requirements for inspection and maintenance of temporary sediment traps. Temporary sediment traps, however, should be inspected at least weekly and after each 0.5-inch or greater rainfall event and repaired immediately. Recommended maintenance includes the following.

  • Ensure that trap sidewalls are vegetated or otherwise stabilized to prevent erosion of the structure and filling of the trap volume.
  • Check spillway for accumulated debris, erosion, and displacement of rock.
  • Immediately address erosion, bulging, or other conditions indicating weakness or potential failure of the containment berm.
  • Remove sediment from the trap when sediment accumulates to the clean out level (50 percent of wet storage depth).

Dispose of dredged sediment at fill areas, soil stockpiles, or other locations where it can be stabilized with vegetation or contained via sediment controls (e.g., silt fencing). Dredged sediment containing hazardous wastes (i.e., from demolition debris, waste sites, etc.) must be characterized and disposed of in accordance with MPCA requirements.

  • When performing sediment cleanout, inspect the trap for accumulated trash and other debris. Keep inflow, outflow, and interior of trap clear of any debris.
  • Prevent erosion at inflow and outflow by installing and maintaining riprap or other appropriate measures.
  • Stabilize or use silt fencing to address upslope areas that are contributing excessive volumes of sediment to the trap and increasing maintenance needs.
  • Maintain original dimensions of trap (grade, depth, etc.) throughout life of practice.

Sediment Basins

The recommended maintenance activities outlined above for sediment traps also apply to sediment basins. Maintenance of sediment basins is necessary when sediment levels reach half of the design volume of the basin. Once this threshold is reached, 72 hours are allotted for proper maintenance to occur. As noted above, MnDOT Specification 2573.3 Part P.2 states: “Drain the basin and remove the sediment when the depth of sediment collected in the basin reaches 50 percent of the storage volume determined by the outfall device. Complete drainage and removal within 72 hours or as soon as field conditions allow access. Remove sediment to the original designed or excavated grade or as necessary to restore the function of the device. Restore stabilized condition of side slopes and access road. Clean out and shape temporary sedimentation basins intended for use as permanent water quality management basins as shown on the plans.”

Additionally, during inspections, evaluate the embankment for signs of seepage, settlement, or slumping. Any problems should be repaired immediately. All woody vegetation (e.g., trees, brush, etc.) should be cleared from the embankment and principal spillway. During sediment cleanout, trash and other debris should be removed from the basin, dewatering device, and riser structure.

Costs

The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2017.

Unit costs related to sediment traps and basins
Link to this table

Bid item Item description Units Average price
2573602/00010 Temporary sediment trap each $605.00
2106501/00010 Excavation – common cubic yards $5.60
2106607/00020 Excavation – subgrade cubic yards $9.59
2106523/00060 Common embankment (CV) cubic yards $2.18
2106521/00070 Granular embankment (CV) cubic yards $8.51


Reference materials

Except where more stringent recommendations are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include:

The following is a list of additional resources that are not specific to Minnesota:


Related pages

This page was last edited on 18 July 2022, at 19:43.