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.
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
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.
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.
Section 14 (Temporary Sediment Basins) of the MPCA Construction Stormwater General Permit (2018) has several requirements regarding sediment basins, including the following.
Location and timing
Additional information on basin drainage is provided in Construction stormwater treatment - dewatering.
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.
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|Flow attenuation||Secondary design benefit|
|Runoff volume reduction||Secondary design benefit|
|Erosion prevention||Little or no design benefit|
|Sediment control||Primary design benefit|
|Nutrient loading||Secondary design benefit|
|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|
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.
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.
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.
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.
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|
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)|
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.
The following design recommendations apply to sediment basins:
While sediment traps differ from basins in design options, sizing, and (typically) permanence, there are some basic construction recommendations common to both BMPs:
The 2018 MnDOT Standard Specifications for Construction includes several specifications relevant to sediment basins and traps.
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.
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.
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.
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.
The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2017.
|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|
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: