Construction stormwater treatment - BMP removal after final stabilization
In general, when a temporary BMP is no longer needed on a construction site, most often after final stabilization has occurred, it must be removed by the contractor. However, some BMPs, such as temporary sediment basins, may be converted to permanent BMPs after construction is complete. Further, many BMPs used to prevent erosion, control sediment, and provide temporary or permanent stabilization of a construction site are often left in place.
While many construction stormwater BMPs must be removed from the site after final stabilization, it is generally acceptable for BMPs made of natural materials (such as fiber logs/rolls, natural fiber erosion control mats, etc.) to be left on the site to decompose. Erosion prevention practices constructed from degradable materials represent the most common class of stormwater construction BMPs that are left in place after construction is complete and the site is stabilized. Perimeter control BMPs constructed from degradable materials may also be left on site in some cases. However, temporary perimeter controls constructed from materials that do not readily decompose, such as silt fences, filter socks, straw bale barriers, sandbag barriers, metal stakes and reinforcement fencing, and other relatively low cost BMPs should be removed but are often forgotten on site after construction is complete.
Contents
Applicability
In general, whether a construction stormwater BMP is removed after the completion of construction and stabilization of the work site primarily depends on (1) the conditions of the contract and applicable permit(s), (2) the intended function and lifespan of the BMP, and (3) the nature of the materials used to construct the BMP.
MPCA’s 2018 Stormwater Construction Inspection Guide states that “all temporary synthetic and structural erosion prevention and sediment control BMPs (such as silt fence) must be removed on the portions of the site for which the Permittee is responsible”, and that BMPs “designed to decompose on site (such as some compost logs) may be left in place.”
Site applicability
Whether they are to be removed or left in place, all BMPs should still be cleaned of excess sediment and other accumulated debris at the completion of construction and final stabilization when they are no longer needed. However, depending on the expected rate of decomposition and intended use of the site, a contractor or site owner may wish to remove and dispose of some BMPs or BMP components in lieu of leaving them in place to decompose. The guidance in this BMP chapter pertains primarily to sites where the standard regulatory procedures for determining whether a temporary construction stormwater BMP should be removed, left in place, or converted to a permanent stormwater management feature.
The table below provides general recommendations for categories and types of temporary BMPs regarding whether they are typically removed or left in place on a construction site after final stabilization.
Typical recommendations for BMP removal for common construction stormwater BMPs.
Link to this table
| BMP category/type | Standard procedure | Applicability and special considerations |
|---|---|---|
| Erosion prevention practices (stabilization practices) | ||
| Temporary/permanent seeding | Left in place | Mulch or other natural temporary cover to protect seeded areas should also be left in place. |
| Mulches | Left in place | Mulch, hydraulic mulch, and any anchoring materials must be degradable. |
| Tackifiers/soil stabilizers | Left in place | If using tackifiers and stabilizers (e.g., polyacrylamide or PAM), ensure they are nontoxic to aquatic life (i.e., not cationic). |
| Erosion control blankets (ECBs) | Left in place | Installed ECBs with emergent vegetation are left in place. May be removed and reused when used as temporary cover without seed on short-duration projects. |
| Turf reinforcement mats (TRMs) | Left in place | Left in place to provide permanent armoring and vegetation support. |
| Erosion prevention practices (other) | ||
| Riprap | Left in place | Typically serves a permanent function for armoring, stabilizing, and protecting soil against erosion. |
| Sediment control practices | ||
| Vehicle tracking BMPs | Removed | Removed after final site stabilization or after no longer needed (e.g., vehicles no longer entering or existing site). |
| Perimeter controls | Varies; most are removed | May be left in place if constructed from degradable materials (e.g., fiber rolls/logs). Silt fencing must be removed after upslope areas are stabilized. |
| Check dams | Varies | May be left in place if constructed from degradable materials (e.g., fiber rolls/logs). Check post-construction stormwater management plan for applicability. |
| Diversion barrier controls | Varies; most are removed | In general, all are removed after stabilization. In special cases, such as where removal may damage structures or foundation soils, cofferdams may be left in place. |
| Inlet protection | Removed | Must be removed after stabilization. |
| Outlet energy dissipation | Varies | Devices designed for temporary, short-term (e.g., less than one year) use must be removed after stabilization. Whether they are left in place as permanent BMPs depends on function and desired aesthetics. Check post-construction stormwater management plan for applicability. |
| Sediment traps/basins | Varies | Temporary sediment traps are removed after stabilization. Temporary sediment basins may be converted to permanent stormwater management basins after stabilization. |
| Stabilized earth/soil berm | Varies | Typically removed after stabilization. In some cases, may be left in place if permanently stabilized with vegetation. Check post-construction stormwater management plan for applicability. |
| Surface water protection practices | ||
| Buffer zones | Left in place | Always preserved on site |
| Construction stormwater treatment practices | ||
| Dewatering (including chemical treatment and sediment filtration) | Removed | Dewatering practices are operated during construction activity and may be removed after construction ceases and site is stabilized. |
| Practices to minimize sediment discharge | ||
| Diversion and working in the dry | Removed | Removed after construction is complete and stream channel and site are stabilized. |
| Temporary stream crossing | Removed | Removed after construction is complete and stream channel and site are stabilized. |
| Stabilized construction exit | Removed | Removed after construction is complete and construction equipment/materials have been removed. Grade and stabilize area after removal. |
Permit applicability
Final stabilization and removal of applicable temporary BMPs must be completing prior to closing out the project permit. At project completion, a Notice of Termination (NOT) will need to be submitted to terminate coverage under the MPCA Construction Stormwater General Permit. The NOT must be submitted within 30 days of completing final stabilization at the project site. The following must be completed (MnDOT, 2006):
- All soils must be stabilized to a uniform cover to achieve 70 percent cover over all exposed areas of the project site.
- Temporary synthetic and structural erosion prevention and sediment control BMPs must be removed.
- All sediment from conveyances and from temporary sediment basins designed to be used a permanent sediment basin will need to be restored to the original design volume and the sediment stabilized.
Section 13 of the MPCA Construction Stormwater General Permit describes conditions for permit termination (submission of the NOT), which includes BMP removal. Regarding the removal of temporary BMPs, Section 13.5 states: “Permittees must remove all temporary synthetic erosion prevention and sediment control BMPs prior to submitting the NOT. Permittees may leave BMPs designed to decompose on-site in place.”
See Costs for information on payment for temporary sediment control devices by MnDOT, including the schedule of payment following BMP removal.
Planning considerations
It is important to consider the cost and time required for BMP removal and disposal when planning the overall project cost and schedule. The MnDOT Standard Specifications for Construction (2018 Edition) Section 2753.3 Part A.1 delegates responsibility of erosion and sediment control BMP removal to the Erosion Control Supervisor, stating: “The following list describes the duties of the Erosion Control Supervisor:
- (2) Ensures proper installation, functionality, and maintenance, clean-up, and removal of all erosion and sediment control Best Management Practices (BMPs) and in accordance with manufacturer’s recommendations.”
Removal or modification procedures
The removal of temporary erosion and sediment control BMPs is not considered a standalone construction stormwater practice; therefore, there are no specific design recommendations for BMP removal, only recommended procedures. In all cases, the removal of devices intended for temporary use shall be performed in accordance with the terms of the contract or as directed by the Engineer.
The following information provides general recommendations on sequencing for several common erosion prevention and sediment control BMPs that are removed from construction sites after final stabilization, when they are no longer needed.
Vehicle tracking BMPs (including temporary concrete washout facilities)
- Remove and dispose of trapped sediment.
- Remove and dispose of materials used to build facility outside of the construction area.
- Backfill any holes or disturbances left after removal.
- Permanently stabilize soil areas disturbed by removal.
Perimeter controls (silt fence)
- Remove and dispose of accumulated sediment.
- Dispose of fencing and posts that are not able to be reused.
- Fill and compact post holes and trenches.
- Grade the area to blend with adjacent ground.
Perimeter controls (straw bale, silt socks, filter berms, and other filtration barriers)
- Remove and dispose of non-degradable materials (including any stakes).
- Remove and dispose of accumulated sediment.
- Re-grade and stabilize as necessary.
Check dams (ditch checks, ditch dikes, etc.)
- Remove and dispose of accumulated sediment.
- Where appropriate, spread rock check dams along channel bottom after channel is vegetated.
- Remove manufactured dikes and check dams after site is stabilized.
- Restore the channel to its original cross-section.
- Stabilize and revegetate any areas disturbed by removal.
Diversion barrier controls (e.g., cofferdams, temporary dikes, etc.)
- Prior to cofferdam removal, stabilize the work area.
- Dewater the cofferdam and discharge sediment-laden water through an approved sediment filtering device.
- Remove the downstream cofferdam first, then remove the upstream cofferdam.
- Remove all parts of the cofferdam with minimal disturbance to the stream channel, stream banks, and any adjacent permanent structures.
- If using floating silt curtains in conjunction with diversion barriers, remove upon the completion of work, being careful not to allow resuspension of sediment, trash, or oil into the water.
- In special cases, such as where removal may damage foundation soils or structures, cofferdams may be left in place; this generally only applies to sheet piling where sheet piles extending below the permanent structure are cut off and left in place.
Storm drain inlet protection
- After surrounding drainage area has been stabilized, remove inlet protection and clean area around inlet including removal of accumulated sediment.
- Re-grade area around inlet as needed.
- Clear inside of inlet of sediment and debris prior to final inspection.
Outlet energy dissipation
- After surrounding drainage area has been stabilized, remove accumulated sediment from the device and dispose.
- Remove the structure if it is not needed to prevent scour erosion after construction.
- Re-grade and stabilize as necessary.
Temporary sediment traps and basins
- After all sediment-producing areas have been stabilized, remove accumulated sediment from the structure and dispose.
- Remove the structure and all unstable sediment.
- Grade the area to blend in with adjacent areas and stabilize as necessary.
Stabilized earth/soil berm
- Determine if berm is necessary for post-construction site drainage.
- Once site reaches final stabilization, remove the berm if necessary, and any accumulated sediment.
- Fill, compact, and vegetate areas of ground disturbance to blend with adjacent ground.
Dewatering (chemical treatment and sediment filtration)
- Remove and dispose of accumulated sediment.
- Properly dispose of treatment chemicals (polymers, flocculants, or other sedimentation treatment chemicals).
- Properly dispose of screening equipment, geotextiles, dewatering bags, fiber filters, and other equipment.
- Remove and dispose of materials used to construct dewatering system
- Backfill any holes or disturbances left after removal.
- Permanently stabilize soil areas disturbed by removal.
Temporary stream diversion
- Do not remove the downstream diversion until all sediment-laden water from the upstream diversion has been filtered through an approved sediment filtering device.
- If using a temporary diversion channel, remove the lining material and then fill and stabilize the channel.
- Stabilize the points of tie-in to the existing stream channel using riprap or other appropriate methods.
- After removing the diversion, revegetate any area disturbed by the removal.
Temporary stream crossing
- Remove temporary stream crossings promptly when no longer needed.
- Remove sediment buildup in the area of the temporary crossing.
- Remove the temporary crossing.
- Restore the stream channel to its original cross-section.
- Stabilize and revegetate any areas disturbed by removal of the temporary crossing.
- Use turf reinforcement matting along banks where flows may cause erosion.
Temporary sediment basins and fiber rolls or logs can be left on site, but should be modified. Temporary sediment basins (conversion to permanent stormwater management basins):
- The contributing drainage areas must be permanently stabilized.
- Clean out sediment from the basin and conveyances.
- Inspect to ensure that side slopes and the volume, containment berm, outlet, and inlets comply with stormwater basin design requirements.
Fiber rolls or logs:
- May be left in place if they are degradable. A common application where fiber rolls/logs are left on site is when they are used as permanent slope breaks (often in conjunction with erosion control blankets).
- If they are removed, dispose of accumulated sediment and fill in any trenches, holes, or depressions to blend in with adjacent ground. Degradable contents, such as compost, may be left in place and spread over the ground to decay.
Inspection and maintenance (final closeout)
Final inspection and closeout of a construction site requires cleanup of all materials related to construction activities, final stabilization to prevent erosion from disturbed areas, and removal of temporary erosion prevention and sediment control BMPs. Final cleanup Section 1407 (Final Cleanup) of the MnDOT Standard Specifications for Construction (2018 Edition) states: “Before requesting final inspection in accordance with 1516.2, “Project Acceptance,” the Contractor shall remove the following from the Project Site and other locations outside of the Project Site used in performing the Work:
- Surplus and discarded materials,
- Equipment,
- Rubbish,
- Temporary structures, and
- Other items not on the Project Site before execution of the Contract.
The Contractor shall also leave the Project Site, including borrow pits, in a condition acceptable to the Engineer. The cost of final cleanup is included in the Contract Unit Prices of the Contract Items.” Final stabilization As discussed above in Applicability, MPCA’s 2018 Stormwater Construction Inspection Guide provides guidelines on conducting final inspection of the site for meeting final stabilization requirements. Regarding BMP removal, the guidance states:
- “Inspect any stabilized areas to ensure that excessive erosion is not occurring. Estimate whether the site has been stabilized with uniform perennial vegetative cover with a density of 70% over the entire pervious area. Temporary BMPs in areas with final stabilization must be removed and sediment must be cleaned out of all conveyances and temporary sediment basins that will be used as permanent water quality management basins. Areas where temporary BMPs have been removed must be stabilized and seeded.”
- Prior to submission of the NOT [Notice of Termination], all temporary synthetic and structural erosion prevention and sediment control BMPs (such as silt fence) must be removed on the portions of the site for which the Permittee is responsible. Best Management Practices designed to decompose on site (such as some compost logs) may be left in place.
Once vegetation has been established and applicable temporary erosion and sediment control BMPs have been removed, the Notice of Termination will need to be submitted to formally close out the project.
Costs
Costs for removal of larger BMPs are typically included in bid pricing. Section 2573.5 (Basis of Payment) of the MnDOT Standard Specifications for Construction (2018 Edition) states: “The contract pay items for stormwater management will include the material, equipment, installation, maintenance, and removal as required by the contract to complete the work. For items of work not included in the schedule of Contract Pay Items, the Department will pay Unit Prices for the relevant items of work. The Department will provide partial payment of temporary sediment control devices properly installed at no greater than 50 percent of the contract unit price for each relevant pay item unless otherwise required by the contract. The Department will pay the remaining partial payment after proper maintenance and removal of the device.” These terms provide an incentive for the contractor to ensure that BMPs are properly removed after the completion of construction.
Reference materials
Except where more stringent requirements are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include the following.