Bridging construction and post-construction stormwater management: Considerations for permanent stormwater control measures

Most construction projects are now required to implement stormwater Best Management Practices (BMPs) to minimize offsite discharges of pollutants during construction, as well as permanent Stormwater Control Measures (SCMs) to minimize offsite discharges of pollutants after construction is complete. Further, an increasing number of redevelopment sites have existing SCMs that may need to be protected, reconfigured, or replaced during construction.

This article focuses on the relationship between construction BMPs and permanent SCMs, considering where SCMs can be leveraged as a BMP during construction and where SCMs require special construction stormwater management considerations. Keep in mind that regulatory constraints vary considerably at the state and local levels. The suggestions provided herein are just that and should always be vetted against all applicable regulations before being implemented on a given project. For the purposes of this article, construction-phase controls are referred to as BMPs and post-construction, permanent controls as SCMs.

Extended detention basins and wet ponds during construction

If an extended detention basin (EDB) or wet pond exists at the site prior to construction, the project team should decide early on whether to use it as a construction stormwater BMP or protect it in place by minimizing turbid stormwater discharges to the facility. If the facility will be protected in place during construction of the rest of the site, the stormwater pollution prevention plan (SWPPP) should be designed to minimize the discharge of sediment into the EDB or wet pond throughout construction. Perimeter sediment control measures, inlet protections, phasing to minimize upgradient disturbances, implementation of temporary and/or permanent stabilization as soon as practicable, diversion ditches, sediment traps and temporary sediment basins (TSBs) located elsewhere within the site are all examples of BMPs that should be considered for use in a treatment train upgradient of the EDB. Construction fencing to exclude construction traffic from the EDB may also be a prudent addition.

If the EDB or wet pond will be expanded during construction or if a new EDB or wet pond is being built, it may make good sense to use it as a TSB during construction. These facilities will be located at low points of the site, and this strategy has the advantage of treating the water where it will naturally drain. In some cases, TSBs may need to be designed to provide more ponding volume and treat greater pollutant loads than EDBs and wet ponds, which are sized for the post-construction water quality capture volume. It is critical to compare the permanent SCM design against the applicable TSB design standards to determine whether the proposed grading for the SCM will provide sufficient ponding volume. If the SCM design does not provide adequate ponding volume for use as a TSB, one approach might be to expand the basin size and then regrade as part of converting to the SCM configuration. Alternatively, an EDB or wet pond with insufficient volume to treat construction stormwater discharges on its own can sometimes be combined with other BMPs to provide sufficient treatment prior to discharge, depending on the regulations that govern TSB design in the area.

EDBs and wet ponds are also designed for different discharge rates and standards than TSBs. For example, many jurisdictions mandate that water within TSBs be discharged from the top of the water column first, which may necessitate the use of a skimmer or similar device, while EDBs are typically designed to discharge through a submerged orifice plate. The rate at which water will be discharged from the SCM outlet structure will be calibrated to the EDB’s volume, which, if smaller than the TSB volume may lead to slower than permissible drain times while the pond is in use as a TSB. The project team will need to evaluate whether the proposed outlet structure for the SCM can be installed early in the process and used as the TSB outlet structure. If the outlet structure is not suitable for use in the temporary configuration, consider further whether to install the permanent outlet structure, block it off, and use an alternative outlet during construction, or use an alternative outlet and wait to install the permanent outlet structure until the project is converting the TSB to serve as a SCM.

In all cases, it is critical that the EDB or wet pond is installed and functioning in accordance with the approved design by the time construction is complete. The SWPPP should include information on all of the tasks that will be required in order to convert from temporary to permanent configuration. Depending on the SWPPP design, the list might include:

• Dredging accumulated sediment.
• Performing additional earthwork to get to the correct pond volume.
• Removing the TSB outlet structure.
• Installing the permanent outlet structure or unblocking the permanent outlet structure.
• Installing or cleaning out other features including the forebays, low flow channel, and micro-pool.
• Installing permanent stabilization measures. EDBs and wet ponds are both likely to have permanent stabilization considerations specific to the fact that the basin has sloped sides and will sometimes or always contain water. Make sure to work with the engineer of the SCM to include all relevant information on permanent stabilization in the SWPPP, such as specialized seed mixes or erosion control blankets.
Removing all construction BMPs once final stabilization has been achieved.

Infiltration stormwater control measures during construction

Infiltration SCMs such as bioswales, sand filters, and permeable interlocking pavers require special considerations in the SWPPP and project phasing. These SCMs are all designed to allow stormwater to percolate through naturally existing soils or through specially designed filtration media at a pre-determined rate. In cases where the design relies on infiltration through existing soils, protection of the infiltration area from compaction by construction equipment wherever possible is essential. SWPPP designers should ensure that access to the future location of the SCM is prohibited using construction fencing or similar. If compaction cannot be avoided due to grading or other constraints, consult with the engineer who designed the SCM to find out how decompaction can be achieved and incorporate that information into the final stabilization section of the SWPPP.

SCMs that use engineered and imported filtration media like sand filters and permeable interlocking pavers are vulnerable to rapid clogging with sediment discharged to the SCM during the construction and final stabilization phases. Consider moving the installation of these SCMs later in the construction schedule, implementing especially robust upgradient control measures, and rapidly implementing final stabilization measures in the area draining to the SCM to minimize this risk. If the SCM already exists at the site and is to be protected in place, use of upgradient treatment trains or diverting stormwater away from the SCM during most of the active construction phase will be critical.

Many infiltration SCMs also have special stabilization requirements as compared with other areas of the project site, such as subgrade preparation, placement depth and order of imported media, seed mixes, soil amendment, and plantings. The author of the SWPPP and the drainage engineer responsible for designing the infiltration SCM need to work together to make sure that the SWPPP incorporates the stabilization requirements specific to the SCM. 

Conclusion

SCMs offer both challenges and opportunities when it comes to managing stormwater during construction. In some cases, existing or proposed SCMs can serve as a BMP for construction stormwater. In other cases, SCMs should be protected from turbid construction stormwater discharges and compaction. In all cases, the project team members responsible for designing the SCM, SWPPP, permanent stabilization/landscaping, and construction schedules need to work together to proactively plan for how the SCM will be managed during construction through to the permanent configuration, and the construction documents delivered by each discipline should incorporate the relevant information.