NPDES Construction Stormwater Permits
The Clean Water Act Section 402 requires anyone disturbing one or more acres of land (or less than one acre if part of a larger common plan of development) to obtain coverage under an NPDES (National Pollutant Discharge Elimination System) construction general permit before initiating earth disturbance. In most states, this is the Construction General Permit (CGP) issued by the state environmental agency (delegated by the EPA to most states). The permit requires development and implementation of a Stormwater Pollution Prevention Plan (SWPPP), installation of best management practices (BMPs) to minimize sediment and other pollutant discharge, regular site inspections, and corrective action when BMPs fail.
Failure to comply with NPDES permit conditions exposes the project owner and contractor to significant penalties: EPA can assess administrative penalties up to $10,000 per day per violation and judicial penalties up to $25,000 per day per violation for negligent violations. Civil enforcement actions and third-party lawsuits under the citizen suit provisions of the Clean Water Act have become more common. A well-designed and faithfully implemented SWPPP is essential risk management for any land-disturbing project.
SWPPP Development
The SWPPP documents the site conditions, the BMPs to be installed, the inspection schedule, and the responsible parties. Key SWPPP elements include: site description (project location, total and disturbed acreage, receiving water bodies, soil types, slopes); existing and proposed topography maps; construction sequence identifying when each phase of disturbance occurs and what BMPs will be installed before disturbance begins; BMP installation details (location, sizing, materials, installation standards referenced to state erosion control manuals or EPA BMP specifications); inspection schedule (typically weekly and within 24 hours of a 0.5-inch or greater rainfall event per EPA CGP); corrective action procedures; and stabilization timeline (temporary stabilization within 14 days of last disturbance; permanent stabilization of disturbed areas within 2 years of completion).
Perimeter Sediment Controls
Silt fence is the most commonly used perimeter BMP for capturing sediment runoff from disturbed areas. It consists of a permeable geotextile fabric attached to wire or wooden posts, installed in a trench with the bottom 6 inches of fabric buried in the ground to prevent undercutting. Silt fence is effective for shallow sheet flow conditions on slopes not exceeding 3:1; it is not effective for concentrated flow, steep slopes, or high velocities. A key installation detail is the j-hook at the end of each silt fence run (turning the ends upslope to prevent end-runs) and proper J-trench installation of the bottom. Silt fence must be inspected after each rain event and repaired or replaced when more than one-third of its height is filled with sediment.
Rock check dams are installed across drainage swales and channels to slow velocity, allowing sediment to settle behind the dam. Rock should be hard, angular stone (riprap) sized to resist movement at design flow; ASTM D 1073 Type D1 or D2 aggregate is commonly specified. Outlet protection at the toe of disturbed slopes and at pipe outfalls prevents scour erosion where concentrated flow discharges onto exposed soil.
Inlet Protection
Storm drain inlet protection prevents sediment from entering the storm drainage system during construction. Multiple BMP types are available depending on inlet type and contributing drainage area. Block and gravel inlet protection uses concrete blocks arranged to form a permeable dam across the inlet opening with coarse gravel behind; this is durable and effective for flat inlets with high contributing areas. Geotextile inlet inserts fit inside the storm drain inlet frame and capture sediment while allowing water to drain; these are easier to install but fill quickly and require frequent cleaning. Silt fence around circular inlets with a pool area for settling is another common approach. Inlet protection must be cleaned or replaced when sediment accumulates to one-half of the storage capacity (typically after each significant rain event).
Sediment Basins
On larger sites with substantial runoff volume, sediment basins (or sediment traps for smaller areas) provide active settling of suspended sediment before discharge. EPA CGP typically requires a sediment basin for sites with 10 or more acres draining to a common point. Basin sizing uses the EPA sizing guidance: 1,800 cubic feet of storage per disturbed acre for basins using the surface area method, or using detailed settling calculations based on design storm runoff and target removal efficiency. Sediment basin design includes a riser structure with perforated risers that allow clarified water above the sediment pool to exit while sediment settles; an emergency spillway sized for the 10-year or 25-year storm peak discharge to protect the embankment from overtopping; and a cleanout access route for equipment to remove accumulated sediment.
Post-Construction Stormwater Management
After construction is complete and sites are stabilized, many jurisdictions require long-term stormwater management features to address the increased runoff quantity and reduced water quality from impervious surfaces. Common post-construction BMPs include detention basins (providing temporary storage of stormwater to reduce peak discharge rates, releasing at pre-development rates), retention basins (permanent wet ponds that provide water quality treatment through settling and biological uptake in addition to detention), bioretention (rain gardens and bioswales that use engineered soil media and plants to filter stormwater and promote infiltration), and permeable pavement (porous asphalt, pervious concrete, or permeable pavers that allow infiltration through the surface into a stone reservoir base). The specific requirements are set by local MS4 (Municipal Separate Storm Sewer System) permit conditions and local stormwater management ordinances.