Erosion Processes and Regulatory Framework
Construction sites are highly vulnerable to soil erosion because vegetation removal exposes bare soil to rainfall and runoff. The erosion process begins with raindrop impact that dislodges soil particles, followed by sheet flow that transports particles downhill. As flow concentrates, rills and gullies form, carrying sediment into drainage systems and waterways. The National Pollutant Discharge Elimination System permit program regulates stormwater discharges from construction sites.
stormwater pollution prevention.
The Clean Water Act requires construction sites disturbing one acre or more to obtain NPDES permit coverage. The permit requires development of a Stormwater Pollution Prevention Plan that identifies potential erosion sources, specifies control measures, and establishes inspection and maintenance procedures. Violations of permit requirements can result in significant fines and project shutdowns.
Erosion Control Best Management Practices
Erosion control BMPs prevent soil particles from being detached by rainfall or runoff. Vegetative stabilization using temporary or permanent seeding establishes plant cover to protect soil surfaces. Seeding with fast-growing annual grasses provides temporary cover during construction. Mulching with straw, wood fiber, or rolled erosion control products protects slopes from raindrop impact until vegetation is established. soil compaction testing. excavation protective systems. groundwater control methods.
Surface roughening through tracking or tillage slows runoff and promotes infiltration on slopes. Erosion control blankets and mats provide immediate protection on steep slopes and channels. Hydromulching applies a mixture of seed, fertilizer, mulch, and binder in a single operation for large areas. The selection of erosion control methods depends on slope steepness, soil type, season, and duration of exposure.
Sediment Control Practices
Sediment control BMPs capture soil particles after they have been transported by runoff. Silt fences are the most widely used sediment control device, consisting of geotextile fabric supported by stakes. The fabric filters sediment from runoff while allowing water to pass through. Proper installation with the fabric trenched into the ground and stakes on the downslope side is essential for effectiveness.
Sediment basins and traps capture runoff from larger areas and allow sediment to settle before discharge. The basin volume must be sufficient to store runoff from a 2-year, 24-hour storm event. Outlet structures control the release rate and prevent resuspension of settled sediment. Sediment removal and disposal is required when the basin capacity is reduced by 50 percent.