15.3 Erosion, Sediment Control, and BMPs
Key Takeaways
- Erosion control prevents soil detachment, while sediment control traps particles after they have been mobilized; good plans use both.
- Construction stormwater permitting under NPDES applies when land disturbance is one acre or more, or less than one acre within a larger common plan of development or sale that ultimately disturbs one acre or more.
- Best management practices must match the flow condition: silt fence is for shallow sheet flow, not high-velocity concentrated flow.
- Temporary BMPs protect the active construction phase, while permanent BMPs such as swales, infiltration areas, outlet protection, and detention features support final site performance.
- Inspection and maintenance are part of the design concept because clogged inlet protection, undermined silt fence, or full sediment basins no longer provide their assumed function.
Erosion Control Is Not the Same as Sediment Control
Erosion control keeps soil from being detached and transported in the first place. Sediment control captures soil after it has already moved. PE Civil WRE questions frequently hinge on that distinction. If a slope is bare and steep, the best first move is stabilization or upslope flow diversion, not simply adding a downstream fence and hoping it catches everything. The governing principle is the erosion control hierarchy: prevent, then divert, then capture.
The physics behind prevention is captured by the Universal Soil Loss Equation (USLE), A = R x K x LS x C x P, where A is average annual soil loss (tons/acre/year), R is rainfall erosivity, K is soil erodibility, LS is the slope length-steepness factor, C is the cover-management factor, and P is the support-practice factor. The two factors an engineer most directly controls during construction are C (mulch, blankets, seeding cut C from near 1.0 on bare soil to below 0.1) and LS (shorter, flatter slopes through phasing and diversion benches).
This is why stabilizing cover and breaking up slope length beat chasing sediment downstream.
Permitting Context
Under the federal National Pollutant Discharge Elimination System (NPDES) program, construction stormwater permit coverage is generally required for sites disturbing one or more acres, and for smaller disturbances that are part of a common plan of development or sale that will ultimately disturb one or more acres. Many states are delegated to administer the program through their own Construction General Permit and may add stricter requirements. Coverage requires preparing and following a Stormwater Pollution Prevention Plan (SWPPP).
For PE purposes, remember that clearing, grading, excavation, stockpiles, dewatering, concrete washout, fueling, and material storage can all generate stormwater pollutants beyond sediment.
BMP Selection Table
| BMP | Primary purpose | Best use | Common misuse |
|---|---|---|---|
| Construction phasing | Reduce exposed soil area | Large staged sites | Disturbing the full site too early |
| Temporary seeding or mulch | Prevent erosion | Inactive disturbed areas | Waiting until final grading only |
| Diversion berm or swale | Route clean water around disturbed soil | Upslope run-on control | Sending concentrated flow onto an unprotected slope |
| Silt fence | Trap sediment from shallow sheet flow | Down-gradient perimeter | Installing across channels or deep flow |
| Inlet protection | Keep sediment out of storm drains | Active paved/graded areas | Blocking the inlet so streets flood |
| Check dam | Slow concentrated ditch flow | Temporary channels/swales | Letting flow bypass the ends |
| Sediment trap or basin | Detain runoff and settle sediment | Larger drainage areas | Forgetting cleanout volume and access |
| Stabilized entrance | Reduce track-out | Construction access points | Treating it as a street-sweeping substitute |
| Outlet protection (riprap apron) | Prevent scour | Pipe and culvert outlets | Ending a high-velocity pipe on bare soil |
Design and Calculation Workflow
- Map disturbance and drainage. Identify disturbed acres, off-site run-on, discharge points, receiving waters, slopes, stockpiles, and existing inlets.
- Keep clean water clean. Divert upslope runoff around disturbed soil; smaller dirty drainage areas are far easier to control.
- Select erosion prevention first. Preserve vegetation, phase grading, stabilize inactive areas within the permit timeframe, and protect steep slopes with blankets or turf reinforcement.
- Add sediment controls at logical collection points. Perimeter controls, inlet protection, traps, and basins go where runoff actually concentrates.
- Check concentrated-flow energy. Channels, pipe outlets, culverts, and downspouts may need lining, check dams, riprap, or energy dissipation.
- Apply sizing criteria. Common rules of thumb: a sediment trap sized near 3,600 ft^3 of storage per acre and a sediment basin near 3,600 ft^3 per acre of drainage (many states specify the figure). Example: at 3,600 ft^3 per disturbed acre, a 4.5-acre area requires 4.5 x 3,600 = 16,200 ft^3.
- Plan inspection and maintenance. Typical permits require inspection at least every 7 days and within 24 hours of a qualifying storm (often 0.5 inch). A BMP that is undermined, full, clogged, or bypassed is failed until repaired.
Temporary vs Permanent BMP Thinking
Temporary construction BMPs control risk while the site is unstable: silt fence, temporary sediment basins, construction entrances, check dams, mulch, temporary seeding, inlet protection. Permanent BMPs are part of the finished drainage and water-quality system: vegetated swales, bioretention areas, detention or retention ponds, infiltration practices, stable channels, and riprap outlet aprons that often serve as both treatment and final landscape.
The exam may describe a BMP that sounds helpful but is hydraulically mismatched. Silt fence traps sheet-flow sediment but must never act as a channel dam, and it is rated for only a small contributing area (commonly about 0.25 acre per 100 ft of fence on slopes flatter than about 2 percent). Inlet protection reduces sediment to the storm sewer, but if it creates unsafe ponding or diverts muddy water around the inlet, it needs reconfiguration. A riprap apron controls outlet scour but does not replace upstream erosion prevention.
A final WRE concept is the trapping (removal) efficiency of a sediment basin, which depends on the ratio of the particle settling velocity to the basin overflow rate (surface loading). Because settling velocity scales with the square of particle diameter (Stokes' law), basins readily capture sand and coarse silt but pass clay-size fines, which is why turbidity, not just visible sediment, often governs discharge limits and why source-control erosion prevention beats end-of-pipe capture. The best PE answer usually combines source control, flow control, sediment capture, and maintenance, mirroring real WRE practice.
A temporary drainage swale on an active construction site is carrying concentrated flow and beginning to erode between storms. Which BMP most directly reduces flow velocity within the swale?
A local standard requires 3,200 ft^3 of sediment-trap storage per disturbed acre. A construction phase drains 3.75 disturbed acres to one trap. What minimum storage volume does the standard require?