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 commonly applies when land disturbance is one acre or more, or less than one acre within a larger common plan of development or sale.
- 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. Sediment control captures soil after it has already moved. PE Civil WRE questions often hinge on that distinction. If a slope is bare and steep, the best first move is usually stabilization or flow diversion, not simply adding a downstream fence and hoping it catches everything.
Construction stormwater controls also have a permitting context. Under the federal National Pollutant Discharge Elimination System framework, 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. States or local agencies may administer the program and may add requirements. For PE purposes, remember that clearing, grading, excavation, stockpiles, dewatering, concrete washout, fueling, and material storage can all create stormwater pollutants.
BMP Selection Table
| BMP | Primary purpose | Best use | Common misuse |
|---|---|---|---|
| Construction phasing | Reduce exposed soil area | Large sites with staged work | 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 to an unprotected slope |
| Silt fence | Trap sediment from shallow sheet flow | Down-gradient perimeter areas | Installing across channels or deep flow paths |
| Inlet protection | Keep sediment out of storm drains | Active paved or graded areas | Blocking the inlet so street flooding occurs |
| Check dam | Slow concentrated ditch flow | Temporary channels and swales | Using where flow will bypass around the ends |
| Sediment trap or basin | Detain runoff and settle sediment | Larger drainage areas | Forgetting cleanout volume and maintenance access |
| Stabilized entrance | Reduce track-out | Construction access points | Treating it as a substitute for street sweeping |
| Outlet protection | Prevent scour | Pipe and culvert outlets | Ending a high-velocity pipe on bare soil |
Calculation and Design 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 when practical. Smaller dirty drainage areas are easier to control.
- Select erosion prevention first. Preserve vegetation, phase grading, stabilize inactive areas, and protect steep slopes.
- Add sediment controls at logical collection points. Perimeter controls, inlet protection, traps, and basins should be placed where runoff actually flows.
- Check concentrated-flow energy. Channels, pipe outlets, culvert outlets, and downspouts may need lining, check dams, riprap, turf reinforcement, or energy dissipation.
- Apply permit or local sizing criteria. If a standard requires 3,600 ft^3 of sediment basin storage per disturbed acre, a 4.5 acre drainage area requires 4.5 x 3,600 = 16,200 ft^3 of storage.
- Plan inspection and maintenance. A BMP that is undermined, full of sediment, clogged, or bypassed should be treated as failed until repaired.
Temporary vs Permanent BMP Thinking
Temporary construction BMPs control risk while the site is unstable. Examples include silt fence, temporary sediment basins, construction entrances, check dams, mulch, temporary seeding, and inlet protection. Permanent BMPs are part of the finished drainage and water-quality system, such as vegetated swales, bioretention areas, detention or retention ponds, infiltration practices, stable channels, and riprap outlet aprons.
The exam may describe a BMP that sounds helpful but is mismatched to the hydraulics. Silt fence can trap sheet-flow sediment, but it should not be used as a dam in a channel. Inlet protection can reduce sediment entering the storm sewer, but if it creates unsafe ponding or diverts muddy water around the inlet, it needs a different configuration. A riprap apron controls scour at an outlet, but it does not replace upstream erosion prevention.
The best PE answer usually combines source control, flow control, sediment capture, and maintenance. That mirrors real WRE practice: prevent detachment where possible, slow the water where needed, settle or filter sediment before discharge, and maintain the system after construction traffic and storms damage it.
A temporary drainage swale on an active construction site is carrying concentrated flow and beginning to erode between storm events. Which BMP is most directly suited to reducing flow velocity within the swale?
A local standard requires 3,200 ft^3 of sediment trap storage per disturbed acre. A phase drains 3.75 disturbed acres to one trap. What minimum storage volume is required by that standard?