CESSWI Erosion Control BMPs 5 — Questions and Answers
Question 1: Under the NPDES Construction General Permit, which condition triggers the requirement to implement 'non-storm water management' erosion controls?
- Discharge of irrigation water or dewatering effluent from a construction site (Correct answer)
- Rain events exceeding 0.5 inches in 24 hours
- Disturbance of more than 5 acres at one time
- Presence of endangered species habitat within 500 feet
Correct answer: Discharge of irrigation water or dewatering effluent from a construction site
Non-stormwater discharges such as irrigation water or construction dewatering effluent require specific management BMPs because they are not precipitation-generated.
Question 2: What is the function of a rock riprap apron installed at the outlet of a pipe slope drain?
- Dissipate flow energy to prevent scour erosion at the discharge point (Correct answer)
- Filter sediment from discharge before it reaches a waterway
- Slow infiltration of concentrated flow into the soil
- Redirect flow back up the slope for reuse
Correct answer: Dissipate flow energy to prevent scour erosion at the discharge point
Riprap aprons at pipe outlets dissipate the kinetic energy of concentrated discharge, preventing scour holes from forming in the receiving soil.
Question 3: Which factor makes soil with high sodium content (sodic soil) particularly challenging for erosion control BMP performance?
- Sodium causes soil particles to disperse, creating highly erodible and poorly draining conditions (Correct answer)
- Sodium accelerates biodegradation of erosion control blankets
- Sodium prevents seed germination regardless of BMP type
- Sodium increases runoff pH, corroding metal BMP components
Correct answer: Sodium causes soil particles to disperse, creating highly erodible and poorly draining conditions
Sodic soils have dispersed, unstable aggregates that break apart easily under rainfall, dramatically increasing erodibility and reducing infiltration rates.
Question 4: An erosion control inspector is reviewing a SWPPP for a linear project (pipeline installation). Which unique erosion challenge must linear projects address that is less common on area grading projects?
- Trench backfill settlement creating concentrated flow paths parallel to the installation corridor (Correct answer)
- Greater total disturbed acreage requiring additional BMPs
- Proximity to residential structures requiring noise controls
- Elevated grading equipment exhaust affecting vegetation establishment
Correct answer: Trench backfill settlement creating concentrated flow paths parallel to the installation corridor
Linear project trenches frequently settle after backfill, creating low-gradient linear channels that concentrate runoff along the corridor and increase erosion risk.
Question 5: What is the primary reason that seeding alone is NOT considered a stabilization BMP immediately after installation?
- Vegetation must germinate and achieve sufficient cover before providing meaningful erosion protection (Correct answer)
- Seeds increase surface roughness, which accelerates runoff velocity
- Seeding requires soil amendments that temporarily reduce infiltration
- Seeds attract wildlife that disturb the soil surface
Correct answer: Vegetation must germinate and achieve sufficient cover before providing meaningful erosion protection
Seeding requires germination and establishment time (often 2–4+ weeks) before it provides measurable erosion protection, so interim measures are needed during that period.
Question 6: When evaluating the adequacy of an erosion control plan, an inspector should verify that BMPs are designed to handle what storm event at minimum?
- A 2-year, 24-hour storm event for temporary BMPs (Correct answer)
- A 100-year, 24-hour storm event for all BMPs
- A 10-year, 1-hour storm event for slopes only
- A 5-year, 6-hour storm event for perimeter controls
Correct answer: A 2-year, 24-hour storm event for temporary BMPs
Most construction stormwater permits and guidance documents require temporary erosion control BMPs to be designed to handle at least a 2-year, 24-hour storm event.
Question 7: Which best describes the difference between erosion prevention BMPs and sediment control BMPs in a construction SWPPP?
- Erosion prevention keeps soil in place at the source; sediment control captures soil after it has been detached and transported (Correct answer)
- Erosion prevention applies only to slopes; sediment control applies only to flat areas
- Erosion prevention is required only during rain events; sediment control operates continuously
- Erosion prevention uses vegetative methods only; sediment control uses structural methods only
Correct answer: Erosion prevention keeps soil in place at the source; sediment control captures soil after it has been detached and transported
Erosion prevention BMPs (mulch, blankets, seeding) stop soil detachment at the source, while sediment control BMPs (silt fence, basins) capture already-mobilized particles before they leave the site.
CESSWI Erosion Control BMPs 5
Under the NPDES Construction General Permit, which condition triggers the requirement to implement 'non-storm water management' erosion controls?