CPESC Sediment Control Best Practices 5 — Questions and Answers
Question 1: Which soil texture is generally MOST erodible under rainfall impact?
- Clay
- Sand
- Silt loam (Correct answer)
- Gravel
Correct answer: Silt loam
Silt loam soils have high erodibility (high K factor in RUSLE) because silt particles are easily detached by raindrop impact yet remain suspended in runoff rather than settling quickly.
Question 2: A vegetated filter strip is MOST effective when:
- Runoff enters as concentrated channelized flow
- Runoff enters as uniform sheet flow across a dense vegetative buffer (Correct answer)
- The slope of the filter strip exceeds 10%
- Soils in the filter strip are highly compacted to reduce infiltration
Correct answer: Runoff enters as uniform sheet flow across a dense vegetative buffer
Vegetated filter strips work by slowing and spreading sheet flow through dense vegetation, allowing sediment to settle and some runoff to infiltrate.
Question 3: What maintenance action should be performed on a sediment basin outlet structure after every significant storm event?
- Replace the geotextile filter fabric on the riser
- Inspect for clogging, structural damage, and verify the principal spillway is functioning (Correct answer)
- Remove all vegetation growing near the embankment
- Add riprap to the basin floor to reduce scour
Correct answer: Inspect for clogging, structural damage, and verify the principal spillway is functioning
Post-storm inspection of the outlet structure ensures it is not clogged or damaged, which could cause basin failure or bypass of stored sediment.
Question 4: Which of the following is a chemical additive approved for use in treating turbid stormwater discharges from construction sites?
- Sodium hypochlorite (bleach)
- Anionic polyacrylamide (PAM) (Correct answer)
- Calcium chloride
- Ferric sulfate at high doses
Correct answer: Anionic polyacrylamide (PAM)
Anionic PAM is widely approved for stormwater turbidity treatment at construction sites because it is non-toxic to aquatic life at application rates and effectively flocculates fine particles.
Question 5: In a sediment basin design, the 'wet storage' volume is intended to:
- Store runoff from the 100-year storm event
- Provide sufficient hydraulic residence time for sediment to settle from design storm runoff (Correct answer)
- Prevent vegetation from colonizing the basin bottom
- Accommodate permanent groundwater infiltration into the soil
Correct answer: Provide sufficient hydraulic residence time for sediment to settle from design storm runoff
Wet storage volume is the permanent pool below the outlet that holds water long enough to allow sediment particles to settle before discharge.
Question 6: When a construction site is located adjacent to a water of the United States (WOTUS), what additional sediment control consideration is MOST critical?
- Increasing the height of perimeter silt fences to 36 inches
- Establishing and protecting a vegetated buffer zone between the disturbed area and the water body (Correct answer)
- Eliminating all stormwater runoff from the site during construction
- Using only biodegradable erosion control products within 100 feet of the water
Correct answer: Establishing and protecting a vegetated buffer zone between the disturbed area and the water body
A vegetated buffer zone acts as a final filter and provides physical separation between construction activity and sensitive waters, helping prevent sediment and pollutants from entering WOTUS.
Question 7: During cold weather construction, which sediment control practice may require special attention because freeze-thaw cycles can compromise its effectiveness?
- Riprap-lined channels
- Silt fence fabric and trench backfill (Correct answer)
- Concrete washout areas
- Sediment basin embankments built with clay
Correct answer: Silt fence fabric and trench backfill
Freeze-thaw cycles can heave the trench backfill and loosen the buried fabric toe of silt fences, creating gaps that allow runoff to undercut the fence.
CPESC Sediment Control Best Practices 5
Which soil texture is generally MOST erodible under rainfall impact?