CESSWI Sediment Control BMPs 4 — Questions and Answers
Question 1: Which of the following is a PRIMARY design consideration when sizing a sediment basin outlet structure?
- The basin must drain completely within 24 hours of storm cessation
- The outlet must handle the 10-year peak flow without overtopping the embankment (Correct answer)
- The outlet pipe diameter must match the inlet ditch width
- The outlet must be constructed of HDPE to prevent corrosion
Correct answer: The outlet must handle the 10-year peak flow without overtopping the embankment
The outlet structure must safely convey the peak flow from the design storm (commonly 10-year) to prevent embankment overtopping and basin failure.
Question 2: What is the function of an anti-seep collar on a sediment basin riser pipe?
- To prevent sediment from entering the outlet pipe
- To stop water from migrating along the pipe through the embankment (Correct answer)
- To reduce pipe vibration during high flow events
- To anchor the riser pipe to prevent uplift
Correct answer: To stop water from migrating along the pipe through the embankment
Anti-seep collars interrupt the flow path of water that could migrate along the outside of the outlet pipe through the embankment, preventing piping failures.
Question 3: In which scenario is a rock berm PREFERABLE to a silt fence for sediment control?
- Sheet flow over flat slopes less than 1%
- Concentrated flow conditions or in drainage channels (Correct answer)
- Areas with established vegetation nearby
- Perimeter control along property lines
Correct answer: Concentrated flow conditions or in drainage channels
Rock berms can handle concentrated flow and higher flow velocities that would overwhelm and breach a silt fence, making them more appropriate for drainage channels.
Question 4: What is the typical maximum spacing between rock check dams in a drainage channel to ensure continuity of sediment capture?
- The downstream dam crest elevation should equal the toe elevation of the upstream dam (Correct answer)
- Spacing should not exceed 50 feet regardless of slope
- Spacing should maintain ponded water 2 feet deep between dams
- Dams should be spaced 100 feet apart on slopes less than 5%
Correct answer: The downstream dam crest elevation should equal the toe elevation of the upstream dam
Check dams should be spaced so that the elevation of the crest of the downstream dam equals the elevation of the toe of the upstream dam, creating a level pool between them.
Question 5: Which factor MOST affects the long-term effectiveness of a sediment basin in removing fine particles?
- Basin depth exceeding 5 feet
- Hydraulic residence time — the longer water stays in the basin, the more settling occurs (Correct answer)
- Use of concrete rather than earthen embankments
- Location of the basin relative to prevailing wind direction
Correct answer: Hydraulic residence time — the longer water stays in the basin, the more settling occurs
Fine sediment removal depends on particle settling velocity and the time water remains in the basin; longer residence time allows finer particles to settle out.
Question 6: During a site inspection, you observe sediment deposited outside the construction boundary. This MOST likely indicates:
- Sediment transport from adjacent undisturbed areas
- Failure or inadequacy of perimeter sediment controls (Correct answer)
- Normal sediment redistribution from wind activity
- Sediment from vehicle tracking on public roads
Correct answer: Failure or inadequacy of perimeter sediment controls
Sediment deposited beyond the construction limit of disturbance is direct evidence that perimeter BMPs have failed to contain site runoff.
Question 7: What is the recommended minimum distance from a sediment trap or basin outlet to a stream or sensitive receiving water?
- 10 feet
- 25 feet
- 50 feet (Correct answer)
- No minimum if outlet is vegetated
Correct answer: 50 feet
A minimum 50-foot buffer between the outlet and receiving waters allows additional settling and energy dissipation before discharge reaches sensitive waterways.
CESSWI Sediment Control BMPs 4
Which of the following is a PRIMARY design consideration when sizing a sediment basin outlet structure?