CEPSCI Sediment Basin and Trap Design 1 — Questions and Answers
Question 1: What is the primary purpose of a temporary sediment basin on a construction site?
- To store construction materials
- To capture and retain sediment-laden runoff, allowing sediment to settle out before water is discharged (Correct answer)
- To provide drinking water for construction workers
- To store stormwater for irrigation
Correct answer: To capture and retain sediment-laden runoff, allowing sediment to settle out before water is discharged
Temporary sediment basins trap stormwater runoff and hold it long enough for suspended sediment to settle to the bottom before the clarified water is released through a controlled outlet.
Question 2: A sediment basin is typically required for drainage areas exceeding:
- 0.5 acres
- 5 acres (Correct answer)
- 10 acres
- 50 acres
Correct answer: 5 acres
Most regulatory guidance requires a sediment basin when the drainage area exceeds 5 acres, where the volume of runoff exceeds the practical capacity of smaller BMPs like sediment traps.
Question 3: The wet storage volume in a sediment basin is designed to:
- Provide habitat for aquatic species
- Hold the accumulated sediment load between cleanout events (Correct answer)
- Store clean water for reuse
- Provide surge capacity during large storm events
Correct answer: Hold the accumulated sediment load between cleanout events
Wet storage volume is the portion of the basin below the permanent pool level that accumulates settled sediment between cleanout operations.
Question 4: The outlet structure of a sediment basin should be designed to:
- Release all captured water as quickly as possible
- Release water slowly from the upper portion of the pool where the water is clearest while retaining the sediment-laden lower water (Correct answer)
- Prevent any water from leaving the basin
- Allow sediment to pass through with the water
Correct answer: Release water slowly from the upper portion of the pool where the water is clearest while retaining the sediment-laden lower water
Outlet structures should discharge from near the water surface where the water is clearest, retaining the deeper, sediment-laden water and allowing sediment to settle before discharge.
Question 5: Surface area is critical in sediment trap design because:
- Surface area should be ignored
- Larger surface area allows more particles to settle per unit time by lowering the overflow rate (Correct answer)
- Only depth matters for sediment trap design
- Only the outlet pipe size matters
Correct answer: Larger surface area allows more particles to settle per unit time by lowering the overflow rate
Surface area is the key design parameter for sediment settling because larger surface area relative to flow rate lowers the overflow rate and increases the proportion of particles that settle out.
Question 6: When should a temporary sediment basin be cleaned out?
- Only when it is completely full
- When sediment accumulation reaches 50% of the designed wet storage volume (Correct answer)
- After every rainfall event
- Never; basins are designed to fill completely
Correct answer: When sediment accumulation reaches 50% of the designed wet storage volume
Sediment basins should be cleaned out when accumulated sediment reaches approximately 50% of the wet storage volume to maintain hydraulic capacity and sediment removal efficiency.
CEPSCI Sediment Basin and Trap Design 1
What is the primary purpose of a temporary sediment basin on a construction site?