CEPSCI Sediment Basin and Trap Design 2 — Questions and Answers
Question 1: A skimmer outlet on a sediment basin is designed to:
- Measure water depth
- Draw water from the surface of the pool to minimize turbulence and discharge the clearest water (Correct answer)
- Filter sediment using a screen
- Prevent fish from entering the basin
Correct answer: Draw water from the surface of the pool to minimize turbulence and discharge the clearest water
A floating skimmer draws water from just below the surface, discharging the clearest, least turbid water while keeping the turbid bottom water retained for further settling.
Question 2: What is the purpose of installing an inlet energy dissipater or baffle in a sediment basin?
- To aerate the water in the basin
- To dissipate the energy of incoming flows and prevent short-circuiting (Correct answer)
- To filter large debris from the basin
- To mark the inlet location
Correct answer: To dissipate the energy of incoming flows and prevent short-circuiting
Inlet energy dissipation structures reduce the velocity of incoming runoff to prevent resuspension of settled sediment and reduce the risk of short-circuiting through the basin.
Question 3: Short-circuiting in a sediment basin means:
- An electrical problem with the outlet pump
- Runoff flowing directly from inlet to outlet without adequate residence time for settling (Correct answer)
- The basin overflowing during a storm
- Sediment blocking the outlet pipe
Correct answer: Runoff flowing directly from inlet to outlet without adequate residence time for settling
Short-circuiting occurs when flow channels directly from the inlet to the outlet, bypassing the settling zone and reducing residence time, which dramatically reduces sediment removal efficiency.
Question 4: The emergency spillway of a sediment basin is designed to:
- Provide the normal discharge path
- Safely pass flows from storms exceeding the design storm without causing basin failure (Correct answer)
- Filter sediment from large storm events
- Prevent all flow from leaving the basin
Correct answer: Safely pass flows from storms exceeding the design storm without causing basin failure
Emergency spillways safely convey overflows from storms exceeding the design capacity, preventing basin failure and uncontrolled breaching that could release stored sediment.
Question 5: Which soil property is most important when constructing the embankment of a sediment basin?
- High permeability to allow infiltration
- Adequate compaction and low permeability to prevent seepage and embankment failure (Correct answer)
- High organic content
- High sand content for drainage
Correct answer: Adequate compaction and low permeability to prevent seepage and embankment failure
Sediment basin embankments must be constructed of properly compacted, low-permeability material to prevent seepage, piping failure, and embankment breaching.
Question 6: A sediment trap differs from a sediment basin primarily in that:
- Traps are permanent and basins are temporary
- Traps are smaller, used for drainage areas under 5 acres, and are simpler in design without a formal outlet structure (Correct answer)
- Traps are more effective at removing sediment
- Basins are always lined with concrete
Correct answer: Traps are smaller, used for drainage areas under 5 acres, and are simpler in design without a formal outlet structure
Sediment traps are simple, small ponding areas typically used for drainage areas under 5 acres, relying on a rock outlet weir rather than the engineered outlet structures required for larger sediment basins.
CEPSCI Sediment Basin and Trap Design 2
A skimmer outlet on a sediment basin is designed to: