CESSWI Stormwater Management Practices 2 — Questions and Answers
Question 1: Which type of detention basin is designed to drain completely between storm events, providing no permanent pool of water?
- Wet pond
- Dry detention basin (Correct answer)
- Constructed wetland
- Retention pond
Correct answer: Dry detention basin
Dry detention basins (also called dry ponds) temporarily store runoff and drain completely between storms, providing only flood control without water quality benefits from a permanent pool.
Question 2: What is the primary purpose of an extended detention basin compared to a standard dry detention basin?
- Increase peak discharge rates
- Provide longer residence time to settle fine sediments (Correct answer)
- Eliminate the need for outlet structures
- Reduce basin footprint
Correct answer: Provide longer residence time to settle fine sediments
Extended detention basins hold runoff for 24–40 hours, allowing finer sediment particles that would not settle in a standard detention period to be removed before release.
Question 3: A bioretention cell (rain garden) treats stormwater primarily through which combination of processes?
- Chlorination and UV disinfection
- Filtration, infiltration, and biological uptake (Correct answer)
- Coagulation and flocculation
- Gravitational settling only
Correct answer: Filtration, infiltration, and biological uptake
Bioretention cells use engineered soil media for filtration, underlying native soil for infiltration, and plants for biological nutrient uptake to achieve pollutant removal.
Question 4: Under the NPDES Construction General Permit, what minimum rainfall threshold typically triggers the requirement to inspect a construction site for BMPs?
- 0.10 inch
- 0.25 inch (Correct answer)
- 0.50 inch
- 1.00 inch
Correct answer: 0.25 inch
Most NPDES Construction General Permits require post-storm inspections following rainfall events of 0.25 inch or greater within a 24-hour period.
Question 5: Which outlet structure configuration is best suited to control two separate design storms (e.g., 2-year and 100-year) from a single detention basin?
- Single orifice plate
- Riser with multiple orifices at different elevations (Correct answer)
- Perforated riser pipe
- Simple overflow weir
Correct answer: Riser with multiple orifices at different elevations
A multi-stage riser with orifices at different elevations independently controls smaller, more frequent storms at lower stages and larger storms at higher stages.
Question 6: What term describes the volume of runoff that must be captured and treated to meet water quality objectives under many state stormwater programs?
- Critical storm volume
- Water quality volume (WQv) (Correct answer)
- Channel protection volume
- Recharge volume
Correct answer: Water quality volume (WQv)
The Water Quality Volume (WQv) is the runoff volume associated with a small, frequent storm (often the 90th percentile event) that must be captured and treated to achieve pollutant load reductions.
Question 7: Permeable pavement systems reduce stormwater runoff volume primarily by:
- Increasing surface slope to accelerate drainage
- Allowing water to infiltrate through voids into underlying storage layers (Correct answer)
- Directing runoff to adjacent vegetated swales only
- Reducing impervious area calculations by 50% by default
Correct answer: Allowing water to infiltrate through voids into underlying storage layers
Permeable pavement contains open-graded voids that allow rainfall to pass through the surface and into a stone reservoir base, where it infiltrates into the subgrade or is slowly released.
CESSWI Stormwater Management Practices 2
Which type of detention basin is designed to drain completely between storm events, providing no permanent pool of water?