CSM Best Management Practices (BMPs) & Design 2 — Questions and Answers
Question 1: Which BMP type removes pollutants primarily through settling and sedimentation rather than biological uptake?
- Vegetated swale
- Wet detention pond (Correct answer)
- Green roof
- Permeable pavement
Correct answer: Wet detention pond
Wet detention ponds rely on extended hydraulic retention time to allow suspended solids and associated pollutants to settle out.
Question 2: When sizing a bioretention cell, the typical ponding depth is limited to avoid anaerobic conditions in the media. What is the commonly accepted maximum ponding depth?
- 6 inches
- 12 inches (Correct answer)
- 18 inches
- 24 inches
Correct answer: 12 inches
Most design guidelines limit bioretention ponding depth to 12 inches to prevent anaerobic conditions that inhibit plant survival and pollutant treatment.
Question 3: A site designer wants to use a dry extended detention basin. Which pollutant removal mechanism is least effective in this BMP compared to a wet pond?
- Gravitational settling of coarse sediment
- Biological nutrient uptake by aquatic plants (Correct answer)
- Reduction of peak discharge rate
- Particle filtration through outlet structures
Correct answer: Biological nutrient uptake by aquatic plants
Dry basins drain between storms and lack a permanent pool, so aquatic vegetation cannot establish and biological nutrient uptake is minimal.
Question 4: Under the 'runoff volume reduction' approach to stormwater management, which BMP best addresses the water quality volume by retaining and infiltrating the first flush?
- Detention pond with a large outlet orifice
- Bioretention with an underdrain
- Infiltration basin sized to drawdown within 72 hours (Correct answer)
- Vegetated filter strip alone
Correct answer: Infiltration basin sized to drawdown within 72 hours
An infiltration basin that fully drawdowns the water quality volume within 72 hours achieves maximum volume reduction by returning runoff to groundwater.
Question 5: Which soil characteristic most directly limits the feasibility of infiltration BMPs at a given site?
- Soil color and organic matter content
- Saturated hydraulic conductivity (Ksat) (Correct answer)
- Soil pH and cation exchange capacity
- Soil temperature and frost depth
Correct answer: Saturated hydraulic conductivity (Ksat)
Saturated hydraulic conductivity determines how quickly water moves through soil; sites with Ksat below 0.5 in/hr are typically poor candidates for infiltration BMPs.
Question 6: A constructed stormwater wetland differs from a wet detention pond primarily in that it:
- Has a deeper permanent pool averaging 4–6 feet
- Incorporates shallow marsh zones with emergent vegetation for enhanced treatment (Correct answer)
- Relies solely on chemical precipitation to remove pollutants
- Uses a lined bottom to prevent groundwater contamination
Correct answer: Incorporates shallow marsh zones with emergent vegetation for enhanced treatment
Stormwater wetlands incorporate shallow emergent marsh zones (typically 0–18 inches deep) that support biological nutrient cycling and additional pollutant removal beyond simple settling.
Question 7: For a permeable pavement system in a cold climate, which design modification is most important to maintain functionality during freeze-thaw cycles?
- Installing a heated sub-base to prevent freezing
- Selecting open-graded aggregate base course material instead of dense-graded (Correct answer)
- Applying impermeable membrane over the surface in winter
- Increasing pavement thickness by a factor of three
Correct answer: Selecting open-graded aggregate base course material instead of dense-graded
Open-graded aggregate base material drains freely and resists frost heave, maintaining the structural integrity and permeability of the system through freeze-thaw cycles.
CSM Best Management Practices (BMPs) & Design 2
Which BMP type removes pollutants primarily through settling and sedimentation rather than biological uptake?