CSM Best Management Practices (BMPs) & Design 3 — Questions and Answers
Question 1: Which outlet structure feature is specifically designed to remove floating pollutants such as trash and oil from a stormwater pond?
- Perforated riser pipe
- Skimmer or reverse slope pipe (Correct answer)
- Anti-seep collar
- Ogee spillway
Correct answer: Skimmer or reverse slope pipe
A skimmer or reverse slope pipe draws water from below the surface, preventing floating debris and oils from passing through the outlet during normal operations.
Question 2: In bioretention design, what is the primary purpose of an internal water storage (IWS) zone?
- To provide additional sediment storage above the media
- To create an anaerobic zone that promotes denitrification and reduces nitrogen loads (Correct answer)
- To increase the hydraulic head driving infiltration into native soils
- To store irrigation water for plant establishment periods
Correct answer: To create an anaerobic zone that promotes denitrification and reduces nitrogen loads
The IWS zone creates saturated, anaerobic conditions in the lower media that support denitrification bacteria, significantly improving total nitrogen removal.
Question 3: When designing a vegetated filter strip, the recommended maximum slope to ensure uniform sheet flow and adequate treatment is:
- 2%
- 5% (Correct answer)
- 10%
- 15%
Correct answer: 5%
Filter strips are typically designed with slopes of 2–5%, with 5% being a common upper limit to maintain sheet flow and prevent channelization that bypasses treatment.
Question 4: A grass swale is being designed to convey stormwater while providing water quality treatment. Which parameter is most critical to verify for adequate treatment residence time?
- Swale length and longitudinal slope (Correct answer)
- Manning's roughness coefficient of concrete lining
- Freeboard above design water surface
- Width-to-depth ratio of the channel cross-section
Correct answer: Swale length and longitudinal slope
Swale length and longitudinal slope together determine flow velocity and hydraulic residence time, which governs the degree of settling and biological treatment achieved.
Question 5: Which factor most strongly influences the selection of plants for a bioretention facility in an arid or semi-arid climate?
- Ability to tolerate both inundation and extended drought periods (Correct answer)
- Capacity to produce seed pods that self-propagate aggressively
- Root depth exceeding 6 feet to access groundwater
- Preference for acidic soil conditions with pH below 5.5
Correct answer: Ability to tolerate both inundation and extended drought periods
Bioretention plants in arid climates experience both flooding during storms and prolonged dry periods between events, requiring species tolerant of both extremes.
Question 6: A pre-treatment forebay is incorporated into a stormwater pond design primarily to:
- Increase the permanent pool volume to improve phosphorus removal
- Trap coarse sediment and debris to protect the main treatment area (Correct answer)
- Aerate incoming stormwater to enhance aerobic decomposition
- Provide emergency overflow capacity during extreme storm events
Correct answer: Trap coarse sediment and debris to protect the main treatment area
A forebay captures coarse sediment and large debris at the inlet, protecting the main pond from premature sedimentation and reducing maintenance frequency.
Question 7: Which BMP configuration best manages high-frequency, low-volume runoff events (the 'water quality storm') while also providing peak flow attenuation for larger design storms?
- A single large detention pond sized for the 100-year event
- A bioretention cell in series with a downstream detention basin (Correct answer)
- An infiltration trench without an overflow bypass
- A grass swale discharging directly to a receiving stream
Correct answer: A bioretention cell in series with a downstream detention basin
Placing a bioretention cell upstream of a detention basin treats small frequent storms for water quality while the basin attenuates peak flows from larger design storms.
CSM Best Management Practices (BMPs) & Design 3
Which outlet structure feature is specifically designed to remove floating pollutants such as trash and oil from a stormwater pond?