CESSWI Stormwater Management Practices 5 — Questions and Answers
Question 1: What is the primary difference between infiltration trenches and infiltration basins as stormwater BMPs?
- Trenches use sand media; basins use gravel
- Trenches are narrow, subsurface stone-filled features; basins are shallow surface depressions (Correct answer)
- Basins require underdrain systems; trenches do not
- Trenches treat larger drainage areas than basins
Correct answer: Trenches are narrow, subsurface stone-filled features; basins are shallow surface depressions
Infiltration trenches are narrow excavations filled with stone aggregate that receive runoff and infiltrate it underground, while infiltration basins are open surface areas that allow ponded water to percolate into the soil.
Question 2: A stormwater management facility that reduces peak discharge rate but does NOT reduce total runoff volume is best described as a:
- Retention pond
- Detention facility (Correct answer)
- Infiltration basin
- Bioretention cell
Correct answer: Detention facility
Detention facilities temporarily store runoff to reduce peak discharge rates but release the entire volume over time, providing no net reduction in total runoff volume.
Question 3: Which stormwater practice provides the greatest co-benefit of groundwater recharge in addition to runoff volume reduction?
- Wet detention pond
- Infiltration-based BMP (trench, basin, or permeable pavement) (Correct answer)
- Extended detention basin
- Green roof
Correct answer: Infiltration-based BMP (trench, basin, or permeable pavement)
Infiltration-based BMPs direct stormwater into the soil profile, directly recharging groundwater and sustaining baseflows in receiving streams, unlike detention or rooftop practices.
Question 4: During inspection of a stormwater BMP maintenance access road, an inspector notes the road surface is eroding and depositing sediment into an adjacent wet pond forebay. The BEST corrective action is to:
- Remove the forebay and expand the main pool
- Stabilize the access road with gravel or pavement and install edge protection (Correct answer)
- Increase mowing frequency along the road
- Reroute pond drainage to bypass the access road
Correct answer: Stabilize the access road with gravel or pavement and install edge protection
The access road should be surfaced with stable aggregate or pavement and provided with edge protection such as rolled erosion control products or riprap to prevent sediment delivery to the pond.
Question 5: The 'first flush' phenomenon in urban stormwater refers to:
- Initial flushing of sanitary sewers during high-flow events
- Disproportionately high pollutant concentrations in early runoff from impervious surfaces (Correct answer)
- The peak discharge occurring at the beginning of a storm event
- First-year performance decline of new stormwater BMPs
Correct answer: Disproportionately high pollutant concentrations in early runoff from impervious surfaces
The first flush describes the elevated concentrations of pollutants (oil, metals, nutrients) mobilized from impervious surfaces by the initial portion of a storm before dilution occurs.
Question 6: Which design element of a stormwater management pond is specifically intended to prevent the release of floating pollutants such as oil and trash during storm events?
- Anti-seep collar
- Submerged outlet or baffle at the outlet structure (Correct answer)
- Riprap outlet apron
- Vegetated littoral shelf
Correct answer: Submerged outlet or baffle at the outlet structure
A submerged outlet pipe or skimmer baffle at the outlet structure draws water from below the surface, preventing floating oil, debris, and trash from being discharged with the outflow.
Question 7: Under the EPA's MS4 (Municipal Separate Storm Sewer System) permit program, which minimum control measure specifically addresses post-construction stormwater management?
- MCM 1 – Public Education and Outreach
- MCM 5 – Post-Construction Stormwater Management in New Development and Redevelopment (Correct answer)
- MCM 3 – Illicit Discharge Detection and Elimination
- MCM 6 – Pollution Prevention/Good Housekeeping
Correct answer: MCM 5 – Post-Construction Stormwater Management in New Development and Redevelopment
MCM 5 requires MS4 permittees to develop and enforce programs to address stormwater from new development and redevelopment projects that disturb one or more acres.
CESSWI Stormwater Management Practices 5
What is the primary difference between infiltration trenches and infiltration basins as stormwater BMPs?