CESSWI Stormwater Management Practices 3 — Questions and Answers
Question 1: In a constructed stormwater wetland, the shallow marsh zone typically operates at what average water depth to support emergent vegetation?
- 0–3 inches
- 6–18 inches (Correct answer)
- 24–36 inches
- 48–60 inches
Correct answer: 6–18 inches
The shallow marsh zone of a constructed wetland is typically maintained at 6–18 inches depth, which supports emergent plants like cattails and bulrushes that provide biological treatment.
Question 2: Which BMP design parameter ensures that sediment trapped in a basin will not be resuspended and discharged during large storm events?
- Freeboard elevation
- Length-to-width ratio (Correct answer)
- Trap efficiency curve
- Anti-seep collar placement
Correct answer: Length-to-width ratio
A length-to-width ratio of 3:1 or greater promotes plug flow, reducing turbulence and short-circuiting that could resuspend settled sediment during high-flow events.
Question 3: A stormwater harvesting system (cistern or rain barrel) primarily reduces which component of the urban water cycle?
- Total dissolved solids in groundwater
- Runoff volume by capturing and reusing rainfall (Correct answer)
- Groundwater table elevation
- Evapotranspiration rates
Correct answer: Runoff volume by capturing and reusing rainfall
Stormwater harvesting captures rooftop runoff for non-potable reuse (irrigation, toilet flushing), directly reducing the volume of stormwater discharged to drainage systems.
Question 4: When inspecting a grassed swale, which condition indicates that the swale is NOT functioning properly?
- Dense, uniform grass coverage at least 4 inches tall
- Rill erosion channels and bare soil visible along the flow path (Correct answer)
- Standing water present 24 hours after a design storm
- Slight sediment deposit at the inlet check dam
Correct answer: Rill erosion channels and bare soil visible along the flow path
Rill erosion and bare soil in a grassed swale indicate that flow velocities are exceeding permissible limits or vegetation is inadequate, causing erosion rather than filtration.
Question 5: The rational method equation Q = CiA calculates peak runoff rate. What does the coefficient 'C' represent?
- Channel flow velocity in ft/s
- Runoff coefficient reflecting land cover and soil type (Correct answer)
- Concentration time in minutes
- Curve number for the watershed
Correct answer: Runoff coefficient reflecting land cover and soil type
The runoff coefficient C (dimensionless, 0–1.0) accounts for the fraction of rainfall that becomes surface runoff based on land use, soil type, and slope.
Question 6: What is the purpose of a forebay in a stormwater pond or wetland design?
- Provide emergency overflow spillway capacity
- Pre-settle coarse sediment near the inlet to protect the main pool (Correct answer)
- Increase the permanent pool volume
- Establish the outlet control elevation
Correct answer: Pre-settle coarse sediment near the inlet to protect the main pool
A forebay is a smaller, deeper cell at the inlet that traps coarse sediment and debris before flow enters the main treatment area, making sediment removal easier and protecting the main pool.
Question 7: Under Low Impact Development (LID) principles, what is the primary goal when managing stormwater from a new development?
- Maximize impervious surface to reduce maintenance
- Mimic pre-development hydrology by managing runoff at the source (Correct answer)
- Pipe all runoff directly to the nearest water body
- Size detention basins to handle the 500-year storm
Correct answer: Mimic pre-development hydrology by managing runoff at the source
LID aims to replicate pre-development hydrology through distributed, small-scale practices that promote infiltration, evapotranspiration, and retention close to where rain falls.
CESSWI Stormwater Management Practices 3
In a constructed stormwater wetland, the shallow marsh zone typically operates at what average water depth to support emergent vegetation?