LARE LARE Drainage System Design 5 — Questions and Answers
Question 1: What is the primary purpose of a check dam installed in a vegetated swale?
- To permanently impound water for habitat creation
- To slow flow velocity, promote infiltration, and trap sediment (Correct answer)
- To increase hydraulic capacity during large storm events
- To redirect flow to an adjacent drainage system
Correct answer: To slow flow velocity, promote infiltration, and trap sediment
Check dams slow stormwater velocity in swales, allowing sediment to settle and promoting infiltration between successive pools.
Question 2: A site requires a 48-inch diameter reinforced concrete pipe (RCP) to convey the 25-year design storm. What minimum cover depth is typically required over the pipe in a roadway application?
- 6 inches
- 12 inches
- 24 inches (Correct answer)
- 36 inches
Correct answer: 24 inches
Most design standards require a minimum of 24 inches of cover over RCP under roadways to prevent pipe failure from surface loads.
Question 3: Which condition creates a 'surcharged' storm sewer system?
- When pipes are flowing at less than 50% capacity
- When hydraulic grade line rises above the pipe crown due to downstream backwater (Correct answer)
- When inlets are clogged with debris during a storm
- When flow velocity drops below the minimum self-cleaning velocity
Correct answer: When hydraulic grade line rises above the pipe crown due to downstream backwater
A surcharged system occurs when the hydraulic grade line exceeds the top of the pipe, causing pipes to flow under pressure rather than as open-channel flow.
Question 4: A landscape architect is designing a low-impact development (LID) site. Which combination of practices best addresses both runoff volume reduction and water quality improvement?
- Increasing pipe sizes and adding more inlets
- Bioretention cells, permeable pavement, and green roofs in series (Correct answer)
- Deep detention basins with large rip-rap outlets
- Wide concrete channels with high-flow velocity
Correct answer: Bioretention cells, permeable pavement, and green roofs in series
Bioretention, permeable pavement, and green roofs each reduce runoff volume and filter pollutants, making them complementary LID strategies.
Question 5: When applying the SCS/NRCS Curve Number method, which land cover and soil type combination produces the highest runoff potential (highest CN)?
- Woods in good condition on Group A soils
- Open space with poor grass cover on Group B soils
- Commercial district on Group D soils (Correct answer)
- Meadow in good condition on Group C soils
Correct answer: Commercial district on Group D soils
Commercial land cover (high imperviousness) combined with Group D soils (lowest infiltration rate) produces the highest CN values, approaching 95.
Question 6: An urban redevelopment project must achieve 'no net increase' in stormwater runoff volume. Which strategy most directly satisfies this requirement?
- Matching post-development peak flow rates to pre-development rates for the 10-year storm only
- Providing on-site volume reduction through infiltration and evapotranspiration equal to the increase in runoff volume (Correct answer)
- Increasing the size of downstream storm sewers to accept additional runoff
- Directing runoff to a regional detention facility off-site
Correct answer: Providing on-site volume reduction through infiltration and evapotranspiration equal to the increase in runoff volume
Volume neutrality requires reducing runoff on-site through infiltration or ET to offset the increase caused by new impervious surfaces.
Question 7: A stormwater swale is designed with a 3:1 (H:V) side slope. What is the primary reason for this design standard?
- To maximize hydraulic capacity of the channel
- To provide safe mowable slopes and reduce erosion potential on channel banks (Correct answer)
- To increase flow velocity and reduce time of concentration
- To comply with floodplain elevation requirements
Correct answer: To provide safe mowable slopes and reduce erosion potential on channel banks
A 3:1 side slope is stable, reduces erosion, and allows maintenance equipment such as mowers to safely operate on channel banks.
LARE LARE Drainage System Design 5
What is the primary purpose of a check dam installed in a vegetated swale?