LARE LARE Drainage System Design 2 — Questions and Answers
Question 1: Which hydraulic concept describes the condition when stormwater runoff exceeds the infiltration capacity of the soil?
- Horton overland flow (Correct answer)
- Interflow
- Baseflow contribution
- Capillary rise
Correct answer: Horton overland flow
Horton overland flow occurs when rainfall intensity exceeds the soil's infiltration capacity, causing surface runoff.
Question 2: In a bioretention cell, what is the primary purpose of the ponding zone?
- To provide temporary storage for incoming stormwater before it infiltrates (Correct answer)
- To permanently retain water for irrigation
- To filter sediment using gravel beds
- To accelerate peak discharge to the downstream system
Correct answer: To provide temporary storage for incoming stormwater before it infiltrates
The ponding zone temporarily stores stormwater above the growing media, allowing it to infiltrate gradually.
Question 3: A designer needs to reduce the peak discharge from a 10-year storm. Which approach directly addresses this goal?
- Increasing impervious surface coverage
- Installing a detention basin with a controlled outlet (Correct answer)
- Increasing pipe slope to accelerate flow
- Removing riparian vegetation along channels
Correct answer: Installing a detention basin with a controlled outlet
A detention basin stores runoff and releases it at a controlled rate, reducing peak discharge to downstream systems.
Question 4: Which Manning's roughness coefficient (n) value best represents a clean, straight natural earth channel?
- 0.010
- 0.025 (Correct answer)
- 0.050
- 0.080
Correct answer: 0.025
A clean, straight natural earth channel typically has a Manning's n of approximately 0.025.
Question 5: What is the primary regulatory intent of the water quality volume (WQv) standard in stormwater design?
- To prevent 100-year flood events
- To capture and treat the first flush of stormwater runoff containing the highest pollutant loads (Correct answer)
- To ensure zero discharge from any site
- To maintain baseflow in streams year-round
Correct answer: To capture and treat the first flush of stormwater runoff containing the highest pollutant loads
WQv standards target the first flush — typically the first 0.5–1 inch of runoff — which carries the greatest pollutant concentrations.
Question 6: On a grading plan, a landscape architect specifies a swale with a 2% longitudinal slope and trapezoidal cross-section. Which site condition would most likely require armoring the swale bottom?
- Sandy loam soils with low clay content (Correct answer)
- Silty clay soils with high cohesion
- Flat terrain adjacent to wetlands
- Areas receiving sheet flow from turf only
Correct answer: Sandy loam soils with low clay content
Sandy loam soils have low cohesion and are highly erodible at typical swale velocities, requiring armoring or erosion protection.
Question 7: Which design parameter is most critical when sizing an infiltration trench to meet groundwater protection requirements?
- Distance to the seasonal high water table (Correct answer)
- Trench length
- Surface area of contributing pavement
- Depth of the aggregate storage layer
Correct answer: Distance to the seasonal high water table
Separation from the seasonal high water table is critical to prevent groundwater contamination and ensure adequate treatment depth.
LARE LARE Drainage System Design 2
Which hydraulic concept describes the condition when stormwater runoff exceeds the infiltration capacity of the soil?