LARE LARE Drainage System Design 3 — Questions and Answers
Question 1: A landscape architect is designing a rain garden on a residential lot with silty clay soils. Which modification is most appropriate to improve performance?
- Increase ponding depth to 24 inches
- Amend native soil with compost and sand to improve infiltration (Correct answer)
- Use a solid bottom liner to prevent seepage
- Reduce the drainage area to less than 50 square feet
Correct answer: Amend native soil with compost and sand to improve infiltration
Amending heavy clay soils with compost and sand increases infiltration rates, which is essential for rain garden function.
Question 2: In the Rational Method (Q = CiA), what does the runoff coefficient 'C' represent?
- The ratio of peak runoff rate to rainfall intensity
- The fraction of rainfall that becomes surface runoff (Correct answer)
- The channel conveyance capacity in cfs
- The time of concentration in minutes
Correct answer: The fraction of rainfall that becomes surface runoff
The runoff coefficient C represents the proportion of rainfall that becomes surface runoff, ranging from 0 (no runoff) to 1 (all rainfall runs off).
Question 3: Which stormwater BMP is best suited to treat highway runoff containing petroleum hydrocarbons and heavy metals?
- Dry detention basin
- Wet extended detention pond
- Vegetated filter strip
- Underground hydrodynamic separator (Correct answer)
Correct answer: Underground hydrodynamic separator
Hydrodynamic separators are effective at removing sediment-bound metals and hydrocarbons from concentrated highway runoff in space-constrained sites.
Question 4: What is the primary purpose of a flared end section (FES) at a culvert outlet?
- To increase pipe capacity by reducing entrance loss
- To reduce outlet velocity and minimize erosion at the discharge point (Correct answer)
- To prevent sediment from entering the pipe
- To allow fish passage through the culvert
Correct answer: To reduce outlet velocity and minimize erosion at the discharge point
A flared end section spreads and slows the discharge, reducing scour and erosion at the culvert outlet.
Question 5: During a grading review, a plan shows a drainage divide running through the center of a proposed parking lot. What does this mean for stormwater design?
- All runoff drains to a single low point and one inlet
- Runoff flows in two directions toward separate drainage systems or outfalls (Correct answer)
- The parking lot is located within a floodplain
- Perforated underdrains are required beneath the lot
Correct answer: Runoff flows in two directions toward separate drainage systems or outfalls
A drainage divide is the high point from which runoff flows in opposite directions, requiring separate stormwater management for each drainage area.
Question 6: Which factor most significantly increases the time of concentration (Tc) for a watershed?
- Increasing the slope of the drainage path
- Increasing the length of the drainage path (Correct answer)
- Converting grassland to impervious pavement
- Removing riparian buffers along streams
Correct answer: Increasing the length of the drainage path
A longer drainage path increases travel time, which directly increases the time of concentration.
Question 7: A retention pond differs from a detention pond primarily because it:
- Discharges all stored water within 72 hours of a storm
- Maintains a permanent pool of water between storm events (Correct answer)
- Is designed only for groundwater recharge
- Uses vegetation rather than open water for treatment
Correct answer: Maintains a permanent pool of water between storm events
A retention pond maintains a permanent pool that provides water quality treatment through settling and biological uptake, while detention ponds drain completely.
LARE LARE Drainage System Design 3
A landscape architect is designing a rain garden on a residential lot with silty clay soils.
Which modification is most appropriate to improve performance?