LARE LARE Drainage System Design 4 — Questions and Answers
Question 1: A stormwater system must pass a 100-year storm event. Which design element is specifically sized to convey this extreme flow?
- Roof drains and downspouts
- The minor conveyance system of inlets and pipes
- The major conveyance system including overland flow paths and open channels (Correct answer)
- Bioretention cells and rain gardens
Correct answer: The major conveyance system including overland flow paths and open channels
The major conveyance system — streets, channels, and overland flow paths — is designed to safely convey extreme storm events like the 100-year storm.
Question 2: Which soil characteristic is most important to measure before designing an infiltration-based BMP such as a permeable pavement system?
- Soil pH
- In-situ saturated hydraulic conductivity (Ksat) (Correct answer)
- Organic matter content
- Cation exchange capacity (CEC)
Correct answer: In-situ saturated hydraulic conductivity (Ksat)
In-situ saturated hydraulic conductivity (Ksat) determines how quickly water moves through the native soil, which directly controls infiltration BMP performance.
Question 3: What is the key hydrologic advantage of green infrastructure over conventional gray infrastructure in urban areas?
- Green infrastructure eliminates the need for any underground pipes
- Green infrastructure restores predevelopment hydrology by promoting infiltration and evapotranspiration (Correct answer)
- Green infrastructure increases peak discharge rates for safer conveyance
- Green infrastructure is cheaper to install than all gray alternatives
Correct answer: Green infrastructure restores predevelopment hydrology by promoting infiltration and evapotranspiration
Green infrastructure mimics natural hydrology by returning water to the ground and atmosphere, reducing runoff volume and restoring watershed water balance.
Question 4: A site's pre-development CN (curve number) is 65 and post-development CN is 85. What is the primary design implication?
- Post-development runoff volume will be significantly lower than pre-development
- Post-development runoff volume will be significantly higher, requiring stormwater management (Correct answer)
- The site is exempt from stormwater regulations
- Soil infiltration rates have improved after development
Correct answer: Post-development runoff volume will be significantly higher, requiring stormwater management
A higher CN indicates more runoff generation, meaning development has increased impervious cover and requires stormwater management to reduce post-development runoff.
Question 5: In open channel hydraulics, critical flow occurs when:
- Flow velocity equals zero
- The Froude number equals 1.0 (Correct answer)
- Manning's roughness coefficient is at its maximum
- Channel slope exceeds 10%
Correct answer: The Froude number equals 1.0
Critical flow occurs when the Froude number (Fr) equals 1.0, representing the transition between subcritical and supercritical flow.
Question 6: A drainage easement on a plat restricts the placement of structures within 20 feet of a drainage channel. A client wants to build a deck within this easement. What is the landscape architect's best response?
- Approve the deck if it is constructed on piers above the ground
- Advise the client that the easement prohibits the deck and coordinate with the governing jurisdiction for any variance (Correct answer)
- Relocate the channel to eliminate the easement conflict
- Design the deck using flood-resistant materials to comply with the easement
Correct answer: Advise the client that the easement prohibits the deck and coordinate with the governing jurisdiction for any variance
Drainage easements are legal encumbrances that restrict construction; the designer must advise the client and work through proper legal and regulatory channels.
Question 7: Which outlet structure configuration best provides both water quality treatment and channel protection for a multi-stage stormwater pond?
- Single large orifice at the pond bottom
- Riser structure with multiple outlets at different elevations controlling different design storms (Correct answer)
- Open weir discharging at the top of the permanent pool
- Perforated pipe underdrain connected directly to the storm sewer
Correct answer: Riser structure with multiple outlets at different elevations controlling different design storms
A multi-stage riser with outlets at different elevations independently controls the water quality volume, channel protection volume, and peak flood control storage.
LARE LARE Drainage System Design 4
A stormwater system must pass a 100-year storm event.
Which design element is specifically sized to convey this extreme flow?