Civil Engineering PE Environmental Engineering and Stormwater Management 1 — Questions and Answers
Question 1: In the rational method Q = CiA, the variable i represents:
- Infiltration rate in inches per hour
- Rainfall intensity for a storm duration equal to the time of concentration (Correct answer)
- Impervious fraction of the watershed
- Inflow rate from upstream drainage
Correct answer: Rainfall intensity for a storm duration equal to the time of concentration
In Q = CiA, i is the design rainfall intensity (in/hr) corresponding to the time of concentration and the selected return period storm.
Question 2: The NRCS Curve Number (CN) method estimates:
- Peak discharge rate only
- Direct runoff depth from storm rainfall depth (Correct answer)
- Evapotranspiration losses only
- Groundwater recharge volume
Correct answer: Direct runoff depth from storm rainfall depth
The CN method calculates direct runoff Q from rainfall P using Q = (P − 0.2S)²/(P + 0.8S), where S is the potential maximum retention based on CN.
Question 3: Time of concentration (Tc) for a watershed is defined as:
- Time from start of rainfall to peak runoff at the outlet
- Travel time from the hydraulically most remote point to the outlet (Correct answer)
- Duration of the design storm event
- Time required to fill the detention basin to the overflow elevation
Correct answer: Travel time from the hydraulically most remote point to the outlet
Tc is the travel time of runoff from the farthest point in the drainage area to the outlet, and it governs the critical storm duration for design.
Question 4: A dry detention basin is designed primarily to:
- Permanently store stormwater for irrigation reuse
- Attenuate peak flows by temporarily storing runoff (Correct answer)
- Remove dissolved pollutants through biological treatment
- Recharge groundwater through infiltration only
Correct answer: Attenuate peak flows by temporarily storing runoff
Dry detention basins temporarily store excess runoff to reduce peak discharge rates, releasing water through a controlled outlet after the storm passes.
Question 5: A typical Manning's roughness coefficient (n) for a natural earthen channel is:
- 0.010–0.013
- 0.025–0.035 (Correct answer)
- 0.050–0.080
- 0.100–0.150
Correct answer: 0.025–0.035
Natural earthen channels have n ≈ 0.025–0.035, considerably higher than smooth concrete linings (n ≈ 0.012–0.015).
Question 6: Hydraulic radius R in Manning's equation is defined as:
- Half the pipe diameter
- Cross-sectional flow area divided by wetted perimeter (Correct answer)
- Wetted perimeter divided by cross-sectional area
- The depth of flow for wide channels
Correct answer: Cross-sectional flow area divided by wetted perimeter
Hydraulic radius R = A/P, where A is the cross-sectional area of flow and P is the wetted perimeter; for a full circular pipe R = D/4.
Civil Engineering PE Environmental Engineering and Stormwater Management 1
In the rational method Q = CiA, the variable i represents: