CESSWI Stormwater Runoff Management 2 — Questions and Answers
Question 1: Which hydrologic parameter represents the fraction of rainfall that becomes surface runoff?
- Runoff coefficient (C) (Correct answer)
- Infiltration rate
- Evapotranspiration rate
- Time of concentration
Correct answer: Runoff coefficient (C)
The runoff coefficient (C) in the Rational Method represents the proportion of rainfall that converts to runoff, ranging from 0 to 1.
Question 2: A detention basin designed to reduce peak discharge by storing runoff temporarily is classified as which type of BMP?
- Retention basin
- Detention basin (Correct answer)
- Bioretention cell
- Constructed wetland
Correct answer: Detention basin
A detention basin temporarily stores runoff and releases it at a controlled rate to reduce peak flows, unlike retention basins which hold water permanently.
Question 3: Under the NPDES Construction General Permit, a Stormwater Pollution Prevention Plan (SWPPP) must be prepared for sites disturbing at least how many acres?
- 0.5 acres
- 1 acre (Correct answer)
- 2 acres
- 5 acres
Correct answer: 1 acre
The NPDES CGP requires a SWPPP for construction sites disturbing 1 or more acres, or smaller sites in a common plan of development exceeding 1 acre.
Question 4: Which factor most directly increases the peak runoff rate from a developed watershed compared to pre-development conditions?
- Increased vegetative cover
- Reduced impervious surface
- Shorter time of concentration (Correct answer)
- Increased soil infiltration capacity
Correct answer: Shorter time of concentration
Development shortens time of concentration by replacing permeable surfaces with impervious ones and adding channels, causing runoff to reach the outlet faster and intensifying peak flows.
Question 5: What is the primary purpose of a level spreader at the outlet of a stormwater conveyance system?
- Increase flow velocity to prevent sedimentation
- Convert concentrated flow to sheet flow (Correct answer)
- Filter sediment from runoff
- Store runoff during peak events
Correct answer: Convert concentrated flow to sheet flow
A level spreader converts concentrated pipe or channel flow into diffuse sheet flow, reducing erosive velocities before runoff enters a vegetated buffer.
Question 6: In the context of green infrastructure, which practice captures rooftop runoff for reuse or infiltration directly at the source?
- Bioswale
- Rain barrel or cistern (Correct answer)
- Dry swale
- Riprap outlet
Correct answer: Rain barrel or cistern
Rain barrels and cisterns capture and store rooftop runoff at the point of generation, reducing the volume that enters the storm drainage system.
Question 7: Which condition is most likely to cause a stormwater detention pond to fail its design function during a storm event?
- Outlet pipe set at the pond bottom elevation
- Vegetation established on embankment slopes
- Sediment accumulation reducing storage volume (Correct answer)
- Emergency spillway installed at the top of the embankment
Correct answer: Sediment accumulation reducing storage volume
Sediment accumulation in a detention pond reduces its live storage volume, decreasing the pond's ability to attenuate peak flows as designed.
CESSWI Stormwater Runoff Management 2
Which hydrologic parameter represents the fraction of rainfall that becomes surface runoff?