CESSWI Stormwater Runoff Management 3 — Questions and Answers
Question 1: Which curve number (CN) value in the TR-55 method indicates the LEAST runoff potential from a given soil and land use combination?
- 98
- 85
- 70
- 35 (Correct answer)
Correct answer: 35
Lower curve numbers indicate greater infiltration and less runoff potential; a CN of 35 represents a highly permeable soil with dense vegetation, while 98 represents nearly impervious surfaces.
Question 2: What term describes the practice of directing rooftop downspouts away from impervious surfaces and toward vegetated areas to promote infiltration?
- Downspout disconnection (Correct answer)
- Cistern overflow routing
- Drywell injection
- Curb elimination
Correct answer: Downspout disconnection
Downspout disconnection redirects roof drainage to lawns, gardens, or rain gardens rather than directly into the storm sewer, reducing runoff volume.
Question 3: A permeable pavement system fails to drain adequately after several years of use. What is the most likely cause?
- Excessive rainfall intensity during storms
- Clogging of pores by fine sediments and debris (Correct answer)
- Structural failure of the aggregate base
- Tree root intrusion into the pavement surface
Correct answer: Clogging of pores by fine sediments and debris
Fine sediment and organic debris accumulate in the pores of permeable pavement over time, reducing infiltration rates and requiring vacuum sweeping maintenance.
Question 4: Which NRCS hydrologic soil group has the HIGHEST runoff potential due to very slow infiltration rates?
- Group A
- Group B
- Group C
- Group D (Correct answer)
Correct answer: Group D
Hydrologic Soil Group D soils (such as clays) have very slow infiltration rates, high runoff potential, and are typically wet, shallow, or have impermeable layers.
Question 5: During a CESSWI inspection, you observe that a site's temporary sediment basin outlet riser is submerged with no freeboard remaining. What is your immediate concern?
- Increased sediment removal efficiency
- Potential overtopping and embankment failure (Correct answer)
- Reduced hydraulic head causing slow drainage
- Sedimentation of the outlet pipe
Correct answer: Potential overtopping and embankment failure
When freeboard is lost, the embankment is at risk of overtopping, which can cause rapid erosion and catastrophic failure of the sediment basin.
Question 6: Which of the following best describes the concept of 'water quality volume' (WQv) in stormwater management?
- The total annual runoff volume from a site
- The volume of runoff from a design storm used to size water quality BMPs (Correct answer)
- The maximum volume a detention pond can hold before overtopping
- The volume of groundwater recharge achieved annually
Correct answer: The volume of runoff from a design storm used to size water quality BMPs
Water quality volume is the runoff volume from a specified small storm (often 1-inch or the 90th percentile event) used to size BMPs for pollutant removal and volume reduction.
Question 7: What is the primary mechanism by which a vegetated filter strip reduces stormwater pollutant loads?
- Chemical precipitation of dissolved pollutants
- Sedimentation and infiltration as sheet flow passes through dense vegetation (Correct answer)
- Biological uptake by plant roots in the filter strip
- Aeration of runoff to oxidize contaminants
Correct answer: Sedimentation and infiltration as sheet flow passes through dense vegetation
Vegetated filter strips slow sheet flow, allowing sediment to settle and runoff to infiltrate, removing particulate-bound pollutants from stormwater.
CESSWI Stormwater Runoff Management 3
Which curve number (CN) value in the TR-55 method indicates the LEAST runoff potential from a given soil and land use combination?