CPESC Erosion Control Methods & Techniques 5 — Questions and Answers
Question 1: A contractor proposes using a plastic sheeting (polyethylene) cover over a stockpile during a storm. What is a key limitation of this approach?
- Plastic sheeting degrades too quickly to provide adequate protection
- Concentrated runoff from the sheeting edges can cause erosion at the stockpile perimeter (Correct answer)
- Plastic sheeting is not approved under any state construction general permit
- The sheeting prevents natural vegetation from establishing on the stockpile
Correct answer: Concentrated runoff from the sheeting edges can cause erosion at the stockpile perimeter
While plastic prevents direct rainfall erosion, runoff sheeting off the cover concentrates at edges, potentially causing significant scour without perimeter controls.
Question 2: Compost blankets applied to slopes offer an advantage over straw mulch because they:
- Decompose more slowly, providing protection for 3–5 years
- Supply nutrients and beneficial microorganisms that enhance vegetation establishment (Correct answer)
- Repel water, preventing infiltration that could cause slope instability
- Are lighter weight, making them easier to apply on steep slopes
Correct answer: Supply nutrients and beneficial microorganisms that enhance vegetation establishment
Compost provides a biologically active medium with nutrients and microbes that improve soil conditions and accelerate vegetation establishment compared to inert straw.
Question 3: What is the primary purpose of a sediment basin as compared to a sediment trap?
- Sediment basins use chemical treatment while traps use only physical settling
- Sediment basins provide greater storage volume for larger drainage areas (Correct answer)
- Sediment traps are permanent structures while basins are temporary
- Sediment basins are required on slopes while traps are used in flat areas only
Correct answer: Sediment basins provide greater storage volume for larger drainage areas
Sediment basins are designed for larger contributing drainage areas (typically >5 acres) requiring greater storage capacity than a sediment trap can provide.
Question 4: Under the NPDES Construction General Permit framework, a 'stabilized' area typically means:
- The area has been seeded regardless of vegetation coverage achieved
- Uniform perennial vegetation with 70% cover, or equivalent non-vegetative stabilization (Correct answer)
- Any area covered with mulch or erosion control blanket regardless of slope
- The area has been graded to final grade and compacted
Correct answer: Uniform perennial vegetation with 70% cover, or equivalent non-vegetative stabilization
Most CGPs define stabilization as 70% uniform perennial vegetation cover or equivalent protection provided by non-vegetative measures.
Question 5: When evaluating the effectiveness of a vegetated filter strip, the most important design parameter is the:
- Species composition of the filter strip vegetation
- Flow path length of sheet flow through the vegetated area (Correct answer)
- Width of the filter strip measured perpendicular to slope
- Soil pH of the filter strip area
Correct answer: Flow path length of sheet flow through the vegetated area
Flow path length determines contact time between runoff and vegetation, directly controlling sediment removal efficiency of the filter strip.
Question 6: Terracing on agricultural slopes primarily reduces erosion by:
- Increasing soil organic matter to improve aggregate stability
- Breaking long slopes into shorter segments and intercepting runoff (Correct answer)
- Compacting soil to reduce infiltration and surface detachment
- Increasing slope gradient to convey runoff more quickly off-site
Correct answer: Breaking long slopes into shorter segments and intercepting runoff
Terraces divide long slopes into shorter segments, reducing slope length (LS factor), intercepting runoff, and conveying it safely to stable outlets.
Question 7: Which condition would require upgrading a standard silt fence to a reinforced silt fence or alternative BMP?
- Contributing drainage area exceeds 0.25 acres per 100 linear feet of fence
- The slope behind the fence is steeper than 3:1
- Anticipated ponding depth behind the fence will exceed 1.5 feet
- All of the above (Correct answer)
Correct answer: All of the above
Silt fence design limitations include drainage area, slope gradient, and ponding depth—exceeding any of these limits warrants a more robust alternative.
CPESC Erosion Control Methods & Techniques 5
A contractor proposes using a plastic sheeting (polyethylene) cover over a stockpile during a storm.
What is a key limitation of this approach?