CPESC BMP Selection 5 — Questions and Answers
Question 1: Which condition would make a silt fence installation INEFFECTIVE regardless of proper installation technique?
- Sandy soil with low clay content
- Concentrated flow channelized toward the fence (Correct answer)
- Slope gradient of 3:1 or less
- Fence installed with 6-inch soil anchor
Correct answer: Concentrated flow channelized toward the fence
Silt fences are designed for sheet flow only; concentrated or channelized flow will overtop or undercut the fence, rendering it ineffective.
Question 2: A project near a cold-water trout stream requires extra protection. Which BMP standard is typically elevated for such sensitive receiving waters?
- Stabilized construction entrance aggregate size
- Sediment basin design storm (larger storm event used for sizing) (Correct answer)
- Height of perimeter silt fence
- Frequency of grading operations
Correct answer: Sediment basin design storm (larger storm event used for sizing)
Near sensitive water bodies, sediment basins are often designed to capture runoff from larger design storms (e.g., 10-year vs. 2-year) to provide additional protection.
Question 3: Which BMP is most appropriate for stabilizing a large stockpile of excavated soil expected to remain in place for more than 14 days?
- Gravel perimeter berm only
- Temporary seeding and erosion control blanket (Correct answer)
- Silt fence on all sides without surface treatment
- Polyethylene sheeting over the entire pile
Correct answer: Temporary seeding and erosion control blanket
Temporary seeding with erosion control blanket provides surface stabilization for stockpiles inactive for more than 14 days, which is a common regulatory threshold.
Question 4: During BMP selection, 'redundancy' means:
- Installing duplicate identical BMPs side by side
- Using backup BMPs so that failure of one does not result in off-site discharge (Correct answer)
- Replacing BMPs more frequently than required
- Documenting BMP locations in the SWPPP twice
Correct answer: Using backup BMPs so that failure of one does not result in off-site discharge
Redundancy in BMP design means layering multiple controls so that if one fails, subsequent BMPs prevent sediment from leaving the site.
Question 5: A temporary diversion dike is installed at the top of a disturbed slope. Its primary function is to:
- Filter sediment from runoff before it reaches the slope
- Intercept upslope runoff and redirect it around the disturbed area (Correct answer)
- Slow runoff velocity at the slope toe
- Measure total runoff volume from the disturbed area
Correct answer: Intercept upslope runoff and redirect it around the disturbed area
A diversion dike at the slope crest intercepts clean runoff from undisturbed upslope areas and routes it around the disturbed zone, reducing erosion potential.
Question 6: Which BMP performance factor is MOST directly linked to the hydraulic residence time within a sediment basin?
- Basin liner permeability
- Sediment particle removal efficiency (Correct answer)
- Structural integrity of the embankment
- Overflow pipe diameter
Correct answer: Sediment particle removal efficiency
Longer hydraulic residence time allows finer particles more time to settle, directly improving the sediment removal efficiency of the basin.
Question 7: When a CPESC evaluates BMP adequacy after a site inspection, which finding would most urgently require immediate corrective action?
- Minor sediment accumulation at 20% of silt fence capacity
- Sediment discharge observed reaching an adjacent stream (Correct answer)
- Temporary seeding germination at 60% coverage
- One missing stake on a 100-foot silt fence run
Correct answer: Sediment discharge observed reaching an adjacent stream
Active sediment discharge to a waterway is an immediate regulatory violation requiring same-day corrective action to prevent further environmental harm.
CPESC BMP Selection 5
Which condition would make a silt fence installation INEFFECTIVE regardless of proper installation technique?