CESSWI Sediment Control BMPs 5 — Questions and Answers
Question 1: A polymer-enhanced sediment control BMP (such as PAM-treated straw wattles) works primarily by:
- Physically filtering particles larger than 75 microns
- Causing fine clay particles to flocculate and settle faster (Correct answer)
- Absorbing sediment into the polymer matrix
- Chemically bonding sediment to the fabric surface
Correct answer: Causing fine clay particles to flocculate and settle faster
Polyacrylamide (PAM) and similar polymers cause fine clay particles to coagulate (flocculate), forming larger aggregates that settle much more rapidly than individual clay particles.
Question 2: Which inspection frequency is typically required for sediment control BMPs during a period of active construction?
- Once per month regardless of weather
- After every rainfall event of 0.5 inches or more, and at least weekly (Correct answer)
- Before the start of each work week only
- Only after storm events exceeding the 10-year design storm
Correct answer: After every rainfall event of 0.5 inches or more, and at least weekly
Most NPDES construction general permits require BMP inspections after any storm event producing 0.5 inches or more of rainfall and at least once per week during active construction.
Question 3: What is the MOST critical location to install sediment controls on a construction site draining toward a wetland?
- At the center of the disturbed area to intercept flow early
- At the perimeter of the disturbed area and at all discharge points near the wetland (Correct answer)
- Along the wetland edge inside the 100-foot buffer zone
- At the top of the slope to prevent rilling
Correct answer: At the perimeter of the disturbed area and at all discharge points near the wetland
Perimeter controls at the downslope boundary and at all potential discharge points near sensitive resources provide the last line of defense before sediment reaches the wetland.
Question 4: Why is geotextile fabric specification (AOS) important when selecting a silt fence material?
- AOS determines the tensile strength of the fabric posts
- AOS (Apparent Opening Size) controls which particle sizes are filtered versus passed through the fabric (Correct answer)
- AOS rating affects the UV resistance of the fabric
- AOS determines how long the fabric will last before replacement
Correct answer: AOS (Apparent Opening Size) controls which particle sizes are filtered versus passed through the fabric
Apparent Opening Size (AOS) specifies the equivalent pore size of the fabric; selecting the correct AOS balances sediment retention with hydraulic conductivity to prevent blowouts.
Question 5: A site inspection reveals that silt fence posts are leaning at more than 30 degrees from vertical after a storm. The CORRECT corrective action is:
- Drive additional posts immediately downslope to brace the leaning posts
- Remove accumulated sediment, pull posts back to vertical, and re-drive or replace (Correct answer)
- Leave the fence in place until the end of construction season
- Add wire backing to support the leaning posts
Correct answer: Remove accumulated sediment, pull posts back to vertical, and re-drive or replace
Leaning posts indicate overloading from sediment accumulation or hydrostatic pressure; the proper fix is to remove sediment and restore the fence to its original installed position.
Question 6: What distinguishes a sediment trap from a sediment basin in terms of drainage area?
- Sediment traps serve areas less than 5 acres; basins are required for 5 acres or more (Correct answer)
- Sediment traps serve areas less than 1 acre; basins are required for larger areas
- Both can serve up to 10 acres; basin is used when depth exceeds 5 feet
- There is no regulatory distinction; the terms are interchangeable
Correct answer: Sediment traps serve areas less than 5 acres; basins are required for 5 acres or more
Sediment traps are typically designed for drainage areas less than 5 acres, while sediment basins with engineered outlet structures are required for larger disturbed areas.
Question 7: Which of the following BEST describes 'construction phasing' as a sediment control strategy?
- Scheduling equipment deliveries to minimize site disturbance
- Limiting the amount of land disturbed at any one time to reduce exposed soil area (Correct answer)
- Phasing contractor payment based on BMP installation completion
- Installing BMPs in phases starting from the site perimeter inward
Correct answer: Limiting the amount of land disturbed at any one time to reduce exposed soil area
Construction phasing limits the total acreage disturbed at any one time, reducing the source area generating sediment-laden runoff and the demand on downstream controls.
CESSWI Sediment Control BMPs 5
A polymer-enhanced sediment control BMP (such as PAM-treated straw wattles) works primarily by: