CESSWI Sediment Control BMPs 2 — Questions and Answers
Question 1: What is the primary failure mode of silt fence installations on slopes exceeding 2%?
- Fabric UV degradation
- Overtopping due to insufficient ponding capacity (Correct answer)
- Post corrosion at soil interface
- Fabric clogging within 30 days
Correct answer: Overtopping due to insufficient ponding capacity
On steeper slopes, flow velocity increases and ponding area behind the fence is insufficient to contain runoff, leading to overtopping rather than filtration.
Question 2: A sediment basin is designed for a 2-acre drainage area. What is the minimum required surface area using the standard 0.01 acre-ft per acre of drainage area design criterion?
- 0.01 acres
- 0.02 acres (Correct answer)
- 0.1 acres
- 0.2 acres
Correct answer: 0.02 acres
The minimum surface area is calculated as 0.01 acre-ft/acre × 2 acres = 0.02 acre-ft of storage, and surface area is derived from this volume requirement.
Question 3: Which sediment control BMP is MOST appropriate for protecting a storm drain inlet in a paved parking lot during active construction?
- Rock check dam
- Inlet protection filter bag or sediment trap (Correct answer)
- Silt fence staked around the inlet
- Straw wattle placed 10 feet upstream
Correct answer: Inlet protection filter bag or sediment trap
Filter bags or inlet sediment traps are specifically designed to fit over or around paved inlet grates, capturing sediment while allowing drainage.
Question 4: When installing a rock check dam, what is the recommended height-to-width ratio for the notch or spillway?
- The notch depth should be 1/3 the dam height and centered
- The notch should be at the same elevation as the top of the dam
- The notch should be 6 inches below the top of the dam edges (Correct answer)
- There is no notch; water overtops uniformly
Correct answer: The notch should be 6 inches below the top of the dam edges
The spillway notch in a rock check dam is typically cut 6 inches below the top edges so that design flow passes through the notch while preventing flanking around the sides.
Question 5: How does a floating turbidity curtain differ from a silt fence in its primary mechanism of sediment control?
- It filters sediment through a geotextile fabric suspended in water
- It creates a physical barrier that settles sediment within an enclosed water area (Correct answer)
- It absorbs sediment using polymer-impregnated fabric
- It deflects surface runoff away from sensitive receiving waters
Correct answer: It creates a physical barrier that settles sediment within an enclosed water area
Floating turbidity curtains act as physical barriers in water bodies, enclosing an area to allow suspended sediment to settle out before water is released.
Question 6: What is the maximum recommended drainage area for a silt fence installation without supplemental measures?
- 0.25 acres per 100 linear feet (Correct answer)
- 0.5 acres per 100 linear feet
- 1 acre per 100 linear feet
- 2 acres per 100 linear feet
Correct answer: 0.25 acres per 100 linear feet
Silt fences are designed for sheet flow conditions draining no more than 0.25 acres per 100 linear feet of fence; larger areas require additional or alternative BMPs.
Question 7: Which condition MOST commonly causes a sediment trap to fail prematurely in performance?
- Trap is located too far from the disturbed area
- Accumulated sediment fills the trap volume, reducing storage capacity (Correct answer)
- Outlet pipe diameter is too large for the drainage area
- Trap liner material is incompatible with local soil chemistry
Correct answer: Accumulated sediment fills the trap volume, reducing storage capacity
When sediment accumulates to 50% of the trap design volume, the trap loses effectiveness and must be cleaned out to restore storage capacity.
CESSWI Sediment Control BMPs 2
What is the primary failure mode of silt fence installations on slopes exceeding 2%?