CPESC Sediment Control Best Practices 2 — Questions and Answers
Question 1: What is the primary purpose of a sediment forebay in a sediment basin?
- To increase water velocity through the basin
- To trap coarse sediment before water enters the main pool (Correct answer)
- To aerate stormwater and reduce turbidity
- To prevent vegetation from establishing in the basin
Correct answer: To trap coarse sediment before water enters the main pool
A sediment forebay traps coarse sediment near the inlet, protecting the main pool and reducing maintenance frequency.
Question 2: Which condition would MOST likely cause a silt fence to fail prematurely?
- Installation on slopes less than 2%
- Geotextile fabric buried 6 inches into the ground
- Concentrated flow directed against the fence (Correct answer)
- Posts spaced 6 feet apart on level ground
Correct answer: Concentrated flow directed against the fence
Silt fences are designed for sheet flow only; concentrated flow creates hydraulic pressure that overwhelms the fabric and causes blowouts.
Question 3: A check dam in a drainage channel should be constructed so that water overtops:
- At the edges of the dam to prevent center scour
- At the center, which should be lower than the edges (Correct answer)
- Uniformly across the entire crest at equal depth
- Only through a pipe outlet installed at the base
Correct answer: At the center, which should be lower than the edges
The center of a check dam is kept lower than the edges so overflow is concentrated at the center, preventing flanking and bank erosion.
Question 4: During a CPESC site inspection after a storm event, you observe sediment accumulation of 12 inches in a 24-inch deep sediment trap. What action is required?
- No action needed until accumulation reaches 18 inches
- Clean out the sediment trap immediately (Correct answer)
- Add more baffles to increase settling efficiency
- Increase the trap's surface area by 50%
Correct answer: Clean out the sediment trap immediately
Standard practice requires cleanout when sediment accumulation reaches half the design depth (typically 50% capacity) to maintain effectiveness.
Question 5: What is the recommended maximum drainage area for a standard rock check dam in a small channel?
- 1 acre
- 5 acres (Correct answer)
- 10 acres
- 25 acres
Correct answer: 5 acres
Rock check dams are typically limited to drainage areas of about 5 acres or less because larger watersheds generate flows that can destabilize the structure.
Question 6: Which factor most directly influences the settling velocity of sediment particles in a basin?
- Particle color and organic content
- Basin length-to-width ratio
- Particle size and specific gravity (Correct answer)
- Water temperature only
Correct answer: Particle size and specific gravity
Settling velocity is governed primarily by particle size and specific gravity, as described by Stokes' Law for laminar flow conditions.
Question 7: A turbid discharge is leaving a construction site during rain. The FIRST corrective action should be to:
- Install a new sediment basin downstream
- Identify and stabilize the sediment source on-site (Correct answer)
- Apply flocculants directly to the receiving water
- Notify the regulatory agency immediately
Correct answer: Identify and stabilize the sediment source on-site
Source control is the first priority; stabilizing bare soil or addressing the erosion source prevents sediment from reaching controls and receiving waters.
CPESC Sediment Control Best Practices 2
What is the primary purpose of a sediment forebay in a sediment basin?