CESSWI Erosion & Sediment Control Principles 5 — Questions and Answers
Question 1: A construction SWPPP must be updated when which of the following occurs?
- A new subcontractor begins work on site
- Site conditions change in a way that affects stormwater discharges (Correct answer)
- The general contractor changes project managers
- Monthly rainfall totals exceed the seasonal average
Correct answer: Site conditions change in a way that affects stormwater discharges
NPDES Construction General Permits require SWPPP amendments whenever there are changes in design, construction, operation, or maintenance that affect the potential for pollutant discharges.
Question 2: Which factor most directly determines the settling velocity of a sediment particle in a quiescent (still) water environment?
- Particle color and mineralogy
- Particle size, density, and shape (Correct answer)
- Particle moisture content at the time of erosion
- Upstream travel distance before entering the basin
Correct answer: Particle size, density, and shape
Settling velocity in still water is governed by Stokes' Law, which relates particle diameter, density, and shape to the rate at which a particle sinks.
Question 3: A slope with a 3:1 (horizontal:vertical) gradient is considered what relative to erosion risk compared to a 4:1 slope?
- Lower risk because a 3:1 slope has a longer horizontal run
- Higher risk because a 3:1 slope is steeper than a 4:1 slope (Correct answer)
- Equal risk because both slopes require the same BMPs
- Lower risk because vegetation establishes more readily on steeper slopes
Correct answer: Higher risk because a 3:1 slope is steeper than a 4:1 slope
A 3:1 slope is steeper than a 4:1 slope (33% vs. 25% grade), resulting in higher flow velocities and greater erosion potential.
Question 4: What is the most common cause of silt fence failure in the field?
- Fabric UV degradation after prolonged sun exposure
- Concentrated flow overtopping or breaching the fence at low points (Correct answer)
- Insufficient post embedment depth in sandy soils
- Excessive sediment accumulation improving barrier performance
Correct answer: Concentrated flow overtopping or breaching the fence at low points
Silt fences most commonly fail when runoff concentrates and overtops or flows around the ends of the fence at topographic low points rather than as sheet flow.
Question 5: During final stabilization inspection, what minimum percentage of vegetative cover is typically required before a CGP Notice of Termination (NOT) can be submitted?
- 50%
- 70% (Correct answer)
- 80%
- 95%
Correct answer: 70%
EPA's Construction General Permit defines final stabilization as achieving 70% vegetative cover density compared to pre-disturbance conditions, or equivalent permanent stabilization.
Question 6: An inspector observes that a rock outlet protection apron at the end of a pipe is missing several stones, exposing bare soil. What is the primary risk of this condition?
- Reduced aesthetic appearance of the outlet
- Scour erosion at the pipe outlet that can undermine the pipe and headwall (Correct answer)
- Increased sediment trapping capacity at the outlet
- Higher infiltration rates due to the coarser soil surface
Correct answer: Scour erosion at the pipe outlet that can undermine the pipe and headwall
Missing outlet protection exposes the soil below and downstream of the pipe to high-velocity discharge, causing scour that can progressively undermine structural components.
Question 7: Which of the following scenarios would require the installation of a temporary diversion dike above a disturbed area?
- Runoff from undisturbed upland areas drains across an active construction zone (Correct answer)
- The disturbed area is located on a flat, poorly drained site
- A sediment basin needs to be dewatered after a storm event
- The construction entrance has become clogged with mud from vehicle traffic
Correct answer: Runoff from undisturbed upland areas drains across an active construction zone
Temporary diversion dikes intercept clean runoff from upland areas before it crosses disturbed ground, preventing it from picking up sediment and complicating stormwater management.
CESSWI Erosion & Sediment Control Principles 5
A construction SWPPP must be updated when which of the following occurs?