CPESC Erosion Control Methods & Techniques 3 — Questions and Answers
Question 1: A level spreader is used in stormwater management to:
- Concentrate flow into a single outlet pipe for controlled discharge
- Convert concentrated flow into sheet flow before entering a vegetated filter (Correct answer)
- Increase flow velocity to prevent sediment accumulation
- Provide temporary storage of runoff during peak storm events
Correct answer: Convert concentrated flow into sheet flow before entering a vegetated filter
A level spreader distributes concentrated channel flow evenly across its full length, converting it to sheet flow for treatment by a vegetated filter strip.
Question 2: When installing silt fence on a slope, the bottom of the fence fabric should be:
- Stapled flush to the soil surface to prevent water from undercutting
- Folded outward (upstream) and buried at least 6 inches in a trench (Correct answer)
- Left loose to allow water to pass under during high-flow events
- Folded inward (downstream) and anchored with wire mesh
Correct answer: Folded outward (upstream) and buried at least 6 inches in a trench
Burying the toe of the fabric in a trench prevents undercutting and ensures runoff must pass through the filter fabric.
Question 3: Runoff from steep erodible slopes is best managed by intercepting it before it accelerates using:
- Outlet protection at the base of the slope
- Diversions and interceptor drains across the slope (Correct answer)
- Increasing ground cover at the toe of the slope only
- Installing a silt fence at the base of the slope
Correct answer: Diversions and interceptor drains across the slope
Interceptor drains and diversions capture runoff before it gains velocity, preventing rill and gully formation on steep slopes.
Question 4: Which seeding technique is best suited for steep rocky slopes where conventional drill seeding is impractical?
- Broadcast seeding followed by disc incorporation
- Hydroseeding with wood fiber mulch and tackifier (Correct answer)
- Drill seeding at double the normal seeding rate
- Sod installation with staples at 1-foot intervals
Correct answer: Hydroseeding with wood fiber mulch and tackifier
Hydroseeding applies seed, mulch, and tackifier in a slurry that can reach inaccessible steep and rocky slopes while anchoring seed in place.
Question 5: The concept of 'soil loss tolerance' (T value) in erosion control represents:
- The maximum storm event size a site can withstand without measurable erosion
- The maximum annual soil loss rate at which soil productivity can be maintained indefinitely (Correct answer)
- The threshold sediment concentration allowable in stormwater discharge
- The minimum ground cover percentage required to prevent sheet erosion
Correct answer: The maximum annual soil loss rate at which soil productivity can be maintained indefinitely
T value represents the maximum rate of soil erosion that can occur while maintaining long-term soil productivity, typically 1–5 tons/acre/year.
Question 6: Fiber rolls (wattles) installed on slopes are most effective when placed:
- Vertically down the slope to channel runoff
- Perpendicular to the slope contour at regular intervals (Correct answer)
- At the base of the slope only to capture sediment
- On top of vegetation to prevent lodging during storms
Correct answer: Perpendicular to the slope contour at regular intervals
Fiber rolls placed on contour perpendicular to the slope intercept sheet flow, reduce velocity, and trap sediment across the entire slope width.
Question 7: What distinguishes rill erosion from sheet erosion?
- Rill erosion occurs only on slopes steeper than 10% while sheet erosion occurs on gentler slopes
- Sheet erosion removes soil uniformly across the surface while rill erosion creates small concentrated channels (Correct answer)
- Rill erosion is caused by wind while sheet erosion is caused by water
- Sheet erosion occurs in forests while rill erosion occurs on agricultural land
Correct answer: Sheet erosion removes soil uniformly across the surface while rill erosion creates small concentrated channels
Sheet erosion removes a relatively uniform thin layer of soil, while rill erosion forms small concentrated channels as overland flow concentrates.
CPESC Erosion Control Methods & Techniques 3
A level spreader is used in stormwater management to: