NETTCP Drainage & Stormwater Management 2 — Questions and Answers
Question 1: What is the function of an energy dissipator placed at the outlet of a culvert?
- To filter pollutants from stormwater before discharge
- To reduce flow velocity and prevent scour erosion at the outlet (Correct answer)
- To measure discharge volumes for regulatory reporting
- To prevent backflow during high water events
Correct answer: To reduce flow velocity and prevent scour erosion at the outlet
Energy dissipators such as riprap aprons or stilling basins reduce high-velocity outlet flow to prevent erosion and scour of the receiving channel.
Question 2: A hydraulic gradient line (HGL) that rises above the crown of a drainage pipe indicates:
- The pipe is flowing partially full under normal conditions
- The pipe is flowing full under pressure (pressure flow) (Correct answer)
- Infiltration of groundwater into the system
- The pipe slope is steeper than the critical slope
Correct answer: The pipe is flowing full under pressure (pressure flow)
When the HGL exceeds the pipe crown elevation, the pipe is under pressure and flowing full, which can cause surcharging and potential surface flooding.
Question 3: Which underdrain configuration is most effective at lowering the water table beneath a highway pavement to prevent frost heave in New England?
- Surface interceptor ditch along the roadway edge
- Longitudinal perforated pipe underdrain with permeable backfill placed at subgrade elevation (Correct answer)
- Impermeable geocomposite barrier beneath the pavement
- Edge of pavement curb and gutter system
Correct answer: Longitudinal perforated pipe underdrain with permeable backfill placed at subgrade elevation
A longitudinal perforated pipe underdrain with permeable aggregate intercepts and removes groundwater at subgrade level, preventing saturation and frost heave.
Question 4: When inspecting a stormwater detention basin, which condition requires immediate corrective action?
- Vegetation growth on the side slopes below the normal water level
- Evidence of overtopping, erosion, or seepage through the embankment (Correct answer)
- Accumulation of sediment in the forebay after a storm event
- Standing water in the basin between storm events
Correct answer: Evidence of overtopping, erosion, or seepage through the embankment
Overtopping, erosion, or seepage through an embankment are signs of structural failure risk that require immediate repair to prevent dam breach.
Question 5: The time of concentration (Tc) in a drainage watershed is defined as:
- The duration of the design storm event in hours
- The time for runoff to travel from the hydraulically most remote point to the design point (Correct answer)
- The lag time between peak rainfall intensity and peak discharge
- The time required for soil infiltration capacity to reach a steady state
Correct answer: The time for runoff to travel from the hydraulically most remote point to the design point
Time of concentration is the travel time for water from the farthest point in the drainage area to reach the outlet, which determines the peak discharge timing.
Question 6: What is the minimum recommended slope for a storm drain pipe to maintain self-cleaning velocity and prevent sediment deposition?
- 0.1% (1 foot per 1,000 feet)
- 0.5% (0.5 feet per 100 feet) (Correct answer)
- 2.0% (2 feet per 100 feet)
- 5.0% (5 feet per 100 feet)
Correct answer: 0.5% (0.5 feet per 100 feet)
A minimum slope of 0.5% is generally required to achieve the minimum 2.5 fps self-cleaning velocity in storm drain pipes to prevent sediment accumulation.
Question 7: Which New England-specific drainage challenge most often requires the use of deep-sump catch basins rather than standard designs?
- High annual rainfall totals exceeding 60 inches
- Freezing and thawing cycles that mobilize sand and road salt into drainage systems (Correct answer)
- Dense urban development limiting land for surface detention
- Proximity to coastal tidal influence
Correct answer: Freezing and thawing cycles that mobilize sand and road salt into drainage systems
New England's freeze-thaw cycles and heavy winter sand/salt application cause significant sediment accumulation in drainage inlets, requiring deep sumps to increase storage capacity between cleaning cycles.
NETTCP Drainage & Stormwater Management 2
What is the function of an energy dissipator placed at the outlet of a culvert?