KDOT Drainage & Storm Water Systems 4 — Questions and Answers
Question 1: What is the purpose of a storm water detention basin in a highway right-of-way?
- To permanently store runoff for irrigation
- To temporarily store runoff and reduce peak discharge rates (Correct answer)
- To filter pollutants using biological processes only
- To increase the time-of-concentration to zero
Correct answer: To temporarily store runoff and reduce peak discharge rates
Detention basins temporarily store storm water to attenuate peak flows, releasing water at a controlled, lower rate to downstream channels.
Question 2: During inspection of a roadside ditch, you notice the ditch lining has separated from the soil and is undermined. What condition does this indicate?
- Proper compaction has settled the lining into place
- Piping or subsurface erosion beneath the ditch lining (Correct answer)
- The lining material has expanded due to freeze-thaw
- Hydrostatic uplift from a high water table
Correct answer: Piping or subsurface erosion beneath the ditch lining
Separation and undermining of ditch lining indicates subsurface piping or erosion where water has moved soil from beneath the lining.
Question 3: What is a 'subdrain' and what is its primary function in highway drainage?
- An open channel that collects surface runoff
- A perforated pipe or aggregate trench that removes subsurface water from pavement layers (Correct answer)
- A detention pond outlet control structure
- A concrete-lined channel beneath a bridge deck
Correct answer: A perforated pipe or aggregate trench that removes subsurface water from pavement layers
A subdrain is a subsurface drainage system using perforated pipe or aggregate to intercept and remove groundwater or perched water from pavement base layers.
Question 4: When inspecting a concrete pipe culvert joint, what defect is most critical to document and report immediately?
- Minor mortar cracking at the joint
- Joint separation with visible soil infiltration or exfiltration (Correct answer)
- Slight discoloration of joint material
- Aggregate exposure at the pipe bell
Correct answer: Joint separation with visible soil infiltration or exfiltration
Joint separation allowing soil migration indicates potential pipe collapse, embankment failure, or sinkhole formation, requiring immediate reporting.
Question 5: What is the KDOT-required minimum slope for roadside ditches to ensure adequate self-cleaning velocity?
- 0.1%
- 0.3% (Correct answer)
- 1.0%
- 2.0%
Correct answer: 0.3%
KDOT generally requires a minimum ditch slope of 0.3% to maintain sufficient flow velocity to prevent sediment deposition and promote self-cleaning.
Question 6: What does 'tailwater depth' control in culvert outlet hydraulics?
- The inlet headwater elevation required for flow
- Whether the outlet is submerged, affecting hydraulic performance (Correct answer)
- The sediment load capacity of the downstream channel
- The design storm return period for the structure
Correct answer: Whether the outlet is submerged, affecting hydraulic performance
Tailwater depth at the outlet determines if the culvert operates under outlet control and whether the exit is submerged, reducing overall hydraulic efficiency.
Question 7: Which erosion control practice is most appropriate for a freshly graded roadside ditch on a KDOT project before vegetation is established?
- Placing concrete paving immediately
- Applying temporary erosion control blankets or erosion control matting (Correct answer)
- Compacting the soil surface to reduce permeability
- Installing permanent riprap for the entire ditch length
Correct answer: Applying temporary erosion control blankets or erosion control matting
Temporary erosion control blankets protect exposed soil from rainfall impact and surface flow until permanent vegetation becomes established.
KDOT Drainage & Storm Water Systems 4
What is the purpose of a storm water detention basin in a highway right-of-way?