CSM CSM Urban Stormwater Systems & Infrastructure 1 — Questions and Answers
Question 1: What is a combined sewer overflow (CSO) and why is it a stormwater concern?
- A CSO occurs when a combined sewer system carrying both sewage and stormwater exceeds capacity, discharging untreated waste to waterways (Correct answer)
- A CSO is a planned release of clean stormwater from municipal retention ponds
- A CSO refers to the overflow of a stormwater detention basin during a 100-year storm
- A CSO is a backup system that routes excess flow to a water treatment plant
Correct answer: A CSO occurs when a combined sewer system carrying both sewage and stormwater exceeds capacity, discharging untreated waste to waterways
Combined sewer systems carry both sanitary sewage and stormwater in one pipe; during heavy rain, excess volume overflows to receiving waters as untreated sewage mixed with stormwater, posing significant water quality and public health risks.
Question 2: What distinguishes a detention pond from a retention pond in stormwater management?
- A detention pond temporarily holds runoff and releases it slowly after a storm; a retention pond permanently holds a standing pool of water (Correct answer)
- A detention pond is lined with concrete; a retention pond uses natural soil
- A detention pond is used only in industrial areas; a retention pond is used in residential areas
- A detention pond removes pollutants chemically; a retention pond removes them biologically
Correct answer: A detention pond temporarily holds runoff and releases it slowly after a storm; a retention pond permanently holds a standing pool of water
Detention ponds (dry ponds) temporarily store runoff to control peak flow and drain completely between storms, while retention ponds (wet ponds) maintain a permanent pool that provides additional water quality treatment.
Question 3: What is an impervious surface and how does it affect the urban stormwater runoff cycle?
- A hard surface that prevents infiltration, increasing runoff volume, velocity, and peak discharge compared to natural land (Correct answer)
- Any surface that is older than 25 years and requires replacement
- A permeable pavement system that allows water to seep into the subgrade
- A surface treated with waterproof sealant for industrial applications
Correct answer: A hard surface that prevents infiltration, increasing runoff volume, velocity, and peak discharge compared to natural land
Impervious surfaces such as roads, rooftops, and parking lots prevent rainfall from infiltrating into the soil, dramatically increasing the volume and peak rate of stormwater runoff and the pollutants it carries.
Question 4: What is the function of a storm drain inlet grate in an urban stormwater system?
- To collect surface runoff from streets and direct it into the underground storm drain conveyance system (Correct answer)
- To filter all pollutants from runoff before it enters the storm drain
- To measure the flow rate entering the storm sewer system
- To prevent backflow from the storm sewer into streets during flooding
Correct answer: To collect surface runoff from streets and direct it into the underground storm drain conveyance system
Storm drain inlets capture surface runoff from streets and parking areas and convey it to the underground storm sewer pipe network for transport to an outfall or treatment facility.
Question 5: What is the purpose of a headwall at a storm drain outfall?
- To stabilize the end of a pipe where it discharges to a channel or receiving water and to prevent erosion around the outlet (Correct answer)
- To measure total stormwater discharge volume from the pipe
- To block stormwater from discharging during dry weather conditions
- To raise the discharge elevation to create a ponding effect
Correct answer: To stabilize the end of a pipe where it discharges to a channel or receiving water and to prevent erosion around the outlet
A headwall is a concrete or masonry structure that supports and protects the end of a storm drain pipe at an outfall, preventing embankment erosion and scour around the pipe outlet.
Question 6: In the context of urban stormwater infrastructure, what is the purpose of an energy dissipater at a pipe outfall?
- To reduce the velocity of discharged water to prevent erosion and scour at the receiving channel or water body (Correct answer)
- To generate hydroelectric power from stormwater flows
- To filter fine sediment particles from the outfall discharge
- To regulate flow rate so that peak discharge does not exceed pre-development conditions
Correct answer: To reduce the velocity of discharged water to prevent erosion and scour at the receiving channel or water body
Energy dissipaters such as riprap aprons or baffled outlets reduce the kinetic energy of high-velocity outfall discharge to protect the receiving channel from scour and erosion.
CSM CSM Urban Stormwater Systems & Infrastructure 1
What is a combined sewer overflow (CSO) and why is it a stormwater concern?