Stormwater
Detention
Learning Objectives:
- Recognize the primary purpose of stormwater detention, which is to manage excessive stormwater runoff and protect the environment and public safety.
- Differentiate between stormwater detention and retention. Detention temporarily holds and releases stormwater, while retention permanently stores it for purposes such as groundwater recharge.
- Learn how to design stormwater detention basins, including considerations for sizing, location, depth, and outlet structures.
Stormwater detention is a critical component of stormwater management in civil engineering, aimed at controlling and mitigating the adverse effects of stormwater runoff on urban and natural environments. It involves the temporary storage of stormwater to reduce peak flow rates and prevent flooding, erosion, and water quality issues. Understanding stormwater detention is crucial for civil engineers involved in urban planning, infrastructure development, and environmental conservation.
The principles of stormwater detention revolve around the control and regulation of runoff from rainfall events. When precipitation occurs, impermeable surfaces in urban areas can lead to rapid runoff, overwhelming natural drainage systems and causing flooding. Stormwater detention aims to mitigate these issues by temporarily storing and then gradually releasing stormwater, preventing downstream flooding and erosion.
Detention basins, ponds, and tanks are common structures used for stormwater detention. These facilities temporarily hold excess water during intense rainfall and then release it slowly, mimicking the natural hydrological cycle and reducing the impact on downstream areas. The detention period allows for the gradual release of water, preventing the sudden and excessive flow that could lead to flooding.
Arbitrarily0, CC BY-SA 3.0, via Wikimedia Commons
A partially-completed stormwater detention system. There is a base layer of stones, covered by fabric, upon which are placed the yellow water detention containers. The containers themselves are covered with fabric, then overlayed with stone. Manholes at three of the four corners handle inlet and outlet stormwater flows.
Solved Example: 9499-01
What is the primary purpose of stormwater detention in a civil engineering context?
A. To increase groundwater recharge
B. To treat stormwater for contaminants
C. To temporarily store and slowly release stormwater
D. To prevent stormwater runoff entirely
Correct Answer: C
Solved Example: 9499-02
Which of the following factors affects the design of a stormwater detention basin?
A. Erosion control methods
B. Groundwater quality testing
C. Hydrologic characteristics of the watershed
D. Stormwater conveyance system capacity
Correct Answer: C
Stormwater Routing
Learning Objectives:
- Understand the fundamental principles of the hydrologic cycle, including precipitation, evaporation, infiltration, and runoff.
- Analyze rainfall data to determine rainfall intensity-duration-frequency (IDF) relationships for stormwater design.
- Interpret hydrographs and hydrograph components to assess stormwater runoff behavior.
- Apply appropriate runoff models (e.g., Rational Method, SCS Curve Number Method) to estimate runoff volumes and peak flow rates for different design scenarios.
- Design stormwater detention and retention basins to control peak flows, attenuate runoff, and manage stormwater quality.
- Design stormwater pipe networks, including pipe sizing, hydraulic analysis, and selection of appropriate materials.
- Apply principles of open channel flow, including the Manning's equation, to calculate flow rates, velocities, and channel dimensions for stormwater conveyance.
- Identify and design stormwater BMPs, such as infiltration basins, bioretention systems, green roofs, and permeable pavements, to promote sustainable stormwater management.
- Understand floodplain mapping, floodplain regulations, and flood hazard mitigation techniques to reduce the risk of flooding.
- Learn methods for controlling and treating stormwater runoff to address water quality concerns, including pollutant removal and erosion control.
- Hydrologic Cycle: Understand the various components of the hydrologic cycle, including precipitation, runoff, infiltration, and evaporation, and their role in stormwater management.
- Rainfall Analysis: Learn how to analyze rainfall data, including intensity-duration-frequency (IDF) curves, to estimate design storm events for engineering purposes.
- Hydrographs: Study hydrographs, which represent the time-dependent variation of stormwater runoff. Learn to create and interpret hydrographs using rainfall data.
- Runoff Models: Understand different runoff models, such as the Rational Method and the Soil Conservation Service (SCS) curve number method, for estimating runoff volumes and peak flows.
- Detention and Retention Basins: Explore the design and function of stormwater detention and retention basins, which store and slow down stormwater runoff to reduce peak flows and prevent flooding.
- Pipe Networks: Learn about stormwater pipe networks and how to design them to efficiently transport runoff from streets and urban areas to appropriate discharge points.
- Open Channel Flow: Understand the principles of open channel flow, including the Manning's equation, to calculate flow rates and velocities in natural and artificial channels.
- Stormwater Management Practices: Familiarize yourself with best management practices (BMPs) for stormwater management, including green infrastructure, permeable pavements, and bio-retention systems.
- Floodplain Management: Learn about floodplain mapping and management to reduce the risk of flooding in urban and rural areas.
- Regulations and Standards: Be aware of relevant regulations, codes, and standards related to stormwater management, such as local stormwater ordinances and the Clean Water Act.
- Environmental Considerations: Consider the environmental impacts of stormwater runoff, including pollutant removal and protection of natural habitats.
- Sustainability: Explore sustainable stormwater management practices that promote water conservation and minimize the impact on ecosystems.
Pinpin, CC BY-SA 3.0, via Wikimedia Commons
Histogram of the flow of the Aude at the Moussan hydrological station, Station code: Y1612020, catchment area: 4838 km², annual flow: 44.2 m³/s.
Solved Example: 9358-01
To estimate the magnitude of a flood peak (Qp) by the formula \[{Q_P} = \dfrac{{124A}}{{\sqrt {A + 10.4}}}\], where A = catchment area in km$^2$. This formula is termed as:
A. Dicken's formula
B. Inglis formula
C. Ryves formula
D. Rational formula
Correct Answer: B
Solved Example: 9358-02
The extent to which the inflow hydrograph gets modified due to the reservoir storage can be computed by a process known as:
A. River routing
B. Channel routing
C. S hydrograph
D. Flood routing or reservoir routing
The process of estimating and figuring out the stage height, volume in storage, and outflow rate from a reservoir or stream reach for a specific flood hydrograph is known as flood routing.The process of using the data of flood flow at one or more upstream sections to determine the flood hydrograph at a stretch of a river is known as flood routing.
Correct Answer: D
Stormwater Quality
Learning Objectives:
- Recognize the significance of managing stormwater quality to protect aquatic ecosystems, safeguard public health, and comply with regulatory requirements.
- Understand the various sources of stormwater pollution, including industrial discharges, construction site runoff, transportation-related contaminants (such as oil and heavy metals), and residential pollutants (such as fertilizers and pesticides).
- Familiarize yourself with the characteristics of common stormwater pollutants, including sediments, nutrients (nitrogen and phosphorus), bacteria, chemicals, heavy metals, and oils and grease.
Stormwater quality refers to the condition and characteristics of rainwater and snowmelt runoff as it flows over surfaces and enters stormwater drainage systems. Managing stormwater quality is a critical aspect of civil engineering, as it involves mitigating the adverse environmental impacts of urban runoff, such as pollution and contamination.
As urbanization increases impervious surfaces, stormwater can accumulate various contaminants such as sediments, nutrients, heavy metals, and hydrocarbons, posing environmental and public health risks. The syllabus typically covers principles related to the characterization of stormwater pollutants, regulatory standards, and best management practices for mitigating adverse impacts. Understanding stormwater quality involves knowledge of sedimentation processes, chemical and biological constituents, and the implementation of stormwater management strategies to enhance water quality in receiving bodies.
Rowan Andrews, CC BY-SA 4.0, via Wikimedia Commons
The image shows how precipitation collects contaminates from roads and houses and washes it into the stormwater drains and into the local bodies of water. The local bodies of water are supporting life and the storm water is contaminating species water quality.