Effective management of stormwater is crucial in urban development to reduce flooding, control runoff, and enhance water quality. Central to this management are systems for water detention and retention, each serving distinct purposes in handling stormwater.
Understanding Stormwater Detention and Retention Systems
Stormwater retention and detention are often confused, yet they represent different practices in stormwater management.
Retention:
Retention systems are designed to hold runoff over the long term, maintaining a permanent pool of water between rainfall events. Water only exits these systems when displaced by new runoff, pumped out for other uses, evaporated, or naturally infiltrated into the soil.
Detention:
In contrast, detention systems temporarily collect rainwater and runoff, gradually releasing all stored water shortly after a storm, without retaining pools of water between events.
Detention Basins
A detention basin, also known as a retarding basin, is an excavated area located on or near tributaries of rivers, streams, lakes, or bays to mitigate flooding and, in some cases, downstream erosion by storing water temporarily. These basins may be referred to as dry ponds or holding ponds if they do not maintain a permanent water pool. When designed to retain some water volume at all times, they are called retention basins. Detention basins primarily manage water quantity and have limited effectiveness in improving water quality unless they include a permanent pool.
Design Considerations for Detention Systems
1- Basin Location:
Detention basin sites can incorporate multiple land uses, such as parking lots, parks, sports fields, road embankments, depressed areas, floodplains, drainage channels, swales, rooftops, and underground storage. Site selection should consider costs, public safety, and maintenance. Collaboration between specialists and engineers is essential to create criteria that meet community recreational needs and flood control objectives.
2- Basic Layout:
A detention basin comprises several key components: an inlet protected by an energy dissipater, a fore bay for sediment trapping, a trickle channel for low flows, multiple storage layers, an outlet for controlled release, and an emergency spillway. Inlets should have trash racks for debris control, and the fore bay must include a sediment baffle system while accommodating first flush runoff volumes. The trickle channel should handle 30-50% of the 2-year peak flood flow, and a width-to-length ratio greater than two is preferable for sedimentation. Proper design coefficients and erosion protection are vital for inlet and outlet performance, with embankment slopes ensuring soil stability.
3- Design Event:
Flood control structures are typically designed for multiple events. Generally, a detention basin should provide sufficient storage to manage the 10-year and 100-year release rates. A freeboard of 6 to 12 inches is necessary, and the emergency spillway must accommodate the maximum probable runoff. If a water quality control basin (WQCB) is present, it should be sized to capture 80% of runoff, ensuring that, on average, 80% of runoff events from the tributary watershed are captured without overtopping. The stored water volume is considered treated through settling in the permanent pool over a 12- to 24-hour drain time.
Environmental and Regulatory Considerations
Stormwater detention and retention systems are essential for complying with environmental regulations:
– Water Quality Standards: These systems help remove pollutants from runoff, ensuring compliance with water quality standards.
– Flood Control Requirements: Well-designed systems mitigate flooding risks, aligning with regulatory floodplain management requirements.
By incorporating these systems into urban planning, municipalities can achieve regulatory compliance while promoting sustainable water management practices.
Conclusion
Stormwater detention and retention systems are critical elements of modern urban infrastructure, addressing runoff management, flood control, and water quality improvement challenges. Through thoughtful design and innovative technologies, these systems contribute to creating resilient and sustainable urban environments.
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