Storage Concept in Urban Runoff Management
Storage is widely used in urban runoff quantity and quality control, providing provision for both peak flow reduction and suspended solids removal. Over decades, the design principles for storage structures have changed and improved due to a better understanding of the urban rainfall-runoff behavior and enhanced model representation and simulation technologies. These structures, like the detention ponds, are now designed and constructed in such a way that they not only control extreme runoff volumes but also prevent water pollution generated from smaller and the “first flush” events. Thus, in managing urban runoff, they have a significant impact on the quantity and quality of the receiving water bodies.
The component of a detention pond that operates to improve the runoff water quality is called the Water Quality Capture Volume (WQCV). From a hydrologic point of view, WQCV is a suitable volume (often expressed in units of watershed depth) that detains water for a long enough period so that a targeted level of pollutant removal is achieved before complete release to the receiving water body. The required WQCV volume, total drain time, and pollutant removal standards vary among stormwater management policies of different communities.
Several empirical or “rule of thumb” techniques are used to estimate and design the WQCV for urban catchments. In this Tutorial, the WQCV component of the detention pond has been designed following the methodology proposed in the Urban Storm Drainage Criteria Manual (USDCM, Volume 3) of the Urban Drainage and Flood Control District (UDFCD), Colorado, USA.
Other than WQCV, detention ponds usually have sufficient storage capacity to capture and detain the extreme event runoff volumes (e.g. 10-year, 50-year, or 100-year runoffs) for a certain duration from the upstream catchment. The pond outlets are designed to limit the release rates in such a way that maximum discharge from them doesn’t exceed the value of the pre-developed condition. Thus, the pond mitigates the adverse impact of increased imperviousness (resulting from the development process) by reducing the peak flow. Note that detention ponds do not reduce total runoff volume. They can only flatten the outflow hydrograph shape and reduce the peak by controlling the discharge rate in a defined and regular manner. To control the discharge from a detention pond, different types of flow control structures like orifice and weir are used in an urban drainage system.
Figure 2.1 shows the schematic of a detention pond and its outlet structures designed to control a WQCV and three design storm runoffs. Section B-B explains the arrangement of the compartments in the pond and estimated maximum water levels under different storm events. The lower prism storage in the pond is used to capture the WQCV and the 2-year storm runoff whereas the upper and larger prism storages capture 10-year and 100-year storm runoffs. Orifices Or1, Or2, and Or3 release flows of WQCV, 2-year storm, and 10-year storm respectively. A transverse weir Wr1 is used to release the flow of 100-year storm runoff. Before the complete release, water must be detained in the pond for a period long enough to meet the discharged water quality criteria. An adequate freeboard is provided on top of the 100-year stormwater level in the pond.
The proposed detention pond in this Tutorial is designed with a similar structure as shown in the above Figure 2.1.