Simple Hydrograph Shapes for Urban Stormwater Water Quality Continuous Analyses
Over the years, a number of different approaches have been used to represent hydrographs in urban areas for drainage design. Unit hydrographs are usually used to represent one inch of runoff and are scaled according to calculated total runoff amounts, and the hydrograph shapes are based on different drainage area characteristics. An actual complex rain distribution is then used to assemble a set of scaled unit hydrographs to represent the total storm event. As an example, the United States Department of Agriculture (USDA) developed different urban area hydrograph shapes which are dependent on expected rainfall distribution patterns. The simplest hydrograph shape is a triangle, while more complex hydrographs have more detailed recession curves and other features.
The need for accurate hydrograph representations for drainage design calculations has long been recognized for both single event design storm and for continuous simulations using long term rainfall records. The events of most interest in drainage design are obviously large and occur infrequently. Actual rainfall and flow records of these events are therefore rare, with little opportunity for verification of flow modeling tools.
Reasonable assumptions based on regional observations of selected large events that have occurred over long periods have therefore been the basis for most drainage design calculations. However, these assumptions and tools may not accurately represent the runoff conditions that occur during more frequent rains of most interest for use in water quality evaluations in urban areas (see Burton and Pitt, 2002; Pitt, 2002 for extensive summaries of the literature). Because these smaller rains are more common, it is likely that significant monitoring records exist that are suitable for calibration and subsequent verification of stormwater models.
This chapter reviews about 550 urban area hydrographs that have been collected at eight locations in four regions of North America, representing different land uses under widely varying rain conditions. Statistical analyses were performed to quantify the important shape factors of the observed hydrographs, applicable for water quality analyses that focus on small and intermediate-sized rains.
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