Simple Hydrograph Shapes for Urban Stormwater Water Quality Analyses.

Robert Pitt and John Voorhees


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. As an example, the 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 have long been recognized for drainage design calculations, for both single event “design storms” 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 runoff conditions that occur during more frequent rains of most interest for use in water quality evaluations in urban areas. 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 paper reviews 550 such observations that have been collected over the past 35 years at four areas representing different land uses and statistically analyses the important shape factors of the observed hydrographs under a range of small to intermediate-sized rains of most interest for water quality analyses.

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