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This paper presents the results of a detailed analysis of storm events in the Dallas area, for the purpose of defining realistic design storms for hydraulic modeling of separate sanitary sewers. The analysis utilizes adjusted radar-rainfall data and analysis tools that allowed the characteristics of thousands of storm cells to be defined. Particular emphasis has been placed on developing depth-area reduction factors (DARF) that can be applied to convert point-rainfall for a range of return periods into average rainfall over basins of various sizes. Although DARF curves have been published in the past, they have not been developed using the spatial resolution provided by today's radar rainfall data and analysis tools, do not consider how DARF may vary with peak rainfall intensity within a storm (i.e., the return period of the event), and are not specific to storms in the Dallas region. Another feature of this study is the separate handling of the two very distinct types of storms that occur in Dallas: cloudbursts and synoptic-scale events. Cloudbursts are short (generally under 3 hours), high-intensity events that cover a limited geographic area, and produce much lower rainfall intensities over larger tributary basins. These are the events in which DARF must be properly applied to avoid generating overly conservative design flows. Synoptic-scale storms are longer events that cover large areas with equal rainfall intensities, but those intensities are much lower than experienced in cloudbursts. In this study, long-term historical rainfall records were processed to exclude cloudburst events and generate custom Intensity/Duration/Frequency (IDF) statistics specifically for developing synoptic-scale design events. Software was written to generate design events from the two sets of IDF data and the multiple DARF curves developed in this study, for any desired combination of return period, storm type, basin size, and other storm variables.

This paper should be of interest to persons responsible for modeling and planning drainage systems or wastewater collection systems that experience high peak wet weather flows. The results of this study should be useful for developing realistic design storms that avoid excessive conservatism and potentially save millions of dollars in sewer improvements.

Document Type: Research Article


Publication date: 2005-01-01

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