WEST Conducting Study to Update Depth-Area Reduction Factors Methodology for Arizona

WEST is pleased to announce it was awarded a contract from the Flood Control District of Maricopa County (FCDMC), Arizona, in cooperation with the Arizona Department of Transportation, to conduct a study to update the depth-area reduction factor (DARF) criteria for Arizona. The project will develop DARF values to convert point rainfall to average areal rainfall, accounting for a comprehensive range of durations, watershed areas, storm frequencies, geographic locations, and seasons for any location within Arizona. The DARF study is a unique and important project with the potential to significantly reduce construction costs for drainage and flood control infrastructure throughout the state.

Engineers use rainfall depths developed by the National Oceanic and Atmospheric Administration (NOAA) and site-specific rainfall studies to complete analyses that identify and evaluate flood hazards. These values are also used to design drainage and flood control facilities (dams, culverts, basins, bridges, etc.). The DARFs currently used in Arizona are based on pre-1980 data of limited spatial resolution compared to the present rainfall database. Storm reconstructions from more recent data suggest that the current DARF used by multiple agencies within the state may result in the overestimation of runoff magnitudes.

The project will be completed in multiple phases, with Phase 1 consisting of a literature review of similar studies and data collection. Based on this work, the WEST team will develop a recommended approach to the DARFs for Arizona. Future phases will develop DARF criteria for each hydrometeorological zone within the state of Arizona and will develop a tool to be used by engineers and hydrologists in performing rainfall-runoff modeling. WEST will conduct technical outreach during the project.

FCDMC selected the WEST team in recognition of specialized experience and technical competence, qualifications, past performance, and capacity to accomplish work, in particular the team’s expertise in developing spatially distributed rainfall estimates for hydrologic applications. In discussing the award, Brian Wahlin, Ph.D., WEST Vice President, commented, “WEST applauds the Flood Control District of Maricopa County and the Arizona Department of Transportation for taking this step to advance our hydrometeorological understanding of storms in the state and to provide more cost-effective flood control and drainage infrastructure.”

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areal rainfall

The Flood Control District was formed 60 years ago to address flooding throughout the county. Despite the desert setting, severe floods occur because of its unique characteristics — especially during summer Monsoon and winter rainstorms.

For more information on the FCDMC, visit: https://www.fcd.maricopa.gov/5308/Flood-Control-District.

To learn about WEST’s flood warning and forecasting and other services supporting local, national, and international initiatives, see: www.westconsultants.com/services/


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