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City of Billings Case Study: Complex floodplain issues

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In 1937, a major flood of Yellowstone River tributaries devastated Billings, Montana. Despite the vulnerability of the community that this event revealed, the floodplain was not accurately delineated for decades. The reason for this delay in mapping was due to two factors: 1) lack of available funding and 2) exceedingly complex flow patterns in the developed floodplain. Recently, however, a rapidly expanding population and an increasing rate of development highlighted the critical need for an accurate floodplain delineation. The City of Billings recognized that only with an accurate floodplain map could they effectively manage the risk to their citizens.

Funding for the mapping project finally came through in 2005, and PBS&J was hired to delineate the 100-year floodplain for the area west of Billings. It became clear almost immediately, however, that the complexity of this mapping exercise would require an innovative modeling approach. Conventional onedimensional flow modeling would not accurately delineate the floodplain in the project area.

For the majority of floodplain studies, one-dimensional hydraulic modeling is appropriate and provides a clear picture of the area impacted by a flooding event. Typical studies involve determining the peak flow for a specific reach of stream, computing the water surface elevation for each cross section, and using topographic maps to delineate the floodplain. Most often, HEC-RAC – a one dimensional, open-channel flow, hydraulic computer model – is issued to determine water surface elevations and is the industry standard for floodplain delineation projects. Developed by the US Army Corps of Engineers, this free tool is accepted by FEMA and is appropriate for most situations.

The floodplain in the vicinity of Billings, however, is not a typical situation. First, the prevalence of manmade structures like ditches, bridges, and canals complicated the hydraulic model. Second, Billings lies on extremely flat terrain without distinct channels, making it very challenging to determine where or how far flood water will spread. This lack of a distinct natural flow pattern and the frequent splitting of flows by manmade structures eliminated HEC-RAS as an option. It was immediately clear to the PBS&J engineers and hydrologists that HEC-RAS would not be capable of accurately characterizing the reality of the situation.

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