Service Description: City of Boulder Estimation of Maximum Possible Debris Dam along Boulder Creek and Fourmile Canyon Creek
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Description: City of Boulder staff analyzed areas of potential debris dams that would create a worst case scenario. The criteria used for these areas included a wide 100 yr floodplain and the presence of potential choke points which included sharp bends in the creek or bridge crossings where debris is most likely to collect. Eight potential debris dam areas along Boulder Creek and four along Fourmile Canyon Creek were identified. It is important to note that additional debris dam areas potentially exist in both drainageways.
Next, staff used GIS to calculate the volume, surface area, and maximum depth of ponding behind the debris dam (an example of which is shown in the illustration below). Maximum depth of ponding was calculated as the depth of water directly upstream of the debris dam when flows would begin to spill around the debris dam.
The findings were input into a spreadsheet from the Colorado Division of Water Resources, Dam Safety Branch, which uses the Froehlich 2008 methodology to determine the peak flows in cubic feet per second of a complete dam failure. In addition, City staff hired Wright Water Engineering (WWE) to cross-check the methodology and results
of the analysis. WWE also compared the results of the Froehlich 2008 methodology with the McDonald & Langridge-Monopolis, 1984. No major errors were found in WWE’s cross-check of the analysis.
Copyright Text: Chris Trice, City of Boulder Public Works/Utilities
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