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FLOOD10504
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Last modified
1/29/2010 10:12:11 AM
Creation date
10/30/2007 10:56:37 AM
Metadata
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Template:
Floodplain Documents
County
Garfield
La Plata
Community
Glenwood Springs, Durango
Basin
Colorado Mainstem
Title
Glenwood Springs / Durango Wildfire Documents
Floodplain - Doc Type
Correspondence
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<br />.. <br /> <br />A technical review ofFLO-2D as a predictive tool for assessing post-fire debris flow <br />hazards in the Coal Seam and Missionary Ridge fire areas. <br />J. Andrew Gleason <br />Colorado Geologic Survey <br /> <br />Introduction <br />The debris flow hazards in areas subjected to wild fires are well documented in <br />Colorado (Cannon et. ai, 2003). Assessing the actual amount of debris associated with <br />these hazards is problematic. This study reviews the debris flow portion of the USGS <br />Administrative Report, POST -FIRE HYDROLOGIC HAZARDS STUDY FOR THE <br />2002 HAYMAN, COAL SEAM, AND MISSIONARY RIDGE WILDFIRES, <br />COLORADO, July 11, 2003 by M.E. Smith and others. Specifically this review <br />addresses phase 2 of the USGS report which utilizes FLO-2D, a commercial hydrologic <br />modeling program, in order to asses the debris flow hazard in the Coal Seam and <br />Missionary Ridge fire areas during a 100 year storm event. <br />FLO-2D is a physical process model that distributes a flood hydrograph over a <br />system of square grid elements. The model uses an input hydro graph, volumetric <br />sediment concentration, and existing topography to route a debris flow from the <br />originating tributary to the depositional zone. FLO-2D numerically routes hyper- <br />concentrated sediment flows as a fluid continuum while predicting areas of inundation, <br />change in bed elevations and maximum debris flow depths (O'Brien, 2002). FLO-2D is <br />on FEMA's list of approved hydraulic models and was used for the USGS study at the <br />request of FEMA. <br />The USGS ran simulations of debris flow events for a 100 year stonn for 26 <br />basins in the Coal Seam fire area and for 25 basins in the Missionary Ridge fire area. A <br />sensitivity analysis was conducted on the flood simulations for each area and was found <br />to be robust (Friedel, pers. com. 2003). This analysis gave confidence that the inflow <br />hydro graphs used for the debris flow simulations were accurate to an acceptable level. <br />This is relevant because the inflow hydrograph is the parameter with the greatest effect <br />on the outflow hydro graph in FLO-2D (O'Brien, 2002). <br />The FLO-2D model requires a representation of the potential flow-surface <br />elevation in a square grid format. A 10 meter DEM was used for the basins in the study. <br />The 100-year, I-hour storm was used to model debris flows. For the Coal Seam area, the <br />100-year, I-hour rainfall was 1.64 inches. For the Missionary Ridge area, the 100-year, 1- <br />hour rainfall was 1.77 inches (Miller and others, 1973). The inputs for the debris flow <br />simulations used laminar flow resistance that was calculated based on the Manning's n <br />value surface roughness for the channels and floodplains. <br />FLO-2D routes the movement of debris flows based on the Bingham plastic <br />model which combines the yield stress and the viscous stress components to define the <br />rheological behavior of debris flows. The yield stress and viscosity are computed as a <br />function of sediment concentration. The volumetric sediment concentration used in the <br />FLO-2D simulations by the USGS was 20-43% based on a literature review. The authors <br />ofthe USGS study are well aware ofthe limitations of the model for predicting debris <br />flow run out distances and address the problems associated with varying sediment <br />concentrations and the effect on viscosity. <br />
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