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<br />* <br /> <br />The following is an example of the limitations of the model and suggested further <br />study by the authors which I address in this report; <br />"Minimum and maximum likelihood areas and depths are not presented in the model <br />output and there is large, unquantified variability in the expected outcome of the 100- <br />year storm. Another limitation of the analyses is that estimated inundation areas and <br />depths from the model output are not verifiable. No post-fire data exist at this time <br />against which to compare the FLO-2D simulations; however, future debris flows from <br />areas within the Missionary Ridge and Coal Seam burn areas where precipitation gages <br />are operating will facilitate verification" (USGS Admin. Report, 2003, p. 39). <br /> <br />Methodology and results for comparison of USGS modeled debris flow depths and <br />measured debris flow events. <br /> <br />Data were collected on basins that were modeled by the USGS using FLO-2D <br />within the Coal Seam and Missionary Ridge Fire areas and that were known to have post- <br />fire debris flow events. Debris flow depths were collected by measuring the height of <br />mud flow lines on trees and boulders from floodplains or from the bottom of channels. <br />Depths were also measured from the bottom of charmels to the height of debris flow <br />levees adjacent to channels when levees were present. Full depths were measured from <br />the bottom of channels even when significant channel incision was apparent. Locations <br />for each measured debris flow depth point were correlated to the mapped locations from <br />the FLO-2D simulations. The measured depth was then compared to the modeled depth <br />for each measurement location. Four data sets were compiled, two for each fire area. The <br />first data set for each fire area consists of numerous debris flow events which occurred <br />during the fall of 2002. Debris flow depth data from two unique storm events were also <br />collected for each area. <br /> <br />Missionary Ridge Fire; Debris Flow Events- Fall 2002 <br /> <br />Debris flow depth data (n=44) were collected on 13 basins within the Missionary <br />Ridge fire area. The depths in Figure 1 were measured from the events of August and <br />September 2002 which had the largest amounts of rainfall and largest recorded debris <br />flow events within the fire perimeter (Thurston, 2002). The FLO-2D simulations used <br />inflow hydro graphs based on the 100 year predicted rainfall event, which in the <br />Missionary Ridge area was 1.77 inches of rain in one hour. The debris flow (~vents from <br />August and September 2002 that were compared to the modeled results were 10 year <br />storm events or less, with a maximum average stoml intensity of 1.25 inches per hour <br />(Thurston, 2002). <br />The line on Figure 1 labeled "Measured = Modeled Data" represents the line <br />above which the measured debris flow depths were greater than the depths predicted by <br />FLO-2D. Seven measured debris flow depths in 6 ofthe basins exceeded the calculated <br />depths for a 100 year debris flow event. All but one of the measurement points that <br />exceeded the model predictions were in channels. 'While this may be an und<::r-estimation <br /> <br />o <br />