eNews July 2017

Final Report on Debris Flow Potential Following Wildfires in Watershed Near Santa Fe Now Available (continued)

Previous studies have established models based on statistical data from past forest basin burn events that estimate the debris-flow probability and intensity as a function of areal burn extent, soil properties, basin morphology, and rainfall from different types of short-duration storms. Models of this kind have been adapted for use in this study by making reasonable assumptions and estimates regarding these significant physical properties. In particular, a series of GIS-produced maps and accompanying data have been produced that show the estimated probability and volume of post-fire debris flows for SFMW, given a 2-, 5-, and 10-year, 30-minute rainfall event following a moderate to high severity wildfire. For their study, they have also hypothesized that watershed basins with slopes greater than 30% are to be identified as the potential debris flow zones.

As expected, this study confirms that increases in storm intensity increases yield in potential debris flow volumes. The highest values are located in the main stream channels, most notably the Santa Fe River. As side tributaries converge and gather further debris, the main channel could carry tens of thousands of cubic meters of debris into the McClure Reservoir, which supplies water for Santa Fe. For a 10-year recurrence storm, the sum of all sub-basin sediment volumes was estimated to be in excess of 100,000 cubic meters. As a result, the authors are confident that if a severe wildfire were to occur in the SFMW, debris flows would have a high probability of occurrence pending a large storm event. The negative impact on the freshwater supply for the city would be significant, and would also entail a considerable expense for the subsequent required dredging of the McClure Reservoir. The results of this study may help provide city and forest managers with an opportunity to prepare for and mitigate potential issues associated with debris flows.