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PURPOSE AND SCOPE The purpose of this study was to identify and characterize the potential post.fire sediment yield rates from drainages in the study area. A total of 93 separate watersheds were evaluated, Another purpose of this study is to prov ide a m,ans for rapid response to post.fire emergency mitigatio" through use of a computerized Geographic Information System (GIS) database. The database includes information, ne'~ed to model the post-fire sediment yield potential fiom a single to... . watershed or series of watersheds depending on the ex,tent of the burn. . ~ ~ METHODS USED A model, Factors Effecting Sediment Yield in the Pacific Southwest, developed by the Pacific Southwest Inter. Agency Committee (PSlAC) 1968 (revised 1991), was used to estimate the sediment yield from 93 watersheds in the study area. The PSIAC method consists of rating a watershed on the basis of nine factors shown in Table I. The possible range of values for each factor is also listed. Table 1. Factors and Rating Ranges used in lhe Pacific Southwest Inter-Agency Committee methodfor estimating sediment yields. (revised, 1991). Factor Ratio!! RI!!!J~ Surface Geology 0-5 Soils 0-10 Climate 0-10 Runoff 0-10 Topography 0-20 Ground Cover .10-10 Land Use -10 - 10 Upland Erosion 0.25 0-25 Channel Erosion and Sediment Transport Elements Considered Rock type, Weathering, Hardness, Fracturing Texture, Salinity, Aggregation, Shrink-swell, Caliche, Organics, Rockiness. Storm frequency, Intensity, Duration, Snow, Freeze. thaw. Volume per unit area, Peak flow per unit area. Steepness of upland slopes. Relief, Fan & Floodplain Development. Vegetation, Litter, Rocks, Understory Development, Management Quality, Grazing Intensity, Logging, Roads, Fire, Roads, Rill & Gully Development Bank & Bed Erosion, Flow depths, Active Headcuts Channel Vegetation / Cover Each watershed was subdivided according 1:0 soil map units delineated by the Forest Service. Each soil map unit was rated by the factors listed in Table 1. Each soil unit received a numerical rating for the present condition, a Low-Intensity Bum, and a High-Intensity Burn. This rating corresponds to the upland sediment yield from that watershed in units of tons per acre, assuming an average sediment density of 90 pounds per cubic foot. ne post-fll'e sediment yield ratings assume a bum OV'::T the entire watershed. The sediment yield factor rating .:beets for this study are contained in Appendix C and available upon request from the NRCS State Office in Salt Lake City, Utah. Sediment yields were calculated at the ';""yon outlet area. Routing of the sediment through the fan lIrea below the apex involves detailed hydrology, !,edimentation, topography and stream mechanics information. 170 The interfluve areas (small triangle-shaped areas between main watersheds) generally do not have a single discrete drainage, so transport of sediment from these areas is generally by overland flow processes without a defined point of impact. These areas involve very small drainages and sediment volume and were not considered in this report. Previously mapped debris flows, debris slides, landslides and other slope failures in each watershed are considered with the sediment yield evaluation. This determined which watershed has a higher relative hazard rating if a wildfire occurred. Partly-detached landslides identified in 1983 were sediment sources for debris flows in 1984 in Utah (Wieczorek and others, 1989).