FLOOD04102 - Laserfiche WebLink

Home Browse Search

',' The Low-Intensity Burn can greatly effect the hydrologic character of a watershed. A study perfonned by the Davis County Public Works Department showed that deterioration of only 17 percent to 45 percent of the vegetation of a watershed tremendously increases the runoff and concluded that watershed conditions are probably the most important variable in modeling mountain hydrology and effectively planning mitigation of flooding (Williams, 1991). PSlAC estimates of sediment yield from an area impacted by a Low-Intensity burn show an increase of 2 times the Present-Condition yield rate given the criteria listed in Table 3 (Low-Intensity Burn Sediment Yield Model, Map-2). Appendix - A lists the Low-Intensity Burn sediment yield rates for each watershed evaluated in the study. IDGH-INTENSITY BURN SEDIMENT YIELD MODEL Criteria used for modelling the sediment yield potential from a High-Intensity Burn are listed in Table 4. Table 4. High-Intensity Burn Criteria, a, Litter burned, some larger sticks remain b, Organic layer consumed (upper 2" at surface) c. Some stumps would burn leaving a hole d, Water repellent layer fonns in areas with abundant organics and very hot burn temperatures The PSIAC data shows that sediment yield from a High- Intensity burn area should be expected to increase an average of approximalely 6 times the present sediment yield rate (Table 2, Map-3), A table showing the sediment yield data for each watershed is contained in Appendix - A, Table 5 lists the watersheds which show the highest potential for producing upland sediment from rangeland areas after a fire. The relative ranking is based on the sum of the yields modeled for the Present Condition, a Low- Intensity Bum, and a High-Intensity Burn. The ranking considers only the sediment resulting from sheet and rill erosion. . Table 5, Relative hazard ranking of watersheds for sediment yield (I ~ Highest Potential) Rank Wshed. Rank Wshed. 1. 32 II. 52 2. 93 12. 80 3. 34 13. 79 4. 48 14, 88 5. 33 15. 58 6. 24 16. 23 7. 59 17. 64 8. 82 18. 46 9. 92 19. 69 10. 91 20. 43 In California, there have been cases where sediment from sheet and rill erosion was a very small portion of the total sedimenl yield from a burned watershed. The major source of sediment and debris in those cases was dry ravel from steep, degraded, fractured granitic and metamorphic rocks (oral comm., Julia Grimm, 1995), The metamorphics which dominate this study area (Fannington Canyon Complex) are not as weathered and degraded as in California. However, the process of dry ravel will likely contribute significantly to the sediment load due to the steep, rugged nature of the watersheds evaluated in this study. The watersheds which contain the highest percentage area of slope failures and a high sediment yield potential are #32, #48, #59, #69 (Rudd Creek) , #80, #82 and #24. These watersheds warrant close attention in the event of a fire. All but Rudd Creek are unnamed watersheds. Watershed numbering starts in the northern study area, increasing numerically to the south (Map - I). DEBRIS FLOW POTENTIAL The PSIAC sediment yield model does nol evaluate the debris flow potential but can indicate where sediment is available for transport during a runoff event. A USGS study completed by Wieczorek & others (1983) showed that most canyons along the Wasatch Range in Davis County had high to very high potential for sediment transport to the fan areas. With this in mind, any degradation of the upper watersheds can greatly increase the debris flow potential. The nonnal sequence of events leading to catastrophic mudflows or debris flows is: (I) high-intensity stonns on (2) water-repellent, frozen, saturated, or otherwise impenneable soils which yield (3) large volumes of water with (4) high velocities to a channel with (5) abundant sediment available for transport (Forest Service, 1978). Slope-failure acreage was calculated by the GIS software for each watershed in the study area. The watersheds with the largest area of slope failures are listed in Table 6. Any of the canyons listed in Table 6 would have a very high potential for a debris flow following a significant size wildfire in the watershed. The Slope Failure Map for Davis and Weber counties (Lowe, 1989), is a valuable tool in targeting zones for emergency mitigation when a burn consumes vegetation on any of the slope failures. The data from this map is included in the GIS database. A burn can alter the hydrologic characteristics of a slope by decreasing infiltration in some areas while concentrating excess runoff flows into rills and gullies or into disturbed scarp areas. Runoff directed into a scarp area of a previously failed slope may reactivate a portion or all of the slope. The watersheds where this has the highest potential of occuring are listed in Table 6. Table 6, Canyons with largest area of slope failures as of 1989, Measured with GIS software. Canyon Aere Watershed Ward Canyon Centerville Parrish Bair Davis Fannington Sheppard Holbrook Steed Rudd Ricks Taylor 556 479 380 320 243 238 147 144 143 138 118 103 81 78 77 61 72 68 65 83 71 69 74 29 173