FLOOD06861 - Laserfiche WebLink

Home Browse Search

sizes, and watershed-rehabilitation measures, Onsite data also were collected for subsequent storms that produced runoff, Monitoring is planned until water and sediment runoff in the bumed . area has retumed to near-normal. For bumed and unbumed basins, data also were collected to: a,) estimate hillslope erosion; b,) estimate channel agradation and degradation; c,) approximate volumes of channel and alluvial-fan deposits; and d,) identify past fire and flood deposit sequences in the alluvial stratigraphy, This monitoring complements sediment studies being conducted by other USGS and USFS scientists and helps validate channel change estimates from aerial photographs taken before and after the fire, Results and Discussion Most efforts have concentrated on monitoring numerous floods and providing preliminary information for hazard-mitigation and forest managers, Results are presented for the storm and flood of July 12, 1996, the effects of the fire on flooding, and assessing watershed-rehabilitation efforts, July 12, 1996 Storm and Flood Maximum rainfall from bucket-survey data for the July 12, 1996 storm was about 80 mm in an hour in the community of Buffalo Creek and headwaters of Spring Creek (fig, 2), Before additional rainstorms, the extent of fresh rill and gully erosion was compared to rainfall-bucket amounts without gaged rainfall data, Hillslope erosion then was used to estimate rainfall in areas without rainfall data, Hillslopes (bumed or unbumed) with sparse vegetation and less than about 25 mm rain had some sediment movement and minimal rill development (fig, 3a), Hillslopesthat received about 50 mm of rain had rills about 75 mm deep and 50 mm wide (fig, 3b), Hillslopes that received about 75 mm of rain had numerous gullies up to 0,5 m deep and a meter wide (fig, 3c), Numerous gullies up to a meter deep and 3 m wide (fig, 3d) were documented about 5 km south of Buffalo Creek near the headwaters of Sand Draw, Spring Creek, Shinglemill Creek, and Spring Gulch, The gullies were used to infer a maximum 1,hr rainfall amount of at least 115 mm, Rill and gully erosion data were used to graphically display comparisons and to draw an isohyetal map (fig, 2), Rainfall amounts decreased rapidly outside the bumed area and the storm covered about 50 to 75 km2, The rainfall isohyetal pattem estimated from NWS-WSR-88 radar by the National weather Service (written commun., 1996) during the July 12, 1996 storm is also shown on figure 2, The NWS estimated the maximum rainfall was about 80 mm, Henz (1998) analyzed Doppler radar signatures and upper-air observations for the July 12 storm, His approach differs from the NWS radar estimate in that interpretations have been validated with ground-truth rainfall data for a network in the Denver area during about the past 15 years, Henz estimated maximum rainfall of about 130 mm in about an hour with the cell located near the head of Spring Creek with similar isohyetal pattems, but oriented slightly different (fig, 4), Henz's preliminary results had the storm footprint located about 3 km southeast of the present center in Spring Creek (Henz Meteorologic Services, written commun" 1997), However, the geomorphic rainfall estimates indicated a lack of rainfall (fig, 3a) and essentially no runoff in Henz's preliminary estimated area of maximum rainfall. Areas having extreme flood runoff (next section) were not located below Henz's preliminary storm footprint. Henz used our results to better locate the storm footprint (fig, 4), Therefore, it appears that geomorphic techniques provide good estimates of rainfall amounts and very good estimates of spatial variability when compared with Henz's estimates from Doppler radar, Henz's rainfall estimates are considered the most reliable for the July 12, 1996 storm because they are based on, The geomorphic rainfall data also provide valuable information to assess the reliability of radar estimated rainfall, which are used for flash-flood forecasting and other purposes, 4 . .