Hillslope Effects on the Character of the Geomorphic Instantaneous Unit Hydrograph for a Burned Landscape (Invited)
Abstract
Predicting flash floods from burned landscapes is complicated by the relatively short temporal and spatial scales typical of rainfall and unsteady flow phenomena. In addition, most of the water is derived from the hillslope portion of a landscape and delivered to the valley portion. Understanding how these two portions of a landscape are connected is essential to developing predictive models of post-fire floods and debris flows. Most DEMs are on the scale of 30-m pixels and cannot resolve the short spatial scale of hillslope drainage networks. A 0.1-m DEM was derived from data collected by using a tripod mounted T-LiDAR system with sub-centimeter resolution for a watershed burned by the 2005 Harvard wildfire in Southern California. Rainfall and runoff were measured at three watershed scales (~ 30 m2, 1000 m2, and to 35,000 m2). A detailed study using these data was performed to determine the effects of the hillslope drainage pattern on the characteristics of the geomorphic instantaneous unit hydrograph (GIUH) and the flood wave dispersion. The effect of vegetation on flow velocities is negligible because of selecting a severely burned landscape. The hillslope portion of the landscape can have flow velocities, uh, and flow distances, Lh, which are different from those of the valley portion (uv and Lv). Thus, the hillslopes cause a geomorphic-kinematic dispersion, DGK, which differs from the geomorphic dispersion, DG. GIUHs and the ratio DGK/DG were computed for watersheds with three different scales, assuming a range of physically realistic ratios of r = uv/uh. The ratio DGK/DG had a value < 1 for certain ratios of r that depended upon watershed scale. The value (rmin) of r for which DGK/DG is a minimum depended primarily on the ratio [var(Lv)/var(Lh)] /[E(Lv)/E(Lh)], where E( ) is the expected value. The skewness of the GIUHs varied from negative to positive depending on r, and the dependence of skewness and DGK/DG on r are connected. Thus, the form of the flood hydrographs is highly dependent on the statistics (such as mean, variance, and skewness) of the flow paths in both the hillslope and channel portion of a watershed, and explain why flood hydrographs are not simply a function of watershed area.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMEP22A..02M
- Keywords:
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- 1804 HYDROLOGY / Catchment;
- 1826 HYDROLOGY / Geomorphology: hillslope;
- 1839 HYDROLOGY / Hydrologic scaling;
- 1874 HYDROLOGY / Ungaged basins