HR: 08:45h
AN: H51M-04    [Abstracts]
TI: Calculation of flow, sediment transport, and channel erosion in burned watersheds prone to debris flows
AU: * Kean, J W
EM: jwkean@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, Denver, CO 80225,
AU: Gartner, J E
EM: jegartner@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, Denver, CO 80225,
AB: The loss of vegetation by fire leaves recently burned watersheds vulnerable to flash flooding and debris flows. Quantifying the flow and sediment transport conditions that lead to the formation of debris flows in these watersheds is essential for improving predictions of the likely occurrence of these events. Unfortunately, conventional water and sediment gaging methods are not well suited for monitoring recently burned watersheds because these methods require measurements that are difficult or impossible to obtain owing to the rapid and potentially dangerous response of burned basins to rainfall. In an effort to provide accurate flow and sediment transport data in burned basins for use in warning systems and for testing models of debris-flow initiation, a fluid- mechanically based method for defining water and sediment stage-discharge relations has been adapted for use in the steep mountain channels characteristic of debris-flow-prone watersheds. The approach uses a fully predictive flow model to convert non-contact measurements of flow stage into estimates of water and sediment discharge. The model does not use empirical roughness coefficients, but rather determines channel roughness directly from detailed field measurements of the topography and the physical roughness elements on the bed and banks of the channel. The model also determines the component of boundary shear stress appropriate for sediment transport calculations on the entire wetted area of the surveyed reach. This calculation gives the model the unique capability of predicting vertical and lateral channel erosion during an event, which is an important component because channels are usually the largest source of sediment exported from burned basins. Completed field tests of the method in four steep mountain channels show the theoretical stage-water discharge relations produced by the model are in excellent agreement with direct measurements of discharge. Additional testing in progress is focused on (1) evaluating model predictions of channel erosion and (2) continued verification of the flow component of the model using surface velocity measurements obtained through video imagery. Initial results from this phase of testing show the model adequately reproduces the reach-averaged cross-sectional geometry of a gully formed during a flash flood on a burned hillslope.
DE: 1810 Debris flow and landslides
DE: 1815 Erosion
DE: 1821 Floods
DE: 1860 Streamflow
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting