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Roughness Coefficient Modifications and Synthetic Rating Curves for the HAND Method

Authors: Damilola Eyelade*, University of California, Santa Barbara, J Michael Johnson, University of California, Santa Barbara
Topics: Water Resources and Hydrology, Remote Sensing, Hazards, Risks, and Disasters
Keywords: Water modelling, Flooding, Risk mitigation, hazards, Rating Curves, DEM
Session Type: Paper
Presentation File: No File Uploaded

The Height Above Nearest Drainage (HAND) methodology and associated products makes it possible to estimate synthetic rating curves (SRCs) using Manning’s equation to estimate water depth for a given discharge, cross sectional area, and channel characteristics including roughness, length and slope. The HAND SRC implementation underpinning operational USA flood forecasting uses a single roughness coefficient for all reaches and water depths, regardless of local hydraulic properties. This typically results in underestimation of depth in lower order reaches and overestimation in higher order stream reaches. We show that a decomposed roughness coefficient including an in channel and over-bank components can partially mitigate these errors. This generalized methodology represents a different approach compared to calibrating the roughness coefficient for specific reaches. A raster dataset generated with the aid of empirical relationships between hydraulic properties results in the demarcation of in channel and over-bank geometry properties. This then serves as input for a composite channel roughness equation that provides unique roughness coefficients for each stage and location along a reach. Subsequently, the ability of this method to adjust HAND SRCs to match actual rating curves from USGS gage stations and flood extents was then evaluated.

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