The unusual channel resistance of the Texas Hill Country and its effect on flood flow predictions

摘要

The importance of hydraulic roughness estimates in modeling river flow cannot be overstated. Spatial and temporal variation of flow resistance can have a dramatic effect on the prediction of floodplain width, depth, and other hydraulic properties. The Texas Hill Country, a physiographic region of uplifted and differentially eroded limestone in south-central Texas, is the site of considerably heterogeneous fluvial forms and processes. Predictions of the spatial extent of floods in this area are difficult given this topographic and fluvial heterogeneity. In this report, we present a new method to extract spatial maps of hydraulic roughness using channel geometry and newly accessible USGS National Water Information System (NWIS) data. We use these data to construct maps of hydraulic roughness at different discharges and to test roughness prediction methods suggested by previous authors. Our method for extracting roughness values is general and should be applicable in most areas of the United States. The patterns of hydraulic resistance around the Texas Hill Country and through its eastern boundary, the Balcones Escarpment, are spatially diverse, ranging from n congruent to 0.016 to 0.213, and no simple spatial pattern of roughness exists. The most unexpected finding is the high values of Manning's roughness value (n) in most areas of the Hill Country, with n values often two or three times greater (n congruent to 0.04-0.06) than what would be expected using common n-estimation techniques (n congruent to 0.01-0.02). This simple finding is of practical importance because the too-low values of Manning's roughness are almost certainly translated into floodplain maps as too narrowly delineated floodplains.