Abstract

This report examines the rational and modified rational methods using rainfall-runoff datasets developed from Texas watersheds for application to Texas Department of Transportation hydraulic design problems. A unified rational method for Texas (URAT) is proposed as an alternative to the conventional method in current use. URAT substitutes land-use specification, slope influence, and probability adjustments to runoff coefficients by a simple area-weighted coefficient based on functional impervious cover. URAT incorporates the effect of slope and probability adjustments in the specification of the watershed time of equivalence, a characteristic time that makes rational peak discharges and regression equation peak discharges equal at some specified recurrence interval. An extended application (example) is presented to illustrate how to use the URAT method for both peak discharge and hydrograph estimation. Several appendices document various approaches examined, and provide an extensive examination into the runoff coefficient.