Use of the rational and modified rational methods for TxDOT hydraulic design

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.<br><br>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.<br><br>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