Volume-based Runoff Coefficeints for Urban Catchments

The rational method is derived as a special case of kinematic wave flow. The definition of runoff coefficient is expanded from the flow-based ratio into volume-based ratio between the volumes under runoff hydrograph and rainfall hyetograph. The new method presented in this paper provides a dimensionless approach to theoretically derive the runoff coefficients for both the conventional two-flow drainage and low-impact cascading drainage systems. The key factors for determining runoff coefficients were identified as the ratios of depression loss and infiltration amount to the design rainfall depth. Although this method provides theoretical values for runoff coefficients, field data or computer simulation are always helpful for further validation.