Optimum Rainfall Interval and Manning’s Roughness Coefficient for Runoff Simulation

To simulate a runoff hydrograph from an overland plane or from a catchment that is subject to natural rainfall, the use of a computer model is essential. To develop such a model, among other parameters, the modeler is required to specify rainfall interval, rainfall data for that interval, and Manning’s roughness coefficient (Manning’s $n$ ) of the plane or catchment surface. To evaluate Manning’s $n$ for runoff simulation, it is common to use the hydrograph fitting technique. With this technique, the simulated hydrographs are actually dependent on the rainfall data and, hence, the rainfall interval. The Manning’s $n$ is, therefore, also dependent on the rainfall interval. In this technical note, using rainfall and runoff data over a concrete plane, a concrete catchment, and an asphalt plane, the optimum rainfall interval and the corresponding optimum Manning’s $n$ have been evaluated. Five rainfall intervals were used in the evaluation, which ranged from $0.25\min \text{to}4.0\min$ . The evaluation shows that: (1) for cases where the rainfall interval is shorter than the time of concentration, the effect of rainfall interval on the optimum Manning’s $n$ is small in that they share a common optimum Manning’s $n$ . The evaluated optimum Manning’s $n$ are all consistent with the published recommended values. (2) For cases where the rainfall interval is longer than the time of concentration, the evaluated optimum Manning’s $n$ are larger than those for cases where the rainfall interval is shorter than the time of concentration. (3) A comparison of simulated hydrographs with observed hydrographs shows that the simulated hydrographs are significantly closer to the observed hydrographs for cases where the rainfall interval is shorter than the time of concentration. Further, for the cases where the rainfall interval is equal to or longer than the time of concentration, there is a tendency for the simulated hydrographs to reach equilibrium, which never occurs in the observed hydrographs. (4) As a balance between the extra effort and space required to collect and store rainfall data of a shorter interval and the goodness of fit between the simulated and observed hydrographs, a rainfall interval equal to about half of the time of concentration is considered optimum.