Use of Catchment Physiographic Factors in Selection of Design Storm and its Effect on Floods Estimated for Ungauged Catchments

Abstract

Hydraulic designs in river engineering are composed of two main aspects such as flood estimation and channel sizing which are essential for safe flood conveyance. Prediction of runoff water in an ungauged catchment area is vital for various practical applications such as design of drainage structure, flood defenses, flood protection works, inflow forecasting and for catchment management tasks (say water allocation) and climate impact analysis. Almost all hydrologic analyses require one or more time-scale parameters as input. Out of this, time of concentration (tc) is the most frequently utilized time parameter [1]. However, modelers are often confused by the number of estimation methods of tC and often select a method without evaluating and comparing its accuracy with other methods [1,2]. For instance, the design of urban storm water drainage systems using the rational method requires an estimate of tc for selecting the design rainfall intensity from the Intensity-Duration-Frequency curves [3]. A study on modifying available equations on estimation of tc to minimize their bias for any particular region of interest was attempted and the methods such as California, Kirpich, Arizona Department of Transport, NRCS were applied to 72 watersheds and sub-watersheds in Khorasan Razavi province, Iran [4] wherein ranking-based selection was adopted for the best performing method.