Effective Discharge Calculation: A Practical Guide

An alluvial river adjusts the dimensions of its channel to the wide range of flows that mobilize its boundary sediments. However, in man)' rivers it has been demonstrated that a single representative discharge can be used to determine a stable channel geometry. The use of a single representative discharge is the foundation of regime and hydraulic geometry theories for detennining morphological characteristics of alluvial channels and rivers. This representative discharge has been given several names by different researchers including dominant discharge, channel forming discharge, effective discharge, and bankfull discharge. This has led to some confusion. The channel forming discharge and the dominant discharge are equivalent and are defined as a theoretical discharge that if maintained indefinitely would result in the same channel geometry as the existing channel subiect to the natural range of flow events. Although conceptually attractive, this definition is not necessarily physically feasible, because bank line vegetation, bank stability and even the bed configuration would be different in a natural stream than in a constant discharge stream. Channel-forming discharge concepts are applicable to stable stream systems, i.e., streams that are neither aggrading or degrading. Channel-forming discharge can be estimated using one of three prescribed methodologies. One such deterministic discharge is the bankfull discharge, which is the discharge that fills the channel to the top of its banks. Another deterministic discharge used to represent the channel-forming discharge is a specified recurrence interval discharge, typically between the mean annual and five-year peak. This report focuses on a third approach to determine the channel-forming discharge, known as the effective discharge. The effective discharge transports the largest fraction of the bed material load. Because of this, the effective discharge can be a good estimator for channel-forming discharge.