Abstract
There are a plethora of analytical approaches to account for the effect of unsteady flow (a.k.a. hysteretic behavior) on the conventionally-built steady rating curves (RCs) used to continuously estimate discharges in open channel flow. One of the most complete correction methods is Fread’s method (Fread, 1975) which is based on fully dynamic one-dimensional wave equation. Proposed herein is a modified Fread’s method which is adjusted to account for the actual geometry of the cross section. This method improves the accuracy associated with the estimation of conveyance factor and energy slope, so it is particularly useful for small to mid-size streams/rivers where the original method’s assumption does not properly hold. The modified Fread’s method is tested for the sites in Clear Creek (Iowa, USA) and Ebro River (Spain) to illustrate the significance of its improvement in discharge estimation. While the degree of improvement is apparent for the conveyance factor because the hydraulic depth is replaced by hydraulic radius, that for the energy slope term specifically depends on the site and event conditions.