An improved discharge prediction method for overbank flows

Two stage channels exist in many natural rivers, and in certain cases multi-stage channels are deliberately formed in order to increase conveyance capacity in times of floods. To predict the stage-discharge relationship, traditional channel division methods are still widely used in practice because of their simplicity. However it is well-known that these methods either under-estimate or over-estimate the channel discharge. More recently a so-called ‘weighted divided channel method’(WDCM) has been proposed by Lambert & Myers (1998) that can improve the estimation of both zonal discharge and the overall discharge. The values of weighting factor (ζ) recommended for use in the WDCM for the Flood Channel Facility (FCF) data with smooth floodplains and rough floodplains are 0.5 and 0.2 respectively. Atabay & Knight (2006) show that the weighting factor (ζ) is not well established, varying with both channel shape and roughness distribution, besides not being physically based. Despite this, the WDCM is a promising approach that is simple to apply. In this paper, a modified WDCM method is presented that gives better predictions for homogeneously roughened channels. In addition, a new method, named the ‘horizontal single method’(HSM) is recommended, based on empirical formulae, that gives good estimates of both zonal and total discharge for homogeneously and heterogeneously roughened compound channels.