A SUITE OF METHODS FOR FLOOD DISCHARGE ESTIMATION ONLY USING AT-SITE STAGE DATA

Abstract

In this study, various discharge estimation methods are tested for their efficacy to reproduce the observed site discharge using at-site cross-section details and the stage hydrograph data recorded at this section. These methods are the Jones formula, the Fread method, the modified Fread method, the refined Jones formula, and the routing methods, which are the VPMS method and the HEC-RAS model. The routing methods were used after assuming an imaginary prismatic cross-section reach, which is characterised by a cross-section same as that of the input section where the stage-hydrograph is recorded. The results are given as simulated discharge hydrograph corresponding to the recorded stage-hydrograph for which the discharge hydrograph is to be estimated, and the comparison of the estimated discharge hydrographs by the considered methods are made using the Nash-Sutcliffe Efficiency (NSE) and the coefficients of correlation criteria. In most of the cases, the VPMS and the Jones formula produced better efficiency comparatively than the Fread method, the modified Fread method, the refined Jones formula and the HEC-RAS routing based solutions. The problem of oscillatory discharge hydrograph estimation by the solution using the refined Jones formula for smaller time-interval data was solved using larger time-interval stage data (of 30 minutes), which resulted in an increased efficiency of the refined Jones formula. However, the improvement may be considered still not significant enough to yield a solution better than that of the Jones formula. The HEC-RAS produced the best discharge hydrograph reproduction for the GH 20 site which is characterized by a mild slope (𝑆0≈0.0002). Most of the discharge estimation methods showed a higher efficiency in almost all the cases, which is as high as >95%. Surprisingly the Jones formula showed a high NSE above 99% in almost all the sites, except for the GH 20 site. The comparison with Petersen-Øverleir (2006) model showed that the improvement brought out by this method in estimating discharge hydrograph is not significant over that estimated by the Jones formula, as the original Jones formula itself already gives a higher NSE estimate > 0.99. Though the NSE estimates of the simulated discharge hydrographs at all the considered gauging sites are very high using the original Jones formula, but it failed to produce a good reproduction of the observed discharge hydrograph for the GH 20 site. Therefore, from the limited study of evaluating the efficacy of the considered at-site stage-hydrograph based discharge estimations methods in reproducing the observed discharge hydrographs at four gauging sites of Chattahoochee River and that at a gauging site of Clear Creek River, both in US, it can be inferred that the original Jones formula serves the purpose of reproducing the observed discharge hydrographs at these sites (except that of GH20) closely. v But for gauging sites characterized by mild bed slope, the routing methods such as the VPMS method or HEC-RAS model could be used for discharge hydrograph estimation corresponding to the recorded stage-hydrograph