DISCHARGE ESTIMATION IN A CHANNEL REACH RECEIVING SIGNIFICANT LATERAL INFLOW USING VPMS AND RCM METHODS

Abstract

The conventional approach of discharge estimation involves the use of the a-priori developed rating curve at a gauging site, the development of which requires the use of measured river flow, against the corresponding observed water level or flow depth. This approach of developing rating curve often proves to be costly and tedious task. To circumvent this approach, Perumal et al. in 2007 and 2010 applied the Variable Parameter Muskingum Stage (VPMS) routing technique at those sites where only the stage measurements and channel characteristics are known. However, the approach did not involve the contribution of lateral flow from the intervening catchment of the considered river reach. In this study, an attempt is made to account for lateral flow contribution along the channel reach while routing using the VPMS method. Silvia et al. in 2017 applied the VPMS method accounting for lateral flow on a 50 km stretch of Tiber River in Central Italy and obtained reasonably acceptable results. This study is an extension of the same method with the only consideration of few constant wave travel times instead of using the travel times which were varying at every routing time interval. The appropriateness of using few wave travel times in the proposed method is also checked by conducting numerical experiments of its application by routing using the proposed method in hypothetical prismatic channel reaches. The routing solutions are compared with the benchmark solutions obtained using the well-known HEC-RAS model by routing the same input hydrograph in the same hypothetical channel reach considering the same lateral flow. Subsequently, both the approaches are compared for their application for the same flood events as used by Silvia et al. in 2017. The Rating Curve Model(RCM) given by Moramarco et al. in 2005 is another technique to obtain the discharge information at a section where only the stage is monitored and discharge is recorded at an upstream section, and which also caters for the intervening catchment lateral inflow in the considered river reach. In this study, the application of the RCM model is tested for the same hypothetical channel section as is being used for testing the VPMS method. The RCM model used in this study involves the use of two ranges of wave travel time; the travel time being constant for the rising limb and varying with time in the recession limb. The method is also applied for different flood events of the Tiber River in a 50 km reach between the Santa Lucia and Ponte Felcino gauging stations and the results v obtained are compared with those obtained from the VPMS method. Finally, a comparative assessment is made for the applicability of both the methods in different flow situations. Overall, this study reveals that the modified VPMS method considering a limited number of travel times while routing a flood wave scores better than the modified RCM method.