Sediment transport modelling of rivers Bharathapuzha & Periyar using HEC- RAS

Abstract

Sediment transport has great importance in flood monitoring as flood control schemes mainly depend upon the peak flood levels. The flood levels may be changed in the same discharge and sometimes the pattern of the flow of natural channels changes due to the siltation and the flood pattern of the region can be changed. The 244 Km long river Periyar is also known as the lifeline of the Indian State Kerala. The Bharathapuzha and the Periyar are the major rivers in Kerala much prone to morphological changes. The river Bharathapuzha also known as the Nila is the second-longest river in Kerala and originated from the Anamalai hills. The major share of the industries in Kerala is situated along the banks of these rivers and human settlement is more along these river banks. It is observed that the morphology of these rivers was greatly affected by the 2018 Kerala flood and the flood pattern of the region along the banks has been changed. Also, the flow-carrying capacity of the river Periyar has been greatly affected by the changes in the morphological alterations. River Bharathapuzha is an interstate river between the states of Kerala and Tamilnadu. Due to the sedimentation and erosion, the morphology of the river Bharathapuzha has changed a lot in recent times and it affected the flow-carrying capacity of the river. During the lean season, it has been observed that the flow in the river is intermittent and dry at a lot of stretches. Hence it is decided to conduct sediment modelling using HEC RAS 6 for studying the changes in the morphology of the river Periyar and Bharathapuzha. The data has been collected from the GDSQ sites of the Central Water Commission at Neeleeswaram (River Periyar) and Mankara (River Bharathapuzha). At both of these stations, the gauge, discharge, sediment, water quality and meteorological observations are conducted throughout the year daily and the site Neeleeswaram is a full climatic station. The flow and sediment data have been collected from 1971 to 2021. Initially, 1-D sediment modelling has been conducted on the rivers Periyar & Bharathapuzha at the stretches near the sites Neeleeswaram & Mankara. The geometry has been created using the actual - observed cross-sections that have been received from the CWC. Quasi-unsteady flow approach has been used for the modelling. The flow series created from the discharge data received from the CWC has been used as the U/S boundary condition and the friction slope calculated from the actual observation at the CWC site was used as the downstream boundary condition of normal depth. The initial conditions for the sediment data were provided from the bed gradation samples from the CWC sites and the boundary condition for the sediment data was given as the sediment time series data collected from the CWC site Neeleeswaram & Mankara. After the successful running of the model, the output of the sediment simulation has been analysed and significant erosion and sedimentation have been observed in the different sections of the river stretch at Periyar. While comparing the results of the River Bharathpuzha model, It is identified as the river was not much proned to the bed erosion.