Flow Modelling in River Confluence using HEC-RAS2D
I Gede Tunas (a), Rudi Herman (a), Yassir Arafat (a)

a) Department of Civil Engineering, Universitas Tadulako, Jalan Soekarno-Hatta Km. 9 Palu, Central Sulawesi, 94117, Indonesia

Abstract

The flow in river confluence is a very interesting phenomenon to study. The flow characteristics are greatly influenced by various factors: the discharge from both rivers, the angle of river confluence, the elevation of the bed of both rivers at the confluence and the concentration of sediment. These factors can affect the flow and configuration of the river bed. This paper aims to study the flow characteristics at the river confluence of Sombe - Lewara River and Palu River, Central Sulawesi Indonesia. The study was performed using HEC-RAS2D Model based on DEM data from LiDAR coverage as the source of model geometry data. Other important data as boundary conditions consist of design discharge at the upstream boundary and normal depth at the downstream boundary. Manning^s roughness coefficient and infiltration rate of the domain are expressed as land cover layer and infiltration layer in RAS Mapper. Previously, calibration was applied to determine the optimal Manning^s roughness coefficient by evaluating the deviation of observed and measured water levels at control points. The results of the study are presented in three parameters: flow depth, velocity and water surface elevation. It is seen that the flow velocity weakens at the flow confluence line and gradually normalizes at a distance of about 30 meters downstream. The maximum velocity on the left and right sides of the flow confluence line reaches 3.5 m/sec and 5.7 m/sec respectively depending on the discharge in each river branch. The flow depth on the right side is greater and the left side is in accordance with the discharge and size of the river cross-section. The conclusion of this study is that the flow velocity and depth are predominantly influenced by the discharge and cross-section geometry.

Keywords: flow velocity- water depth- water surface elevation, RAS Mapper

Topic: Hydrology