Prediction of a design flood-discharge that caused sedimentation in the river mouth of Batang Anai

Civil Engineering, University of Andalas, Padang

^* Corresponding author: mas_mera@eng.unand.ac.id

Abstract

Batang Anai is a river that crosses through four administrative regions in the West Sumatra Province with a catchment area of about 498 km². The upstream is a steep topography because it is located in west part of the Marapi mount. While the middle and lower reaches are mild since in the lowlands. The flow empties into the Indian Ocean. These conditions make this river meandering in the middle and lower reaches. The river mouth is influenced by the tide in which resists sedimentation toward the ocean. As a result, the capacity of the river cross-section decreases which eventually causes flooding. In 2013, the lower reach was dredged. In 2015, however, the river mouth was covered back by sedimentation by two-meter height in some points. Bathymetry was measured before and after dredging. This study is conducted to determine a design flood-discharge that caused sedimentation based on the 2015 bathymetry data by simulation using the SMS software, i.e. the RMA2 module for modelling hydrodynamics and the SED2D module for modelling sedimentation. The attention is paid to see both flow patterns and sedimentation profiles. The model scenarios consider by either including or excluding the Batang Kandis flow. Batang Kandis is a tributary of Batang Anai which empties into near its river mouth. The design flood-discharges for simulation are varied according to the return periods of 2, 5, 10, 25 and 50 years. The bathymetry data used for simulation are the 2013 one after dredging. All the simulation results are then compared to the 2015 bathymetry data. The hydrodynamic model in which the Batang Kandis flow included gives velocities close to those from the field for all return periods. The results of the sediment model using the flood flow with a 5-year return period, that’s Batang Anai is 1060 m³/s and Batang Kandis is 268.5 m³/s, show that the sedimentation profiles are closed to the 2015 bathymetric measurement data.