Flood is occurring more frequently in Kuching nowadays due to the impact of climate change and rapid urbanization. The only discharge outlet for Sarawak River Basin currently is at Kuching Barrage and Shiplock. Sarawak State Government had decided to build Matang Bypass Channel from Sarawak River’s “Oxbow” to Batang Salak River for mitigating the flooding issues within Sarawak River Basin. Matang Bypass Channel had a bottom width of 250 m, 500 m reserve width and 8 Km in length. Flow behaviour with two discharge outlets during low tides are unknown yet. Therefore, this research is carried out to study Sarawak River flow behaviour after construction of Matang Bypass Channel using InfoWorks River Simulation (RS). Rainfall data used is January 2018. Four scenarios investigated are 1) Open two gates at Matang Bypass Channel opens and all gates at Kuching Barrage, 2) Open all gates at Matang Bypass Channel and Kuching Barrage, 3) Open gates at Matang Bypass Channel, but close all gates at Kuching Barrage, 4) Close all gates at Matang Bypass Channel, but open all gates at Kuching Barrage. Results revealed that when water gates are opened, sea water has the potential to backflow into Sarawak River basin through Kuching Barrage since sea level at Kuching Barrage discharge outlet is always 0.5 m higher than Matang Bypass Channel discharge outlet. When the gates at Matang Bypass Channel are fully opened and Kuching Barrage are closed, Kuching Barrage will retain the excess water and the river water will only be discharged into ocean through Matang Bypass Channel. In contrast, as the gates at Matang Bypass Channel are closed and at Kuching Barrage are fully opened, Matang Bypass Channel will store the excess water and river water will be discharged through Kuching Barrage alone.
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