This study focuses on the impact of climate change on Sarawak River flow due to heavy rainfall and the backflow of sea level. The sea level around Sarawak is expected to rise at 3.6 mm/year. When surface runoff meets seawater backflow, especially during king tides, determining the flood inundation level at low-lying regions along the Sarawak River is critical to prepare appropriate mitigation measures. The hydrodynamic model was developed using Infoworks River Simulation (RS) software. The flow of the Sarawak River is modeled from Git Station to Muara Tebas port. Various data are input into the hydrodynamic model for calibration and validation, including boundary conditions, river networks, ground models, river cross-section, water level, and rainfall data. The simulation was carried out for the years 2050 and 2080. The flood maps were generated to depict flood depth, extent, and submerged areas. In 2020, the backflow of seawater did not overwhelm Kuching City, which is located within the Sarawak River Basin. However, many low-lying locations along Sarawak River are expected to be inundated by 2050, with the flood depth ranging from 1 to 4 m. In 2080, the average increment of flood depth is predicted to be 1 m especially for low-lying areas compared to year 2050. Hence, the relevant authorities must take proactive efforts to manage the growing flood challenges caused by climate change, particularly in low-lying areas.

The Bengoh Dam was constructed to secure reliable raw water supply to Kuching and its surrounding areas until 2030. However, the changes in river morphology along the Sarawak Kiri River after the Bengoh Dam construction remain unknown. This research was conducted to study the impact of Bengoh Dam construction on erosion and sedimentation along the Sarawak Kiri River using InfoWorks River Simulation (RS). The novelty is to identify and predict the short- and long-term geomorphology changes along the Sarawak Kiri River over the course of a year. Two scenarios were created, namely, a) with Bengoh Dam b) without Bengoh dam. The model with the Bengoh Dam scenario will be further investigated for ten years of simulation. The five locations selected for comparison and discussion are Bengoh Dam, Bengoh, Danu, Git Village and Batu Kitang. After the dam was built, sediment transport would be hindered, resulting in riverbed alterations. These changes are influenced by the location and proximity to the Bengoh Dam. Simulation results revealed that a huge amount of sediment will be accumulated behind Bengoh Dam. Both scenarios showed that there would be erosion at Bengoh, immediately after the Bengoh Dam, due to a phenomenon known as “Hungry Water.” It is novel to discover that the sedimentation rate at Danu, Git and Batu Kitang, which are located downstream of the Bengoh Dam, has drastically decreased. The capacity of the Batu Kitang Submersible Weir (BKSW) reservoir is unaffected by the constructed Bengoh Dam; hence no dredging must ensure the security of Kuching’s water supply.