The Effect of Discharge, Sediment Transport, Tidal and Land Subsidence in The River Flood Risk Management: Case Study of Sayung River

Abstract Indonesia’s extensive coastline and tropical climate make it highly vulnerable to flooding, particularly in coastal regions like the Sayung River in Central Java. This study focuses on flood drivers such as land subsidence, sedimentation, and tidal factors, which have caused severe damage to areas like Semarang and Demak. Three analyses were conducted: the first employed an unsteady flow model with upstream discharge (Q25) and downstream tidal input. The second used a quasi-unsteady flow model with daily discharge and tidal input. The third was a 2D simulation predicting conditions for the 10th, 20th, and 50th years, factoring in tides, subsidence, sea level rise, and sedimentation. Sediment control measures for the 10th year include embankment construction and channel maintenance, while medium- and long-term solutions involve implementing a polder system with a weir, spillway, reservoir (+4.85 million m 3 ), and pump capacity (+52.45 m 3 /s). These integrated approaches provide critical insights for flood management in coastal regions threatened by climate change and land degradation.