Mapping of flood hazard induced by land subsidence in Semarang City, Indonesia, using hydraulic and spatial models

Frequent floodings in Semarang City have generated increasing damages and losses in property and life quality. The cause of flooding is related to the coupled impacts of land subsidence, hydraulics hazards along with poor drainage and water retention systems. This paper studies the most recent flooding hazards caused by hydrological origins (i.e., river discharge, tidal) and land subsidence. In the study, riverine origin of flooding is simulated with the help of HEC-RAS 2D, while the tidal origin is simulated to high highest water level. However, due to the absence of the most recent topographic data, the role of land subsidence is measured by estimating the vertical changes of digital elevation model taken from Sentinel 1A. Flooding extent, in terms of depth and coverage, is verified based on satellite imagery Sentinel-2 which is cloud-processed using Google Earth Engine (GEE) and field survey. Fluvial flood is simulated with several boundary condition scenarios using combinations of 5-, 25-, or 50-year return periods of flood which is integrated with mean sea level (MSL) or high highest water level (HHWL) tides. Those boundary conditions are then incorporated into different terrains, namely LiDAR, DEMNAS, and TerraSAR DEM, to see how different digital elevation models (DEMs) can impact model sensitivity. By overlaying model outputs and land cover map, it can be concluded that settlements and water bodies are among the most potentially affected areas, covering up to 17 km2. This study is expected to help policymakers make a primary assessment of combined tidal and fluvial flood hazard through mitigation and adaptation measures.