Hydrogeology Analysis and Water Balance Calculations in Effort to Mitigate Flood at Semarang City, Central Java Province

The main concern of an urban / city area hazard is flood, one of them is Semarang city, with several conditions related to surface and subsurface. The presented methods consist of hydrogeological approach, where analyzing subsurface conditions relate to hydrogeology system, and numerical methods such as water balance which provide values of flowing water (Baseflow) and water overflow (Runoff). The research provides an overview of surface conditions in general such as topography, river area, and land use, which directly affect flooding. The topography profile in lowland zone indicates a catchment area, following with 10 watersheds. High usage of land use will reduce the percentage of permeable area, resulting in low infiltration capacity and volume of subsurface baseflow. In terms of the hydrogeological system and groundwater level, Semarang City composed by distribution of shallow alluvial deposits and Damar Formation’s aquifer, where the water table level ranges from 4 – 22 meters above the sea level. The water balance based on climatological data from all urban rainfall stations indicates unbalance, due to excess runoff from water surplus not equal than infiltration rate. Urban flood mitigation includes infiltration wells and wetlands, where the placement of mitigation zones considers subsurface hydrogeological conditions, and their positions relate to surface conditions.