Flood routing model using integration of Delft3D and GIS (case study: Tanggul watershed, Jember)

Flood disaster is one of the most common disasters in Jember District. One that is often affected is in 10% of the area of Jember District, namely the Tanggul watershed with an area of 345,875 km2. The condition of Tanggul River, with a length of 23.50 km, which tends to narrow and meander can reduce flow especially when the river discharge is enlarged due to rain. If rainfall is above 100 mm on the upstream part of the watershed, then the downstream of the Tanggul River with a length of ± 10.19 km that passes through Paseban Village, Kraton Village, and Kencong Village are often unable to withstand the discharge which will overflow the surrounding area. The history of flood occurences in the Tanggul watershed in the last 25 years had occurred 27 times, including 9 times which resulting in breakdown of the embankment. According to records from the Lumajang Public Works Office, the latest flood event in Tanggul River was occurred on December 22, 2018 with water elevation reaches 3.5 m and a discharge of 749,153 m3. This resulted in a broken embankment with a length of 60 m and a height of 7.2 m from the riverbed. Mitigation efforts such as flood routing to get an overview and information on overtopping points is needed to be done in order to minimize losses. This research aims to simulate flood using integration of the main programs DELFT3D and GIS. The flood routing model is obtained by utilizing a spatial model of analysis in Geographic Information Systems (GIS) to determine critical embankment or overtopping points and hydraulic flow simulation with DELFT3D program. The modeling results are visually compared with the coordinates of the flood events in the field that prepared by the Lumajang Public Works Office. Based on the 1000-year return period, the overtopping point occurs at ± 12.8 km from the downstream, namely Kencong District and ± 11.8 km from the downstream, namely Paseban District. In the end, this model can be used as a framework for the Tanggul watershed flood routing model to look for the effectiveness of various non-technical flood control alternatives such as land conservation and technical such as river normalization.