By Djoko Purnomo – (Thesis M.Si Sriwijaya University, 2013). Supervised by Prof. Charlotte de Fraiture, PhD, M.Sc (UNESCO-IHE), Prof. Dr. Robiyanto H. Susanto, M.Agr.Sc (Sriwijaya University), F.X. Suryadi, PhD, MSc (UNESCO-IHE).

Palembang is the capital city of South Sumatra Province. As a big city rapid population growth is inevitable. New areas were reclaimed for urban development and city infrastructure including Lambidaro River Basin. The area of Lambidaro River Basin is about 6,388 ha and most of them are lowland area. The topography is relatively flat and low, especially in the downstream part of the river which its river mouth ends to Musi River.

By its nature, some of the areas are always inundated and some developed areas have been the subject of flooding almost every year due to insufficient both urban drainage and flood protection infrastructures system, therefore some improvements are necessary to deal with those problems. This research was carried out by DUFLOW modelling and ArcGIS. It aims to analyse the existing drainage and flood protection of Lambidaro River System, to discuss the alternative flood mitigation measures, and to evaluate the alternative measures based on technical effectiveness and cost estimation.

The methodology of this research consists of preparation, field work and data acquisition, data analysis and modelling, results analysis, and conclusions and recommendations. Preparation phase mainly consists of literature study by reviewing journals, reports, books, etc. related to the topic. Field work and data acquisition phase includes compiling the spatial data and hydrological data from several offices related to the study, and also collecting primary data through field measurement such as river discharge and water level fluctuation.

Data analysis and modelling phase comprises of spatial and hydrological data analysis. Spatial analysis was done by watershed delineation from Digital Elevation Model (DEM) of Palembang City to generate watershed and sub-watershed of Lambidaro River System. Meanwhile, hydrological data analysis consists of rainfall and water level fluctuation analysis which are the input of boundary condition of DUFLOW Model besides river discharge.

Prior to DUFLOW Modelling development, model calibration was done and resulted in water level fluctuation that close to the data gained from field measurement, meaning that the DUFLOW modelling is appropriate for further scenarios development. In this study, Lambidaro River System was divided into six segments, namely Man-made Canal, Natural River 1, Natural River 2, Karangsari, Tanjung Rawa, and Poligon segment.

Three scenarios were enhanced in this study. Firstly, existing scenario consisted of three schemes, namely scheme with no rainfall, scheme with design rainfall 4% and scheme with design rainfall 2% of occurrence. Existing scenario without rainfall scheme was done to evaluate the impact of maximum water level fluctuation in Musi River. The result was flooding in Man-made Canal, Karangsari, Tanjung Rawa, and Poligon segments. Existing scenario with design rainfall of 4% and 2% schemes were modelled to find out the effect of those design rainfall to the increasing flooding. The result shows that flooding occurs in all segments with the total area is increasing moderately from scheme 1 (without rainfall) of 801 ha to 1,352 ha of scenario with design rainfall 4% of occurrence, and then slightly rising to 1,425 ha of scenario with design rainfall 2%.

The second scenario was existing condition with the improvement measures of the urban drainage and flood protection. Based on the modelling, increasing water depth by dredging or deepening of all of the segments did not give significant impact for reducing the water level. Then measures partially of the segments were evaluated and resulted in heightening the levee for Man-made canal and Karangsari, widening and heightening the river embankment for Natural River 1, widening of Natural River 2, functioning the gate of Poligon segment, and constructing flaps gate and heightening the levee for Tanjung Rawa segment.

The last scenario was aimed to evaluate the drainage system with design rainfall 2% of occurrence in responds to land use change, land subsidence and sea level rise for 50 years ahead. This scenario resulted in the increase of flooding areas into 1,508 ha. The proposed measures to implement were the same as those of second scenario with the additional increase of the river embankment on Man-made canal, Natural River 1, Natural River 2, Karangsari, and Tanjung Rawa segments.

Flood alleviation in Lambidaro River Basin in Palembang City should consider future condition in responds to land use change, land subsidence, and sea level rise. To determine some preferred alternatives of flood mitigation measures, besides technical evaluation, holistic economic and social analysis are necessary to conduct for policy making.

Keywords: Urban drainage, flood modelling, DUFLOW modelling, flood protection, flood mitigation measures