By Lazuardi, M.Sc. – (Thesis M.Sc. Unesco-IHE, Delft, Netherland, 2014), Supervised by Prof. Charlotte de Fraiture Ph.D., M.Sc., Prof. Dr. Ir. Robiyanto H. Susanto, M.Agr.Sc. and F.X. Suryadi, Ph.D., M.Sc.
As one of the main cities in Indonesia with the quite rapidly development, Palembang City generally is not also spared from urban problems as well as issues on urban drainage systems which related directly to the flooding problem. Flooding problem caused by urban development that lead to land use change contribute to an impact on the incidence of flooding. Moreover, geographically, Palembang City has a low elevation and relatively flat further increase the potential for flooding occurrence. Some areas are experiencing flooding as well as Tanah Merah drainage system, the study area which has an area of 256 ha, is also enhance by other factors including: urban drainage channel that rarely get maintenance, sedimentation from erosion and accumulation of garbage in the bottom of the channel, high intensity of the rainfall, bottle necks in the banks of drainage channels and a land use change in the river basin area due to development of Soekarno Hatta Street and other public facilities.
Hence, the research that has four objectives, namely to determine the current characteristic of Tanah Merah Drainage system; to simulate the possible future impacts caused by land use changes; to evaluate the proposed scenarios by government in order to improve the hydraulic performance of the urban drainage system and to figure out and model the alternative solutions, is highly essential conducted in order to participate in scientifically formulation of flood management measures.
The combination of spatial analysis through ArcGIS and Hydrological Analysis by DUFLOW Modelling Studio applied as research methodology to achieve the research objectives. The resulted stages from preliminary phase to fieldwork and data compilation are input of the combined modelling analysis. Furthermore, the spatial analysis through ArcGIS applied as a method of watershed delineation to determine the location of the research, hydrological condition of the area, flow direction, flow accumulation, and watershed boundaries. The next step is to combine the layout map as the results from the previous step along with the characteristics of the hydrology data for later in the process of DUFLOW Modelling Studio (DMS). In DMS, the hydraulic model establishes to obtain flood elevation in every cross section of the drainage system. While, the last step conducted is delineation of flood plain boundaries by applying Arc-GIS based on calculated flood elevation in every point at cross section by considering the DEM.
In order to obtain the research objectives, three scenarios would be designed namely: Scenario 1 of the current conditions; Scenario 2 of the current condition coupled with improvement measures by the Government; and the third Scenario is alternative solution which is government’s measures combined with improvement measures in related to nature insightful. However, prior to the development scenarios been tested using DMS and ArcGIS, calibration of the accuracy of the modeling is conducted by using primary data, secondary data and supplementary data. Calibration is important step in modelling in order to gain the optimum accuracy of modelling result which can be used in the modelling phase. This step done through comparing water level fluctuation resulted from modelling with water level fluctuation resulted from field measurement.
Scenario 1, Modelling for the current conditions is executed by involving the influence of the highest tidal fluctuation and maximum rainfall. Three schemes prepared in determining the different variations of the current conditions specifically the first scheme is about drainage conditions affected by the highest tidal fluctuation coupled with a design rainfall event with a return period of 25 years and second scheme explore the influence of both event with a return period of 50 years. As for the third scheme, the development of a second scheme is then coupled with land use changes. In current conditions, the flooding that occurred amongst three schemes occurs mostly in middle area which has a low elevation. The maximum height of flooding occurred on the second and third schemes with a height of 0.90 m, total flooded area 21 ha and duration of 2-8 hours.
Scenario 2, is exploring the measures taken by the government in addressing flood in this region. These measures will be modeled to identify the effectiveness of such measures. Government measures such as improvement of culvert dimension, developing river embankment (lining), dredging/deepening and constructing Retention Pond. Through the steps that have been modeled in this scenario is in fact demonstrated ineffectiveness in dealing with flooding. The government’s measures only scale down intensity, duration and area of flooding but could not cope with flooding entirety. Eventually, the inundation area indicates a decrease in the area and heights of inundation that reach up to only 0.5 m with a total area of 10 ha and duration of 3 hours.
Scenario 3, the last scenario, proposed as the response of flood management to minimize the impact of technical solution on the environment. Building with Nature approach is applied against the government’s measures such as channelization of urban drainage using natural materials and changes in the construction of retention pond to the construction of natural dike. These step proved to be effective to manage flooding through the Duflow Modelling Studio. The water level of 1.82 m+MSL on surface elevation can be blocked by the addition of 0.5 m dike while 1 m natural dike adequate for localize water inside the natural dike.
Furthermore, in anticipation to the occurrence of extreme flood events that exceed the expected flood events as well as complementary to the flood management measures, especially with regard to their impact on housing it’s important to applied flood resistant housing concepts to minimize the damage to the buildings. These measures include increasing structure/building above the floodplain, utilize design and structure materials.
Keywords: Building with Nature, DUFLOW modelling, flooding, GIS, urban drainage.