by Anna Mariana Situngkir (Thesis M.Sc Unesco-IHE, Delft, Netherland, 2016) supervised by Prof. Dr. Charlotte de Fraiture, M.Sc, Prof. Dr. Ir. Robiyanto H. Susanto M.Agr.Sc, and Dr. F.X. Suryadi, MSC
Pesawaran Regency is an under development Regency located at South of Lampung. Gedongtataan as the capital city of Pesawaran Regency, which is situated at Way Semah Sub River Basin is where the development efforts are focused. Gedongtataan is affected by floods every year especially along the road and during rainy season and this should be addressed immediately if development is to continue without hindrance. To assess the performance of the existing drainage, the study uses Duflow Modelling Studio. Based on the observations, the drains are clogged by solid waste, grass weed, and silt, they are also affected by a lack maintenance, and lack of drainage along the road. Therefore, it is assumed that the lack of maintenance influences the canals’ roughness. Based on the results of existing drainage studies performed by Duflow Modelling Studio, it shows that there are inundated sections where existing drainage has insufficient capacity to cope with the water discharge. Therefore, it is a safe assumption that the cause of flood in Gedongtataan is the insufficient drainage capacity and also its poor condition. Beside poor drainage conditions, flood in Gedongtataan is also exacerbated by increased runoff from upstream due to deforestation which causes the overflow of the river. In order to deal with the flood that occurs in Gedongtataan, the drainage system should be improved to support the development.
Besides analyzing existing condition, Duflow was also used to assess performance extreme condition. In the existing condition, the upstream boundary will be the discharge of the river and the downstream boundary will be water level in the river. Meanwhile, in the extreme condition, the upstream boundary will be extreme value of rainfall runoff obtained from a Gumbel Distribution calculation with a return period 50 years and wastewater discharge and the downstream boundary will be the water level of the river. The extreme condition also considers the land use changes so the paved area will increase from 35% to 60%. Duflow will also generate water level in both the existing condition and the extreme condition. Based on the water level, the result shows that there are inundated sections in the existing condition with inundation depths between 0.1 m and 0.5 m. On the other hand, the water level of each section in extreme condition increases and the inundated section becomes larger than that of the existing condition. Furthermore, the result of both the existing and the extreme condition
in Duflow modelling will be overlaid with topography map produced in Are GIS 10.1 in order to better visualize the areas where flooding occurs.
To overcome the flood, some scenarios were proposed and developed in Duflow Modelling based on extreme condition. It is assumed that if the scenarios can overcome the flood in extreme condition therefore the flood in existing condition can be reduced. The developed scenarios are Scenario 1. Cleaning canals, Scenario 2. Dyke construction and canal widening, Scenario 3. Resizing canals, and Scenario 4. Utilizing paddy field as temporary retention pond. Scenario 1. Cleaning canals is done to remove solid waste, grass weed, and siltin order to improvethe roughness of the drainage canals by increasing Chezy Coefficient from 30 mm/s to 45 mm/s and increasing the perimeter area. Scenario 2.Dyke construction and canal widening those on the canals to increase the water level and water discharge. However, dykes tend to breach due to overtopping. Scenario 3.Widening and deepening canals to increase water depth and to improve water discharge. Scenario 4. Utilizing paddy field as temporary retention pond. Based on the volume, the retention pond can store 159,000 1113 water, while the volume of water during flooding is 104,000 m3 therefore the paddy field can store water during flooding. This scenario should also reduce the flood significantly, but it needs to give incentive to the farmers and to build ring dyke surrounding the field to keep water not goes to surrounding areas. Therefore, the best scenario is a resizing canal which is a combination between deepening and widening of canals.
Analysis of the impact of people dumping solid waste into the canal on flooding is also included in scenario one. The solid waste inside the canals clog the water flow and reduce the perimeter area of the cross sections therefore the water discharge reduces and less water is transported by the canals and the areas surrounding the canals will be inundated easily during heavy or intense rain.
The impact of land use change due to development will increase the built up area which lead to an increase of impervious land. This causes the areas that can absorb water from rainfall to reduce therefore the runoff becomes higher. Moreover, paddy held areas which functioned to hold water and to slow the runoff also reduce. If the runoff becomes greater, the areas tend to flood more easily and the occurrence of the flood is more frequent.
Based on Duflow modelling result, the best scenario to overcome the flood is the proposed scenario which will reduce flood optimally, that is a resizing canals which is a combination of widening and deepening canals. It is recommended that the flood protection should involve the societies participatory to build their awareness and belonging of the canals so they will not throw away the solid waste into canals and maintain the canals. Government also needs to control the land use changes during development. The social aspect needs to be developed further and also requires to consideration of the cost benefit to support good decision making regarding flood protection development.
Keywords: Urban Drainage, Flood Protection, Duflow, Gedongtataan