Flood management has so far focused on increasing river capacity while rainwater management in watersheds (DAS) as a source of water is still neglected, so it is necessary to develop a method of improving the function of the watershed by formulating water storage needs. In addition, the current (structural) flood control is more about controlling surface runoff or quantity of rainwater by increasing or restoring the function of the watershed, namely collecting and storing rainwater, in the form of building reservoirs upstream of the watershed. The water reservoir functions to manage the quantity of rainwater by controlling the peak discharge during flood events, and as the most effective water storage facility for regulating water. Water reservoirs can be in the form of detention basins, retention basins, retarding basins, and small dams.
This study intends to formulate the capacity requirements of the watershed reservoir volume, through the determination of the optimal watershed storage volume with the volume of the reservoir as a function of surface runoff. The purpose of this study is to determine the surface runoff of various flood re-periods, determine the volume of the reservoir as a function of surface runoff, determine the most optimal watershed storage volume, and formulate the capacity requirements of the watershed reservoir volume.
The research was carried out in the Jatigede watershed covering an area of 1,468.22 km2, located in Garut Regency and Sumedang Regency, West Java Province. The Jatigede watershed has 92 sub-watersheds and is upstream of the Cimanuk watershed.
The data used are primary data (river discharge measurement) and secondary data (average daily rainfall in 2011-2020, soil type maps in 2020, land use maps in 2011 and 2020), where primary data are used as research calibration. Analysis of flood surface runoff using the soil conservation service (SCN) curve number (CN) method, and the HEC-HMS flood discharge model. Surface runoff discharge with HEC-HMS is converted to the spool volume of all Sub-watersheds on segments 1 and segment 2 of the Jatigede watershed. Concentration time was used in the analysis. The spool volume of each Sub-watershed is converted so that the surface runoff depth of various periods is obtained. The formulation of the capacity needs of the spool volume is obtained from the results of the analysis of the most optimal spool volume based on the slope of the river. The formulation results obtained were validated with the volume of storage from the heat-HMS surface runoff conversion, validation was carried out in segment 3 of the Jatigede watershed (Java Island), Jangkok Watershed (Lombok Island), Tanggek Watershed (Lombok Island), and Jeran Watershed (Sumbawa Island).
The results of the formulation of the capacity requirements of the watershed storage volume found that the formulation of the upstream watershed storage volume (VTHD) produced the most optimal spool volume equation based on the slope of the river, the Q10 re-period for the slope of 0.1 and the Q25 re-period for the slope of 0.01. A representative equation for calculating the volume of the upstream reservoir of the watershed (VTHD) based on 2 (two) slopes of the river, a slope of 0.1 with the equation VTHD = A × (0.0011 CN3.5276), and a slope of 0.01 with the equation VTHD = A × (0.0036 CN3.2332). VTHD validation was carried out in segment 3 of the Jatigede watershed (Java Island), the Jangkok watershed and the Tanggek watershed (Lombok Island), and the Jeran watershed (Sumbawa Island). The average VTHD accuracy rate for slope 0.1 was 71.11%, and slope 0.01 was 71.31%. This suggests the VTHD model can be applied to calculate the capacity requirements of water reservoirs upstream of the watershed. The validation results show that VTHD is more effectively built or applied to a watershed area of fewer than 25 km2 to control watershed surface runoff, which is possible by utilizing river borders.
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