EVALUATION LIFETIME SERVICE AND CAPACITY OF THE KADUMALIK DAM RESERVOIR

The dam is a construction of water structures built across the river. The function of the dam is to hold and store water, both rain water, river water and water from the lake which will then form a reservoir. Kadumalik Dam is one of the projects of the Directorate General of Water Resources (SDA) of the Ministry of Public Works and Public Housing (PUPR) which will be built in 2020 on an area of 500 Ha. The purpose of building Kadumalik Dam is to meet the irrigation needs of the Irrigation Area (D.I) and meet diverse water needs, but the dam can also pose a great risk as well, namely the risk of accumulation of sedimentation. Sedimentation is one of the factors that is very influential in dam management. A large sedimentation rate will speed up the service life of the reservoir so that the planned reservoir function can be disrupted. Therefore this research is important to know how much the dam capacity and how old the dam service. This study uses analytical calculation methods for sediment transport based on observations and calculation methods with efforts to handle sedimentation. Based on the results of the analysis, the sedimentation rate in the Kadumalik Reservoir is 2.71 tons / ha / year and is classified as an erosion hazard class I or very light erosion rate, calculation of the method with sedimentation handling measures results in the analysis of the Sediment Control Building (BPS) can control 2.2 million m3 of sediment, so that it can extend the reservoir service life to 50 years.


Introduction
The dam is a construction of water structures built across the river. The function of the dam is to hold and store water, both rain water, river water and water from the lake which will then form a reservoir. Reservoirs can be interpreted as a container or place formed by containment. Reservoirs can play a role in meeting the needs of raw water for drinking water, distribution of irrigation water to paddy fields, hydropower (PLTA), flood control, and tourism [1], [2], [3].
Kadumalik Dam is one of the projects of the Directorate General of Water Resources (SDA) of the Ministry of Public Works and Public Housing (PUPR) which will be built in 2021 on an area of 500 Ha. The purpose of the construction of the Kadumalik Dam is to meet the irrigation needs of the Cilutung Irrigation Area (D.I) and meet diverse needs [4].
Dam construction aims to meet diverse water needs, but dams can also pose a great risk, namely the risk of sedimentation [2]. Sedimentation is one of the factors that is very influential in dam management [2]. A large sedimentation rate will speed up the service life of the reservoir so that the planned reservoir function can be disrupted [2], [3]. This study aims to determine how much capacity and age of dam services by evaluating the rate of sedimentation that occurs in the reservoir with the existing analytical methods of sediment transport, and the method of calculation by handling sedimentation in the Kadumalik Reservoir (DTA) catchment area ( Figure 1 ) [5], [6], [7].

Method
The steps taken to predict the sedimentation rate in the Kadumalik Reservoir are as follows [8]:

Sediment Analysis Results
Hydrological processes, directly or indirectly, are associated with erosion, sediment transport and sediment deposition in downstream areas. Changes in land use and watershed management practices also affect erosion, sedimentation, and in turn, will affect water quality [9], [5].
Sedimentation can be known from the amount of erosion in an area. One formula that can be used to predict the amount of erosion is a model developed by Wischmeier and Smith (1978), commonly known as the Universal Soil Loss Equation (USLE). This model is considered the most popular method and is widely used to predict the magnitude of erosion [2], [10], [9], [6].
By using the USLE equation it can be predicted the average rate of erosion of a particular parcel of land, on a steep slope and with certain rainfall patterns, for each type of crop and management measures (soil conservation measures) that are or may be undertaken. The equation used groups various physical (and management) parameters that affect the rate of erosion into six main parameters, with the following equation [2], [10], [9], [6]: With the USLE equation above, and by obtaining factor values from the erosion parameters then the amount of land erosion can be calculated [2], [10], [9], [6], [7]. To find out the level of erosion hazard in certain areas can be seen based on the erosion hazard class qualifications as presented in Table 1.  The sediment transport rate in the river consists of two parts, namely suspension sediment transport and elevated sediment transport. For the calculation of the elevated sediment rate can be calculated by multiplying the sediment concentration with the average river discharge, while for the basic sediment transport it can be calculated by the MPM and Einstein equations [2], [10], [7], [8], [11].
The equation for calculating elevated sediment is given as follows: Based on calculations using the MPM method, the sediment transport rate at the Kadumalik Dam planned location is 2.38 million m3 / year or 6.11 mm / year. As for the Einstein method, a sediment transport rate of 2.61 million m3 / year or 6.72 mm / year was obtained. In subsequent calculations, the results of the Einstein method are used [2], [10], [4].
To calculate the amount of sediment trapped in a reservoir, the following equation is used: Based on the results of sedimentation analysis, it appears that sedimentation will reach intake elevation between the 9th and 10th years. Calculation results can be seen in Table 2. [12], [4].

Table 2. Kadumalik Reservoir Sedimentation Calculation
Based on the results of the analysis in Table 2, it can be seen that the service life of the dam only reaches the age of 10 years, this shows that the service life of the Kadumalik Reservoir is very short. Therefore, efforts should be made to handle sedimentation in the Kadumalik Reservoir catchment area. One of the efforts to handle this can be done by building a BPS (Sediment Control Building). The results of the sedimentation calculations with the efforts are listed in Table 3 [4].

Potential Deposits
The location of the dam in the watershed, is determined based on the comparison of the volume of reservoirs in 2 alternative dam locations. The potential of the reservoir is calculated based on a topographic map by first calculating the area of each contour line of the reservoir. This area was calculated with the help of CAD software. The storage volume is calculated using the truncated cone formula for inundation pits between two contour planes [12], [4], [13], [14], [7], [8], [11].