No abstract

Mrica Reservoir is one of many reservoirs located in Central Java that experienced a considerably high sedimentation during the last ten years. This condition has caused a rapid decrease in reservoir capacity. Various countermeasures have been introduced to reduce the rate of the reservoir sedimentation through catchment management and reservoir operation by means of flushing and/or dredging. However, the sedimentation remains intensive so that the fulfillment of water demand for electrical power generation was seriously affected. This paper presents the results of evaluation on the dynamics of the purpose of this research is to evaluate the sediment balance of the Mrica Reservoir based on two different scenarios, i.e. the existing condition and another certain type of reservoir management. The study on sediment balance was carried out by estimating the sediment inflow applying sheet erosion method in combination with the analysis of sediment rating curve. The measurement of the deposited sediment rate in the reservoir was conducted through the periodic echo sounding, whereas identification of the number of sediment that has been released from the reservoir was carried out through the observation on both flushing and dredging activities. The results show that during the last decade, the rate of the sediment inflow was approximately 5.869 MCM/year, whereas the released sediment from the reservoir was 4.097 MCM/year. In order to maintain the reservoir capacity, therefore, at least 1.772 MCM/year should be released from the reservoir by means of either flushing or dredging. Sedimentation management may prolong the reservoir's service life to exceed the design life. Without sediment management, the lifetime of the reservoir would have finished by 2016, whereas with the proper management the lifetime may be extended to 2025.

Bantul Regency is a district in Yogyakarta Province which has geographic, geological, hydrological, and demographic characteristics that are likely to cause drought. Drought event in Bantul Regency may have significant impacts on various aspects in line with the characteristics of drought impacts which are complex and cross-sectoral. This study addresses to analyze the level of risk of drought with observation units in 75 villages in the Bantul Regency. The risk analysis was carried out by comparing the time period of the 10 years, i.e. 2008 and 2018 to observe the shift of risk areas of drought in Bantul Regency. The research was conducted using quantitative research methods with quantitative descriptive and mapping analysis. The analysis steps are drought hazard analysis, vulnerability analysis, and drought risk analysis. The analysis shows that during the last 10 years, Kabupaten Bantul has been experiencing an increasing number of villages classified as high risk of drought, both in urban and rural areas. In 2008 there were 15 villages (20%) and increased to 21 villages (28%) in 2018 that were classified as very very high level. Meanwhile, in 2008 there were 30 villages (40%) in 2008 and increased to 32 villages (42.7%) in 2018 that were classified as very high level. It caused by the increasing probability of drought as well as vulnerability. The analysis results can be used as input for stakeholders to take mitigation and anticipation actions to reduce the impact of drought based on the spatial characteristics of the risk areas.

Human survival depends on water availability as water is a basic needs. As population increases, water becomes more limited. One of the ways to prevent draught is collecting water in retention basin. Tambaksari Multipurpose Retention Basin serves to provide basic water demand and irrigation water for the local community. Water availability and water demand should be equal, so a simulation of the operating patter of the retention basin is necessary to distribute water equally and determining the failure level of the retention basin in performing its task. In the present study, the retention basin operating pattern was simulated using Standard Operating Rule (SOR) method. The present study used secondary data, including population data, rainfall data, climatological data and technical data of the retention basin. The size of DAS Tambaksari is 0.41 km² while the size of the irrigated land is 12 Ha. The rainfall data was from 2008-2018 obtained from two rain stations, population data of Tambaksari Village was from 2008-2019, and the climatological data was from 2014-2019. The simulation result of the retention basin operating pattern using SOR method showed that the final collection wasn’t under minimum collection and failure happened in the operation of the retention basin for one year. . The optimal ability of the target release is 70.83% for the reliability of irrigation needs and the level of reliability for raw water needs of 58.33%. Population projection analysis to determine the population in 20 years showed that the future population of 4044 required 3.861 l/person/s of water. Irrigation demands for rice-rice-palawija planting pattern showed that irrigation intake was 11.89 l/s. Water balance determined the balance between inflow and outflow. The value for average water availability was 0.021 m³/s while total average water demand was 0.016 m³/s. therefore, the water availability in partial intake wasn’t able to meet the irrigation and basic water demands. Keywords: discharge, retention basin operation simulation, SOR

Sedimentasi waduk merupakan permasalahan global yang sangat penting dalam perencanaan waduk karena menyebabkan penurunan kapasitas tampungan waduk. Dalam menghitung sedimentasi waduk, biasanya meliputi sedimen yang masuk, sedimen yang keluar, dan endapan sedimen di dalam waduk, sehingga didapatkan imbangan sedimen yang terjadi. Namun dalam praktik di lapangan, tidak semua melakukan ketiga perhitungan tersebut, karena pertimbangan biaya pelaksanaan yang cukup mahal. Untuk mengatasi hal tersebut, perhitungan sedimentasi dilakukan dengan pendekatan konsep efisiensi tangkapan (trap efficiency) sedimen. Selama ini pendekatan konsep ini didasarkan pada hasil penelitian di luar negeri dan tingkat akurasi masih belum optimal. Tujuan penelitian ini untuk mengetahui tingkat akurasi dari beberapa metode prediksi efisiensi tangkapan sedimen. Penelitian ini dilakukan di Waduk Mrica, Kabupaten Banjarnegara. Data-data yang diperlukan berupa data sekunder, seperti: data debit inflow, data rating curve debit sedimen, data pengukuran echosounding, data analisa butiran sedimen dasar waduk, dan data teknis waduk. Tahapan yang dilakukan dalam penelitian ini, diantaranya: menghitung sedimen yang masuk, sedimen yang mengendap, efisiensi tangkapan, dan mendapatkan tingkat akurasi dari beberapa metode yang ditinjau. Kriteria statistik dari metode terpilih harus memiliki nilai korelasi > 0,6 dan error < 20%. Imbangan sedimen di Waduk Mrica menunjukkan bahwa laju sedimen yang masuk sebesar 6,001 juta m3/tahun. Sedimen rerata yang mengendap sebesar 3,850 juta m3/tahun, sehingga sedimen yang dikeluarkan rerata sebesar 2,151 juta m3/tahun. Tingkat akurasi dari metode prediksi efisiensi tangkapan bervariasi dari sedang sampai kuat. Metode prediksi efisiensi tangkapan sedimen Brune, Harbor dkk, dan Jotihiprakash dan Garg dianggap paling representatif untuk digunakan pada Waduk Mrica.