Floods that occur in the Upper Citarum River Basin are generally caused by flood discharge that exceeds the capacity of the Citarum River. Dayeuhkolot is one of the areas that is often flooded in the Upper Citarum River Basin. The Citarum River Authority has made efforts to handle floods in Upper Citarum by constructing the Nanjung Tunnel. When floods occur, Nanjung Tunnel will function to divide the water partially into the tunnel and through the Curug Jompong. SOBEK Model was utilized to find out the effect of the Nanjung Tunnel against flood inundation in Dayeuhkolot. This flood modelling also accommodates other efforts that have been made by BBWS Citarum such as the Cieunteung Pond and Cisangkuy Floodway. A 20-year return period was used for modelling. From the results of the modelling, it was found that difference in flood inundation area with and without the Nanjung Tunnel was 12.5 Ha or 3.5 %, with the duration of the overall inundation being reduced by 4.9 hours. Nanjung Tunnel does not have a great influence on the area of inundation, but it has an effect on reducing the duration of floods that occur in Dayeuhkolot.

The 2020 drought led to a reduction in the water supply of the Citarum cascade reservoirs (Saguling, Cirata, and Juanda) due the lowering intensity of rainfall in the catchments of the three reservoirs. Rainfall in late 2020 through February 2021 was much lower than the predicted and historical averages. This caused the water level of the Citarum cascade reservoirs fell to be below normal at the end of December 2020. To meet the water demand, an increase in inflow enhancement is needed, hence the implementation of Weather Modification Technology (WMT) which was carried out on March 12th - April 21st, 2021 in the Citarum watershed, West Java Province. WMT is applied for increasing or decreasing the amount of rainfall by intervening in the cloud growth process, which is carried out by seeding NaCl powder on clouds using an airplane. The purpose of monitoring and evaluation (M&E) of WMT implementation is to collect data, analyze, and evaluate the hydrological conditions of the Citarum cascade reservoirs during WMT implementation. The purpose of M&E is to assess the success of additional reservoir volume due to rainfall during WMT implementation. The M&E is conducted by collecting daily data of the rainfall, water level, and hydrological data of the reservoirs. The data collected each day is used as the basis for the seeding strategy on that day. After the WMT implementation activities are completed, an evaluation of the overall WMT implementation results is continued, which includes analyzing rainfall data, river water level, as well as the inflow, volume, and water level of the reservoir. The results of this study show that WMT implementation could not raise the water level of the three reservoirs to the Normal Operating Limit, but it has succeeded in increasing the reservoir volume by approximately 270 million m3 for Saguling, 201 million m3 for Cirata, and 59 million m3 for Juanda.Keywords:   Reservoir, Citarum, Weather, Modification, Technology ABSTRAKKemarau tahun 2020 mengakibatkan berkurangnya pasokan air waduk kaskade Citarum (Saguling, Cirata, dan Juanda) akibat berkurangnya intensitas air hujan yang jatuh pada daerah aliran sungai ketiga waduk. Curah hujan pada akhir 2020 hingga Februari 2021 jauh lebih rendah dari rata-rata historis dan prediksinya. Hal tersebut mengakibatkan kondisi muka air waduk kaskade Citarum turun hingga dibawah Normal pada akhir Desember 2020. Untuk memenuhi kebutuhan air dibutuhkan peningkatan inflow, melalui penerapan Teknologi Modifikasi Cuaca (TMC) yang telah dilaksanakan pada 12 Maret – 21 April 2021 di DAS Citarum, Provinsi Jawa Barat. TMC diterapkan untuk menambah atau mengurangi curah hujan dengan melakukan intervensi pada proses pertumbuhan awan, yang dilakukan dengan penyemaian serbuk NaCl pada awan menggunakan pesawat. Maksud dilakukannya monitoring dan evaluasi (Monev) terhadap penerapan TMC adalah untuk pengumpulan data, analisis, dan evaluasi kondisi hidrologi waduk kaskade Citarum selama pelaksanaan TMC. Tujuan Monev sendiri adalah untuk menilai keberhasilan tambahan volume waduk akibat curah hujan selama pelaksanaan TMC. Monev TMC dilakukan dengan pengumpulan data setiap hari baik data hujan, Tinggi Muka Air (TMA), maupun data hidrologi waduk. Data yang terkumpul digunakan sebagai dasar untuk strategi penyemaian pada hari itu. Setelah kegiatan penerapan TMC selesai, dilanjutkan evaluasi hasil penerapan TMC secara keseluruhan yang mencakup analisa data hujan, TMA sungai, inflow, volume dan TMA waduk. Hasil penerapan TMC pada kajian ini tidak dapat menaikkan TMA ketiga waduk hingga Batas Operasi Normal, namun telah berhasil menambah volume waduk sekitar 270 juta m3 untuk Saguling, 201 juta m3 untuk Cirata dan 59 juta m3 untuk Juanda.Kata Kunci: Modifikasi, Cuaca, Teknologi, Waduk, Citarum

AbstrakTujuan penelitian ini adalah untukmerencanakan dimensi tubuh bendungan yang secara teknis layak untuk dibangun, serta aman terhadap stabilitas. Debit banjir rencana yang digunakan adalah sebesar 1.152 meter 3 /dt dengan periode ulang 1000 tahun. Berdasarkan analisis yang telah dilakukan, didapatkan elevasi muka air banjir rencana adalah + 303,22 meter, tinggi jagaan yang didapatkan sebesar 3,0 meter, elevasi puncak tubuh bendungan didapatkan + 306,22 meter, tinggi bendungan yang didapat dengan mengurangkan tinggi elevasi puncak tubuh bendungan dengan elevasi dasar bendungan adalah 56,16 meter, lebar mercu puncak bendungan sebesar 11,0 meter, kemiringan lereng urugan bagian hulu sebesar 1 : 3 dan untuk lereng urugan bagian hilir adalah 1 : 2,25. Menggunakan skema formasi garis depresi dengan drainase kaki dengan cara grafis, tidak terjadi rembesan yang keluar tubuh bendungan dan nilai kapasitas debit aliran filtrasi dari perhitungan didapatkanQ = 9,553 x 10 -5 meter 3 /dtk < 2 % Qinflow rerata = 5,785 meter 3 /dtk, bendungan aman terhadap aliran filtrasi. Stabilitas tubuh bendungan terhadap longsor dengan nilai faktor aman pada berbagai kondisi yaitu bendungan kosong dan terisi penuh baik dalam keadaan normal maupun terjadi gempa, untuk bendungan tipe urugan dengan menghitung beban tubuh bendungan, momen geser, tekanan air pori, serta beban gempa. Menggunakan metode irisan bidang luncur Fellenius, nilai yang didapatkan diatas angka aman (1,2) maka bendungan yang direncanakan aman terhadap longsor. AbstractThe purpose of this study was to plan the dimensions of the dam body that is technically feasible to build, as well as secure the stability. Flood discharge plan used amounted to 1,152 meters cubic/ sec with a return period of 1000 years. Based on the analysis that has been done, get a plan of flood water level is + 303,22 meters, surveillance obtained high of 3,0 meters, the body of the dam crest elevation gained + 306,22 meters high dam that is obtained by subtracting the height elevation of the peak body dam with a base elevation of the dam is 56,16 meters, width of the dam crest beacon of 11.0 meters, rockfill the upstream slope of 1:3 and for rockfill downstream slope is 1: 2,.25. Using the scheme of depression line formation with drainage leg with a graphic way, do not occur out of the body dam seepage and flow filtration capacity value of the calculation, Q = 9,553 x 10 -5 meters cubic / sec, smallest than 2% Qinflow mean = 5,785 mmeters cubic / sec, dam secure against flow filtration. The stability of the main dam of landslide with the value of safety factor in a variety of conditions, namely dams both empty and fully charged in a normal state or an earthquake, for rockfill dam by calculating the weight of the dam, sliding moment, pore water pressure and earthquake loads. Using the method of slices Fellenius glide plane, the value obtained above safe rate (1,2), the planned dam safety to landslides