The lahar flow  that occurred after the 2012  Gamalama Volcano  eruption, leading to the eastern valley flowing downstream  through the Togurara River to the center of Ternate City and Sultan Babullah Airport. To overcome the potential of  lahar  flow in the river since 2013 - 2016, several sabo dams and building facilities have been built. Based on the results of the calculation of deposits that potentially become lahar flows in 2016, the built-in capacity of the Sabodam has not been able to control the volume of sediment in the upstream of the river, then in the year 2017, 2018 constructed several additional Sabodam.  In order to determine the effectiveness, feasibility and conditions of the completeness of Sabodam, is done field assessment on 25 ~ 27 September 2018, the method used was a mathematical approach based on sabo technology. The assessment was in the form of a field survey, simple measurements, interviews with the Sabodam management agency and local residents related to the lahar flow that had occurred. With the capacity of several additional Sabodams built in series, the average river bed slope was originally 9.09% to 6.83%. After the construction of Sabodam, the maximum lahar flow was once as high as 7 m, but now it decreases do 4.2 m.   As the slope of the Togurara River slopes progressively, the lahar flow rate and its destructive power are reduced, so that Ternate City and Sultan Babullah Airport are spared from lahar disaster.Keywords: Lahar flow, sabodam, sabo technology, slope of riverbed, Togurara River.

On Thursday, July 14, 2022, a flash flood occurred in Pati Regency, Central Java, caused by heavy rainfall with an intensity of up to 147.5 mm/day on the slopes of Mount Muria. resulting several rivers in Pati Regency overflowed, causing damage to embankments and triggering a flash flood that devastated residential areas and agricultural land. In this study, a simulation was conducted to estimate the extent of the affected area by flash floods in the Sat River, Pati Regency, in the future. The study utilized HEC-RAS version 6.2, which is capable of accurately simulating non-Newtonian fluid behavior, thus enabling precise simulation of flash floods. The simulation results revealed that the flash flood in the Sat River, Pati Regency, had an impacted area of 655 hectares with an average water depth of 3.25 meters for a 2-year return period. For a 25-year return period, the impacted area increased to 1,017 hectares with an average water depth of 3.04 meters, while for a 50-year return period, the impacted area reached 1,091 hectares with an average water depth of 3.01 meters. This research provides a better understanding of the impact of flash floods in the Sat River, Pati Regency, and can serve as a reference for future disaster mitigation efforts. By knowing the extent of the affected area and the average water depth, more effective preventive and management measures can be implemented to mitigate the significant losses caused by flash floods.Keywords:       flash flood, flood mitigation, flood simulation, HEC-RAS, hydraulic analysis, Pati Regency  ABSTRAKPada Kamis, 14 Juli 2022, banjir bandang terjadi di Kabupaten Pati, Provinsi Jawa Tengah yang disebabkan oleh curah hujan dengan intensitas mencapai 147,5 mm/hari di lereng Gunung Muria. Akibatnya, sejumlah sungai di Kabupaten Pati meluap, menyebabkan kerusakan pada tanggul dan memicu banjir bandang yang merusak permukiman penduduk serta lahan persawahan. Dalam penelitian ini, dilakukan simulasi dengan tujuan untuk memperkirakan luas area yang terdampak banjir bandang di Sungai Sat, Kabupaten Pati, di masa depan. Studi ini menggunakan perangkat lunak HEC-RAS versi 6.2 yang memiliki kemampuan untuk melakukan simulasi fluida dengan sifat non-Newtonian, sehingga simulasi banjir bandang dapat dilakukan secara akurat. Hasil dari simulasi menunjukkan bahwa banjir bandang di Sungai Sat, Kabupaten Pati, memiliki luas terdampak sebesar 655 hektar dengan kedalaman air rata-rata mencapai 3,25 meter untuk periode ulang 2 tahunan. Pada periode ulang 25 tahunan, luas dampak meningkat menjadi 1.017 hektar dengan kedalaman air rata-rata sebesar 3,04 meter, sedangkan untuk periode ulang 50 tahunan, luas dampak mencapai 1.091 hektar dengan kedalaman air rata-rata sebesar 3,01 meter. Penelitian ini memberikan pemahaman yang lebih baik mengenai dampak banjir bandang di Sungai Sat, Kabupaten Pati, dan dapat digunakan sebagai acuan dalam upaya mitigasi bencana di masa depan. Dengan mengetahui luas area yang terdampak dan kedalaman air rata-ratanya, langkah-langkah pencegahan dan penanganan yang lebih baik dapat diambil untuk menghindari kerugian besar yang ditimbulkan oleh banjir bandang.Kata Kunci:            Analisis Hidrolika, Banjir Bandang , HEC-RAS, Kabupaten Pati, Mitigasi Banjir, Simulasi Banjir

Flash floods frequently occur in Indonesia. Flash flood can be caused by natural and human factors. This paper discusses the causes and mitigation of flash flood using a case study in Banyuwangi. On June 22th 2018, flash floods occurred in the residential areas of Garit, Karang Asem, and Bangunrejo Hamlets. Flash floods were triggered by heavy rainfall the day before, which reached 90 mm/day. The three hamlets are located in Alasmalang Village, Singojuruh Subdistrict, Banyuwangi District. The study began with a field survey, analysis of the damage, and the causes of a flash flood, and its alternative treatments. The suspected source of the material was the landslide material, approximately two million cubic meters from the Pendil Mountain. Some of the material were carried away by the river flooding caused flash floods in the three hamlets. The small width of the bridge was not able to be passed by trees and other flash flood material, so water overflowed the roads and houses. Based on the results of the study, it is recommended for disaster mitigation in the form of structural and non-structural actions. Structural measure in the form of bridge elevation and construction of sediment retaining weirs, while non-structural actions are in the form of an early warning system and dissemination of flash flood hazards.

The debris flow that happen on the of Mount Merapi is really hard to be seen, therefore, it is necessary to conduct laboratory-scale simulations to know when debris flows will happen as regard to rainfall intensity and the slope of Mount of Merapi. This research examines the correlation between the slope and the potential for debris flow at 25 mm/h rainfall intensity. This will be a reference for early warning of landslides on Mount of Merapi. This research uses a tool such as flume that sized 3 x 5 x 0,15 m as a model of slope of Mount of Merapi, and artificial rainfall apparatus as the rain simulator. The simulation is conducted using five years rainfall intensity of 25 mm/h in combination of slope i.e. 15, 20, 25, 30 and 35 degrees whereas the material used to represent the sediment is in form of sand taken from Gendol River upstream with 4,75 mm passing mesh sieves. The result of this simulation is the steeper the slope is, the faster the duration for the rain to cause debris flow. This research can be continued with change variation of rainfall intensity to understand the debris flows behavior. Keywords: Debris flow, Mount of Merapi, laboratory test, rainfall intensity, flume model