Search

ABSTRACT 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 ABSTRAK Pada 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

Steel slag is one of the results of the rest of smelting iron ore/ steel. To reduce the abundance of steel slag, this study seeks to examine utilization as a substitute for gravel in the concrete used for construction sabodam.The durability of concrete on the outer layer was analyzed by using the results of several studies related.The most influential factor is the compressive strength of bump (impact) as a representation of the clash due to lava rock.It will be discussed about the potential use and durability. To analyze the stability of the sabodam, researcher use SNI 2851: 2015 about Sediment Retention Building Design. The parameter used is the stability of the tilting, sliding and bearing capacity. For alternative use sabodams core material, researcher used split stones and steel slagconcrete. The flood scenarios can be divided into two, namely flood plain and flood lava. Based on the results obtained, the potential for instability is found in the parameters of carrying capacity. Thesabodamwhich using split stones, the bearing capacity of the land has a minimum strength is worth -0.98 kN/m2 (for flood) and -0.43 kN/m2 (for lava flood), while sabodam with steel slag has a minimum strength of 1.74 kN/m2 (for flood) and 1.57kN/m2 (for lava flood) which means notensile stresses occured. The conclusion is the concrete with steel slag can be used as material for sabodam.

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.