PENDEKATAN BANJIR LAHAR DINGIN DENGAN SIMULASI IRIC : MORPHO2DH

. Gajah Wong River is one of the major rivers in Yogyakarta which originates from the slopes of Merapi Mount. Cold lava flood is the process of transporting excess sediment due to high-intensity rain on the remaining lava on the slopes of the volcano. This study aims to analyze the impact of erosion and sedimentation due to the cold lava flood of Merapi Mount on the retaining wall of the Gajah Wong River cliff with a sediment discharge ratio approach. The method used in this study is a 2-dimensional numerical analysis method using the iRIC: Morpho2DH application to analyze the flow velocity and changes in the elevation of the river cross-section. The research resulted that the maximum flow velocity that occurs always increases from the existing simulation without running sediment to the normal sediment discharge ratio simulation until the sediment discharge ratio simulation is 2 times, then decreases in the simulation of the sediment discharge ratio 3 times. River cross-section areas that have the risk of erosion and sedimentation due to maximum flow velocity are the cross-section GW152 in the upstream and the crosssection GW145-GW143 in the downstream. In the cross-section GW152, the maximum erosion depth is 8.16 cm on the left cliff and 15.59 cm on the right cliff and the maximum sedimentation height is 6.74 cm on the left cliff and 24.92 cm on the right cliff. Whereas in the cross-section GW145-GW143, the maximum erosion depth is 8.00 cm on the left cliff and 11.77 cm on the right cliff and the maximum sedimentation height is 4.05 cm on the left cliff and 71.17 cm on the right cliff.