Analisis Debit Banjir Menggunakan Hidrograf Satuan Sintetik Nakayasu Terhadap DAS Lesti

The Lesti Watershed (DAS) has a flat to mountainous topography with the highest elevation of the sub-watershed located at the top of Mount Semeru and the outlet of the Lesti River at the Sengguruh Dam. The morphological conditions of the river are irregular and generally do not have river banks (dykes) with an average river depth of between 5-20 meters. The problems that occur in the Lesti Watershed include increasing the population around the Lesti Watershed, land damage that causes erosion, water shortages in the dry season and flooding in the rainy season, etc. There are 3 (three) methods used to minimize or overcome problems that occur in the Lesti River, namely the Gamma Ⅰ, Nakayasu & Snyder method. In this study, the authors used the Nakayasu Synthetic Unit Hydrograph method. From the calculation results, the total design flood discharge with a return period of 10 and 25 years, the highest results are 278.2891 m3/s for a return period of 10 years and 305.2449 m3/s for a return period of 25 years. it is known that the highest intensity of rainfall occurs in the initial 5-6 hours in 24 hours, after passing 6 hours the intensity of rainfall begins to decrease, so that it can be used as material for consideration to address the increasing amount of water discharge that will occur so that later it will not become a problem and dangerous for the area around the Lesti river watershed