PERENCANAAN KAPASITAS KOLAM RETENSI (Studi Kasus : Sungai Tenggang - Sringin, Kota Semarang)

The city of Semarang is undergoing changes in land use in coastal areas, affecting water and soil resources and rendering the region susceptible to floods. Planning is conducted in the Sungai Tenggang and Sungai Sringin watershed areas, as there are potential locations with a total inundation area of 2,804 km2 in the downstream part of these rivers. The total area of the Tenggang and Sringin watersheds is 58.66 km2. Flood discharge analysis involves summing the analysis results using the HSS Nakayasu method and calculating the base flow for each river. The designed flood discharge, Q20th, is 150.01 m3/second for the Tenggang River and 68.52 m3/second for the Sringin River. Based on the designed flood discharge, a retention pond with an area of 2,804 km2, a pond depth of 2 meters, and a dead storage depth of 2 meters is obtained. A pump is planned as an outlet with a 20 m3/second capacity for 13 hours. On the inlet side of the Tenggang River, a weir with a height of 2.1 meters and a spillway length of 15 meters is planned, while on the Sringin River, a weir with a height of 2.4 meters and a spillway length of 8 meters is planned to prevent backwater from the retention pond. Complementary structures include a retaining wall using reinforced concrete with a cantilever type, 5.7 meters in height, and a width of 3.5 meters, under two conditions: sloping ground behind the retaining wall and flat ground without slope.