DESIGN FLOOD OF THE MAHAKAM LAKE CASCADE FOR REDUCING THE FLOOD DOWNSTREAM

Abstract

The Mahakam lake cascade consists of Jempang, Melintang, and Semayang lakes. The cascade lakes have a complex hydrological system; the three lakes merged with the Mahakam River when flooding. Generally, analysis like that focuses on each lake; however, the cascade lakes are analyzed as a system in this research. In the dry season, the water is receding, and in the riparian part is becoming dry land even it is only left on the depth channel that is flooded. The extreme recede condition happened in 1997 during 192 days; however, the longest extreme flood condition lasted for 63 days in 2006. The lake function, which should be able to reduce the flood, cannot work optimally. Floods in this area happened every rainy season. This research intends to analyze the design flood that can still be stored by the storage capacity in each cascade lake (Jempang, Melintang, and Semayang). Therefore, the lake function for reducing the flood in this area can be optimized. The methodology consists of measuring water level depth in the lake multiplying by the lake section area, then conversed into discharge using the rational method. The lake boundaries were determined by spatial analyses that is GIS application. The result shows that the design flood in each lake is varied and depends on each lake capacity. The capacity of the Jempang lake is only for the design flood Q25, about 388.33 m3/s; the Melintang lake can only store the design flood Q10 that is about 140.10 m3/s, and the Semayang lake is only able to store the design flood Q25 that is about 82.05 m3/s. This result can be used as a reference to design the optimization model for reducing the flood downstream.