MODELLING SPILLWAY FOR FLOOD CONTROL OPTIMIZATION IN EMBANKMENT DRY DAM

Purpose: The primary purposes of this present study are to show and compare 3 (three) different types of spillways, namely normal, differential, and labyrinth, by means of passing exceptional flood events with acceptable safety and margin for the dam and spillway while maintaining a hydraulic safety point of view.   Theoretical reference: Ciawi dry dam is a homogenous type of dam with an inclined wet core built in the Ciliwung River, Jakarta, Indonesia. Its purpose is to retain 50 years of floods and cut the peak of floods as part of the Jakarta flood control system. It has been functioning since the year 2022 with another dry dam in the same River basin called the Sukamahi dry dam.   Method: The flood control capacity of the dry dam with a spillway and gateless bottom outlets designed at the riverbed level is investigated with an analytical approach using a theoretical formula. Numerous design discharges were verified and compared for specific scenarios engaging the operation of bottom outlets reviewed based on flood control and the safety of the dam. In addition, a spillways type comparison is made for a scenario with one gate opening and one gate closing of the bottom outlet using PMF design discharge to confirm the satisfaction of dry dam outlet capacity.   Result and Conclusion: The best design choice that can be recommended for the worst scenario is the Labyrinth spillway with a width of 155m and an angle of 18o while the spillway span width is kept to the same existing normal spillway width. This proposed design improvement option with labyrinth spillway for the PMF flood discharge still has a freeboard of 1.17 m allowance to the crest of the dam in the closed gate condition, 1.88 m of freeboard in 1 gate opened condition, and 2.55 m of freeboard while both gates are opened.   Implication of research: This present study evaluates the performance of a dry dam design outlet in flood control by assessing the reservoir capacity to compare the existing built normal spillway with the other types of spillways, namely differential and labyrinth. It will recommend an alternative type of spillway for the Ciawi dry dam or another dam to face global climate change and a future challenge.   Originality/value: The comprehensive investigation of the limitations and benefits of comparing 3 different types of spillways: normal, differential and labyrinth, with a combination scenario of 2 gates opening and closing in the embankment type of dry dam. This proposed type of spillway could be used for upgrading works and even for a new dam.