Distribution scenario of rainfall to control excessive discharge in Ciliwung river

Abstract

Ciliwung river is originated from Bogar region, West Java, and at the lower part it flows through Jakarta. The length of Ciliwung river is about 144 km and the sub catchment area is 337 km2. Flood and inundation threatens Jakarta every year. The flood is controlled mainly by West Floodway Canal which was first constructed in 1920's, in 1973 was reviewed and designed for Q100 = 290 ems at Manggarai gate and 400 ems at the estuary, lately in 2007 was reviewed again and found that the design discharge Q too became 570 ems at Manggarai gate and 700 ems at the estuary, it was doubled in 34 years, a significant increase. However, the urbanization in Ciliwung River is very fast, from around 40% in 2000 to 60 % in 2009 . This condition results in the increase of runoff which finally increases the discharge flowing through the drainage, the river and the West Flood way Canal system. And this discharge will be higher than the capacity . Combined with the condition that around 40% of Jakarta is located in flood plain area, which means that the area is lower than sea level high tide and suffering from landsubsidence , Jakarta is highly potential to suffer from flood and inundation. So, it is necessary to control the run-off. Considering that the West Floodway Canal seems very difficult to make its capacity more and more bigger every time because unavailability of space, so an approach to manage the obvious flood risk must be prepared. An approach to control the run-off is by allocating rainfall in each sub-sub catchment, this concept can be explained as follows : the discharge flowing through the West Floodway Canal is as much as its capacity then the excessive discharge will be retained in sub-sub catchment and infiltrated. The allocation of rainfall is carried out by mathematical model in 2010 by Directorate of Water Resources Utilization, Directorate General of Water Resources . The model is based on 2009 landuse, present capacity of Manggarai gate = 450 ems, no reservoir along Ciliwung river, and capacity of Ciliwung West Floodway Canal is 80% equal to 395 ems. The model shows that the rainfall intensity is 1,861 mm, 0100 = 758 ems thus the excessive discharge equals to 363 ems. In order to control the excessive discharge, the rainfall must be allocated in Ciliwung West Floodway Canal is 1487 mmm, retained in ponds is 323 mm, seepage well is 8 mm and infiltrated 43 mm .