The effect of underdrain box storage (UBS) as an instrument for reducingwater runoff in Mardika residential areas at Ambon City

Land use change due to housing development can damage water catchment areas indirectly. This condition results in reduced rainwater catchment areas which cause rainwater to collect in existing drainage channels.Underdrain Box Storage is a means to collect rainwater and soak it into the ground. Rainwater that falls on the roof of the house is not flowed down into the gutter or the yard of the house, but it is flowed by using water channels into the underdrain box storage (UBS) so as to reduce the amount of runoff that occurs. Based on the results of the analysis of the combination of the correlation curve between Fr and Q2 / (h15.g)1/2, it is found that the regression model Q2 / (h15.g)1/2= 0.25757Fr2 - 0.612Fr + 0.0064 and between Fr and a / h12 in the model regression a/h12 = 0.1Fr2 with the formula of flow through the hole with Cd the discharge coefficient of the contraction vein flow equation at the bottom of the tank is Q = Cd. (2gh)1/2, found the discharge coefficient (Cd) with a value of 0.85-0,96 with the regression model Cd = 0.6473.h0.144. The area of the hole for the underdrain storage box has 2 "(inch) as a diameter and the filling time which is planned to speed up the entry of water into the reservoir is 2 minutes due to the small diameter of the holes, so that the required number of holes is 12 with a distance between the holes is 13.5 - 14 meters. If there rain happened for 1 hour, the underdrain box system can reduce flooding / inundation by 25.47% for the 2-year plan rainfall 11.44% for the 5-year plan rainfall and 7.35% for the 10-year plan rainfall of the total water volume rain.