Effect of the Spillway Elevation on Flood Attenuation in the Bendo Dam

Bendo dam is a multi-purpose dam which used as a flood control, irrigation, raw water, also tour place. Moreover, in this study intends to know the safety of initial dam planning and determine the effect of the spillway elevation on flood attenuation in the Bendo Dam with discharge planned. In this research is fully calculated the maximum flood discharge also the safe freeboard and compare the amount of inflow and outflow in different elevation. This analysis used unit hydrograph synthetic nakayasu as a design discharge calculation and newton raphson method as a flood routing calculation. The location in this analysis is taken at Bendo Dam Ponorogo. The data needed in this calculation was rainfall data from 1996 until 2016 in Ponorogo area also earth map in Ponorogo area from internet. In this analysis, will be calculated effect of flood attenuation with the increasing of spillway 2 m, starting from 214.6 m until peak dam are 224 m. The result of this analysis, spillway elevation are used in Bendo Dam 218.6 m was safe to build. Then, the effect of spillway elevation is increase while the elevation increase also. In return period 1000 years the lowest value was 27.863% at elevation 214.6 m and keep increasing into 29.543% at elevation 216.6 m, 30.982% at elevation 218.6 m and maximum was 32.490% at elevation 220.6 m. Then, at PMF discharge, the lowest value was 24.949% at elevation 214.6 m and keep increasing into 26.390% at elevation 216.6 m, 27.908% at elevation 218.6 m and maximum was 29.492% at elevation 220.6 m