DAM BREAK ANALYSIS A CASE STUDY

Abstract

Batutegi dam is an earthen dam. The dam is located on Way Sekampung River in Lampung Province, which is the southern most province of the Sumatra Island, the western most large island of Indonesia Archipelago. The simulation of dam break flood wave due to a hypothetical failure of Batutegi Dam using the BOSS DAMBRK Model developed by Dr. D L Freed is presented here. The data available for the analysis are various design floods for dam, PMF hydrograph, spillway rating curve, elevation area capacity table for reservoir, cross-sectional details of various downstream reaches etc. More emphasis is given to analyse the sensitivity of dam break flood wave characteristics (stage and discharge) due to the changes in dam breach parameters (time taken for breach formation, inflow hydrograph to the reservoir, final size of breach and breach slope). The option of reservoir storage routing to compute outflow hydrograph from reservoir for different combinations of breach parameters and option of sub-critical dynamic routing for routing this flood hydrograph through the downstream channel is used. The study of Batutegi Dam Break Analysis involves : • Identification of the inflow hydrograph to the reservoir at the time of failure • Flood routing of the hydrograph through the reservoir • Development of the failure condition of the structure • Calculating the outflow hydrograph as a result of the failure of structure • Modeling the movement of the flood wave downstream to determine travel time, maximum water level reached and peak discharge. Sensitivity analysis has been carried out for the breach parameters such as breach development time, breach width and breach slope to investigate the effect of change in values of each parameter on water levels, peak discharge and travel time of peak flow through the river. For sensitivity analysis of each dam break variable on the resulting dam break flood discharge on the downstream of the dam, various runs have been made using Dambrk iii Abstract - DAM BREAK ANALYSIS A CASE STUDY Model. Analysis has been repeated by applying four design inflow hydrographs and no inflow to the reservoir condition with various values of three dam breach parameters. The result of sensitivity analysis of breach parameters shows, that all the breach parameters are sensitive with regard to peak outflow discharge. An increase in flood inflow hydrograph, in final breach width, the breach slope have an effect to increase the peak outflow discharge. However an increase in breach development time has an effect to decrease the peak outflow discharge through breach formation Also, from various studies, it is seen that changes in inflow hydrograph to the reservoir have insignificant/slight effect on the flood wave characteristics. It may be because of large surface area and storage capacity of the reservoir when compared to the total inflow to reservoir. The following points can be summarize from this study : a. Two scenarios of dam failure are applied. First is dam failure due to overtopping and second dam failure due to piping. The results of both scenarios show that dam failure due to overtopping has greater adverse effect on the down stream area than the piping failure. b. Flood stage and peak discharge values are increasing along the river, when related to the increase in the breach parameters except the time taken in breach formation. c. Based on results of breach parameter sensitivity analySis, selected values of breach parameters are used to simulate the PMF inflow hydrograph to the Batutegi reservoir. The result shows the maximum peak discharge just below dam site as 78,610 rOsec and at the end of available cross section (about 163.650 km) downstream of dam only 5,000 m3/sec. d. According to the flood surface elevation along the river down stream of dam site, in most of the river reaches the water levels are more than the bank flooding levels. This indicates that at many locations, the river flood wave is over the bank. e. The time duration of flooding over the flood plain area along the downstream of dam site varies. The duration time varies from 3.2 hr to the maximum of 100.96 hr. The long duration of inundation time occurs in flat reaches of the river.