Analysis of Flood Early Warning Time in The Jragung River System Sri Puji Lestari (a), Dr. Ir. Istiarto, M. Eng, Ph.D (a), Prof. Dr. Ir. Bambang Triatmodjo (a)
(a) Master in Engineering in Natural Disaster Management, Engineering Faculty, Universitas Gadjah Mada, Indonesia
The area of flood inundation in Demak Regency has reached 13000 Ha. One of the reason was the overflow of the Jragung River system (BBWS, 2010). This flood has caused direct losses (damaged and flooded paddy fields) and indirect losses (obstructed traffic due to flooded/interrupted roads). Structural flood mitigation efforts have been carried out on the Jragung River system through the Jragung River System Flood Control activities which include river normalization activities (river channel improvement), cliff elevation, and elevation of structural buildings that cross the river. However, non-structural control efforts need to be made to maximize mitigation efforts, for example through a flood early warning system. In this study, analysis of early warning time of the flood in the Jragung River system will be carried out in an effort to reduce losses and damage. The location in this study is the watershed of the Jragung River upstream system with the control points used in this study are Jragung Dam and Guntur Dam. This study includes hydrological modeling using HEC-HMS 4.2.1 software (Hydrologic Engineering Center - Hydrologic Modeling System) and hydraulic modeling using the HEC-RAS (Hydrologic Engineering Center-River Analysis System) software. The results of the analysis based on the 2, 5, 10, 20, 25, 50, 100, and 1000 years return period shows that the flood discharge Q50, Q100, and Q1000 has the potential to cause flooding in the lower reaches of the Jragung River system. Based on the results of the warning time analysis, for Q50 flood discharge, the maximum potential warning time (Twp) is 5.3 hours, the recognition time (Tr) is 2.4 hours, and the maximum mitigation time (Tw) is 2.9 hours. for Q100 flood discharge, the maximum warning time potential (Twp) is 5.8 hours, the recognition time (Tr) is 1.8 hours, and the maximum mitigation time (Tw) is 4 hours. Whereas for Q1000 flood discharge, the maximum potential warning time (Twp) is 8.7 hours, the recognition time (Tr) is 1.2 hours, and the maximum mitigation time (Tw) is 7.5 hours. All evacuation procedures can be carried out at the maximum mitigation time. The results of this study are expected to be an input for the government to provide flood early warning for communities along the Jragung River system, for example by installing flood alarms.
Keywords: Jargung River, the maximum potential warning time, the recognition time, the maximum mitigation time
Topic: International Symposium of Civil, Environmental, and Infrastructure Engineering