Kajian Efektifitas Pengendalian Banjir di DAS Garang

Abstract

ABSTRACT ANALYSIS FOR EFFECTIVENESS OF FLOOD CONTROL IN GARANG WATERSHED Semarang is capital city of Central Java, Indonesia that suffers water resources problems every year including drought in dry season and flood in wet season. Garang watershed is a watershed that most of its region in Semarang city and less of its region in Semarang municipality and Kendal municipality. The upstream of Garang watershed is from Mount. Ungaran and the downstream of Garang watershed is in Java Sea. According to its geographical condition, Garang watershed is important key for water resources aspect in Semarang city and surrounding areas. This analysis is concentrated ini flood control in Garang watershed with return period of flood design is 100 year return period. Flood control in this analysis is concentrated in effectiveness of an infrastructure in reducing flood discharge that flows through rivers in Garang watershed especially through west floodway in di downstream of Garang watershed. the data for this analysis are geomorphological data of Garang watershed and its main river such as Garang river, Kreo river and Kripik river. Another important data is hidrological data like, rain fall data, water surface data and tidal data in Java Sea. The next step is including hidrological analys, manually with Gama-1 method, then calibration for flood discharge with actual data from AWLR Panjangan. After calibration, we make mathematical model for flood routing in two method. Flood routing with hidological method we used HEC-HMS software and flood routing with hydraulic method, using HEC-RAS software. The analysis created some scenario flood routing to analyze the effectiveness. The analysis also including effectiveness flood reducement, cost effectiveness, the connection between area, volume of storage with reducing flood discharge. And we also analyze the connection between hidrological method and hydraulic method in flood routing based on simulation result using the two methods. From the analysis, we know that combination of Jatibarang Dam, Garang Dam and Mundingan Dam is effective storage to reduce flood (935,14 m³/s) and effective in term of cost and social aspect with 0,58 m³/s for every 1 billion rupiah. This analysis also mentioned that area and volume of storage have linear connection with flood control. This analysis also showed the connection between hydrology formula and hydraulic formula in flood routing that hydrology formula is simplification of hydraulic formula with abandoning acceleration and pressure aspect. Finally, after analysis completed, we can use it for reference to know effectiveness of flood control in Garang watershed and more over we can use it for reference for next study in the future.