Abstract
Sumbangtimun Village and Kandangan Village, in the lower part of Bengawan Solo River, are regularly flooded almost every year. These flood prone area are situated in the river meander with low natural levee. Losses due to flood damage were estimated to reach 2.5 billion rupiah in the year 2007/2008 with the greatest losses from agricultural sector. These losses were caused by flooding that comes early before the harvest time. Surround dike to prevent flood inundation is recommended by the river authority. Regarding different land uses, i.e., rice fields, tree crops plantation, settlement, with different inundation tolerant, a set of mini polder is proposed to overcome the problem of inundation and its water management. The polder system is attributed by surrounded dike, local drainage system and pond retention with or without pumping system. The mini polder system that is equipped with lateral gate as flood control structure is proposed to solve the problem of inundation in the agriculture and settlement areas. Hydraulic mathematical models (HEC-RAS and Pond Pack) for simulating flow characteristics in the flood prone area are employed to determine the design of an appropriate mini polder system and its operational procedures. In addition of technical aspects, the design is also reviewed for compliance of the non-technical aspects including agricultural, economic and environmental in the local communities. According to the result of frequency analysis, flood discharge in the Bengawan Solo River was 1,500 m 3 /s for 2 year return period (Q2) and 2,525 m3/s for 10 year return period (Q10). Flood water elevation based on 2 year flood return period was obtained at +17.50 m or inundation depth average was around 1 m. The area that inundated at this condition was around 250.48 ha and the inundation duration was in 2 days. Flood water elevation based on 10 year flood return period was at +19.05 m or inundation depth average was around 2.5 m. The area that inundated at this condition was 382.58 ha with the inundation duration was in 3 days. The use of storage area and lateral structures models can describe flood phenomena in the modeled flood prone area more accura.
Highlights
Bengawan Solo River is the longest river in Java Island (± 600 km length) that flows started from Wonogiri, in the south part of Central Java Province, passing through the north part of East Java Province, and its mouth is in Java Sea at Ujung Pangkah
Along the floodplain of Lower Bengawan Solo River that forming wide and meandering river channel, there are some settlement and agriculture area that are inundated during high flood but in times of drought, they need irrigation water that can be pumped from the river
The proposed mini polder system should meet several objectives, i.e. this system should work as flood protection during high flood; it could give possibility to allow river water flows to the area for irrigation purposes during normal flow; the cost of construction and operation should not expensive, and it should be equipped with reasonable drainage system and pumping station
Summary
Bengawan Solo River is the longest river in Java Island (± 600 km length) that flows started from Wonogiri, in the south part of Central Java Province, passing through the north part of East Java Province, and its mouth is in Java Sea at Ujung Pangkah. Along the floodplain of Lower Bengawan Solo River that forming wide and meandering river channel, there are some settlement and agriculture area that are inundated during high flood but in times of drought, they need irrigation water that can be pumped from the river.
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