Irrigation & Drainage Systems Engineering

Mini Polders, as an Alternative of Flood Management in the Lower Bengawan Solo River

Abstract

Wignyosukarto BS1*, Mawandha HG2 and Jayadi R3

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.