MINI POLDERS, AS AN ALTERNATIVE OF FLOOD MANAGEMENT IN THE LOWER BENGAWAN SOLO RIVER

摘要

Sumbangtimun Village and Kandangan Village (Java, Indonesia), in the lower part ofBengawan Solo River, are fl ooded almost every year, as they are situated in the river meanderwith low natural levee. Losses due to fl ood damage were estimated to reach 2.5 billion rupiah(11530 Indonesian Rupiah = 1 US Dollar) in the year 2007/2008 with the greatest lossesfrom agricultural sector. These losses were caused by fl ooding before the crop harvest. Theriver authority recommended construction of a dike to prevent fl ood inundation. There is aproposal to develop a set of mini polders to have mixed land uses such as rice fi elds, treecrops plantation, settlement, with different inundation tolerance. The mini polder system wouldbe equipped with lateral gates for fl ood control. Hydraulic mathematical models (HEC-RASand Pond Pack) for simulating fl ow characteristics in the fl ood prone area are employed todetermine the design of an appropriate mini polder system and its operational procedures. Inaddition of technical aspects, the design is also reviewed for compliance of the non-technicalaspects including agricultural, economic and environmental in the local communities.According to the result of frequency analysis, fl ood discharge in the Bengawan Solo Riverwas 1,500m3/s for 2 year return period (Q2) and 2,525m3/s for 10 year return period (Q10).The model predicted the water elevation for a 2-year fl ood as +17.50 m (average inundationdepth of 1 m). The area inundated would be 250.48 ha for 2 days. Flood water elevation basedon 10 year fl ood return period was at +19.05 m or inundation depth average was around 2.5m. The area that inundated at this condition was 382.58 ha with the inundation duration was in3 days. The use of storage area and lateral structures models can describe fl ood phenomenain the modeled fl ood prone area more accurate.