The Bengawan Solo River has been famous in Indonesia for centuries and is often mentioned in folk story and traditional literature. A large number of researchers have studied the Bengawan Solo River and its basin in different aspects including hydrological, socio and cultural aspects.
Java Island, where the Bengawan Solo river basin is situated, typically has two seasons like other regions in Indonesia, i.e. rainy and dry season. The rainy season commonly occurs during October to April, and the dry season during the remainder of the year. The River Bengawan Solo has a monsoonal character with the highest discharge recognized during the four to five months of the rainy season (Hoekstra, 1989). During December to March, humid air brought to Java Island by W-NW monsoon dominates, producing abundant rainfall on the island (Hoekstra, 1988). In addition, river flow in the upper catchment of Bengawan Solo river basin has been found to be increasing significantly, whereas in the middle and lower catchment a moderate diminution has occurred (Pawitan and Haryani, 2012).
A historical study regarding big flood event in Bengawan Solo River Basin in 1966 (case study in Surakarta city), was carried out by Taqobalallah (2009). The study investigated the impact of Bengawan Solo flood in 1966 in Surakarta city and community’s respond to the flood by interviewing old people witnessing the flood. The study revealed that in 1966, Bengawan Solo River had problem with sedimentation and siltation. Moreover, the embankment constructed by Dutch colonial along the river was mostly damaged. Based on the information gathered from the old witnesses, the government and community at that time worked together to evacuate the flood victim, provide public kitchen for supplying food to the flood victim, and rebuild the damaged embankment after the flood water get shrink.
Other research regarding the Bengawan Solo River has concerned the method to choose suitable rainfall gauging stations for predicting flood discharge (Sobriyah et al, 2001). This research showed that the data at rainfall gauging stations at Jurug, Kajangan, Ketonggo, Babat, and Bojonegoro was suitable to be used for predicting flood discharge on Bengawan Solo River. In addition, Sobriyah (2005) also conducted research to estimate flood discharge in Bengawan Solo River Basin using rainfall data of years 1985, 1990, 1991, 1996 and 1997 at Jurug, Kajangan, Ketonggo, Babat and Bojonegoro raingauge stations. The flood estimation was carried out using a grid-based Rational Method with O’Donnell and Muskingum ‘ Cunge routing (Sobriyah, 2005). This research was restricted to the current climate.
At the end of 2007 and beginning of 2008, parts of the Bengawan Solo river basin were affected by flooding which many casualties and cost about 852.89 billion Rupiahs (Indonesian currency, source: Indonesia Ministry of Public Work, 2008). The flood event was reported by mass media, both paper-based and online (BBWS Bengawan Solo, 2008; Kompas, 3 March 2008 edition). Based on these reports, the flood was caused by several factors as follows:
1. High rainfall intensity (about 80 ‘ 135 mm) which was distributed uniformly across most regions in the basin;
2. The capacity of the Bengawan Solo River was exceeded: this is the second largest recorded event, after the 1966 flood;
3. The flood which struck regions of Madiun and Ngawi (located in the middle of Bengawan Solo River Basin) was the largest since 1963;
4. Around the Lower Bengawan Solo River Basin (regions of Bojonegoro, Babat, Lamongan and Gresik), the stream water level was the highest since 1963 (15.97 m);
5. Poor solid waste management in the flood affected areas: the uneducated community commonly throw the garbage into the river which diminishes the river function for discharging water and sediment.
6. Broken embankment along the flood prone areas.
According to a book written by journalists and scientists joining a group called Kompas Expedition Team (Kompas, 2008) the Bengawan Solo river basin has been degraded from the upstream area to downstream. It is explained in the book that the basin degradation is caused by several factors, such as reduction of forest area, reduced vegetation cover to prevent erosion in upstream area, conversion of landuse and uncontrolled waste disposal. In addition, the local community cultivates seasonal plants such as peanut, corn, and cassava along the river bank which causes erosion flowing into the stream during the wet season. This condition triggers sedimentation along the river, from upper catchment areas (Wonogiri, Sukoharjo, Solo, Sragen, Ngawi) to the lower one (Blora, Bojonegoro, Lamongan and Gresik). Furthermore, sedimentation becomes a serious problem in Wonogiri Dam which is situated in the upstream area of Bengawan Solo river basin. This sedimentation problem also affects the performance of hydropower system utilized in the dam.
Beside the sedimentation problem, the river basin also faces a catastrophic water pollution problem, because the Bengawan Solo River now has become a giant waste reservoir, either for domestic waste or industrial waste. Animal carcasses and waste from animal husbandry are disposed of in the river. The river damage is also now aggravated by sand mining activities practiced by local poor communities along the river.
Another scientific report concerning the Bengawan Solo River was written by Hidayat (2009). Based on several sources, the report concluded that climate change has impacted the flood risk in Java Island including the Bengawan Solo River Basin (Hidayat, 2009). However, the report did not discuss in detail how climate change might influence flood events in the basin.
UN-Habitat (2009) also conducted a study reporting on damage to the Upper Bengawan River Basin which was struck by flooding since the last 3 years, and in particular the city of Surakarta (Solo). The report claims that climate change has caused a changing extreme rainfall pattern affecting the city. This has triggered flooding which covered about 45 % of Solo city and gave severe impact mostly in slum settlements along the river banks (UN-Habitat, 2009).
In conclusion, research regarding flooding and related subjects in the Bengawan Solo River Basin by scholars is very limited. However, mass media has actively informed the occurrence of flood.
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