Wlingi and Lodoyo reservoirs in the Brantas River basin, Indonesia, provide numerous benefits including reliable irrigation water supply, flood control, power generation, fisheries and recreation. The function of both reservoirs particularly in relation to flood control has declined due to severe sedimentation that has reduced their storage capacities. The sedimentation in Wlingi and Lodoyo reservoirs is mainly caused by sediment inflow from the areas most affected by ejecta from eruptions of Mt. Kelud, one of the most active volcanoes in Indonesia. The main objective of this research is to assess the sedimentation problem in Wlingi and Lodoyo reservoirs, particularly as they are affected by eruptions of Mt Kelud. We performed reservoir bathymetric surveys and field surveys after the most recent eruption of Mt. Kelud in February 2014 and compared the results with surveys undertaken before the eruption. The assessment revealed that both reservoirs were severely affected by the 2014 eruption. The effective storage capacity of Wlingi reservoir in March 2013 was 2.01 Mm3and the survey in May 2015 indicated that the effective storage of Wlingi reservoir had decreased to 1.01 Mm3. Similarly, the effective storage capacity of Lodoyo reservoir in March 2013 was 2.72 Mm3, reduced to 1.33 Mm3in May 2015. These findings underpin the analysis of the impacts of the secondary disaster due to reservoir sedimentation following the volcanic eruption and the implications for mitigating and managing the risks for sustainable use of reservoirs to control floods, supply water, generate electricity, etc. To cope with the extreme sedimentation problem in Wlingi and Lodoyo reservoirs, diverse sediment management strategies have been applied in these reservoirs and their catchments. However sediment disaster management strategies for both reservoirs, an integral part of the Mt. Kelud Volcanic Disaster Mitigation Plan, require continuous maintenance and recurrent operations, and ongoing evaluation and improvement.

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This study identifies the rainfall intensity that causes the flood along the arterial road connecting Malang to Surabaya, East Java, Indonesia, estimates flood occurrences based on the recorded rainfall data, and proposes the normalization of the side ditch to reduce flood occurrences. The Nakayasu synthetic unit hydrograph was used to analyze the runoff discharge, and the Hydrologic Engineering Center’s River Analysis System software was used to analyze the water level profile of the side ditch. The regression method was used to determine the relationship between the rainfall intensity and inundation depth along the arterial road. Analysis results show that floods occur if the rainfall intensity is ≥1.01 year return period. To estimate the flood inundation depth, simple linear regression was conducted herein. Furthermore, it was observed that flood occurrences can be avoided by normalizing the side ditch cross section with a fully rectangular shape having a channel bottom elevation equal to the existing elevation.

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