STUDY OF SEDIMENT DISTRIBUTION IN THE AMERORO RESERVOIR AND MORPHOLOGICAL CHANGES IN THE DOWNSTREAM RIVER OF THE AMERORO RESERVOIR, SOUTHEAST SULAWESI

• Main Author: Rachmatullah, Indradjati
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/79961
• Institution: Institut Teknologi Bandung

The presence of numerous reservoirs in Indonesia significantly contributes to meeting the demand for raw water, both for irrigation and drinking water. Sedimentation in reservoirs is influenced by various factors such as climate change, land cover changes, and others. The increase in sedimentation in a reservoir can reduce its useful lifespan. The Ameroro Reservoir is in Konawe Regency, Southeast Sulawesi, where the population growth rate is 2.03%, which is relatively high in Indonesia. The Ameroro River, as the main river in the Ameroro Watershed, with an area of 374.8 km2 and a main river length of 45.16 km with an average slope of 2.65%, has two main structures crossing it: the Ameroro Reservoir and Ameroro Weir. Both structures primarily serve to supply water for rice farming, the main commodity in Konawe Regency. This study aims to demonstrate and analyse the sedimentation distribution in the Ameroro Reservoir during its operation and the morphological changes in the Ameroro River before and after the Ameroro Reservoir operates. The modelling of the Ameroro River and the crossing structures uses the HEC-RAS 2D program with Sacramento discharge model input, having a correlation value of 62.77% and NSE of 0.3937 against observed discharge over 10 years. For sediment transport modelling, data on riverbed measurements in 2019 and 2022, including the sediment's grain size distribution from direct sampling in the river, were used. The sediment transport modelling depicting changes in the riverbed approximates measurements using the Meyer-Peter-Muller function with the VanRijn fall velocity function, achieving an error of 15.96%. The simulation results show morphological changes in the river: (a) the average water level of +105 m with an average inundation area at STA 13872 in the upstream river and STA 4446 at the inlet, or 7.7 km from the upstream river length from the Reservoir axis and 4.5 km from the inlet river length; (b) changes in the riverbed when the Reservoir is in operation are more significant than the conditions before the Reservoir operates. The sedimentation rate entering the Reservoir is 1,192,015 m3/year, with 42.47% of the total entering the dead storage compartment. Calculations of the Reservoir's useful life using the Brune curve method and the sediment density factor at each time show a useful life of 85 years compared to the planned 107 years, deviating by 22 years. According to the weighting method for determining alternatives for sedimentation handling in the Reservoir, watershed conservation is found to be the best alternative for maintaining the operating Reservoir. This study is expected to assist in the operational and maintenance aspects of the Ameroro Reservoir.