Simulating bathymetric changes in reservoirs due to sedimentation

Abstract: Currently sedimentation is one of the major issues to deal with, for professionals, as it causes continuous loss of storage and hinder the intended purposes of a dam. More research and case-specific studies are required to understand the behavior of sediment transport mechanism in order to propose remedial measures. Sakuma dam on Tenryu River in Japan is one of the largest dams in Japan and it is rapidly losing its storage capacity due to sedimentation. The dam started its operations in 1957. To study the bathymetric changes upstream of the dam, a mathematical modeling approach was selected using HEC RAS to simulate the existing changes and predict future trends. Before going into the detailed modeling, a literature review has been made about different sediment-related studies on the river and Sakuma dam to get deeper insight and to build a conceptual model. Then a reach of about 32 km of Tenryu River, from Hiraoka dam to Sakuma dam was modeled. Google Earth and AutoCAD were used to extract the geometrical data of Tenryu River to be used as input in HEC-RAS. Other data about flow and sedimentation were obtained from the Department of Civil Engineering, University of Tokyo. As necessary, simplifications and assumptions were also made, and sometimes data was extracted indirectly. After initial data input, it was required to do calibration and validation of the model from 1957 to 2004, when the data were available. Once the model was validated, prediction of future bathymetric changes was also made through model simulation. For this prediction it was assumed that the existing flow data could be recycled. Predictive simulation shows that the dam would probably not serve its intended purpose after year 2035 ± 5. Therefore it is recommended to employ some suitable remedial measures to remove sediment and to prevent loss of storage in order to increase the useful age of the dam. Some prospective study options are also identified at the end.