Hydrodynamic Simulation of Flow Structure in a Stratified Reservoir in Japan

We receive a variety of benefits from dams and reservoirs such as flood control, water supply, recreation, etc. On the other hand, they sometimes cause problems on the river environment. For example, prolonged discharge of high turbidity water to the downstream of reservoirs is one of such problems. The benefits and the problems are, however, mutually related, since such environmental issues are generated from the function of water storage in reservoirs especially during flood events. We are, therefore, expected to take possible measures to reduce the higher turbidity situations in downstream rivers. For that purpose, numerical hydrodynamic simulations based on data obtained in field measurements were conducted in Isawa Dam Reservoir in Japan in a stratified season.

The numerical model used in this study is Fantom 3-D develop by Shintani & Nakayama (2009), which is an environmental hydrodynamic simulator for lakes, reservoirs and ocean. Flow dynamics are solved with hydrostatic approximation with two-equation turbulence models such as k-epsilon or k-omega models. The model has been developed under the idea of object oriented programing for easier application to a variety of environmental hydraulic issues. One of the advantages from the coding is locally refined Cartesian grid system with flexible treatment of bathymetry data. The model is also equipped with wet and dry scheme to express water level variation of the water bodies.

The model was applied to Isawa Dam Reservoir, located in the Kitakami River System in the northeast part of Japan Main Island. Water flow condition was first measured in the reservoir prior to simulation in this study. The measurement was done from the end of August through September, 2016. Middle scale flood that did not destroy the stratification occurred in the first half of the measurement period, while in the latter half of the measurement, initiation of mixing regime that might lead to whole circulation was observed.

The results of the simulation were not able to express the layered flow structure revealed in the measurement, but the flow pattern of horizontal velocity induced by the wind observed in the first half of the measurement period of the epilimnion of the reservoir is able to reproduce roughly. Distributions of vertical velocity and eddy viscosity that seem to correspond to the surface mixing which seen in the second half of the measurement period were identified.