Vertical Structure of Velocity Distributions Measured in a Thermally Stratified Reservoir

In this study, field measurement was carried out in Isawa Dam Reservoir, located in the Kitakami River system in the northern area of Japan, on the flow structure under a thermally stratified condition. The administrator and the local people of the reservoir have concerns about prolonged discharge of higher turbidity water into the downstream, and it is necessary to find solutions to prevention of higher turbidity discharge from the dam. For that purpose, this study made analysis on flow structure in the reservoir that influences on behavior of fine suspended sediment.

The measurement was conducted in the beginning of autumn, when the stratification was about to be destroyed by the cooling weather condition. The duration of the measurement was about one month from the end of August through September, 2016. Vertical structure of velocity in the reservoir was continuously measured with ADCP deployed on the bottom around the center of the reservoir through that period. The measuring interval was 20 minutes, and the vertical resolution was 1m. Water temperature was also monitored twice a day around the downstream end of the reservoir from the water surface to the same hight of the ADCP installation.

From those observation results, layered structures of flow velocity were found in the both components of horizontal and vertical velocity. However, the respective components showed different structures and behaviors with each other. We defined “velocline”, which corresponds to a depth at which vertical distribution of the velocity component exhibits apparently large changes. The depth of the velocline of horizontal velocity was close to the thermocline, but vertical displacement was large and frequent. On the other hand, there were two veloclines whose depths were several meters above and below the thermocline, and movement of the veloclines were much smaller than that of horizontal component. In the latter half of the measurement period, scattered velocity variations were observed in the epilimnion, which were presumably caused by the turnover mixing initiated by surface water cooling.