WATER TEMPERATURE IN THE DOWNSTREAM OF MASAKI DAM AND NUMERICAL EXAMINATION OF IMPROVMENT PLAN

This research is concerned with unnatural rise of water temperature occurring in the so-called `decayed reach' downstream of Masaki Dam, in Tokushima Pref., Japan, where the low water discharge has been remarkably reduced due to water diversion for power generation. Characteristic of water temperature in the decayed reach is described and analyzed based on the results of field observation executed in August 2002 and 2003. It is shown that the water temperature may raises as much as 8°C from the one in the state of nature. It is also pointed out that sensible heat transfer, short-wave radiation, and lateral heat transfer from ground water under exposed gravel bars seem to dominantly affect the water temperature of the observation reach. A one-dimensional mathematical model to simulate the variation of stream water temperature is presented and identified especially with regard to the decayed reach of the Masaki Dam. Effect of water temperature improvement plan for the dam is quantitatively examined using the present model.