INFLUENCE OF WATER CONTROL FACILITIES ON THERMAL STRATIFICATION OF OGOUCHI RESERVOIR FOR 58 YEARS

Selective withdrawal (SW) systems regulate the temperature of water released downstream of dam while vertical curtains (VC) prevent phytoplanktons from the shallow upstream reaches to disperse into the main reservoir. These water control facilities can affect the reservoir’s temperature distribution. Through long term analysis of Ogouchi’s temperature data together with numerical simulation, this paper explains the variation in temperature profiles during the three distinct periods in the reservoir’s operation namely, (1) deep penstock withdrawal (DPW), (2) purely SW and (3) combined SW and VC. Schmidt’s Stability Index (SSI) and Thermocline Strength Index (TSI) were computed to define thermal stratification. DPW promotes mixing and makes the reservoir generally warmer while SW creates a strong thermocline (high TSI) and when combined with VC, increases thermal stability (high SSI). Using 3-D hydrodynamic simulation, the conditions were successfully replicated. Results showed that outflow location and facilities largely affect the temperature and velocity profiles and the mixing and transport of river water within the reservoir.