Minimizing Downstream Temperatures Through Modeling of Dam Operations

摘要

In this article, the temperature dynamics in the McNary Dam forebay and turbine intakes were numerically studied using a CFD model. The model captured the general observed diurnal stratification. Moreover, the model was able to reproduce the period of time when the maximum temperature in the forebay and in the gatewells were reached. The model was used to quantify the impact of atmospheric, operational and structural changes on the water temperature in the forebay and intake units. According to the numerical results: 1.The model reproduced the measured temperature profiles in the forebay for an extreme atmospheric condition observed July 9,2015. Gatewell temperatures were below the upper incipient lethal temperature for salmon. Although the air temperature in July 2015 was 6.5 C higher than on Aug. 18, 2004, gatewell temperatures were only about 0.5 C higher. Numerical results seem to indicate that powerhouse operation with the northern units can minimize the transport of the warm surface water into the units, resulting in less potentially damaging temperatures for salmonids migrating downstream; 2. Temperature within the southern gatewells increased about 0.2 C when 60% of the river discharge was spilled and the northern units were closed; 3. Operating with TSWs, the temperature in the gatewells of Units 11 and 14 increased by about 0.5 C; and 4. The presence of a forebay curtain upstream of the powerhouse reduced the temperature within the gatewells by about 1 to 2 C, preventing fish from entering the "resistance zone" in which their behavioral mechanisms allow them to survive short-term in extreme temperatures. Based on the earlier efforts the Corps try to run the units at lower discharges and space active units apart in an attempt to reduce their ability to be efficient at drawing down the warmer surface water. By doing this, withdrawals seem to be more lateral and less vertical in nature. The operators at the project call this a saw tooth pattern. Looking at the operation on July 9, this seems to be the case.