6.3 Industrial effects

The original PG&E power plant at Moss Landing began operations in 1952 [1]. Intake cooling for seven generators was supplied by water from Moss Landing Harbor (Figure 2). Of the original generators two discharged their 40% effluent through a pipe into Monterey Bay just south of the harbor entrance about 200 m offshore. Five of the original generators discharged about 60% of the heated effluent approximately 0.5 km inland from the H1B; however, this was stopped in 1995 when these units were retired. In 1998, Duke Energy assumed operation of the power plant and has upgraded two of the original units and has added two new turbine generators. The mean temperature of the effluent is approximately 11°C higher than the intake temperature. Intake rates increased from the original design rate of 1.4 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>3</sup> /min, to almost 3.0 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>3</sup> /min circa 1980 [11]. Presently, the average and maximum expected intake flow rates for the original units are approximately 1.8 and 2.3 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>3</sup> /min. The two new generators add almost 1.0 10<sup>3</sup> m<sup>3</sup> /min to the flow. The effluent is discharged directly at the head of Monterey Submarine Canyon through the existing underground piping.

The power plant intake provides a continuous landward flow independent of the tide. It has no effect on the exchange of waters in ES itself unlike the previous situation through the now abandoned slough outfall. However, it is still interesting to compare the power plant coolant water flow with the tidal prism of ES. For an intake rate of 3.0 <sup>10</sup><sup>3</sup> <sup>m</sup><sup>3</sup> /min, the total volume over a half tidal day (12.4 h) is 2.2 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> . If we use our estimate of the tidal prism of 6.2 <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> (1993), then the coolant water flow represents 35% of the ES tidal prism. Although the plant intake and discharge has no effect on the Slough, it dominates the water budget of

Moss Landing Harbor. The present intake rate through the harbor entrance with a cross-sectional area of 300 m<sup>2</sup> , decreases the ebb current speed by roughly 15 cm/s or about 10% of the observed maximum ebb current speeds under the H1B.
