Figure 3.4-3. Seminoe Reservoir CE-QUAL-W2 Model Grid

Figure 3.4-3. Seminoe Reservoir CE-QUAL-W2 Model Grid

Three mechanisms were identified that may change water column temperature stratification under Project operations: 1) Seminoe Reservoir water surface elevation changes due to daily generation and pumping operations; 2) water being withdrawn from Seminoe Reservoir during pumping operations; and 3) water being released to Seminoe Reservoir during power generation. Daily water surface elevation changes due to Project operations vary from less than 7 inches under high reservoir elevation conditions to approximately 20 inches under extremely low reservoir elevation conditions. Because Seminoe Reservoir’s normal maximum temperature stratification under existing conditions occurs in the summer when the reservoir elevation is high, any elevation changes due to Project operations are minimal and do not impact the naturally occurring thermal stratification. In the winter, when reservoir elevations are low, the elevation changes due to Project operations are larger, however, the water column is thermally well mixed (i.e., destratified) and, therefore, changes from Project operations are negligible, if at all. Consequently, elevation changes due to Project operations are not expected to impact naturally occurring thermal stratification or destratification.

The second possible mechanism for changing water temperature stratification is that water is withdrawn from Seminoe Reservoir by the inlet/outlet structure during pumping operations. Model sensitivity runs indicate that when water is withdrawn, the withdrawal action pulls water from model layers (elevations) at and above the location of the inlet/outlet structure. When temperature stratification is naturally present during the summer, withdrawing water from layers at and above the inlet/outlet structure pulls in slightly warmer waters instead of only the deeper, cooler waters near the inlet/outlet structure. When warmer water is then released back from the upper reservoir into the colder water at the inlet/outlet structure elevation in Seminoe Reservoir, it is more buoyant and rises closer to the surface which can change the water column temperature stratification. Although it is difficult to specifically isolate the effects of this process in water column temperature changes, sensitivity runs indicate that such effects are relatively minor in comparison to the effects of water release operations (power generation).

The third possible mechanism for changing water temperature stratification is water being released to Seminoe Reservoir by the inlet/outlet structure during power generation. Water release operations produce an exit velocity and corresponding momentum that temporally mix the temperature over a portion of the water column. Seminoe Reservoir model with Project operations suggests that this mechanism is the most significant. Water released from the upper reservoir to Seminoe Reservoir for 10 hours per day at 12,500 cfs produces a temporal temperature destratification in a portion of the water column immediately in front of the inlet/outlet structure and surrounding areas.

Seminoe Reservoir Dam Outflow

Results from Seminoe Reservoir model were analyzed to assess potential temperature changes in the water released by Seminoe Dam penstocks into Kortes Reservoir. Seminoe Reservoir model indicates that Project operations would result in a slight increase in the temperature of the water released by Seminoe Dam penstocks into Kortes Reservoir. The mechanism identified for Project operations increasing the penstock water temperature is