Preliminary Model Simulation for NFFR for a Special Gaming Scenario
325-cfs Seneca Instream Flow in July-August and
Caribou Powerhouses Closed
3/21/2005, S. Tu and B. McGurk
The 2105 License Group (2105LG) is investigating various alternatives to maintain daily mean water temperatures in the NFFR at 20 ºC or less. Twenty-three alternatives are being analyzed. One alternative, referred to as the ‘modified operation’ scenario, assesses the effectiveness of higher cold-water releases through the low-elevation gate of Canyon Dam while proportionally reducing draft from Lake Almanor through Butt Valley and Caribou powerhouses.
As a first step, PG&E was requested to produce screening information for the full scale ‘modified operation’ scenario. This scenario is hereby termed a ‘special gaming scenario’ because, although the scenario was useful to evaluate background conditions, the 2105LG was not actually proposing an alternative that called for the virtual shutdown of all the powerhouses on the river for July and August. The specific objective of the first step is:
Determine the magnitude of flow required through the Canyon Dam low-level gate that would meet 20 ºC or less for the NFFR from Belden to Poe, assuming no generation water was released from Butt Valley and Caribou powerhouses
Five SNTEMP models have been developed under the various FERC licenses (FERC 2105, FERC 1962 and FERC 2107). These models are capable of simulating the daily mean temperature profile in the river channel from the release point down to the next facility under prescribed environmental conditions. Each of these models is hydraulically and thermally connected in that the inflow condition to the downstream reach is connected to the prediction from the upstream reach (and the powerhouse if it is on-line). For this screening simulation, we used the environmental setting representative of a typical July and August conditions, i.e., a median value at which the frequency of occurrence is at the 50% exceedance level. The following assumptions are made in the modeling:
1. A normal year condition for both meteorological and hydrological input
2. The minimum streamflow release is assumed constant for the entire July-August period
3. Baseline case is the existing Prattville intake configuration with reservoir operation and instream flow schedule based on the 2004 Settlement Agreement (the only exception to the Agreement is that the blending of water through use of the upper and lower gate at Canyon Dam is not considered),
4. Reservoir modeling (MITEMP) for Lake Almanor is not considered at this stage. Instead, the release temperatures at Canyon Dam are assumed the same as those predicted under the baseline case,
5. In conjunction with Assumption 4, the storage in Lake Almanor is assumed unchanged from the baseline case regardless of the higher releases,
6. Butt Valley and Caribou powerhouses are assumed shut down,
7. Minimum streamflow release schedules for each stream reach are based on the pertinent license conditions, i.e., Settlement Agreement for Seneca and Belden Reaches under FERC 2105 (except July and August), the second 5-year phase plan for Rock Creek and Cresta Reaches under FERC 1962 license, and a tentative 150-cfs constant release for Poe under FERC 2107,
8. Water (including all tributaries and an estimated 90-130 cfs from the East Branch) in excess of the required minimum streamflow at each downstream dam is passed through the respective powerhouse, but the powerhouse flow is small: 180-210 cfs through Belden, about 260-330 cfs through Rock Creek, 390-440 cfs through Cresta, and about 580-640 cfs through Poe, and
9. Atmospheric heating through downstream reservoirs (Belden Forebay, Rock Creek, Cresta and Poe Reservoirs) is not considered
A 325-cfs Seneca flow release was selected and simulated. The selection of 325-cfs was based on the minimum streamflow release in the Cresta Reach under the second 5-year plan in the FERC #1962 license. The simulated daily mean temperature profiles for the baseline and 325-cfs Seneca Release case are shown in Figures 1 and 2 for a normal July and a normal August, respectively. From these initial simulations, it ‘appeared’ that the entire NFFR water temperatures could be maintained below 20 ºC under the 325-cfs Seneca release scenario.
However, subsequent examination of model assumptions revealed that Assumption 9 above is not actually true for the special gaming scenario. Assumption 9 is normally true and is based on the actual observation when the powerhouses are in full operation. For instance, Belden Powerhouse normally discharges a flow of 1000-2400 cfs to Rock Creek Reservoir. When the flow is reduced to 325 cfs, the corresponding travel time through the reservoir increases significantly and therefore prolongs the solar exposure time to the water. Solar heating would yield a more pronounced temperature increase when the reservoir water is colder relative to the normal, ‘warmer’ water condition. To demonstrate the significance of this heating, the SNTEMP model for the Belden Reach (SNTEMP-Belden) is extended downstream to include Rock Creek Reservoir. The extended SNTEMP-Belden model was calibrated with actual data using one of the special Caribou tests conducted in July 18-25 of 2003 when indeed colder water entered Rock Creek Reservoir. The extended model result is highlighted with an enclosed circle. Generally, a 1.5 ºC warming is predicted in Rock Creek Reservoir for the 325-cfs high Seneca release scenario.
Using a simplistic approach to account for this problem, all downstream predicted temperature profile could be shifted upward by approximately 1.5 ºC to account for the neglected Rock Creek Reservoir warming. With such a shift, the temperatures in Rock Creek and Cresta Reaches still appear likely to remain below 20 ºC. However, the temperature profile in the lower Poe Reach is likely to exceed the 20 ºC under this special gaming scenario once the Rock Creek Resevoir warming is incorporated. Further extension of the reservoir warming issue to include the other downstream reservoirs (such as Cresta and Poe Reservoirs) was not considered in this analysis, but would likely further raise the water temperature.
Figure 1 Simulated Daily Mean Temperature Profiles for Baseline Case and 325-cfs Seneca Release Case – Normal July
Figure 2. Simulated Daily Mean Temperature Profiles for Baseline Case and 325-cfs Seneca Release Case – Normal August