Temperature Exceedence Results at Belden Dam with Butt Valley PH

Flow Reduction and Variable Seneca Flows – Bechtel Modeling

B. McGurk and S. Tu, 050321

2105 Relicensing Proceedings

Objective:

In the March 4, 2105 sub-committee meeting, results were presented showing temperature conditions at Belden Dam and temperature conditions in Lake Almanor and in Butt Valley Reservoir that resulted from high (400 cfs) July and August releases at Canyon Dam. As a result of that presentation and subsequent discussion, a request was made to evaluate an alternative to reduce large releases at Canyon Dam that used more water than was required for temperature control. Based on the March 4 Bechtel modeling result, the new Canyon Dam release were set at a level of 200 cfs and 400 cfs for July and August, respectively. With these Canyon Dam releases, the task was to determine the corresponding reduction for the draft of warmer water through Butt Valley PH and then Caribou 1&2 to a point that could achieve a Belden Dam temperature of 19^○C at a frequency exceedance level of 70% in July and August. A 70% exceedance level represents a condition under which the predicted temperatures would be equal to or below the specified target level 70% of the time.

Results:

Bechtel completed the new 70% analysis using several assumptions to evaluate various alternatives, as shown in the tables below. As a base case and with the flows specified in the 2004 Settlement Agreement, the % temperature exceedance for the 19 ºC at Belden Dam target was determined to be 18% for July and 0% for August, as shown below. The exceedance was then determined using a 200 or 400 cfs release at Canyon Dam for July and August, respectively, and reducing the flow through Butt Valley Powerhouse, Caribou 1 and 2 powerhouses. The 200 and 400 cfs case, presented in March 4, 2005 meeting, does not cause any decrease in total flow through the Belden, Rock Creek, Cresta, and Poe PHs. The results of this case show a 19 ºC Belden Dam temperature exceedance of 50% and 30% for July and August, respectively.

The latest requested modeling by Bechtel showed that, with high Seneca flows and further reduction in flow through Butt Valley and Caribou 1 & 2 PHs, temperature can be controlled 70% of the time to result in 19^○C or cooler water being released into the Belden reach. The July case shows that a 400 cfs decrease in flow through the Caribous along with a 200 cfs release at Canyon Dam is needed to achieve the 70% exceedance at 19^○C. The August case shows that a 800 cfs decrease through the Caribous along with the release of 400 cfs at Canyon Dam is needed to achieve the 70% exceedance. The July case results in a 260 cfs decrease in water delivery through the system, and the August case results in a 460 cfs decrease through the system.

Flows at Canyon Dam, Caribou 1 &2 and Belden Dam

For Various Temperature Exceedance Levels in July

Flows Released at Canyon Dam Low-elevation Gate	Flows at Caribou Powerhouses	Flows into Belden Reservoir	% of 19ºC Temperature Exceedance at Belden Dam	Remark
60 cfs	Settlement Agreement	Settlement Agreement	18%	Base case
200 cfs	Settlement Agreement minus 140 cfs	Settlement Agreement	50%	Presented on March 4, 2005
200 cfs	Settlement Agreement minus 400 cfs	Settlement Agreement minus 260 cfs	70%	Requested Task

Flows at Canyon Dam, Caribou 1 &2 and Belden Dam

For Various Temperature Exceedance Levels in August

Flows Released at Canyon Dam Low-elevation Gate	Flows at Caribou Powerhouses	Flows into Belden Reservoir	% of 19ºC Temperature Exceedance at Belden Dam	Remark
60 cfs	Settlement Agreement	Settlement Agreement	0%	Base case
400 cfs	Settlement Agreement minus 340 cfs	Settlement Agreement	30%	Presented on March 4, 2005
400 cfs	Settlement Agreement minus 800 cfs	Settlement Agreement minus 460 cfs	70%	Requested Task

Implications:

These flow adjustments have repercussions to both surface water levels in Lake Almanor and to power production from the six powerhouses in the system. Current operations under the Settlement Agreement and under informal agreements between PG&E and the Almanor Lake Level Committee of Plumas County typically call for minimal releases from Almanor in June unless excessively large inflows are occurring. By delaying significant draft until early July, lake level is kept high, benefiting recreation values and maximizing the cold water pool in Lake Almanor. Moderate-to-high draft rates occur during July and August, depending on many factors, and high draft rates occur in September and October. The high draft rates are necessary to meet the terms of the Western Canal Contract.

Under the proposed reduction of draft in July and August, about 44.6 thousand acre feet (TAF) of water cannot be withdrawn from the Lake. Because draft rates are already high in September and October to meet the Western Canal Contract, no additional capacity is available to increase the rate by 44.6 TAF. Therefore, the 44.6 TAF would have to be drafted in June, lowering the lake level by about 2 feet in June. This could impact recreation values and would reduce the cold water pool in the Lake, leaving less available during the hotter July and August months.

By reducing the draft in August by 800 cfs through Butt Valley PH and the Caribous, those powerhouses have a 50% reduction in power output in a critical month. Belden, Rock Creek, Cresta, and Poe have about a 25% reduction in generation output. In July, the generation reduction is about 25% and 13%, respectively. This water is shifted to June, a time of lower need and lower value. This change in timing of generation has serious implications to power operations.