Last week, in consultation with the Oregon Department of Environmental Quality and Warm Spring Tribal Natural Resources, PGE was granted flexibility to adjust the outflow schedule this year, if needed, to keep discharge temperatures below natural thermal potential.

 

This is a transition year for water temperature management. This means that although cold water from the Metolius River has been accumulating in the bottom of the reservoir, it is not accumulated to the depth it will be at mid-summer in future years. Because of the latent warmer water in the surface strata, the portion of the flow taken from the surface is warmer now than it will be in future years. It’s also warmer than the conditions studied and modeled to determine the appropriate discharges.

 

On Saturday, July 24, the percentage of the water leaving the reservoir from the bottom, cool-water intake was increased to lower discharge temperatures. This change was made in response to new information showing that the discharge was 1 degree Centigrade (1.8 degrees Fahrenheit) above the target temperature at the Madras USGS Gauge below the Regulating Dam at River Mile 100.

 

This was not a straightforward control change. As noted in my July 22 post, we began adding bottom water on July 1 to cool the water being discharged down the Deschutes River. The water quality rules called for 15 percent of the flow to be withdrawn from the deep, cooler portion of the reservoir throughout July, with monthly changes to cool the mix from Aug. 1 (30 percent cold) to Oct. 1 (50 percent cold).

 

Unfortunately, the computer controls for the two withdrawal elevations were programmed to work on a month-long water budget. When the change was made last Saturday, the computer attempted to apply the new top-bottom split retroactively to the entire month by opening the bottom withdrawal much more than needed for the increased percentage of bottom water requested for the remainder of July.

 

The result was that for two days the discharge was much colder than anticipated. Monday afternoon, the target was reduced, with the knowledge that the computer will apply the change retroactively and discharge significantly more cold water during the last five days of the month than 15 percent, but not so much as to reduce our ability to cool waters in September and October. On Aug. 1, the pre-programmed schedule will resume with 30 percent bottom withdrawal as required by Blend 17.

 

PGE engineers and technicians are investigating plans to increase the flexibility in the control system to allow refinement of the blends under special circumstances.

 

Don Ratliff,

PGE Senior Biologist

This entry was posted on Tuesday, July 27th, 2010 at 10:39 am and is filed under Metolius River & tributaries. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.