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Reference: Vermeyen, T.B. (1997). "Modifying Reservoir Release Temperatures Using Temperature Control Curtains" Proceedings of Theme D: Energy and Water: Sustainable Development, 27th IAHR Congress, San Francisco, CA, August 10-15, 1997.



Modifying Reservoir Release Temperatures Using Temperature Control Curtains

TRACY B. VERMEYEN, P.E.

US Bureau of Reclamation, Denver, CO USA




ABSTRACT

Reclamation (Bureau of Reclamation) has constructed four temperature control curtains to reduce release water temperature at structures in the Sacramento and Trinity River drainages in northern California. These curtains provide selective withdrawal at intake structures, control topography induced mixing, and control interfacial shear mixing associated with plunging density currents entering reservoirs. Comprehensive field monitoring has been conducted to measure curtain performance characteristics. Monitoring included continuous temperature profiling, and velocity profiling using an ADCP (acoustic Doppler current profiler). This paper presents and summarizes performance data collected near curtains in Lewiston and Whiskeytown Reservoirs.

BACKGROUND

During the late 1980s, extended drought in northern California created potentially life threatening conditions for endangered salmon species inhabiting the Sacramento River. Summer and early fall river water temperatures threatened to exceed critical levels for sustaining juvenile salmon populations. High release water temperatures from the reservoirs, coupled with natural in-stream warming, threatened to make downstream waters too warm for egg incubation and juvenile fish survival. As a result, California's water resources agencies and the National Marine Fisheries Service imposed a maximum temperature of 56 F in the Sacramento River below Keswick Dam. To comply with temperature requirements, Reclamation began an aggressive program to construct selective withdrawal features that would yield cold water releases.

DESCRIPTION OF RESERVOIR SYSTEM

Water from the Trinity River Basin is diverted to the Sacramento River Basin through two tunnels and three reservoirs. Trinity River water is diverted from Lewiston Reservoir through Clear Creek Tunnel to the Judge Francis Carr Powerplant and discharged into Whiskeytown Reservoir. From there, water flows through the reservoir and into the Spring Creek Tunnel and through Spring Creek Powerplant. Spring Creek Powerplant releases water into Keswick Reservoir, where it combines with water released from Shasta Lake. Water released from Keswick Dam enters the upper Sacramento River. Over the course of this diversion and prior to curtain installation, Trinity River water temperatures commonly increased 10 to 13 F. To provide cold water releases, Reclamation engineers chose to install temperature control curtains in Lewiston and Whiskeytown Reservoirs; a total of four curtains were constructed. Curtains allow project operators to manage hydropower operations while controlling the temperature of water releases.

CURTAIN DESIGN

When a reservoir is thermally stratified, water can be selectively withdrawn from distinct horizontal layers. The vertical position and thickness of the withdrawal layer depends on several factors:

To develop a strategy for providing selective withdrawal, Reclamation engineers conducted a VE (value engineering) study to develop cost-effective selective withdrawal options. During the VE study, temperature control curtains were found to offer potential cost savings compared to structural modifications to existing intake structures. Physical model studies were conducted by Reclamation's Water Resources Research Laboratory to determine both reservoir and river responses to curtain installations in Lewiston and Whiskeytown Reservoirs (Johnson1993 and Vermeyen 1993).

The VE team recommended three sites for potential curtain installations. In Lewiston Reservoir, a small, shallow and weakly stratified impoundment, a curtain was recommended to provide selective withdrawal for a near-surface intake to the Clear Creek Tunnel. In Whiskeytown Reservoir, two curtains were recommended: 1) the Carr Powerplant tailrace curtain would minimize interfacial shear mixing which occurs when cold water entering the reservoir plunges below the epilimnion, and 2) a second curtain would provide selective withdrawal for the Spring Creek Tunnel intake.

Model and prototype performance of the Lewiston Reservoir curtain and the Carr Powerplant tailrace curtain will be summarized in this paper.

LEWISTON RESERVOIR CURTAIN MODELING

A physical model study indicated that a curtain surrounding the Clear Creek Tunnel intake was ineffective because substantial mixing with the epilimnetic water occurred upstream of the curtain when cold water passed through a restricted cross section in the reservoir. Mixing occurred in a shear zone, which developed between the withdrawal layer and the deeper epilimnion that formed upstream of the curtain. Locating the temperature control curtain upstream from the "narrows" reduced interfacial shear mixing by lowering curtain approach velocities. In addition, warming caused by mixing in the narrows was reduced by isolating this mixing zone from the main reservoir body, which limited the supply of warm water to the mixing zone. The recommended curtain design called for an 830-ft-long, 35-ft-deep, surface-suspended curtain.

CARR TAILRACE CURTAIN MODELING

A physical model study was conducted to develop a curtain design for Whiskeytown Reservoir that minimized mixing associated with plunging inflows. The model study indicated that a curtain located 2 miles downstream from the Carr Powerplant would effectively limit interfacial shear mixing. The recommended curtain design called for a 600-ft-long, 40-ft-deep, surface suspended curtain.

INSTALLING THE LEWISTON AND WHISKEYTOWN CURTAINS

In the summer of 1992, crews installed a temperature control curtain in Lewiston Reservoir based on the model study research. Total time for engineering, procurement, and construction of the Lewiston Reservoir curtain was five months. Reclamation's Northern California Area Office was responsible for the design and construction of the curtain. Costs for this curtain totaled $650,000.

Two temperature control curtains were installed in Whiskeytown Reservoir during the summer of 1993. The Carr tailrace curtain was fabricated and installed in one month at a cost of $500,000. A 100-ft-deep, 2,400-ft-long surface suspended curtain which enclosing the Spring Creek Tunnel intake was installed over a four-month period at a cost of $1.8 million.

figure

LEWISTON RESERVOIR CURTAIN PERFORMANCE

Lewiston curtain performance was evaluated by analyzing temperature data collected in the Clear Creek Tunnel intake. In 1992, data collected before and after curtain installation indicated that for similar operational conditions the average temperature of water entering the Clear Creek Tunnel was reduced by about 2.5 F after the curtain installation.

figure

During August and September 1994, curtain performance was evaluated for three types of power operations at Trinity and Carr Powerplants (see figure 2a): 1) during days 225 through 228, baseload power releases were held constant at 3,200 ft3/s; 2) days 229 through 236 had partial peaking power operations, during which flows fluctuated between 1,800 and 3,500 ft3/s. A comparison of Trinity Dam outflow and Clear Creek Tunnel temperatures showed a consistent 3.5F temperature gain through the reservoir for days 220 through 243 regardless of the operations (figure 1). However, when operations were changed on day 244, a steady increase in outflow temperature was observed. After day 254, temperature gain through Lewiston Reservoir had stabilized at 6.5 F. This additional 3F temperature gain occurred because warm water accumulated upstream and downstream from the curtain during no-flow periods. Then, during peaking operations, warm water was entrained and released. Warm water accumulation is shown in figure 2c, which is a plot of the hourly temperature profiles collected 150 ft upstream of the curtain.

Figure 2b is an isovel plot of ADCP (acoustic Doppler current profiler) data collected 200 ft upstream from the curtain for the same time period that temperature profiles were collected. Figure 2b illustrates several interesting selective withdrawal characteristics. For days 225 through 228, variations in the upper limit of withdrawal are caused by diurnal fluctuations in the epilimnion thickness. During the day, the epilimnion expands and forces the withdrawal zone to narrow. Conversely, after sunset, the epilimnion contracts and the withdrawal zone expands upward. For days 228 to 236, the expansion and contraction of the withdrawal zone was magnified by peaking flows during daylight hours.

Temperature profiles shown in figure 2c illustrate how the partial and full peaking power operations generated strong mixing upstream from the curtain. Mixing was strong enough to break down the thermal stratification. Two forms of mixing were responsible for the break down: 1) when peaking was initiated, the withdrawal layer extended through the thermocline and entrained epilimnetic water, and 2) plunging inflows generated interfacial shear mixing where the cold, denser water plunged beneath the epilimnion. When flow rates were reduced, reservoir stratification was quickly reestablished. These data and increased temperatures in the Clear Creek Tunnel intake lead to the recommendation that full peaking operations be avoided during periods when release temperature restrictions are in effect.

CARR TAILRACE CURTAIN PERFORMANCE

Curtain performance was evaluated by analyzing temperature data collected in the tailrace below Carr Powerplant and temperature profiles collected downstream from the curtain. The main objective was to determine the reduction of inflow warming attributed to the curtain. In May 1994, temperature profiles collected before and after curtain installation showed dramatic modifications to the reservoir stratification. After curtain installation, the temperature of water flowing into the hypolimnion was reduced from 56 to 53 F. The upstream epilimnion was reduced to a depth of 10 to 15 ft, and the downstream epilimnion expanded to a depth of 20 to 25 ft. In August 1994, the two curtains reduced the overall temperature gain of water routed through Whiskeytown Reservoir by 4 F compared to pre-curtain temperatures collected in August 1988. The majority of the temperature reduction was attributed to the Carr tailrace curtain.

Plots in figure 3 illustrate the hydraulic performance of the Carr tailrace curtain. The data were collected over an 11-day period when the power operations were baseload for 4 days and partial peaking for 7 days (figure 3a). Figure 3b shows ADCP data collected 150 ft downstream from the curtain. For baseload operations, velocities varied with diurnal fluctuations in underflow temperatures. For partial peaking operations, velocity variations were caused by displacement of the thermocline (figure 3c). When diversion flow rates through Carr and Spring Creek powerplants were increased a surface seiche formed. The seiche displaced the thermocline upward near the tailrace curtain and downward at the Spring Creek curtain. Conversely, when flow rates were reduced, the seiche quickly subsided. These variations to the thermal stratification did not adversely affect the performance of the Carr tailrace or the Spring Creek intake curtains. It was is notable that during reduced flow periods the curtain did not appear to hydraulically control the underflow; water was flowing into the hypolimnion as a density current.

figure

CONCLUSIONS

REFERENCES

Vermeyen, T.B. and Johnson, P.L. (1993). "Hydraulic Performance of a Flexible Curtain Used for Selective Withdrawal: A Physical Model and Prototype Comparison." Proceedings of the Hydraulics Division ASCE National Conference, San Francisco, CA, July 25-30, 1993.

Johnson, P.L. and Vermeyen, T.B. (1993). "Flexible Curtain Structure for Control of Vertical Reservoir Mixing Generated by Plunging Inflows," Proceedings of the Hydraulics Division ASCE National Conference, San Francisco, CA, July 25-30, 1993.

Last reviewed: 11/18/04