Evaluation of possible temperature fluctuations from proposed power modifications at Hoover Dam

There are several planned alternatives for increasing the generation capacity of Hoover Dam to help meet peak power demands. These alternatives include: (a) uprating the existing generating units, (b) replacing or adding one or more generating units and (c) adding reversible pumped-storage hydroelectric units. Since the existing generators are at the end of their economic life and have to be replaced, their uprating has been scheduled as routine maintenance. This will increase the generating capacity of the Hoover Dam powerplant from 1240 MW to 1810 MW, but the anticipated capacity for meeting power demand is 2300 MW. Therefore, modifications (alternatives B and C) are also being considered to obtain an additional 500 MW from Hoover Dam. The proposed modification of Hoover Dam will alter the existing daily discharge regime, but because of water requirements downstream, the total volume of water discharged over an annual period will remain the same. To meet peak power demands with the proposed alternatives, the daily discharge cycle will be changed to longer periods of low flow (evening-early morning)and shorter periods of peak flow (midafternoon-dusk). The peak discharge rate will increase to 76,000 ft3.sec-1 (Table l), but minimum flows of 2000 ft3.sec-1 will be maintained with alternatives A and B when the elevation of Lake Mohave is below 630 ft. Since the water of Lake Mohave extends to the tail race of Hoover Dam when lake elevations are greater than 630 ft., minimum flows of 2000 ft3.sec-1 will be unnecessary and hence zero discharge at night may occur. With reversible pumped-storage (alternative C), some of the water used for generating during peak power demands would also be pumped back to Lake Mead at night at a rate of up to 25,000 ft3.sec-1. This will cause reverse flows in the river section, and could pull Lake Mohave water to the dam. Hoover Dam has two sets of intake gates located at elevation 1045 ft. and 900 ft. on the four intake towers. The upper gates (1045 ft.), with few exceptions, have not been used since 1954, but, with the addition of generating units (alternative B) or the installation of reversible pumped-storage units (alternative C), one upper gate on the Arizona Tower would be used in conjunction with the four lower gates to facilitate the higher flows. We have previously reported (Paulson, Baker and Deacon 1980) that the discharge temperature could increase and undergo daily fluctuation due to withdrawal of increasing amounts of warmer water form Lake Mead at higher peak discharge. The U.S. Water and Power Resources Service therefore initiated this investigation to determine to what extent discharge temperature from Hoover Dam and thermal stratification in Lake Mead would change with discharges under the following conditions: (i) all four intakes on the upper gates (ii) all four intakes on the lower gates, (iii) from a combined use of one upper gate and four lower gates