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should be perfected without provision for such re-regulating storage, it may in time become necessary to add the Bullshead Reservoir to the Boulder Canyon project. Increased power output will make such addition self-supporting.

The discharges necessitated by power requirements will assure an adequate annual irrigation supply at all times.

SILT

The Colorado River board adopted a figure of 137,000 acre-feet as the annual silt load of the Colorado River compared with a previous bureau estimate of 80,000 acre-feet. The estimates of upstream development contemplate reservoirs with a total capacity of 11,000,000 acre-feet, of which some 8,000,000 acre-feet would be in power reservoirs principally at the lower ends of the main tributaries. Construction of the Bridge Canyon Dam is included with an active storage capacity of 1,000,000 acre-feet.

The upper reservoirs will materially reduce silt flow while Bridge Canyon would almost completely stop silt flow into Boulder Canyon. The silt accumulation during the repayment period has been assumed at 3,000,000 acre-feet with a maximum deposit at a level slightly below the average storage level.

POWER OUTPUT

Plant efficiency.

The following efficiencies have been assumed in all operations:

Penstocks.
Turbines.
Generators..
Transformers..

Per cent

97 90 96 99

Net over-all efficiency

83 Firm power.

Run-off records on Colorado River extend over a period of 37 years from 95 to 1932 No other stream in this locality has longer records. Great Salt Lake obtains most of its inflow from the western slope of mountains, the eastern side of which drain to the Colorado River. Inflow to Great Salt Lake can be estimated for the past 78 years from known lake levels, known lake area, estimated evaporation rates, and allowances for increasing depletions for irrigation uses. From such estimates it appears that there have been three periods of low run-off in 78 years, each of which closely approximated that of 1900–1905, inclusive, in total run-off for the 6-year period, one of these having a total 6 per cent below and the other 4 per cent above the late period.

There are, furthermore, other periods with conditions only moderately better. Firm power should, therefore, be based on the output obtainable in the low run-off period immediately following 1900. It has been decided that a maximum monthly shortage of 10 per cent in firm power output will be permissible.

With declining storage, increasing outflow is necessary for full power output. When such depletion is carried too far, undesirable results obtain in that the maximum power shortage becomes unduly high, irrigation shortages are invited, and total power output during the critical period is depressed by the continuance of low heads. To meet this situation, an empirical rule has been adopted for a maximum permissible draft of 15,000 second-feet under 1938 conditions whenever the storage level is below 15,000,000 acre-feet; and of 14,000 second-feet under 1988 conditions whenever the storage level falls below 15,000,000 acre-feet.

The resulting firm power for various maximum high-water levels is as follows:

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Dump power output.

Except during the critical periods of low run-off, large amounts of power may be produced in addition to the firm power. This report assumes power installations such that the firm power, if generated at a uniform rate would constitute 65 per cent of the installation, in effect permitting a load factor as low as 65 per cent. Plant capacity at any time not needed for the production of firm power may be utilized to produce dump power provided a suitable market be found.

A detailed study of the relation of accumulated precipitation in winter and early spring to the run-off in the succeeding summer produced results permitting a rather extensive use of flood storage capacity for power production without encroaching upon the primary purpose of such storage for flood control and without endangering firm power output in case a protracted period of low run-off should develop. The details of these provisions are so complicated that they will not be reproduced here but may be consulted in Exhibit H. Heights of dam considered.

The Colorado River Board report was based on consideration of a dam having a top elevation (exclusive of parapet) of 1,207 with ordinary high-water level fixed at elevation 1,197, a 550-foot raise of the low-water surface of the river. It has now been concluded that the 10-foot freeboard heretofore contemplated may safely be reduced to 3 feet, in view of the extremely rare occasions when the upper portion of the flood-control storage will be utilized. As the raise in water level has become the more commonly used term in designating the size of dam, that term will also here be used. The three levels considered are as follows:

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Summary of power output.

The results of the computations on the bases heretofore outlined are herein summarized.

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Reservoir capacity

-acre-feet- 27,000,000 24,000,000 29,500,000 26,500,000 30, 500,000 27,500,000

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Reservoir operations and related data bearing on the reservoir with 575-foot raise in water surface are presented as Plates 1 and 2. (Omitted]

The power analysis indicates a large amount of dump power available over a major portion of time provided adequate equipment be installed for its generation. If contractors for firm power should so coordinate their sources of power as to be able to utilize Boulder Canyon power at a 100 per cent load factor, no dump power would be available except through the provision of additional power units. The price obtainable for dump power may not warrant such an undertaking. In view of these uncertainties it is inadvisable to take dump power into consideration in determining relative advantages for different heights of dam.

Determinations of firm power output have necessitated extensive estimates in base data. Due caution dictates that the results be scaled down at least until new and additional data obtainable only in the course of a number of years of observation, particularly of stream flow and rainfall, shall confirm or alter the estimates made. It is proposed that the comparison be based on firm power output 10 per cent less than that computed for conditions of 1938. Firm power output under 1938 conditions:

Raise in water sur

face

Computed firm
power output

Adopted firm
power output

Adopted annual output

of firm power

Feet 557 575 582

Ilorse power
608, 000
650, 000
663, 000

Ilorse porer
547, 000
585, 000
597, 000

Kilowatt-hours
3, 600, 000, 000
3, 800, 000, 000
3, 900, 000, 000

COST OF POWER OBTAINABLE BY RAISING DAM

The selling price of falling water has tentatively been announced at 1.63 mills per kilowatt-hour of firm power. Estimates have been made on the same basis, of the increase in construction and annual costs occasioned by raising the dam. These costs have been compared with the increase in output, in the following table:

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The above table indicates a fast-mounting cost for the increased energy obtainable with a raise in height of dam, but with such costs still well below the cost with the 550-foot or 557-foot dam, even though the height be increased to 582 feet.

INTERFERENCE WITH BRIDGE CANYON SITE

At the head of the Boulder Canyon Reservoir lies the Bridge Canyon dam site, considered the most desirable of the sites on that section of the river. River level there is at elevation 1,207 feet, with 10,000 second-feet flowing.

The extent of interference by Boulder Canyon is presented in the following table:

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With a 557-foot dam there is negligible encroachment. With the 575-foot and 582-foot dams, the maximum encroachment is 16 feet and 23 feet, respectively, this result obtaining when a full reservoir at Boulder Canyon would be concurrent with a low discharge at Bridge Canyon.

Stream-flow conditions at Bridge Canyon resemble conditions at Bright Angel more nearly than at other stations and at that station there is a rise of 25 feet in water level for a discharge of 150,000 secondfeet. The still-water level with a full reservoir for a 582-foot dam would then roughly equal the level obtaining at present with a flood of 150,000 second-feet.

With the 575-foot and 582-foot dams there would be a backwater effect whenever the reservoir would be full. With very high floods this effect might amount to as much as 3 feet for the 575-foot dam and as high as 8 feet for the 582-foot dam.

From the standpoint of power production at Bridge Canyon there would then be a material, though minor reduction in power output with a 575-foot dam at Boulder Canyon, increasing rapidly with higher dams. From the standpoint of design, to care for flood levels, the Bridge Canyon power site would not be greatly affected with heights of dam at Boulder Canyon up to 582 feet.

CONCLUSIONS

1. For adequate flood control a capacity of 9,500,000 acre-feet should be reserved under 1938 conditions of upstream development declining to 4,000,000 acre-feet in the 50-year repayment period to 1988.

2. Firm power output obtainable upon completion of Boulder Canyon Dam in 1938, with a 10 per cent reduction on account of uncertainties in available base data, would be as follows:

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Same dam as reported by Colorado River Board, with freeboard reduced 7 feet, raising water level accordingly.

It is estimated that these outputs can be maintained throughout the repayment period, particularly if other power dams are constructed along Colorado River.

3. Large amounts of dump power can be produced in seasons of high run-off without encroaching on flood control, irrigation, or the production of firm power. The cost and market for this power are so uncertain that no income should now be counted on from this source.

4. Raising the dam reduces the average cost of firm power output. There are indications, however, that this will not hold true for heights much beyond 582 feet.

5. For Boulder Canyon dams up to 575 feet, the power value of Bridge Canyon dam site would be affected but little and construction cost at Bridge Canyon would not be appreciably increased. For greater heights interference progresses steadily.

E. B. DEBLER.

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