Measurements at Burlington, Iowa (40 miles above Keokuk).

36, 100 Double floats...... Lieut. G. K. Warren, United States Army.

Maj. F. U. Farquhar, Corps of Engineers, United
States Army.

1 In 1866 there was no standard gauge at Burlington and this gauge reading is approximate only as compared with the present gauge, which was established in 1872.

The observations listed above, purposely selected as showing the least discharge of record, have led the engineers of the development to establish in their calculations a minimum discharge of 20,000 second feet. Other observations of minimum discharge, made in 1906 under the direction of Mr. Cooper by a current meter, may serve to show the safe margin of surplus power often available over the amount calculated on the basis of the above minimum.

Measurements at Nashville, Iowa (6 miles above Keokuk).

The waters at Keokuk can not accurately be called turbulent. They are not hurled over hidden bowlders and irregular rock with the speed of a Niagara. The river has much less velocity and presents rather the smooth appearance of water running down an in

clined surface (pl.). An observer with a short metal sounding rod can hear the clear ring of the solid rock bottom all the way across. The regularity of the bottom and the rock of which it is made are charted on the facsimile of the United States Government map shown opposite page 200.

A well constructed concrete dam, power house, dry-dock, and lock on such foundations should last as long as the old Roman concrete work made of natural cement, a great deal of which has been standing 2,000 years and is still in good condition where not destroyed by the hand of man.

STORAGE OF WATER.

In the interests of navigation below the dam, particularly during the open period from March to December in each year, there are certain restrictions placed upon the complete interruption of the flow of the river. The Mississippi River Power Co., successor to the privileges and franchises of the Keokuk & Hamilton Water Power Co., is allowed to cut down the flow only during the night to 15,000 cubic feet per second for two hours, 10,000 cubic feet per second for six hours and 5,000 cubic feet per second during the balance of the time between sunset and sunrise. If the wheels do not pass the above amounts then the deficiency must be made up by letting water through the gates.

From an operating point of view these restrictions are not a gross handicap, for the storage of water during the night, when the load is light, will still be possible for use during the next day when the load is heavy.

In backing up this water, the amount of which can of course be regulated as will be seen from the description of the dam construction below, in the ultimate development, there will be formed a lake from 3 to 5 miles wide and about 40 miles long, overflowing the lowlands and thereby changing the topography of the country immediately adjacent to the river. It will also submerge the Government canal mentioned above which is being supplanted by the new lock and dry dock.

DETAILS OF THE DAM CONSTRUCTION.

Resting on the solid river bottom described, the plant is being built out from the bluffs on either side almost a mile apart. The construction is handled by two distinct organizations-the Illinois division building the dam, the Iowa division the locks and dry dock. Each construction plant consists of a concrete-mixing plant, a stonecrushing plant, a central power plant supplying compressed air and

electric current to the works, a machine shop, a carpenter shop, warehouses for storing cement, warehouses for miscellaneous materials, and various other structures.

The methods of construction, if not unprecedented, may at least be interesting to such as are not familiar with hydraulic engineering work:

A cofferdam consisting of a rectangular timber crib structure, loaded with stone and made water tight by means of clay puddle, is built around a section of the dam about 1,000 feet long. The water is then pumped out of this cofferdam. In this space thus pumped dry is excavated a trench in the solid rock, on which the dam is founded. The piers and arches forming the bridge and the bottom part of the dam between these piers are then built. After this the cofferdam is removed and another section is cofferdammed and the bridge built in the same way. This continues until the bridge is extended all the way from the Illinois shore to the junction with the power house on the Iowa side. This will leave 119 large openings between the bridge piers, through which the water passes unobstructed. These openings will finally be closed off, a few at a time, by means of steel gates, and the balance of the concrete part of the dam will be placed behind these steel gates, gradually raising the crest of the dam until it has reached its full height.

The dam, including abutments, is being built 4,568 feet long, or about seven-eighths of a mile. The spillway section is 4,278 feet in length. The height above the river bed is about 32 feet and its base is 42 feet wide. The upstream face is vertical. The downstream face is an ogee curve, the upper portion a parabola over which the water will spill, the lower portion an arc of a circle which will throw the water away from the toe of the dam. On the top of the spillway are being placed the steel floodgates, one for each opening, 30 feet wide and 11 feet high, supported by concrete piers. These piers are 6 feet thick and are built integral with the dam. The piers also support an arched bridge, from which the gates will be operated by electric hoists. By manipulating these gates the water above the dam may be maintained at a nearly constant level at all seasons.

The dam is being built entirely of massive concrete without reenforcement. It is being locked into the rock bed of the river by potholes and other excavations and is practically a monolith. All concrete, except at specially isolated places, is machine mixed, carried from the mixing plant to the point of use in large buckets by trains running on the completed portion of the dam, where a cantilever crane picks up the buckets of concrete from the cars, carries them out and dumps the contents into the forms.