Until some thirty years or so ago hydro-electric power stations were built only on the banks of rivers where there was a sudden and a deep fall in the stream, giving a high head to the turbines, as at Niagara. The invention of a special turbine, however, made it possible to use a comparatively low head for obtaining electrical energy. The first serious attempt to obtain electrical power on a large scale with a low head was made by the Mississippi River Power Company. In 1913 the company completed a power station at Keokuk, on the upper reaches of the Mississippi River, in the State of Iowa. At that date it could claim to be the largest power plant so far erected.
There are no falls on the Mississippi, only a few rapids. The project was made possible by harnessing Des Moines Rapids. In a stretch of twelve miles these rapids have a total fall of some 23 feet. It was decided to use these rapids because the bluffs, or cliffs, are closer to the river here than they are at any other point in its entire length. This made it possible to build a dam at the foot of the rapids, and so create a large pool above it, from which water could be diverted into the intakes and drive the turbines by its momentum. The hard stone bluffs prevent any possibility of the water flooding the surrounding country. Then, again, the river bed at this point consists of hard, blue limestone, an ideal foundation on which to build a dam.
Briefly, the plan of operations was to build out from the eastern bank of the Mississippi, near Des Moines Rapids, a great dam 4,649 feet long. Connected to it at one end, and running horizontally to it downstream, is the power house. Just below the power house, but also joined to it, is the lock, and beyond this again the dry dock. The dam, the foundations of the power house, the ice fender that stretches upstream from its eastern corner, and the lock form one continuous structure of concrete two and a half miles long.
On either bank plants were set up, so that the work could be started from both sides simultaneously. This initial work was a formidable task. Fifteen miles of standard-gauge railway track had to be laid down. Rolling stock, consisting of nineteen powerful locomotives and 142 heavy wagons, was brought to the scene of operations. A powerful pumping plant, for driving the water out of the cofferdams, had also to be installed, as well as timber works, and huge concrete and stone-crushing plants. Then arrangements had to be made for housing an army of 2,500 men, who toiled for a period of two years and a half, through excessive heat in summer and blinding snowstorms in winter.
Because of the refractory character of the Mississippi the dam was built in the form of a bridge. With its abutments it is 4,649 feet long, 42 feet wide at the bottom, 29 feet wide at the top and 53 feet high. It is composed of 119 arched spans, measuring 30 feet the clear, the piers being 6 feet thick. The spillway sections are formed of arches 30 feet long and 32 feet high, with steel gates mounted on top of the spillway, the gates being 11 feet high and 32 feet wide. The height of the water in the pool above the dam is regulated by the spillway gates.
The foundations for the dam were obtained by the use of cofferdams or cribshuge wooden boxes without tops or bottoms. They were made of heavy timbers, criss-crossed and weighted with stone. They measured 24 feet in length and 16 feet in width, and contained 22,000 feet of timber. After they had been built they were placed on a raft and floated out to the selected site. Careful soundings were then taken to ascertain the exact conformation of the rocky bed of the stream. The bottom timbers of the crib were then shaped to conform with the uneven surface of the river's bottom ; then the whole was sunk and the water was removed by powerful pumps.
After some eight cofferdams had been sunk work on the dam was begun. Although the bed of the stream was composed of hard limestone, it was excavated to a depth of about five feet, and into this trench the piers were built up of steel forms filled with concrete.
After the concrete had firmly set, the steel forms were taken down to be set up again farther out into the river, as new foundations were obtained. As the piers, with their arches, were finished, rails were laid on them to carry a cantilever crane by which the concrete was dumped into the steel forms as they rose. This giant crane had a reach of 240 feet and was at the time the largest of its kind ever erected. After this long line of piers and arches had been finished, the openings between them were closed by concrete spillways and steel gates. The spillways were built up 5 feet at a time. To have built them to their full height at once would have meant that by the time half of them were up the water would be running through the other half so fast that the work there would be difficult.
Proof Against Ice Blocks
While work was progressing on the dam, men were busy on the western bank erecting the power house and the lock by which vessels could pass up and down the stream. Securing the foundations for the power house proved a difficult piece of work. This structure has a base measurement of 1,718 feet by 132 feet, so that a cofferdam of immense size was needed. Over 3,000,000 feet of heavy timber were requisitioned for the cofferdam, and for additional strength its sides were lined with thousands of stout steel plates. As an extra precaution, strong banks of earth, reinforced with heavy stones, were built round it.
These plates and earthen embankment were needed not so much to keep out the water as to make the structure strong enough to resist the pressure of ice and the force of the floods. Scarcely had the cofferdam been completed when it was subjected to a fierce attack. A winter of extraordinary severity had made the ice in the river much thicker than usual, and when it broke up the river was virtually a moving mass of ice. Carried down the stream by the force of the current, the ice struck the cofferdam head on, squarely and firmly. The great blocks of ice, many of them weighing tons, broke into thousands of pieces, and piled themselves against the cofferdam to a height of 30 feet above the top of the wall. But the cofferdam held firm, and no damage was done.
With the melting ice, however, came floods, and a month later the engineers were called upon to wage a stern battle against the rising waters that threatened to swamp the great cofferdam. The engineers were ready, however, for the attack, thousands of sacks of sand having been prepared and loaded into cars, to be hurried, if necessary, to the danger zone. The attack came at two o'clock one Sunday morning. Suddenly, without warning, a storm of considerable force came sweeping straight down the river.
This blew the water into waves of extraordinary height for an inland river, and, worse still, the waters began to rise at an alarming rate. A gang of men held in readiness for such an emergency was at once hurried to the scene and instructed to keep the water out of the cofferdam by placing the sandbags on top of the parapet. The men worked with feverish activity against great odds. The wall was only 2 feet wide, and with water dashing over their feet and the wind blowing with hurricane force, they found it difficult to keep their footing. Two hours passed, and hundreds of sandbags had been placed in position, when it was seen that the water was gaining the upper hand, and pouring over the wall in many places. Messengers were sent post-haste in all directions, and in a comparatively short space of time a hundred more men had arrived upon the scene. More sandbags, and still more, were hurriedly passed along the wave-washed wall, which it was necessary to guard at all costs, for it protected work that had cost enormous sums of money. It was a stern fight, and just as the men were ready to drop from sheer exhaustion the gale subsided and the waters began to fall. The engineers and their brave men had won, but no fewer than 5,000 sandbags had been required.
At the western end of the dam, at right angles to it, is the power house, which extends downstream for a distance of 1,718 feet. The width of the building is 132 ft. 10 in., and its total height is 177 ft. 6 in. It contains four floors, on the first of which are placed the thirty 10,000 horse-power generators and their auxiliaries. On the three floors above are the oil switches and electrical accessories.
The most wonderful part of the building is its substructure, which is one vast monolithic mass, extending 70 feet from the limestone bed of the river to the generator floor. Within this mass are formed thirty vertical cylindrical chambers for the turbines. Each chamber has conduits for the inflow of the water above and outlets for the exit of the water below. There are four circular intakes for each turbine. These converge into a scroll chamber, 39 feet in diameter, which is formed with a spiral floor, so shaped that the water impinges on every point of the circumference of the turbine with equal velocity and equal impulse. After the water has passed through the turbines, it emerges through a draught tube which is 18 feet in diameter at the top. At the bottom it is oblong in shape, this portion measuring 22 ft. 8 in. in height and 42 ft. 2 in. long. The bottom of the draught tubes and of the tailrace is 25 feet below the bottom of the river.
Huge Timber Boom
The turbines were specially designed and built to suit the conditions. The normal capacity of each is 10,000 horsepower, and the overload capacity is 13,500 horse-power. At 57.7 revolutions a minute they have shown an efficiency of 86 per cent. This high efficiency is due, in part, to the construction of the intake and of the spirally-shaped scroll chamber. Because of the comparatively low head (the rating is based on a head of 32 feet), it is necessary to pass great quantities of water through the turbines to secure the required power : hence their great size and weight. Each turbine has a diameter of 16 ft. 2 in., weighs 73 tons and carries twenty blades.
Extending from the upper western corner of the power house, and swinging in an easy curve to the Iowa shore, is the ice fender, a concrete structure 2,325 feet long, which is composed of twenty-nine spans with 10-feet piers and 60-feet openings. It is 16 feet wide at the bottom and 8 feet wide at the top. At its inshore end is a floating boom 300 feet long, of heavy timbers. The water flows into the forebay through the openings between the piers, and the ice and driftwood are stopped by the upper portion of the structure. During the navigation season the floating boom is swung back against the shore line, to permit the passage of vessels.
From the inshore end of the ice fender, down to the dry dock, there has been built a massive sea wall, ranging in height from 45 feet to 73 feet, and 1,110 feet long. Its function is to protect the adjacent tracks of the railway. The power house, ice fender and sea wall mark the boundaries of a large forebay, from , which the water flows through inlets in the western wall of the power house for the operation of the turbines. At the downstream end of the forebay is the lock, which is 110 feet wide, 400 feet long and has a lift of 40 feet.
At the time of its inception the Keokuk enterprise was the only development of its kind on a large scale in the heart of the United States. Its output of 300,000 horse-power was equal to that of all the five hydro-electric plants then operating at Niagara. By long-distance transmission lines electrical energy is carried to towns and cities a hundred miles and more away. All told, a sum of £5,000,000 was spent on the building of the power station.
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