Newspapers / Zebulon Record (Zebulon, N.C.) / Jan. 15, 1937, edition 1 / Page 19
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Truing to OUTGUESS the RIVERS I I By Dr. Frank Thone AMERICA’S great rivers are rap idly being won away from the ranks of the unemployed. Boulder Dam has gone to work; its “juice” now pours over the wires clear to the Pacific shore, and be fore long the million thirsty throats of Los Angeles will be wet with Colorado river water. Norris Dam is generating light for thousands of homes in Ten nessee Valley cities and countrysides Day and night the walls of the Bonne ville and Grand Coulee dams rise higher—and even in advance of their completion are generating more than a little political heat. ' Most of us, when we behold these wonders that our hands have wrought, focus our attention on the dams them selves, and on the giant power plants at their feet. We give a thought, per haps, to the tremendous artificial lakes they have backed up into what were empty canyons only a little while ago, and we bestow a passing look on the masses of churned water pouring back into the channels below. Few ot us, however, give that dis charged water more than a second glance. Its work is finished, it has yielded its energy for conversion into electricity; it is now only a liquid ex haust. an aquatic ash, differing from other power-plant waste only in that it obligingly goes to the dump itself, in stead of having to be hauled there Engineers, though, do not take quite so careless and Inconsequential a view of that spent water. These fussy folk, trained to think quantitatively where the rest of us only guess qualitatively, still have questions to ask about that water even after they have squeezed all the electric juice out of rt that they can get on a paying basis, and after they have taken toll of it for city use and irrigation. CO long as water moves, it is develop- mg energy— doing work on some thing. It is the engineer’s business to know about energy and work—how much, where applied, what doing. So he looks downstream from the dam and asks his questions. But he doesn’t always get his an swers on the spot. Frequently he has to take the river indoors with him. in to his testing laboratory, and repeat the questions there. Os course it isn’t possible to take suah mighty streams as the Colorado or the Columbia into even the biggest en gineering laboratory But for at least some of the answers a small copy of the river will do almost as well as the original, just as marine engineers ca® figure out the "best lines for an ocean liner or a battleship by towing a* model fcT ■» #"kl ( , HBnßjßnaß' “ I through a long tank, or predict the mile-high behavior of a Zeppelin or a new-type airplane by putting small replicas of them into a wind tunnel. So when engineers began asking themselves, a short while ago, what the Colorado was going to do with it self and its channel after it has “gone through the works” at Boulder Dam, they did not try to answer it at Boul der. They went back to Washington, D. C., and in the laboratories of the National Bureau of Standards they built a wooden river. It didn’t look at all like the Colorado at Boulder Can yon, to be sure, but it did ineorpoiate the particular factors that were giving concern to the engineers. What they most wanted to know, in the particular downstream problem at Boulder Dam, was what the Colorado waters would do to the bottom ol the river. Would the “scour” be faster or slower, now that the water gushes forth without the heavy load of silt it bore in the days before the dam was built? For the water that comes out at the foot of the dam at Boulder—or of any dam. for that matter —is not the same as water in pre-dam days. A rapidly flowing river always carried along with it a considerable amount of mineral particles—sand and silt. The faster it flows the larger are the individual par ticles that it can carry, and also the larger is the total load. When a dam is thrown across such a river, creating an artificial lake, the river behaves just as it does when it flows into the still water of a natural lake, or of the sea. It loses its velocity, and thus become* unable to carry its load of mineral particles. The larger ones drop out first and the finer par ticles later. A delta is formed; the reservoir be gins to silt up. The water becomes very much clearer than it was in the old original stream; only the invisibly fine particles, that are in what is tech nically known as the colloid state, re main in suspension as the water leaves the dam. The Columbia river below Grand Ceulee dam. as it will look after the dam is completed. What will the silt-freed waters do to this channel? ... At left, sand from the Colorado river as seen under the microscope. CILTING up is something that hap- pens to all dams. When a dam created reservoir has become complete ly silted up, the dam of course is use less. The lifetime of a dam is measured, among other things, by the number ol years it will take for the silting-up process to complete itself. Engineers figure lifetime for dams, as they do for roads, skyscrapers, bridges, battleships and all the other works of man. They set these figures off against the cost and usefulness of the dams, when they are deciding whether given projects are economically justified. With this silt out of it, what will the re-cleared water do when it rushes out of the enormous tailraces of the power plants? The Bureau of Standards engineers built their wooden river to find that out. Technically, the apparatus is called a Hum*. But modern “flume” differs by only a letter from the ancient Latin “(lumen,’’ which means a river, so that to call the contrivance a wooden river is not just a bit of fancy. The Bureau of Standards flume is 40 feet long. 20 inches wide On eight feet of its bottom, bottom sand from the Colorado river was placed, and watei was flowed over that at various speeds and with varying amounts of stuff al ready in it. The muddy water that originally coursed through the canyor. was imitated in the laboratory by churning up fine particles of clay in the supply tank from which it was pumped to the flume, until 3Vi per cent of the weight of the water was made up ot clay. The work of the miniature river on its bed with clear water flowing was compared with its work when muddy water was used. Muddy water was found quite appreciably less efficient m causing scour on the sand bed. To cause the same amount of scour as clear water, it had to flow 10 per cent faster than clear water under similar condi tions. Sands somewhat coarser than those of the Colorado river bed were tried in the flume also, and it was found that with these the necessary increase in the velocity of muddy water was consider ably greater, running as high as 25 per cent. The difficult problem now facing the engineers is to correlate the results ob tained with the little wooden river in the laboratory with conditions in the actual Colorado river, whicb is about 350 feet wide and 120 feet deep below the dam and travels about 10 times as fast as the laboratory river, yet has the same sand bed. The exact answer to this question is not possible under the present state of hydraulic knowledge, but the results of this investigation indicate that a greater scouring away of the sand bed may be expected in the Colorado when clear water flows over it than was the case with the original muddy river. It is expected also that this scouring will progress gradually downstream. While the results of this specific ex periment will not apply directly to con ditions below the other big dams now under construction or already in use, the general principle that clear water is a better “picker-upper” than muddy water will doubtless occupy the atten tion of engineers who stand at damsites and look downstream elsewhere. Where a river flows over a rocky bot tom. the increased scouring effect of clear water will be a matter of less concern; but there are very few large rivers of this nature. Most of them, like the Colorado, have beds of sand; others are muddy-bottomed. These can expect downstream deepening if they are dammed. -if^ ■ iD&Bk sSXSfiB*? HlfplpP M H ■■ Studying stream actios through the glam sides of as artificial river at the University of Minnesota.
Zebulon Record (Zebulon, N.C.)
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Jan. 15, 1937, edition 1
19
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