Dam (hydrology)

“ You are counting on the torrential water from the hills to keep up the level when you build your dam in the gorge. It strikes me that it would be a lot better to divert all that muddy stuff by a big spillway, so as not to clog your reservoir with silt. Then, instead of putting your dam where you have planned, you might spend a little more money on caisson work and throw a low, masonry core dam about a mile downstream so as to control the deeper alluvial currents which are constant the year around. ”

“ By some it was urged that drainage is necessary to lessen the chance of the earthwork slipping. Others contended that any drain which penetrates the dam must facilitate the percolation of water from the reservoir. It is clear that some of the speakers regarded the dam downstream of the core-wall as being partly staunch, and some as being wholly support. If for any reason there seems a chance of water leaking through the core-wall, it is desirable to regard the earth-filling next to it as staunch. ”

“ A dam must be built at the lower end of the bog, in such a manner as seems necessary from the location and force of the water running through the main ditch. If the main ditch is a brook which carries a large amount of surplus water, the dam must be very strongly and thoroughly built; but if, on the other hand, it is simply a ditch filled by springs or small brooks found in the bog, a simple dam can be thrown up at slight cost; although care must be taken to make it strong enough, so that the high water in winter or spring will not carry it away and leave the vines unprotected from the frost. ”

“ This extra flow disturbed both the colour and the temperature of the clear amber water of the pond. It lifted heavily against the ice, pressed up the tunnels to the very edge of the dry chamber of the lodge, and thrust ponderously at the outlets of the dam. Understanding the peril, the wise little dam-builders sallied forth in a flurry, and with [Pg 249] skilful tooth and claw lost no time in enlarging the outlets. They were much too intelligent to let the flood escape by a single outlet, lest the concentrated flow should become too heavy for them to control it. ”

“ There can be no injury to the dam from a moderate underflow if suitable precautions are take to let the water come to the surface at the down-stream toe of the dam through sand, gravel and broken rock deposits, so that it can not displace the earth particles with which it comes in contact in its passage under the dam. ”

“ If the current is gentle, the dam is made perfectly straight; but if rapid, it is constructed with a considerable curve, the convex side being towards the upper part of the stream. The materials employed are drift wood, green willows, birch, and poplar; these are placed horizontally, and kept down by mud and stones. So strong do these dams become, that they are capable of resisting a considerable force both of water and ice; for generally the wood, taking root, shoots upwards, and forms ultimately a thick hedge. ”

“ They also fill up all the gaps between them with mud, and so hard do they work that by the time the dam is finished it is often two hundred yards long, fifteen or even twenty feet thick at the bottom, and six or eight feet high. And when the river runs swiftly, they are clever enough to make their dam in the form of a curve, so that it may be better able to resist the force of the current. ”

“ The capability of the dam to maintain the waters of the lake at a sufficient level in the dry season depends upon their not finding a ready way either through the dam itself or below it. The construction of the dam is believed to guarantee its own practical impermeability. Not only is there a puddled core, but the mud, sand, and rocks of which the principal mass will be composed will be laid down in the manner best calculated to secure compactness. ”

“ Dams may be formed entirely of an embankment of earth and stone. Their base should not be less than their height, with increased thickness where pressure from a current has to be resisted. Shallow dams may be formed by driving two rows of plank across the line of the dam, and filling in the inclosed space with rammed clay. ”

“ If such a dam is sufficiently strong, and is built upon sound and moderately rough rock, it will always be incapable of sliding. Assuming also that it is incapable of crushing under its own weight and the pressure of the water, it must, in order to fail entirely, turn over on its outer toe, or upon the outer face at some higher level. It may do this in virtue of horizontal water-pressure alone, or of such pressure combined with upward pressure from intrusive water at its base or in any higher horizontal plane. ”

“ Dams situated upon the main drainage lines of large catchment areas are usually built of stone or concrete masonry, and designed with large sluiceways and spillways for the discharge both of silt and flood waters. It need scarcely be remarked that, as a rule, such sites are wholly unsuited to earthwork construction. It is said, however, that “every rule has at least one exception,” and this may be true of those relating to dam sites, as will appear later under the head of new types. ”

“ The dam, also named after Colonel Roosevelt, then President of the United States, floods two valleys, one twelve and the other fifteen miles long and each from one to three miles wide. The reservoir is nearly two hundred feet deep on the average. It is two hundred and eighty feet high, and the thickness of the dam ranges from one hundred and seventy-five feet at the bottom to twenty feet at the top, where its length is one thousand and eighty feet. Massive iron gates weighing sixty thousand pounds guard the outlet of the flood. ”

“ These rubble dams were well tested when the high Nile ran over them; and on work being resumed in November, after the fall of the river, water-tight sandbag dams or sudds were made around the site of the Dam foundation in the still waters above the rubble dams, and pumps were fixed to lay dry the bed of the river. This was the most exciting time in the whole stage of the operations, for no one could predict whether it would be possible to dry the bed, or whether the water would not pour through the fissured rock in altogether overwhelming volumes. ”

“ The dam is a loosely constructed barrier, seldom in water over three feet deep, and generally rising but little above the surface. As the parts of trees of which it is built are of kinds which easily take root from rods or wands, as willow, birch, and alder, these sometimes shoot up and spread, growing into a kind of hedge, which, catching débris coming down stream, constantly increases the strength and size of the structure. Slight and low as the beaver dam is, it is marvellous that a brute should construct it, and its size is by no means contemptible. ”

“ Across one of the many streams a dam of great boulders is laid, so that about the same amount of water is constantly kept running into a channel. These channels are often very long, they skirt steep slopes and are generally cut into the earth, sometimes into the rock; sometimes a little aqueduct is built of planks, mud and earth, supported by bamboo and other poles that stand in the valley. ”

“ The dam is more often built of branches laid parallel to the current with their butts pointing up stream, and weighted down with mud and stones; thus layer after layer is added until the structure rises to the desired height and strength. Some dams contain hundreds of tons of material. They are usually built upon a solid bottom, not of rock—though big, stationary boulders often are included in the construction for the extra support they furnish. ”

“ The runnel which feeds it descends from the snow-capped peak to the south, and copious supplies trickle through the soppy margin at the base of the dripping hills around. The surplus escapes through a head to the north, where a gated dam is thrown across to raise the level, and to regulate the water-power. ”

“ An abundance of water delivered by gravity under high pressure is usually regarded as one of the essential factors in hydraulic-fill dam building, but it is not essential that there be a large continuous flow. The Lake Frances Dam, recently constructed for the Bay Counties Co., of California, by J. D. Schuyler, is 75 ft. high, 1,340 ft. long on top, and contains 280,000 cu. yds. The dam was built up by materials sluiced by water that was forced by a centrifugal pump through a 12-in. ”

“ This dam constitutes one bank of the stream which receives the valley waters, and is about thirty-eight feet high, and half a mile in length; the elevation, however, gradually diminishes from the centre to the extremities. The mineral waters of the valley contain, as we have intimated, an immense quantity of lime, which is deposited with astonishing rapidity in the state of a simple carbonate, (especially in those places where the water has much motion,) producing those mineral forms called stalactites and stalagmites. ”

“ Even overnight, even in a few short hours, a stream has been known to swell from sudden rains or snow, bringing the water with a rush down steep mountain sides and carrying with it the logs that were left strewn on the slopes or near the bank. Men work with feverish haste to roll the logs into the stream. The whole is swept into the dam, the trigger is released at the right moment and the rush of water with its freight of logs sweeps through the open gate with a mighty roar, carrying its cargo for miles on down to the river. ”

“ As regards the choice of position for the dam of a reservoir, supposing that it is intended to impound the water by throwing an obstruction across a valley, it may be premised that to impound the largest quantity of water with the minimum outlay, the most favorable conditions are present where a more or less broad valley flanked by steep hills suddenly narrows at its lower end, forming a gorge which can be obstructed by a comparatively short dam. ”

“ The rise and fall of the water causes the float to rise and fall accordingly, and the float, in turn, opens and closes the gate, so as to render the escape of the water from under the dam sections proportionate to the height of water in the stream. The purpose of this opening is to control the height of the leaves of the dam, and not the water under the dam. If the dam is intended to be set at half the elevation of the full height of the leaves, this device properly adjusted would only allow enough upward pressure under the leaves to raise them to that height. ”

“ Dams are found close together like that when a beaver colony has occupied a stream for years unmolested. The food-wood above the first dam being cut off, they move down stream; for the beaver always cuts on the banks above his dam, and lets the current work for him in transportation. Sometimes, when the banks are such that a pond cannot be made, three or four dams will be built close together, the back-water of one reaching up to the one above, like a series of locks on a canal. ”

“ It is also more easily raised and strengthened—though this operation has also been effected on masonry dams—in case, for instance, of the silting up of the reservoir, a process which is slow in 164 England, but not so slow when water containing much silt is dealt with, as in the case of irrigation reservoirs in India. Sometimes a dam consists of a wall of masonry or concrete with earth behind it as a support. Whatever kind of dam is used, its construction always demands very great care. Serious disasters, with much loss of life, have occurred owing to failures of dams. ”

“ Since the strength of an earth dam depends upon two factors, namely, its weight and frictional resistance to sliding, the effect of water upon different materials entering into an earth structure should be most carefully considered. This will therefore occupy a large place in these pages. An earth embankment founded upon rock may become saturated by water forced up into it from below through cracks and fissures, reducing its lower [Pg 13] stratum to a state of muddy sludge, on which the upper part, however sound in itself, would slide. ”

“ North of the dam the river surface was at least two hundred feet higher than to the south. It lay above the dam like a placid canal, with low palisades its western bank and a high dyke built up along the eastern city side. The water went in spillways through the dam, forming again into the old natural river below it and flowing with it to the south.
The dam was not over a mile or so from Larry's window; in his time it might have been the western end of Christopher Street. The moonlight shone on the massive metal of it: the water spilled through it in a dozen shining cascades. ”

“ Possibly the reasons are that there is a prejudice against them, and well there may be, since they are usually so shallow that the water becomes impure, and since it is not generally realized that a substantial dam can be erected by the use of earth alone. If it is thought advisable not to allow the animals to go to the pool, it may be fenced, since it is not expensive to lay a pipe in the dam, when it is being constructed, on a level with the bottom of the pool, the outer end of the pipe being furnished with a ball and cock to regulate the flow of water into the trough. ”

“ The form may be made to suit the configuration of the dam site. When the earthwork requires it, they may be lined with various materials to secure water-tightness. Often such dams are made composite in character, partly of earth and partly of masonry or some other material. They are also frequently accompanied by numerous accessories, such as settling-basins, aerating devices and covers, which present a diversity in form and appearance. ”