As the first step in the improvement of a water power is the construction of the dam, so the first consideration in that work is strength and durability. While the builder of a mill is bound to consult economy in the means he employs in obtaining a sufficient fall for the purposes of his business, and diverting from its natural channel that portion of the stream which is to be made directly available, the economy which does not provide a sure resistance to sudden floods is very apt to prove the worst form of extravagance in the end.
The tremendous power which a stream acquires in time of high water is seldom properly estimated when it is seen moving tranquilly within its usual bounds. When the day of disaster comes, the trifling neglect which brought it on is regretted, but too late for remedy. Our object in the present volume is to offer a few suggestions which may be of practical value to our readers, in regard to the methods by which a dam may be economically built and yet be of effectual service and the greatest attainable durability. The power of water being by far the cheapest motive power which can be applied to manufacturing purposes, it is important to inquire how it may be employed with the least expense, without sacrifice of strength and utility.
Material and Form of Dams
The weirs or dams thrown across the beds of rivers have been constructed in a great variety of shapes and of different materials, some of them too costly for general use in a country where small mills are chiefly needed. In cases where the supply of water is large and a high fall is not demanded, a temporary dam composed of boulder stones is sometimes thrown across the stream in a diagonal or slanting direction, and of length considerably greater than its breadth. The water is thus partly forced into the conduit or race above the dam, and the remainder passes over the surface of the dam in a shallow sheet. Being hastily and cheaply built, a dam of this kind may be repaired without much outlay, but the inconvenience of doing this after every heavy rise of the stream is a material drawback on its value.
In contrast with this comparatively rude species of dam are those of more solid structure, substantially built of stone, and stretched across the river in the form of a bow, the curve being against the current the middle of the dam, in other words, being higher up the stream than the two ends. A dam of this sort, if provided with massive stone abutments, presents a firm resistance to the onset of a flood, and will stand any test ordinarily experienced.
It may be made with a gentle slope from the crest both up and down the stream ; or with a steep descent on each side, making its walls almost perpendicular ; or again with either a steep or sloping front on the upper side and on the lower a curved apron, the wall rounding downward from the top like the lower half of the letter 0, by which arrangement the fall is made gradual and its force abated.
In a stream of moderate size, a form of weir has sometimes been adopted resembling the letter V, with the apex or point directed up-stream. If built upon piles, with a frame of timber forming an inclined plane upon the face of the dam, and filled up with gravel surmounted by a mass of boulder stones well packed in, the dam will be nearly impenetrable by water. The position of the two arms of the V distributes the force of the water in passing over, and as the currents descending from either side tend toward the centre of the stream, the banks are less liable to be washed away. If timber is abundant, the frame, instead of having a uniform slope downward on the face of the dam, may be made in a series of steps like a wide stairway, breaking the water into cascades. The piles for such a dam may be placed at right angles with the current, stayed and covered with plank, and made watertight with sheet piling supported by foot piles. Constructed in other respects like the one last described, a dam of this kind will possess great durability and admit of no leakage.
An undue accumulation of water above the dam may be remedied by a channel and sluice gate in one of the side walls, by which the surplus water may be drawn off before reaching the crest of the dam. A self-adjusting dam of heavy planks strongly framed together is sometimes stretched across the stream, connected by hinges to the crest of the permanent dam, and held in an upright position by weights passing over wheels on the abutments. In case of a flood the weights give way partially to the increased pressure and the auxiliary dam is let down toward a horizontal posiiton, allowing the water to pass unobstructed. In place of an appendage of this kind, movable flash boards are often used, being held in place by pins and other supports along the brink of the dam, and tightly fitted to each other. In time of low water, the flash boards are of important service in obtaining sufficient head. When the stream rises, the boards are removed (though the supports may often remain) and the crest of the main dam being below high water mark, the surplus water escapes freely.
A cheap and substantial dam may be made, where timber is abundant, by laying a foundation of logs of considerable size, which are placed lengthwise of the stream and close together, forming a sort of corduroy road, extending from bank to bank. (See Illustration at top of page)
If the bottom is soft, the logs should be carefully fitted down and adapted to the inequalities of the bed, and if placed as deep as possible they will be less liable to decay by exposure in time of low water. The breastwork of the dam is built near the up-stream side of this foundation, the logs extending from under it down-stream, and serving as an apron to receive the waste water as it comes over. The rafters and coverings of the dam form an inclined plane on the up-stream side, and extend over the upper ends of the logs, protecting the foundation from being undermined by the water working beneath it.
In a region well timbered, and where the stream has a rock or other solid bottom, a log dam of the following description has advantages in point of cheapness, strength and durability. A series of large logs are placed in line, one at the end of another, at the down-stream face of the dam, the loose rubbish being carefully cleaned out, and hollow places filled with short logs to support the main foundation firmly. The logs used for the foundation tier should be as long and large as can conveniently be procured. A series of short logs are then laid upon this tier, with their butt ends resting on the foundation logs and their top ends on the bed of the river, pointing up stream, the distance between them being six or eight feet. Upon these a second tier of long logs is placed, parallel with the foundation tier, but a little farther up stream.
A second set of ties is laid with the butt ends on the second tier of logs and the top ends on the ground beside the first set. This second set of ties being a trifle shorter than the first, room is left to place a log of moderate size across the ends of the first ties. This will serve as a support for skids upon which to roll up the third tier of large logs. The logs should be notched where they cross and the ends resting on the ground firmly secured in order to impart the necessary strength to the whole structure.
If properly built, the front of the dam will rise considerably faster than the rear, and will at the same time incline up stream, so that its form will resemble a portion of an arch, the foot of the ties being the center and the breast of the dam the circumference. Beside the series of large logs in front, a second and even a third series of smaller size, running parallel with it across the stream, may be placed in the angles formed by the ties, which should be notched where they cross the logs ; and the three series of logs should range in hight so that the covering of the dam will form an inclined plane not too steep for the length of the incline, or the whole fabric may slide down stream when the pressure of the water is brought to bear. Either logs or rafters may be used in constructing the covering. If the former, they should be close together, and chinked with moss, pounded cedar bark,*or other suitable material. If rafters are used, they may be placed about three feet apart and planked crossways, the thickest planks being used at the bottom and at the crest of the dam.