The crib dam is peculiarly adapted to streams which have a comparatively narrow channel, with a high bank on each side although the latter condition is not indispensable, as any deficiency in this respect, if the shape of the country is not extremely unfavorable, can be made up by constructing an artificial levee or embankment. The structure of this dam is of the nature of crib-work throughout, logs being the material used in every part, although stone, gravel, clay and brush are employed in filling at various points, as hereafter described.
The dimensions of the dam shown are nearly as follows : length of span, fifty feet, the logs in each of the two sections being about thirty feet long, giving ample margin for notching at each end ; cribs on each side twenty feet square, the logs of which they are built from twenty-two to twenty-five feet long and the hight of the cribs from twentyfive to thirty feet. The dam itself is twenty-five feet high, the cribs being carried up three or four logs above the top of the dam.
In building a dam of this description, the whole structure, including both the cribs and the Y shaped connection between them, are begun and carried up together. The apron, however, is first put down, consisting of a layer of logs placed closely side by side from bank to bank, with the butt ends down stream, and the limbs lopped off up to the point where the dam is to rest upon the apron. Above this, the limbs may be left on the trunks, as they extend into the earth which is filled in above the dam. The front of the apron should extend three or four feet forward of the cribs, as shown in the cut.
The logs used in building the apron, and also the cribs and the dam itself, should be, if possible, at least one foot in diameter, in order to give the proper degree of weight, strength and solidity to the fabric.
Having completed the apron, the next step is to lay the foundation of the wings and central portion of the dam.
The first log of the crib on each side should be firmly pinned to the apron ; or the foundation of the crib may be laid two or three feet deeper than the apron, in which case it will not be necessary to fasten them together. The cribs are each to be set into the bank, which will thus enclose them on three sides, as appears in our illustration. Thus situated, it is scarcely within the bounds of possibility for the cribs to be moved from their position ; and if their connection with the dam is made firm and secure, the strength of the fabric, aided by the peculiar shape which it presents to the current on the up-stream side, will resist almost any conceivable pressure of water.
In building up the cribs and the dam, the logs are to be notched and saddled wherever they meet that is, at the four corners of each crib, at the points where the timbers of the dam enter the crib, and at the middle of the dam where the two sides of the angle or V intersect. This angle is of course pointed up-stream, the proper distance from the centre or place of intersection to the downstream edge of the apron being about twenty feet.
The pressure of the current upon the harrow-shaped structure thus presented to it will of course tend to spread the two wings or cribs apart; but if the latter are well grounded, filled and supported, and the logs in every part of the dam carefully notched upon each other, the force of the current will have no perceptible effect.
Binders are to be inserted in each half of the dam as the work progresses, one for every second course of logs being sufficient, although one for each course is still more effectual. Small trees or saplings may be used for this purpose with the limbs and brush left on, the butts resting between the logs of the dam and the tops forming a part of the filling on the up-stream side. In the engraving, the ends of these binders may be seen between the courses of logs forming the V, the tops of course being covered up and invisible.
The cribs are to be filled with stone and gravel, and if these materials are scarce, a moderate proportion of clay may also be introduced. The up-stream side of the V is to be covered with upright planking, which will extend from the top log down to the apron. Planks ten inches wide and two inches thick are suitable for this purpose, and they should be placed close together and either pinned or spiked to the logs, as convenience may dictate. The planking is cut away at the points where the binders occur, sufficiently to admit the ends of the binders, which rest upon the horizontal logs and are notched to them as already described.
Fillings and Embankment
The filling on the up stream side, against the planking, completes the building of the dam. For this purpose, any convenient material may be used, whether stone, gravel, clay or brush, or all together. The filling should slope gradually from the crest of the dam, extending up stream a distance of not less than twenty-five feet, in order that all risk of the washing or undermining of the dam may be avoided.
If the banks of the stream are too low to enclose the cribs to a sufficient hight to make them secure in their position, an artificial embankment must be constructed, covering three sides of the crib and extending from the stream unti it reaches ground of the same height as the top of the dam. This embankment should be made wide and substantial, and compactly built of stone or earth. It is important that the material should be of such a nature that the water will not penetrate it, as the destruction or serious injury of the dam may occur in consequence of a very small outlet. The main force of the stream is brought to bear, of course, upon the dam itself; but in time of high water there will be more or less pressure upon the levee, which should accordingly be made as secure as circumstances will allow.