Rip Rap Dam

The construction of a " rip-rap dam," which is the term commonly applied to a dam of the description here shown, is begun by throwing an embankment of earth across the stream (space being left in the middle of the stream for the waste- way or chute) carrying it up to a hight of about
eight feet in the center and sloping it as shown in the cut, quite steeply on the down-stream and more gradually on the up-stream side. The dam here illustrated has an extent between the foot of the up-stream and that of the downstream slope, of from thirty to forty feet, and from one bank of the stream to the other of a little over seventy feet. Of the latter distance, twelve feet in the middle of the stream is occupied by the chute (in which the waste-gate is placed as hereafter described) leaving a distance of thirty feet on each side, from the frame-work of the chute to the bank.

It is not intended, of course, that the water should at any time flow over a dam of this sort, the escape of surplus water being provided for by the chute. The two slopes of the embankment do not meet at the top in such a way as to form a sharp ridge or crest, but the summit is leveled off so as to give a nearly flat surface about four feet in width, extending the whole length of the dam from each bank until it reaches the chute.

Earthwork Embankment

In constructing the embankment, the framework of the chute is to be set in position and strongly planked on the interior side, where the water is to pass, before the earth is filled in at that point except that a fill about two feet in depth is made on which the floor of the chute is to rest. This floor is laid upon a frame of heavy sills and cross-timbers, the planks of which it is composed extending lengthwise of the stream, and projecting at the down-stream end some eighteen inches beyond the face of the dam, in order that the current of water, as it issues from the chute, may be carried beyond and clear of the embankment beneath. The tendency of the water to wash away the foundation of the dam is thus avoided. This is an important point, as the result of neglect in this particular will be the speedy undermining of the chute and caving in of the wall of earth and rock on either side.

The earth-work having been completed, the dam is now to be "rip-rapped" from end to end. This process consists in laying two courses of stone, one above the other ordinary cobble-stones being a suitable material for the purpose over the whole surface of the embankment. The stones are placed on their edges, in the manner in which a gutter is paved, and laid as compactly together as possible, so as to give the entire dam a strong and durable face on both slopes and along the crest. The united depth of the two courses of stone will be about twenty inches.

If three instead of two courts are laid, additional strength will be gained, and the dam will be all the more secure from the effects of any accidental inroad of water. The rip-rapping should not be confined to the dam itself, but extend along the banks on both sides of the stream, a short distance above and below the dam, as shown in the cut. This will prevent the banks from being worn away or washed out, and protect the dam from injury, to which it would otherwise be constantly liable.

The up-stream slope of the dam is covered with earth from the base about two feet upward, reaching to the floor of the chute.

Inlet and Millrace

Our engraving represents, in addition to the dam, the inlet and part of the channel of the mill-race, on the further bank of the stream. The corners of the banks at the point where the water enters the race should be rip-rapped in the same manner as the darn, to secure them from being washed away and caved in by the continual action of the current. The exact distance to which the sides of the race at this point should be covered with stone will be determined by the shape of the bank, character of the soil, swiftness and force of the current, and other considerations which vary in different localities. The matter will be easily regulated by the exercise of a fair degree of judgment; but in general it is best to err, if at all, on the safe side. A little extra precaution, resulting in perfect security, is better than a falling short which may lead to damage and destruction in time of flood.

The construction of the chute and waste-gate is a matter in which, of course, some measure of skill in the carpenter's and millwright's trade will be in demand. The heavy timbers required are the sills and cross-timbers of the floor, the upright posts, the inclined or slanting beams which follow the direction of the slope of the dam up and down stream, and the timbers connecting them at the top, which
will be as long as the crest of the dam is wide. The posts are mortised into the sills below and into the beams above, and their lengths are so arranged as to give the proper slant to the inclined beams, parallel with the face of the dam.

Wastegate

For the construction and operating of the gate, a number of methods are in use. A very simple arrangement is that in which the gate is raised and lowered by the use of a lever inserted into holes in the standard to which the gate is attached. A chain and windlass may also be used, the manner of their application being so obvious as to require no minute description. Still another form of gate is found
very useful, in which the gate is made in sections, each section swinging on a horizontal axle resting on journals near the bottom of the gate, so that it can be let down like the tail-board of a cart when desired, and raised with equal ease whenever necessary.

The division of the gate into sections, or as it were, into several narrow gates, each acting independently of the other, is found expedient on account of the great force it would be necessary to apply to raise and lower the entire gate in the manner described. The
gates fall in the up-stream direction, their own weight assisting the process when they are lowered, and the force of the current helping to raise them sometimes more powerfully than is desired when the chute is to be closed.