Water Power for the Farm and Country Home

by David R. Cooper

Dam of E. B. Miner, Oriskany Falls, N. Y.

Main dam at left; flood spillway at right

The bed of the stream at the site selected for the dam is composed of solidly packed gravel. It was not considered advisable to lay timbers on such a foundation, so a ditch about two feet deep and one and one-half feet wide was dug across the creek bed and filled with concrete, to which a heavy timber was securely bolted, to form the upstream sill for the super-structure. The downstream side was supported on a sill of heavy timber whose ends were embedded in the concrete walls, or abutments, at either end of the dam and whose middle portion was supported by posts, spaced six feet apart, which in turn rested on large blocks of concrete placed in the bed of the creek. This downstream sill was about two and one-half feet higher than the upstream sill. A horizontal floor of double plank extending twelve feet downstream from the upstream sill and supported by the concrete foundations under the downstream sill formed an apron for the water to fall on. This prevents back-washing under the dam. A double layer of heavy plank was then fastened on the two sills, forming a sloping face on the water side of the dam. On the upper edge of this plank-facing, at the crest of the dam, are placed flashboards, one foot high and extending the full length of the dam, thirty-six feet, but divided into six sections, each six feet long. Each of these sections is hinged by the lower edge to the crest of the dam, while the upper edge is held from tipping over by chains fastened to cast-iron lugs located about halfway down the planking. The chain is held in these lugs by pins which are connected by rod and chain to a capstan, or spindle, located at one end of the dam, and are so arranged that by turning the spindle the pins will be drawn successively, thereby letting the flashboards down one at a time. The idea of this arrangement is that, when a flood is rising, the capstan may be turned with a heavy lever crank, winding up the chain and pulling down the flashboards one at a time, to give more space for the flood to pass through so as to prevent the water upstream from the dam from rising too high. This plan has prevented the washing away of Mr. Miner’s power house on several occasions.

Farm Power House on Oriskany Creek

Dam in left background; tail-race in right foreground

The sloping face of the dam receives the direct pressure of the water and transfers it to the sills, which in turn transfer it to the concrete foundation. The reason for sloping the upstream face of the dam is that the pressure of water is always normal, or perpendicular, to the surface against which it presses; therefore, if the face of the dam is sloping, the pressure is downward, rather than outward, as would be the case with a vertical face. This results in greater stability for the dam, due to the lessened tendency to tip over. With a dam of this type the higher the water rises against or over it, the more nearly vertical is the line of pressure, and the dam is held tightly down on its foundation instead of tending to tip over. It follows that the flatter the face of the dam the more stable it will be. Mr. Miner’s dam raised the water about four feet.

But in spite of his provision for floods, Mr. Miner did not want to be under the necessity of letting down his dam for every freshet, so he provided an additional permanent spillway. This is a simple concrete barrier, or wall, which flanks one end of the dam. In plan it was built at an angle with the dam proper, and extends downstream along the side of the natural bank. It was built with its crest a few inches higher than the main dam, so that during periods of ordinary flow the surplus water all passes over the main dam, but as soon as the creek rises a few inches over the main dam, water begins to flow over this extra spillway, which, being about forty feet long, will discharge a considerable volume although the water flowing over it is only a few inches in depth.

This spillway is strengthened on the downstream end by a concrete abutment, which consists of a simple heavy block of concrete extending above the top of the spillway. A similar abutment flanks the upstream end and also constitutes an abutment for one end of the main dam. The other end of the main dam is set against the opposite bank of the creek and is protected from washing and is strengthened by a similar concrete abutment.

It was considered desirable to place the [little power house] away from the main channel of the stream, so an earth embankment was built, extending from the downstream end of the flood spillway, a distance of about sixty feet. This embankment, or dyke, is curved in such manner as to divert the water behind it across a low place to a safe distance from the main channel. Some excavating had to be done behind this embankment in order to secure a channel of sufficient depth to prevent the water from freezing to the bottom and to provide a smooth channel of approach to the power house. This diversion of the water to one side from the main channel prevents the accumulation of debris and silt, which is a hindrance to the proper operation of a waterwheel. The pool thus formed is called a “forebay” and is very quiet water. The velocity of the water flowing through it is so slight that it will not carry much debris.