Fig. 38.—Large Outlet.
Fig. 39.—Large Outlet.
It is almost essential to the efficiency of drains, that there be fall enough beyond the outlet to allow of the quick flow of the water discharged. At the outlet, must be deposited whatever earth is brought down by the drains; and, in many cases, the outlet must be at a swamp or pond. If no decided fall can be obtained at the outlet, there must be care to provide and keep an open ditch or passage, so that the drainage-water may not be dammed back in the drains. It is advised, even, to follow down the bank of a stream or river, so as to obtain sufficient fall, rather than to have the outlet flooded, or back-water in the drains. Still, there may be cases where it will be impossible to have an outlet that shall be always above the level of the river or pond which may receive the drainage water. If the outlet must be so situated as to be at times overflowed, great care should be taken to excavate a place at the outlet, into which any deposits brought down by the drain, may fall. If the outlet be level with the ground beyond it, the smallest quantity of earth will operate as a dam to keep back the water. Therefore, at the outlet, in such cases, a small well of brick or stonework should be constructed, into which the water should pour. There, even if the water stand above the outlet, will be deposited the earth brought along in the drain. This well must at times, when the water is low, be cleared of its contents, and kept ready for its work.
The effect of back-water in drains cannot ordinarily be injurious, except as it raises the water higher in the land, and occasions deposits of earthy matter, and so obstructs the drains. We have in mind now, the common case of water temporarily raised, by Winter flowage or by Summer freshets.
It should be remembered that even when the outlet is under water, if there is any current in the stream into which the drain empties, there must be some current in the drain also; and even if the drain discharge into a still pond, there must be a current greater or less, as water from a level higher than the surface of the pond, presses into the drains. Generally, then, under the most unfavorable circumstances, we may expect to have some flow of water through the pipes, and rarely an utter stagnation. If, then, the tiles be carefully laid, so as to admit only well-filtered water, there can be but little deposit in the drain; and a temporary stagnation, even, will not injure them, and a trifling flow will keep them clean. Much will depend, as to the obstruction of drains, in this, and indeed in all cases, upon the internal smoothness, and upon the nice adjustment of the pipes. In case of the drainage of marshes, and other lands subject to sudden flood, a flap, or gate, is used to exclude the water of flowage, until counterbalanced by the drainage-water in the pipes.
Fig. 40.—Outlet Pipe with Flap to Exclude Flood-water.