Mr. Thomas Arkell, in a paper read before the Society of Arts, in 1855, says, on this point:
"The pressure due to a head of water of four or five feet, may be imagined from the force with which water will come through the crevices of a hatch, with that depth of water above it. Now, there is the same pressure of water to enter the vacuum in the pipe-drain, as there is against the hatches, supposing the land to be full to the surface."
We do not find any intimation that there is any error in the view advanced by the learned gentleman quoted; and if there is none, we have an explanation of the faculty which water seems to have, of finding its way into drainpipes. Yet, we feel bound to confess, that, aside from authority, we should have supposed that the pressure due to a column of pure water, would be essentially lessened, by the interposition of solid matter between its particles.
CHAPTER XX
EFFECT OF DRAINAGE ON STREAMS AND RIVERS.
Drainage Hastens the Supply to the Streams, and thus Creates Freshets.—Effect of Drainage on Meadows below; on Water Privileges.—Conflict of Manufacturing and Agricultural Interests.—English Opinions and Facts.—Uses of Drainage Water.—Irrigation.—Drainage Water for Stock.—How used by Mr. Mechi.
The effect of drainage upon streams and rivers, has, perhaps, little to interest merely practical men, in this country, at present; but the time will soon arrive, when mill-owners and land-owners will be compelled to investigate the subject. Men unaccustomed to minute investigation, are slow to appreciate the great effects produced by apparently small causes; and it may seem to many, that the operations of drainage for agriculture, are too insignificant in their details, perceptibly to affect the flow of mill-streams and rivers. A moment's thought will convince the most skeptical, that the thorough-drainage of the wet lands, even of a New England township, must produce sensible effects upon the streams which convey its surplus water toward the sea.
In making investigations to ascertain what quantity of water may be relied upon to supply a reservoir, whether natural or artificial, for the use of a town or city, a survey is first taken of the district of territory which naturally is drained into the reservoir, and thus the number of square miles of surface is ascertained. Then the rain-tables are consulted, and the fall of rain upon the surveyed district is computed. The ascertained proportion of rain-fall, which usually goes off by evaporation, is then deducted, which leaves with sufficient accuracy, the amount of water which flows both upon the surface, and through the soil, to the reservoir. With proper deductions for waste by freshets, when the water will overflow the reservoir, and for other known losses, a reliable estimate is readily made, in advance, of the quantity of water supplied to the reservoir.
Now, these reservoirs Nature has placed in all our valleys, in the form of lakes and ponds, and the drainage into them is by natural springs and streams; and the annual amount of the water thus naturally flowing into them may be readily computed, if the area within their head-waters be known. If the earth's surface were, like iron, impervious to water, the rain-water would come in torrents down the hill-sides, and along the gentle declivities, into the streams, creating freshets and inundations in a few hours. But instead of that, the soft showers fall, often on the open, thirsty soil, and so are gradually absorbed. A part of the rain-water is there held, until it returns by evaporation, to the clouds, while a part slowly percolates downward, finding its way into swamps and springy plains, and finally, after days or weeks of wandering, slowly, but surely, finds its outlet in the stream or pond.
If now, this surplus of water, this part which cannot be evaporated, and must therefore, sooner or later, enter the stream or pond, be, by artificial channels, carried directly to its destination, without the delay of filtration through swamps and clay-banks; the effect of rain to raise the streams and ponds, must be more sudden and immediate. Agricultural drains furnish those artificial channels. The flat and mossy swamp, which before retained the water until the Midsummer drought, and then slowly parted with it, by evaporation or gradual filtration, now, by thorough-drainage, in two or three days at most, sends all its surplus water onward to the natural stream. The stagnant clay-beds, which formerly, by slow degrees, allowed the water to filter through them to the wayside ditch, and then to the river, now, by drainage, contribute their proportion, in a few hours, to swell the stream. Thus, evaporation is lessened, and the amount of water which enters the natural channels largely increased; and, what is of more importance, the water which flows from the land is sent at once, after its fall from the heavens, into the streams. This produces upon the mill-streams a two-fold effect; first, to raise sudden freshets to overflow the dams, and sweep away the mills; and, secondly, to dry up their supply in dry seasons, and to diminish their water-power.
Upon the low meadows which border the streams, the effects of the drainage of lands above them are various, according to their position. In many cases, it must subject them to inundation by Summer freshets, and must require for their protection, catch-waters and embankments, and large facilities for drainage.