Imágenes de páginas

a little above where the water makes its appearance. The drain must be low enough at the mouth to allow of cutting entirely through the layer of sand or gravel that carries the water, or much will escape under the drain. It is of little use to run drains end wise into banks, for the purpose of drainage, though it is sometimes done successfully when the object is only to obtain a supply of stockwater.


Fig. 38. In the drainage of swamps, or small basin-like depressions, it is customary to cut a main drain through the center, at a depth sufficient effectually to drain the lowest point, in the direction, for example, from 4 to the top of the bill, 1. Then other drains, as at 6, 6, 6, 7, which empty into the first from both sides, com. mencing as near as may be to the edge of the swamp to catch the water in its descent from the higher lands. Without these side drains, or a drain encircling such depressions to a greater or lesser extent, they frequently continue wet and cold, notwithstanding the existence of a good central drain or ditch.

Where there is a basin-shaped field, as in the annexed cut (Fig. 39), in which

[graphic][subsumed][subsumed][subsumed][ocr errors]

; Fig. 39. 1 represents a clay soil, 2 a bed of hard-pan, 3, 4 and 5 different layers of rock and shales, 6 gravel, drains may be cut centering at 7, like those at G, G, G, G, in Fig. 40 (next page), at H, cut through the stra'a into a pit or well; and, if necessary, minor drains may be cut leading into those figured.

In thorough draining, sufficient fall having been obtained from the lowest point of the land to be drained, that becomes the proper starting point. If the field

has a regular descent toward one of its sides, along that side the main drain is carried, and all the minor drains start from and run parallel one to another. If the lowest part of the land to be thoroughly drained be not along one of its sides, the main drain is carried along the lowest place, whether straight or otherwise, and the minor drains start from it on both sides. If the direction of the minor drains, be at right angles to the main drain, it is better to curve the end of the minor drain for a few feet, where it enters the main, so that its current may not be across that of the main drain, but partly in the same direction. The fewer main drains and general outlets to a field, the better. In the drainage of billsides, it has been a question whether the parallel drains should be carried down

FiG 40. the lines of greatest descent, or obliquely to it; but longer experience has settled the question, where the tiles are used, in favor of the line of greatest descent, or, in other words, running the minor drains straight down the slope.

One should think that a question apparently so self-evident would require no argument. But we find in the works of the various writers on this subject, that a great diversity of opinion exists. One party insists that if a drain be cut across the foot of the hill, as at 1 in Fig. 41, it will completely drain not only the stra

[ocr errors][merged small]

Fræ. 41. tum 3, but also that indicated by 2, and all above it; and, therefore, object to moking drains in the direction of the greatest descent. Another party would make a drain to carry off the water from each stratum which would crop out

from the hillside. But, in order to drain land effectually, it is essentially necessary that we have a correct idea of the sources from which the water is derived that is to be carried off; whether the water is directly from the clouds, or is derived from fields enjoying a greater elevation, and sloping toward it, so that the water comes down, like on a roof, from the other fields ; or whether it comes up in springs, which find vent in particular spots, as indicated at 7, Fig. 38. If the water is not derived from the adjoining fields but from the clouds direct, a differ. ent mode of draining is required than would be if the water came from higher fields. When lands are situated midway on an undrained slope, from which the water spreads over the surface of the land, such a system must be adopted as will not only drain the field in question, but also to cut off the supply of water from the bigher fields.

One thing must be borne in mind, that water runs down hill, and does not run so as to spread laterally. From the fact that water always seeks the lowest level by force of gravitation, and drains are simply lower levels to conduct the surplus water away, in order to decide correctly what direction a drain should have, it is not only necessary to have a correct idea of the sources of water, and the superposition of strata, but a definite idea as to the special office the drain is to perform so as to carry off the surplus water and drain the land.

As before stated, drains should be dug up and down the slope, as from 1 to 2, Fig. 41. Suppose a man has a field lying on a slope, which he wishes to drain. If he lay out his drains thirty feet apart, and cut them up and down the line of greatest descent, it is very evident that the drains will then intersect all the strata, and bear away the water from all of them. But, if he lay out his drains the same distance apart across the line of greatest descent, the lower drain will receive the water from the thirty feet next above it; the next drain from the thirty feet next above that and so on; thus compelling the water to traverse or percolate through thirty feet of soil before reaching a drain. But in the other case, the water will traverse a distance of fifteen feet only to find a conduit. The line of the greatest fall is the only line in which the drain is relatively lower than the land on either side of it. The water must be disposed of which rests upon the impervious strata, whether it has found its way there from fields or strata above, or whether it is water from the clouds, and has recently found its way there. But, in order to drain a field lying on a slope, with higher lands above it, it is, perhaps, as well to cut the upper di ain across the line of greatest descent, and lead it, as a sub-main, down the line of greatest descent, at the side or center of the field, to the outlet. This answers the purpose, as these drains significantly have been termed, of mere catch-waters.

Now, looking at the operation of draios across the slope, and supposing that each drain is draining the breadth next above it, we will suppose the drain to be running full of water. What is there to prevent the water from passing out of that drain in its progress, at every point of the tiles, and so saturating the breadth below it? Drain pipes afford the same facility for water to soak out at the lower side, as to enter on the upper, and there is the same law of gravitation to operate in each case. Mr. Denton gives instances in which he has observed, where drains were carried across the slope, in Warwickshire, lines of moisture at a regular distance below the drains. He could ascertain, he says, the depth of the drain itself, by taking the difference of hight between the line of the drain at the surface, and that of the line of moisture beneath il* He says again :

" I recently had an opportunity, in Scotland, of gauging the quantity of water traveling along an important drain carried obliquely across the fall, when I ascer. tained with certainty, that, although the land through which it passed was comparatively full of water, the drain actually lost more than it gained in a passage of several chains through it."

So far as authority goes, there seems, with the exception of some advocates of the Keythorpe system, of which an account has been given, to be very little difference of opinion. Mr. Denton says:

“With respect to the direction of drains, I believe very little difference of opinion exists. All the most successful drainers concur in the line of the steepest descent, as essential to effective and economical drainage. Certain exceptions are recognized in the west of England; but I believe it will be found, as practice extends in that quarter, that the exceptions have been allowed in error.

In another place, he says:

“ The very general concurrence in the adoption of the line of greatest descent, as the proper course for the minor drains in soils free from rock, would almost lead me to declare this as an incontrovertible principle.”

We will suppose A, B, Fig. 42, to represent a portion of the higher field above. Then the catch water or drain across the line of greatest descent will be represented by A, H, E, H, B; and when the nature of the ground will admit, or should there be a depression toward the center of the field, the catch-water may be led from E to J, as a sub-main, being some distance below J, the main drain. The minor drains then should run parallel, or nearly so, to E, J.

Where the distance from E to J is considerable, it is always advisable to run the minor drains F, F, F, etc., into sub.mains, G, G, G, G. In draining a piece of land, situated like that represented in Fig. 41, which would involve the cutting of ditches to the depth of eight or ten feet between 1 and 2, so as to have the

* French on Drainage.

drains of a proper depth at 3, it will be found advisable to lead the minor drains into a sub.main from 4 to 3, and then commence a new series of drains between 2 and I, and lead them into another sub-main at 1.


FIG. 42. Some good drainers advise, that when works stop on a slope, a drain called a kender should connect the tops of the minor drains, thus preventing the water lying between the upper sub.main, A, E, B, of Fig. 42, and the minor drains F, F, F, F, etc., ftom passing down into the ground between the minor drains, and also relieving the minor drains from the pressure of the water above them, and by which they will the more easily become clogged than when protected. However, when the sub.main is dug above the minor drains, as in the figure, the necessity of headers is very slight, except when the quantity and pressure of water is sufficient to cause it to flow over the sub.main.

Even the sub-main will not drain the slope above it entirely. Capillary attraction, and the resistance offered to the descent of the water will prevent the submain from bringing about a complete drainage. The cuttings of our railways and high banks of rivers show that no depth of ditch can remove the moisture from a very considerable distance. This part of the subject has been more fully discussed in the Chapter on Distance of Drains.

The sub-main draining the highest portion of the slope should be independent of all minor drains and branches, for being directly in contact with the head of water from ab ve, it will necessarily curry down more mud and silt, and have a tendency, if allowed, to choke up the minor drains.

« AnteriorContinuar »