Farm drainage Part 13

[Ill.u.s.tration: Fig. 33--FLAT-BOTTOMED PIPE-TILE.]

From an English report on the drainage of towns, the following, which ill.u.s.trates this point, is taken:

"It was found that a large proportion of sewers were constructed with flat bottoms, which, when there was a small discharge, spread the water, increased the friction, r.e.t.a.r.ded the flow, and acc.u.mulated deposit. It was ascertained, that by the subst.i.tution of circular sewers of the same width, with the same inclination and the same run of water, the amount of deposit was reduced more than one-half."

THE SIZE OF TILES.

Is a matter of much importance, whether we regard the efficiency and durability of our work, or economy in completing it. The cost of tiles, and the freight of them, increase rapidly with their size, and it is, therefore, well to use the smallest that will effect the object in view.

Tiles should be large enough, as a first proposition, to carry off, in a reasonable time, all the surplus water that may fall upon the land.

Here, the English rules will not be safe for us; for, although England has many more rainy days than we have, yet we have, in general, a greater fall of rain--more inches of water from the clouds in the year.

Instead of their eternal drizzle, we have thunder showers in Summer, and in Spring and Autumn north-east storms, when the windows of heaven are opened, and a deluge, except in duration, bursts upon us. Then, at the North, the Winter snows cover the fields until April, when they suddenly dissolve, often under heavy showers of rain, and planting time is at once upon us. It is desirable that all the snow and rain-water should pa.s.s through the soil into the drains, instead of overflowing the surface, so as to save the elements of fertility with which such water abounds, and also to prevent the washing of the soil. We require, then, a greater capacity of drainage, larger tiles, than do the English, for our drains must do a greater work than theirs, and in less time.

There are several other general considerations that should be noticed, before we attempt to define the particular size for any location.

Several small drains are usually discharged into one main drain. This main should have sufficient capacity to conduct all the water that may be expected to enter it, and no more. If the small drains overflow it, the main will be liable to be burst, or the land about it filled with water, gushing from it at the joints; especially, if the small drains come down a hill side, so as to give a great pressure, or head of water.

On the other hand, if the main be larger than is necessary, there is the useless expense of larger tiles than were required. The capacity of pipes to convey water, depends, other things being equal, upon their size; but here the word size has a meaning which should be kept clearly in mind.

The capacity of round water-pipes is in proportion to the squares of their diameters.

A one-inch pipe carries one inch (circular, not square) of water, but a two-inch pipe carries not two inches only, but twice two, or four inches of water; a three-inch pipe carries three times three, or nine inches; and a four-inch pipe, sixteen inches. Thus we see, that under the same conditions as to fall, directness, smoothness, and the like, a four-inch pipe carries just four times as much water as a two-inch pipe. In fact, it will carry more than this proportion, because _friction_, which is an important element in all such calculations, is greater in proportion to the smaller size of the pipe.

VELOCITY is another essential element to be noticed in determining the amount of water which may be discharged through a pipe of given diameter. Velocity, again, depends on several conditions. Water runs faster down a steep hill than down a gentle declivity. This is due to the weight of the water, or, in other words, to gravitation, and operates whether the water be at large on the ground, or confined in a pipe, and it operates alike whether the water in a pipe fill its bore or not.

But, again, the velocity of water in a pipe depends on the pressure, or head of water, behind it, and there is, perhaps, no definite limit to the quant.i.ty of water that may be forced through a given orifice. More water, for instance, is often forced through the pipe of a fire-engine in full play, in ten minutes, than would run through a pipe of the same diameter, lying nearly level in the ground, in ten hours.

In ordinary aqueducts, for supplying water, and not for drainage, it is desirable to have a high pressure upon the pipes to ensure a rapid flow; but in drainage, a careful distinction must be made between velocity induced by gravitation, and velocity induced by pressure. If induced by the former merely, the pipe through which the water is swiftly running, if not quite full, may still receive water at every joint, while, if the velocity be induced by pressure, the pipe must be already full. It can then receive no more, and must lose water at the joints, and wet the land through which it pa.s.ses, instead of draining it.

So that although we should find that the mains might carry a vast quant.i.ty of water admitted by minor drains from high elevations, yet we should bear in mind, that drains when full can perform no ordinary office of drainage. If there is more than the pressure of four feet head of water behind; the pipes, if they pa.s.sed through a pond of water, at four feet deep, must lose and not receive water at the joints.

The capacity of a pipe to convey water depends, then, not only on its size, but on its inclination or fall--a pipe running down a considerable descent having much greater capacity than one of the same size lying nearly level. This fact should be borne in mind even in laying single drains; for it is obvious that if the drain lie along a sandy plain, for instance, extending down a springy hill-side, and then, as is usually the case, along a lower plain again, to its outlet at some stream, it may collect as much water as will fill it before it reaches the lower level. Its stream rushes swiftly down the descent, and when it reaches the plain, there is not sufficient fall to carry it away by its natural gravitation. It will still rush onward to its outlet, urged by the pressure from behind; but, with such pressure, it will, as we have seen, instead of draining the land, suffuse it with water.

FRICTION,

as has already been suggested, is an element that much interferes with exact calculations as to the relative capacity of water-pipes of various dimensions, and this depends upon several circ.u.mstances, such as smoothness, and exactness of form, and directness. The smoother, the more regular in form, and the straighter the drain, the more water will it convey. Thus, in some recent English experiments,

"it was found that, with pipes of the same diameter, exact.i.tude of form was of more importance than smoothness of surface; that gla.s.s pipes, which had a wavy surface, discharged less water, at the same inclinations, than Staffordshire stone-ware clay pipes, which were of perfectly exact construction. By pa.s.sing pipes of the same clay--the common red clay--under a second pressure, obtained by a machine at an extra expense of about eighteen pence per thousand, whilst the pipe was half dry, very superior exact.i.tude of form was obtained, and by means of this exact.i.tude, and with nearly the same diameters, an increased discharge of water of one-fourth was effected within the same time."

So all sudden turns or angles increase friction and r.e.t.a.r.d velocity, and thus lessen the capacity of the drain--a topic which may be more properly considered under the head of the junction of drains.

"On a large scale, it was found that when equal quant.i.ties of water were running direct, at a rate of 90 seconds, with a turn at right-angles, the discharge was only effected in 140 seconds; whilst, with a turn or junction with a gentle curve, the discharge was effected in 100 seconds."

We are indebted to Messrs. Shedd & Edson for the following valuable tables showing the capacity of water-pipes, with the accompanying suggestions:

"DISCHARGE OF WATER THROUGH PIPES.

"The following tables of discharge are founded on the experiments made by Mr. Smeaton, and have been compared with those by Henry Law, and with the rules of Weisbach and D'Aubuisson. The conditions under which such experiments are made may be so essentially different in each case, that few experiments give results coincident with each other, or with the deductions of theory: and in applying these tables to practice, it is quite likely that the discharge of a pipe of a certain area, at a certain inclination, may be quite unlike the discharge found to be due to those conditions by this table, and that difference may be owing partly to greater or less roughness on the inside of the pipe, unequal flow of water through the joints into the pipe, crookedness of the pipes, want of accuracy in their being placed, so that the fall may not be uniform throughout, or the ends of the pipes may be shoved a little to one side, so that the continuity of the channel is partially broken; and, indeed, from various other causes, all of which may occur in any practical case, unless great care is taken to avoid it, and some of which may occur in almost any case.

"We have endeavored to so construct the tables that, in the ordinary practice of draining, the discharge given may approximate to the truth for a well laid drain, subject even to considerable friction. The experiments of Mr. Smeaton, which we have adopted as the basis of these tables, gave a less quant.i.ty discharged, under certain conditions, than given under similar conditions by other tables. This result is probably due to a greater amount of friction in the pipes used by Smeaton. The curves of friction resemble, very nearly, parabolic curves, but are not quite so sharp near the origin.

"We propose, during the coming season, to inst.i.tute some careful experiments, to ascertain the friction due to our own drain-pipe.

Water can get into the drain-pipe very freely at the joints, as may be seen by a simple calculation. It is impossible to place the ends so closely together, in laying, as to make a tight joint on account of roughness in the clay, twisting in burning, &c.; and the opening thus made will usually average about one-tenth of an inch on the whole circ.u.mference, which is, on the inside of a two-inch pipe, six inches--making six-tenths of a square inch opening for the entrance of water at each joint.

"In a lateral drain 200 feet long, the pipes being thirteen inches long, there will be 184 joints, each joint having an opening of six-tenth square inch area; in 184 joints there is an aggregate area of 110 square inches; the area of the opening at the end of a two-inch pipe is about three inches; 110 square inches inlet to three inches outlet; thirty-seven times as much water can flow in as can flow out. There is, then, no need for the water to go through the pores of the pipe; and the fact is, we think, quite fortunate, for the pa.s.sage of water through the pores would in no case be sufficient to benefit the land to much extent. We tried an experiment, by stopping one end of an ordinary drain-pipe and filling it with water. At the end of sixty-five hours, water still stood in the pipe three-fourths of an inch deep. About half the water first put into the pipe had run out at the end of twenty-four hours. If the pipe was stopped at both ends and plunged four feet deep in water, it would undoubtedly fill in a short time; but such a test is an unfair one, for no drain could be doing service, over which water could collect to the depth of four feet."

1-1/2-INCH DRAIN-PIPE.

Area: 1.76709 inches.

==================================================================== FALL VELOCITY DISCHARGE FALL VELOCITY DISCHARGE in per second in gallons in per second in gallons 100 feet. in feet. in 24 hours. 100 feet. in feet. in 24 hours.

---------+----------+------------++---------+----------+------------ ft. in. ft. in. 0.3 0.71 5630.87 5.3 3.75 29704.51 0.6 1.04 8248.03 5.6 3.84 30454.28 0.9 1.29 10230.73 5.9 3.93 31168.06 1.0 1.52 12054.81 6.0 4.00 31723.21 1.3 1.74 13799.59 6.3 4.10 32516.36 1.6 1.91 15147.83 6.6 4.18 33150.76 1.9 2.10 16654.68 6.9 4.25 33705.91 2.0 2.26 17923.61 7.0 4.33 34340.38 2.3 2.41 19113.23 7.3 4.41 34974.85 2.6 2.56 20302.86 7.6 4.49 35609.30 2.9 2.69 21333.86 7.9 4.56 36154.45 3.0 2.83 22444.17 8.0 4.65 36878.23 3.3 2.94 23150.71 8.3 4.71 37354.08 3.6 3.06 24268.25 8.6 4.79 37988.55 3.9 3.16 25061.34 8.9 4.85 38464.40 4.0 3.28 26013.03 9.0 4.91 38940.25 4.3 3.38 26806.11 9.3 4.98 39495.39 4.6 3.46 27440.58 9.6 5.04 39971.24 4.9 3.56 28233.66 9.9 5.10 40447.10 5.0 3.65 28947.43 10.0 5.16 40922.93 ====================================================================

==================================================================== 2-INCH DRAIN-PIPE. 3-INCH DRAIN-PIPE.

---------+----------+------------++---------+----------+------------ FALL VELOCITY DISCHARGE FALL VELOCITY DISCHARGE in per second in gallons in per second in gallons 100 feet. in feet. in 24 hours. 100 feet. in feet. in 24 hours.

---------+----------+------------++---------+----------+------------ ft. in. ft. in. 0.3 0.79 10575.4 0.3 0.90 24687.2 0.6 1.16 15528.4 0.6 1.33 36482.2 0.9 1.50 20079.9 0.9 1.66 45534.2 1.0 1.71 22891.1 1.0 1.94 53214.7 1.3 1.94 25970.0 1.3 2.19 60072.2 1.6 2.16 28915.1 1.6 2.43 66655.5 1.9 2.35 31458.5 1.9 2.63 72141.5 2.0 2.53 33868.1 2.0 2.83 77627.6 2.3 2.69 36009.9 2.3 3.00 82290.7 2.6 2.83 37884.0 2.6 3.16 86679.6 2.9 2.97 39758.2 2.9 3.31 90794.1 3.0 3.11 41632.4 3.0 3.47 95182.9 3.3 3.24 43372.6 3.3 3.60 98748.9 3.6 3.36 44979.0 3.6 3.74 102589.1 3.9 3.48 46585.4 3.9 3.87 106155.0 4.0 3.59 48057.9 4.0 3.99 109446.7 4.3 3.70 49530.5 4.3 4.11 112738.3 4.6 3.80 50869.1 4.6 4.23 116029.9 4.9 3.91 52341.6 4.9 4.34 119047.3 5.0 4.02 53814.1 5.0 4.46 122338.9 5.3 4.11 55018.9 5.3 4.57 125356.2 5.6 4.22 56491.5 5.6 4.68 128373.5 5.9 4.31 57696.3 5.9 4.78 131116.6 6.0 4.40 58901.1 6.0 4.89 134133.9 6.3 4.49 60105.9 6.3 4.98 136602.6 6.6 4.58 61309.7 6.6 5.08 139345.6 6.9 4.66 62381.6 6.9 5.18 142088.7 7.0 4.74 63452.5 7.0 5.27 144557.4 7.3 4.83 64667.3 7.3 5.37 147306.4 7.6 4.91 65728.3 7.6 5.46 150069.1 7.9 4.99 66799.2 7.9 5.55 152237.8 8.0 5.07 67870.1 8.0 5.64 154706.6 8.3 5.15 68941.0 8.3 5.73 157175.3 8.6 5.23 70011.9 8.6 5.82 159644.0 8.9 5.31 71082.8 8.9 5.91 162112.7 9.0 5.38 72019.9 9.0 5.99 164313.2 9.3 5.46 73090.9 9.3 6.07 166501.6 9.6 5.53 74027.9 9.6 6.16 168970.3 9.9 5.60 74965.0 9.9 6.24 171164.7 10.0 5.67 75902.0 10.0 6.32 173359.1 ====================================================================

==================================================================== 4-INCH DRAIN-PIPE. 5-INCH DRAIN-PIPE.

---------+----------+------------++---------+----------+------------ FALL VELOCITY DISCHARGE FALL VELOCITY DISCHARGE in per second in gallons in per second in gallons 100 feet. in feet. in 24 hours. 100 feet. in feet. in 24 hours.

---------+----------+------------++---------+----------+------------ ft. in. ft. in. 0.3 1.08 43697.6 0.3 1.13 99584.2 0.6 1.50 60691.2 0.6 1.57 138362.4 0.9 1.83 74043.2 0.9 1.90 167442.6 1.0 2.13 86181.4 1.0 2.20 193881.0 1.3 2.38 96296.6 1.3 2.45 215912.9 1.6 2.61 105602.6 1.6 2.70 237944.9 1.9 2.81 113694.8 1.9 2.90 255569.5 2.0 3.00 121382.3 2.0 3.10 273195.9 2.3 3.19 129089.9 2.3 3.29 289940.1 2.6 3.36 135948.2 2.6 3.46 304921.9 2.9 3.53 142826.5 2.9 3.64 320784.9 3.0 3.68 148895.7 3.0 3.80 334885.4 3.3 3.82 154560.2 3.3 3.96 348974.8 3.6 3.96 160224.7 3.6 4.11 362204.9 3.9 4.10 165889.2 3.9 4.26 375424.1 4.0 4.24 171553.7 4.0 4.40 387762.1 4.3 4.37 176813.6 4.3 4.52 398337.5 4.6 4.50 182073.5 4.6 4.66 410675.3 4.9 4.62 186928.3 4.9 4.78 421250.6 5.0 4.75 192188.7 5.0 4.90 430825.0 5.3 4.86 196639.4 5.3 5.02 442401.3 5.6 4.97 201090.1 5.6 5.14 452976.6 5.9 5.09 205945.3 5.9 5.25 462670.6 6.0 5.20 210396.0 6.0 5.37 473246.0 6.3 5.30 214442.1 6.3 5.49 483820.4 6.6 5.41 218892.8 6.6 5.60 493514.6 6.9 5.51 222938.8 6.9 5.70 502327.4 7.0 5.61 226984.9 7.0 5.80 511140.2 7.3 5.71 231031.0 7.3 5.90 520052.0 7.6 5.81 235077.1 7.6 6.00 528766.5 7.9 5.91 239123.2 7.9 6.10 537578.7 8.0 6.01 243169.2 8.0 6.20 546391.5 8.3 6.10 246810.7 8.3 6.30 555204.5 8.6 6.19 250452.2 8.6 6.40 564017.0 8.9 6.28 255493.7 8.9 6.49 571948.0 9.0 6.37 257735.2 9.0 6.58 579880.0 9.3 6.45 260971.9 9.3 6.66 586930.2 9.6 6.54 264603.1 9.6 6.75 594861.4 9.9 6.63 268254.9 9.9 6.84 602793.2 10.0 6.71 271491.8 10.0 6.93 610723.8 ====================================================================

8-INCH DRAIN-PIPE.

Area: 50.2640 inches.

==================================================================== FALL VELOCITY DISCHARGE FALL VELOCITY DISCHARGE in per second in gallons in per second in gallons 100 feet. in feet. in 24 hours. 100 feet. in feet. in 24 hours.

---------+----------+------------++---------+----------+------------ ft. in. ft. in. 0.3 1.23 277487.7 5.3 5.35 1206959.3 0.6 1.65 372239.7 5.6 5.47 1234031.3 0.9 2.01 453455.7 5.9 5.59 1261103.3 1.0 2.33 525647.7 6.0 5.71 1288175.3 1.3 2.60 586559.7 6.3 5.83 1315247.3 1.6 2.85 642959.6 6.6 5.95 1343838.9 1.9 3.08 694847.6 6.9 6.07 1369391.3 2.0 3.30 744479.7 7.0 6.17 1391951.2 2.3 3.50 789599.6 7.3 6.27 1414531.1 2.6 3.70 844719.7 7.6 6.39 1441583.2 2.9 3.89 877583.5 7.9 6.50 1466399.3 3.0 4.05 913679.5 8.0 6.60 1488959.2 3.3 4.21 949775.6 8.3 6.70 1511539.1 3.6 4.37 971658.7 8.6 6.80 1534099.0 3.9 4.53 920447.4 8.9 6.90 1556658.9 4.0 4.67 1055551.4 9.0 7.00 1579199.3 4.3 4.81 1086135.4 9.3 7.10 1601759.2 4.6 4.95 1116718.7 9.6 7.20 1624319.1 4.9 5.08 1146047.4 9.9 7.29 1644622.1 5.0 5.22 1177631.3 10.0 7.38 1664927.1 ====================================================================

HOW WATER ENTERS THE TILES.

How water enters the tiles, is a question which all persons unaccustomed to the operation of tile-draining usually ask at the outset. In brief, it may be answered, that it enters both at the joints and through the pores of the burnt clay, but mostly at the joints.

Mr. Parkes expresses the opinion, based upon careful observation, that five hundred times as much water enters at the crevices as through the pores of the tiles! If this be so, we may as well, for all practical purposes, regard the water as all entering at the joints. In several experiments which we have attempted, we have found the quant.i.ty of water that enters through the pores to be quite too small to be of much practical account.

Tiles differ so much in porosity, that it is difficult to make experiments that can be satisfactory--soft-burnt tiles being, like pale bricks, quite pervious, and hard-burnt tiles being nearly or quite impervious. The amount of pressure upon the clay in moulding also affects the density and porosity of tiles.

Water should enter at the bottom of the tiles, and not at the top. It is a well-known fact in draining, that the deepest drain flows first and longest. A familiar ill.u.s.tration will make this point evident. If a cask or deep box be filled with sand, with one hole near the bottom and another half way to the top, these holes will represent the tiles in a drain. If water be poured into the sand, it will pa.s.s downward to the bottom of the vessel, and will not flow out of either hole till the sand be saturated up to the lower hole, and then it will flow out there. If, now, water be poured in faster than the lower hole can discharge it, the vessel will be filled higher, till it will run out at both holes. It is manifest, however, that it will first cease to flow from the upper orifice. There is in the soil a line of water, called the "water-line,"

or "water-table;" and this, in drained land, is at about the level of the bottom of the tiles. As the rain falls it descends, as in the vessel; and as the water rises, it enters the tiles at the bottom, and never at the top, unless there is more than can pa.s.s out of the soil by the lower openings (the crevices and pores) into the tiles. It is well always to interrupt the direct descent of water by percolation from the surface to the top of the tiles, because, in pa.s.sing so short a distance in the soil, the water is not sufficiently filtered, especially in soil so recently disturbed, but is likely to carry with it not only valuable elements of fertility, but also particles of sand, which may obstruct the drain. This is prevented by placing above the tiles (after they are covered a few inches with gravel, sand, or other porous soil) compact clay, if convenient. If not, a furrow each side of the drain, or a heaping-up of the soil over the drain, when finished, will turn aside the surface-water, and prevent such injury.

In the estimates as to the area of the openings between pipes, it should be considered that the s.p.a.ces between the pipes are not, in fact, clean openings of one-tenth of an inch, but are partially closed by earthy particles, and that water enters them by no means as rapidly as it would enter the clean pipes before they are covered. Although the rain-fall in England is much less in quant.i.ty and much more regular than in this country, yet it is believed that the use of two-inch pipes will be found abundantly sufficient for the admission and conveyance of any quant.i.ty of water that it may be necessary to carry off by drainage in common soils. In extraordinary cases, as where the land drained is a swamp, or reservoir for water which falls on the hills around, larger pipes must be used.

In many places in England "tops and bottoms," or horse-shoe tiles, are still preferred by farmers, upon the idea that they admit the water more readily; but their use is continued only by those who have never made trial of pipes. No scientific drainer uses any but pipes in England, and the million of acres well drained with them, is pretty good evidence of their sufficiency. In this country, horse-shoe tiles have been much used in Western New York, and have been found to answer a good purpose; and so it may be said of the sole-pipes. Indeed, it is believed that no instance is to be found on record in America of the failure of tile drains, from the inability of the water to gain admission at the joints.

It may be interesting in this connection to state, that water is 815 times heavier than air. Here is a drain at four feet depth in the ground, filled only with air, and open at the end so that the air can go out. Above this open s.p.a.ce is four feet of earth saturated with water.

What is the pressure of the water upon the tiles?

Mr. Thomas Arkell, in a communication to the Society of Arts, in England, says--

"The pressure due to a head of water 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 of water to the surface."