Calculating Diameter of Mains for Water Scheme. The diameters for discharging a given quantity of water at the end of each branch with the ends open cannot be determined by a direct method. The simplest method is to draw a section (see fig. 532) to a convenient scale; for instance, 100 ft. horizontal and 20 ft. vertical to 1 in. On this section set out an assumed hydraulic gradient. Assume the length of the branches to be from D to A 600 ft., and from E to B 300 ft., and the discharge 40 gal. per minute simultaneously at the end of each branch with their ends full open. The dotted lines in the section are the assumed mean hydraulic gradient of the mains and branches, which must be set out so that the pipes at no point are above the gradient. From the hydraulic gradients the frictional loss and the pressure can be scaled off direct without any calculation. The loss of head due to frictional loss at any point on the mains or branches is found by scaling off the distance that the mean hydraulic gradient is below the level line of the water surface in the reservoir. The pressure at any point in feethead is found by scaling the distance that the pipe is below the hydraulic mean gradient, or in pounds per square inch by multiplying the height that the hydraulic mean gradient is above the pipe by 434. The hydraulic mean gradient in the section shows a loss of head due to friction from the reservoir to point D of 80 ft., on the branch D to A a loss of 120 ft., from D to E a loss of 60 ft., branch E to B a loss of 80 ft., and from E to C a loss of 160 ft. The quantity in gallons per minute that is to be delivered at D= 40+ 40+40= 120 gal., and at E 40+40=80 gal. The quantity that has to be discharged and the head being known, the diameter of pipes that will be required in order to deliver this quantity at the different points on the system can be determined by Box's rule. By this rule the diameter of pipe required for delivering 120 gallons per minute at D will be 5 1202 x 333-3 ÷3=301 in., say 3 in. diameter, which is the nearest stock 80 diameter; from D to E 402 x 1166.6 160 402 × 200 120 ÷3=2171 in., say 21 in.; diameter of branch D to A ÷ 3 = 1.614 in., say 1 in.; and for branch EB 3=1.524 in., say 11⁄2 in. V = G The approximate velocities are calculated by the following rule, where V is the velocity in feet per second, G discharge in gallous 2× d2' per minute, and d the diameter of pipe in inches. By this rule the velocity 120 from the reservoir to point D will be -6.66 ft. per second, D to E 6.4 2 x 32 ft., E to C 3.948 ft., D to A 6.53 ft., and EB 8.889 ft. per second. With the ends of the pipes full open discharging the above quantities there will be no head or pressure at the discharge, the whole of the head being absorbed in friction. * This size main and the two other sizes given, viz. 24-in. and 1-in. mains, are only given for sake of example, and are not usually stocked by makers; 2-in. and 3-in. are the usual sizes; some engineers never use less than 3-in. CHAPTER XXIII. LAND DRAINAGE. THE drainage of land is to ensure that the land to be drained will be in a moderately dry state, not too dry in the hot seasons, but free from floods and soddenness or bogginess. It is an essential to agriculture. The two great authorities upon the subject were Mr Smith of Deanston, whose system still retains the name of Deanstonising, and Mr Parks. Both these gentlemen aimed at the same object, and with more or less success, but, strange to say, they differed materially in detail. First of all we shall consider the system of Mr Smith. He advised that in all land drainage there should be frequent drains 10 ft. to 24 ft. apart. That these drains should be shallow, not deeper than 2 ft. 6 in. That parallel drains should be placed at intervals either in wet or dry ground. That the minor drain should be laid on the slopes of, and the main in the hollows of, depression; and finally, that stone drains were preferable to tile drains or pipes. It will be seen now how widely Mr Parks differed from these principles. He advised that the drains should be less frequent, say 20-50 ft. apart. They should be deeper also, say about 4 ft. He had no doubt a special object in view when recommending deep draining, which was a twofold one,—that is, to get rid of all surface water, yet keep the subsoil water below the limits of capillary attraction. He advocated the use of pipes for drains. It is since the principles of land drainage were enunciated by these pioneers that the science has been fully developed. In fact, companies have been formed for the purpose, with parliamentary assistance; for example, the Yorkshire Land Drainage Company, 1843. Subsequently legislation stepped in, such as the Public Moneys Drainage Acts and the Private Moneys Drainage Acts, which were passed during the last century, and naturally the science obtained great influence. Now the main object of land drainage is, of course, essentially to draw off surface water, and to prevent it lying stagnant. But it by no means ends here. That is only quite a primary object. All soils are more or less retentive. This retentiveness depends principally upon the closeness of the molecules which compose the soil. Obviously dense clay will absorb and hold more water than loam, and, in turn, loam will hold more than a sandy soil. Now when the soil is fully charged with moisture it naturally cannot hold any more. The result is stagnant water at the surface; and if this is not removed by a suitable system of drainage, it will naturally sooner or later evaporate. This makes the earth cold, because evaporation abstracts the heat from the earth. Another bad effect of stagnant water is that it shuts out the warming effect of the sun's rays. Again, when the sun goes down and evaporation ceases, the surface water cools below the temperature of the subsoil; gravity then causes it to sink, and it will, in turn, cool the subsoil. Drainage will alter all this. The warm spring rains which give so much benefit to agriculture are not immediately evaporated as before, but will give up their heat to the soil; the result will be a harvest of quite 14 days earlier; in fact it has been proved so. Drained soil in midsummer would be about 2° Fahr. warmer than an undrained one. Now anyone who has studied rainfall statistics will see that not far off 77 per cent. of this when left to itself will be evaporated.* It is from the undrained clay soils that the greatest evaporation takes place. Loose soils do not lose very much as a matter of fact; of the rainfall is the average loss due to evaporation (= 66 per cent.). We can now safely conclude from what we have said that land draining is altogether an advantageous procedure when carried out in accordance with correct principles. The principal ones are as follows: 1. Less evaporation. 2. Aeration of the soil. 3. Increased depth of nitrification. 4. Earlier harvest. 5. Facilitated tillage. 6. Improvement of pasture. 7. General healthiness of the surrounding lands. Of course in some places natural drainage will occur. It is called arterial drainage, and may be assisted, however, by artificial drains. It will occur especially in oolite and chalk, mountain limestone and trap rock where near the surface. We, however, are dealing with lands which have no natural drainage and are excessively wet. The first thing the engineer has to do is to find out why they are wet. Having found out, then find out how much of the land is wet and how much dry. It will not all be bad probably. *Or otherwise lost, as pointed out in a later chapter. A frequent source of wetness is when porous soil obtains water from a distance. Such soil, of course, wants careful draining. Of course the only real way of finding out if a land does want draining is by experience and the natural unhealthy look of the land. The most common cause is an impervious subsoil. Where this is the cause, patches of the land will show a dark look in the spring during wet weather, which will disappear in dry. These patches occur more often than not on slopes. They will, of course, show the wet ground. Regarding the time when such work would be most successfully carried on, the autumn and winter are the best times. Clay, however, may have to wait till Adit Drain Surface Soil Pervious Impervious FIG. 534. Low lands are perhaps the first kind which should gain our attention. Lands near river estuaries are the principal kind. Provided they are above high-water level of ordinary spring tides, surface drainage is a simple matter, and consists of cutting ordinary minor and main drains in the natural hollows, providing small sluices for regulating purposes, the drains in plan being very similar to the branches of a tree. The junctions only want careful attention; pitching with dry rubble may be necessary. Now, where the surface has a fair slope, the system of mains and branches no longer holds good. Catchwater drains, figs. 533, must be formed. By properly placed sluices, irrigation is made easy and efficient in dry seasons. Reservoirs may also be formed. Sometimes, however, an upland district is not so easily dealt with as may be supposed. Say we have a case as in fig. 534. From the saturated substrata we have to bring the water to the surface by means of a special drain called an adit drain. Internal springs would receive similar treatment. Again, in all drainage works the capacity of the drains has to be considered when deciding upon their size. Drains too large add to expense; those too small add still more expense in rectifying the evil. Hydraulic laws must be taken into consideration, because drains having a good fall do not need such a large cross section as their flatter neighbours. Parallel drains should be aimed at, and right-angled junctions. These, however, must in no case be opposite each other. In addition to these considerations, the nature of the subsoil largely controls the position of the drains. For instance, take fig. 535. This will demand frequent drains, which would be well if carried below the clay. But when the clay is deep |