resorted to. Again, on a gravitational scheme the land may be so low or the floods so abnormal that a special pumping plant has to be put down to assist in removing the flood waters from the lower levels. In any scheme of this kind it is usual to construct catchwater drains the whole way round the area concerned, so as to get rid of the water from the uplands once for all and lead it to a separate outfall. This drain is termed a marsh fence, and if the low land happens to be traversed by a natural watercourse it may deliver into it if the water likely to come down will not overcharge it. The drains of such low-lying lands are surface drains of large dimensions, as shown in fig. 541, and must be carefully calculated in respect to their discharging capacity from existing rainfall records. It is well to bear in mind in such calculations that an abnormal rainfall does not necessarily cause floods in low lands; on the other hand, it is long-continued rainfalls which do the damage, and heavy rains coming on after continued bad weather. Various amounts of rainfall can be considered when calculating the maximum discharge required. To calculate on the maximum would be unnecessary, owing to the usual short duration and unlikely occurrence of such falls. Consequently a mean must be arrived at, and in such a case we cannot do better than follow the practice of the early engineers in this direction, who usually calculated on a continuous fall of a quarter of an inch of rainfall in twenty-four hours, making no deductions for any losses by evaporation or otherwise. All springs should be treated as before advised, and outfalls provided for them. The next question is the level of the sill of the sluices, which will be the starting-point of the whole scheme, because upon the level of this point will depend the capacity and duration of discharge of the waters, together with the amount of ebb tide and the depth of water on the sluice sill. It is the best practice where possible to aim at a continual discharge of six hours, viz. three hours on the ebb and three hours on the flood tide, which will, under ordinary circumstances, necessitate the sill of the sluice being 2 ft. above the lowest springtide level. Even then, if there is a wind backing up the tide, this will be reduced, but, on the other hand, a good pressure at the back from the drainage area will increase it. Again, the minimum level of the sluice sill below the general level of the land must not be less than 2 ft., because 18 in. is the minimum depth allowable between the surface of water in the drains and the land, while allowing always 6 in. of water on the sluice sill. Hence we find that we must provide for 4 ft. fall between the land and the lowest springtide level before we can start the design, and the amount stated is little enough.

The water in the drains should never stand higher than 18 in. below the surface-level if the land is to be properly dry, and 2 ft. is advisable. In heavy clay, 2 ft. 6 in. would be better, and 3 ft. is quite the minimum in friable loam, because it is essential that the under-drains should run clear, and always have their ends above water-level. Hence we see that, with the levels generally at the engineer's disposal, in such works great accuracy is required in setting out the works, hence the form of channel demands attention, so that the greatest discharging capacity with the least fall is obtained, and the reader should refer to the question under Hydraulics. At the same time the form of channel referred to therein of a depth of 1 to a width of 2 is inadvisable in practical land drains, which usually have very flat slopes. A well-constructed main drain will flow efficiently with a fall of 3 in. per mile if properly laid out, and flat slopes do not appear to be any real impediment to the flow; on the other hand, such a form of drain is of the utmost service

in dealing with floods, inasmuch as the increasing width of section at the top provides a reservoir for flood water, which is a great consideration in pumping schemes. The slope of the sides will, of course, vary according to the soil through which they are cut. They must be flatter than the natural slope; and where a stiff soil tops a lighter one and the drain extends down to the latter, the sides have very often to be pitched or otherwise treated to prevent them falling in, no matter what slope they have. Besides the main and subsidiary drains and the catchwater drains referred to, in reclaimed land there is generally a drain on the landward side of the bank and at the base of it. Its purpose is to intercept any leakage through the bank and to deal with water thrown over in very severe storms, although the bank should be so designed that neither (especially the latter) should occur, but this ditch is usually kept about 20 ft. away from the actual foot of the slope, and is about 10 ft. wide at the base of excavation. The reason for removing it back is because, if placed at the actual foot of the bank, it would be likely to induce percolation, or cause it to slip, and the latter contingency is reduced to a minimum by using the 20 ft. referred to as a roadway. The drain itself is called a delph. The excavation also makes material for the bank. The depth of water allowed in the delph is usually 3 to 4 ft., and the whole depth 5 to 6 ft., but in sandy soil only a very small drain is allowed because of the liability of bank and drain to slipping. The rule given by Mr Beazley (in his treatise on the reclamation of land) for the distance between bank and delph is "care should be taken that the minimum breadth of undisturbed surface is such that the inner slope of the bank if produced would nowhere intersect the nearest side of the excavation forming the delph-and even this as a minimum in cases only where it is not possible to give a greater breadth." The sluices at the end of the drain are usually made of heavy masonry. The doors are self-acting, and usually of the canal lock-gate type, meeting together when closed at an angle of 130°. There is generally a bridge over the sluice, which may have one or more openings according to circumstances. The piers and abutments are generally extended on the seaward side a distance from two to three times the width of a single opening. They have pointed cutwaters and grooves for draw-doors, which are raised by chains and rack-andpinion motions, or may be stony sluices, of the form in fig. 506A. It is these which regulate the height of water in the drains. Small sluices have doors of the flap-valve type, but are very often of elaborate construction, so as to respond to the slightest back pressure. By reason of the nature of the land on which these works are executed, the foundations (often on alluvial deposits) are frequently most difficult, and piling, and in sand sheet piling, are generally resorted to. The site of the sluices is also an important factor because of the liability of sand to drift and silt up the outfall, and training-walls have often to be erected along the channel from sluice to low water, especially if the foreland* is flat and marshy. It is not necessary that sluices should always discharge into the sea. They often, in fact, discharge into a river.

The site, however, is again important, and the discharge should take place on a concave bend. The reason for this is that there is always deep water at these points, and silting up is less likely, especially when the river is low. The outflow from the sluice must be parallel or nearly so to the ebb-tide current. Professor Rankine, in his Civil Engineering, pp. 727 and 728, says in reference to draining low lands :

:

*The foreland is that stretch of ground between the foot of the bank and the low-water mark of ordinary spring tides.

"The best mode of draining a district of this sort is by means of a canal extending completely through it, which acts alternatively as a reservoir and a channel. The top water-level of the canal is to be fixed so as to give sufficient declivity to the branch drains. Its low water-level will be above that of low water of neap tides to the extent of 1/15 part of the rise of such tides. The space contained in the canal between those levels is the reservoir room."

When levels have been taken and the best position for discharge decided upon, we then lay down the proposed lines of drains on the plan. It is then necessary to find the required size of each, and the necessary amount of earthworks, etc., upon which to base an estimate.

These items should be tabulated as follows:-
:-

5. Discharge period per day (hours).

6. Quantity of water due to rainfall in cub. ft. per second.

10. Area drained by each minor drain in acres and decimals.

14. Discharging capacity, using Equation 293. (This may be reduced at its middle length to take only half the quantity by altering the size given in No. 11.)

15. Number of subsidiary drains.

16. Fall

17. Dimensions of subsidiary drains, allowing continual discharge and using Equation 293, and considering the rainfall supplied by each of the minor drains.

ft.

18. Size of minor drains.

19. Area of land occupied by actual drains.

20. Sluices-number of; width of; depth of water on; velocity through, per second.

CHAPTER XXIV.

PUMPING MACHINERY.

Ar first sight, perhaps, a chapter on pumping engines may seem to be out of place in a book of this sort, and more a matter for mechanical engineers. A little consideration will, however, soon dispel that idea, because the modern municipal engineer will in nine cases out of ten have pumps under his direct charge to oversee, test, and possibly to advise extensions.

Perhaps the simplest automatic pump, or rather a machine for lifting water, is the hydraulic ram. It is illustrated in fig. 555. Its object is to raise water to a considerable height by using

the potential energy of water at a smaller height. At A is attached the supply pipe, called the drive pipe, 10 to 20 ft. long; it is connected with the supply tank. The bell F contains air to act as a cushion. There is a valve B, having a weight keeping the valve open normally. Suppose water starts to flow out of this valve, it acquires Delivery velocity, increasing, too, SO much as to overcome the weight and close the valve. Rushing onward, it gains considerable pressure, so much so that the ball valve lifts and the water flows up the supply pipe. Now the valve B falls again and the process is repeated. The action of the hydraulic ram illustrates the conversion of potential energy into kinetic energy, kinetic into pressure, and pressure, again, to potential (vide Hydraulics). Now, referring to the line diagram fig. 556, we have H = delivery head and h = supply head; then the efficiency of the apparatus

Ball

E.

A

FIG. 555.

(345)

The quantity of water which will run to waste by way of the dash valve B

qH
nh

n being as in Equation 345.

(346)

(351)

while the content of the air vessels should be equal to 0055d2 H. The diameters D and d are in ins. (Molesworth's Tables) . The fall may be anything from 10 ft. to 18 in., but above 10 ft. frictional

effects cause jar and wear to become very excessive.

Rams may be used coupled together also. Hydraulic rams can be well recommended, but their use is not widely taken advantage of. They are very simple, rarely get out of order, are cheap, and work months on end in out-of-the-way places without attention. But they must in the first instance be well designed and fitted. All valves should be of gun-metal, all joints faced, and a snifting valve provided to supply the air which is of necessity absorbed by the water. In fixing, they should be placed below ground to avoid the effects of frost, and a strainer must in all cases be provided. Regarding actual pumps, they may be divided up as

1. Lift pumps. In a lift pump the water is drawn through the suction pipe on the upstroke of the bucket, forced through the bucket valve as the pump descends, and lifted again on the upstroke. A diagram of a bucket pump appears in fig. 557. It is about the only form of really efficient bucket pump, and depends for it principally upon the arrangement of small inlet valves.

2. Plunger or force pumps. 3. Centrifugal pumps.

Plunger pumps are pumps in which water is drawn through a suction pipe, and is displaced by the action of a plunger or piston which forces it through the delivery valve.