Inclination and Scouring Action.

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ly influenced by the irregularities and roughnesses of the channel, the friction being thereby greatly increased, and the extent of surface with which it comes in contact, proportionately retards its velocity. Drains should not be made of a greater width and depth than is actually required to carry off the highest average quantity of water passing into them.

In considering the amount of fall or inclination to be given to the bed of a drain, it would seem at first sight as if too great an inclination could not be given in order to make it discharge its contents in the shortest possible time, and to wash out all solid filth which with a slow current has a tendency to deposit; but here the scouring action must be taken into consideration, and its effect upon the bottom and sides of the drain must not be overlooked.

In Neville's Hydraulic Tables, &c., it is stated, that "the mean velocity of a stream must not be too quick, and should be so determined as to suit the tenacity and resistance of the channel, otherwise the bed and banks will change continually, unless artificially protected. It should not exceed 25 feet per minute in soft alluvial deposits, and 40 feet per minute in clayey beds. It is true that Neville is here speaking of rivers, the conditions of which are widely different from drains."

A surface velocity of two feet per second will, as a rule, be found sufficient for practical purposes. The surface velocity of a stream may be easily ascertained by measuring a given length along the bank between two fixed points, then throwing a cork or piece of light wood into the stream, the time taken for it to pass from point to point as checked by the second hand of an ordinary watch will give the surface velocity. Summed up

shortly, therefore, the conditions to be observed are, that the drain shall be able to carry off with sufficient velocity, the maximum quantity of water or sewage likely to pass into it, but that the velocity shall not be so great as to endanger the bottom or sides.

The efficiency of town drainage depends in a great degree upon its outfall. This may be either into a river, canal, or tidal creek, or into the jullas and rice-fields, and in the latter case it follows the natural fall of the country.

Where the drainage falls into a running stream, it is usually easily managed and uninterrupted, except by exceptional causes, such as unusually high floods. When into a canal, tidal creek, or river, it is necessarily intermittent, and the influx of tide water has to be provided against; but where its outfall is into the rice-fields or jullas, the storm-water and sewage is often headed back, and the drains remain brimful during the rainy season, the spill water only passing off. This lasts till the water-level sinks by the drawing off of the water from the fields and by evaporation and subsoil percolation.

As the water in the rice-fields is a necessity of the cultivation, we cannot (even were it practicable) propose to drain off these lands; the only way, therefore, to meet the difficulty is to carry the main drainage channel between embankments through the fields and jullas till we reach some natural watercourse, at a sufficiently low level, to permit of the water of the drains falling into it. If the watercourse be tidal, a sluice or tidevalve must be provided. If no such natural facilities present themselves, the water must be lifted by steam or wind-power as in the Fen Counties of England and the Lowlands of Holland.

Drainage of Fen Lands.

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It has often been a matter of surprise to me that such a comparatively inexpensive method of pumping water for drainage or irrigation purposes has not been introduced into this country. In no part of India have I ever seen a wind-mill. In the present day steam-power has almost entirely superseded windpower in large drainage works; but I can well recollect when wind-engines were in universal use in the Fen Counties. Littleport Fen, with which I was familiar as a youth, having an area of 28,000 acres drained by wind-engines, now superseded by two steam engines of 30 and 40-horsepower. The following account of the great Bedford level drainage will illustrate what great results may be attained by utilising natural forces:—

The Bedford level is a vast tract of about 400,000 acres of lowland, extending into the counties of Northamptonshire, Huntingdonshire, Cambridgeshire, Lincolnshire, Norfolk, and Suffolk. It was formerly dry land, but, from natural convulsions and other causes, it gradually became a vast morass. As early as 1436, the idea of draining these fens engaged attention. Large sums were expended in attempts to embank and reclaim them, but all of which ended in failure, until in 1634, William, Earl of Bedford, undertook to drain them on the understanding that 95,000 acres of the reclaimed land were to become his. For three years he prosecuted his labours, expending in the attempt £100,000, but again failure was the result. In 1649, the Earl, nothing daunted, again commenced operations, and after outlaying £300,000 in draining, embanking, and protecting the land, met this time with success. In 1664 a corporate body was formed for the management of the reclaimed lands,

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Sluice-gates or Tide Valves

which is still in existence. The Fen lands are intersected by numerous channels, some of them being navigable for over twenty miles. In draining the marsh, the water was raised by wind-engines. In some cases these engines turn a perpetual screw, fitting into a semicircular trough, inclined at an angle of about 30°, the lower part dipping into the water below, and discharging by its revolution the water into the higher level. By this form of engine the friction of pumps and wearing out of machinery are reduced to a minimum, and the mills require little attendance, but work night and day as the wind blows. I have heard high praise of this form of engine from natives of Schleswig Holstein, where it is in constant use, and it was used with great effect in emptying the dry docks at Shanghai. The Dutch, who carry on this system of drainage extensively in their lowlands, employ scoop-wheels worked by wind-engines; and where the lift of water is not more than seven feet, they are very effective.

Where the drainage falls into a tidal stream or creek, and the point of outfall is below highwater marks, the action becomes intermittent,—that is to say, the drainage water can only pass into the stream at those periods of the ebb and flood when the water-level of the stream is below the level of the sill of the outfall, or at least sufficiently low to be overcome by the head of water in the drain, and sluice-gates or tide valves are necessary to prevent the tide water from entering the drain. The sluices will open to discharge drainage when the ebb-tide has fallen sufficiently to reduce the outside pressure below that exerted by the water in the drain, and will remain open until the in-coming flood has again risen to the same point, when they will be closed

Liability to Derangement.

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Where the water in the

by the pressure of the tide. drain is much higher than the flood-level outside, a considerable portion may be discharged through a valve in the upper part of the sluice or by side valves in the masonry; and as soon as the water outside has fallen so far that the head of water inside can overcome the friction of the hinge and the outside pressure, it will begin to discharge, although the tide may still be above the level of the sill or floor.

No matter of what construction, or however well-constructed and well fitted sluice-gates may be, there is always some weak point about them, some liability to accident or derangement, which renders them a source of constant anxiety and necessitates constant attention. If the material be iron, it corrodes; if wood, it swells or shrinks, and rots under the influence of alternate wetting and drying.

If self-acting, they get jammed and choked with floating rubbish, grass, trees, &c. If worked by hand, they are liable to neglect by watchers, who forget to open or close them at proper times.

Where drainage outfalls pass through or communicate with rice-fields, they require, whether self-acting or not, careful watching, as in dry seasons the raiyats will force them open to admit the river water to irrigate their fields. I have in my mind, whilst writing this, two large and important so-called self-acting sluice gates communicating with the river Hooghly, which are a source of constant trouble, and which have never been anything else during my many years' acquaintance with them.

One is an upright gate or tide-valve hung by hinges at the top, and abutting perpendicularly against masonry