1558

THEORY AND PRACTICE OF ENGINEERING.

Book II.

After the quantity of water has been ascertained by meteorological and other surveys,the proportions and dimensions of all the drains must be considered, and care must betaken that their sectional areas and velocities are rightly calculated, so that the main drainwhich, like a great artery, is to receive them all, should not be overcharged, or in dangerof overflowing. To facilitate the current, the slope of the sides of the drains or water-courses should have one and a quarter of base to one of perpendicular height, and theirbreadth at the bottom should be equal to two-thirds the depth of the water they con-tain.

When drains are cut through bogs, they may have a greater slope than when loose sandor gravel is to be operated upon, the fibres of the plants that constitute the peat or bogresisting considerably the action of running water.

When rivers which pass through low grounds are to be embanked or confined, that thefloods they bring down may not inundate the adjoining lands, care must be taken to makethe banks sufficiently strong, as the force increasing as the embankment is raised, in conse-quence of the stream not being able to expand itself in proportion to the increase of water,the more it is allowed to spread, the less occasion is there for strong barriers. The slopeof such embankments should not be less than twice their height, and three times is neces-sary when the rivers they confine are affected by the tide, or subject to the force of the seawave: the thickness at top should not be less than 5 or 6 feet, and the inside slopeshould be one perpendicular to one of base: for sea-banks, where the waves do not risehigher than 4 feet, the thickness at the top may be 6 feet, the slope on the land side 1£feet for every 1 foot in height, and on the sea-side 4 feet to 1 foot in height. The slope onthe land side must vary with the height of the waves, and should be increased to two anda half base to one of perpendicular height: the slope on the water side should be stillmore in proportion to the rise of the tide; for every increase of a foot in height the slopemust be greater, so that when the waves mount to 10 feet in height on the water-side, itshould be 10 feet for every foot in height.

When the earth with which the embankments are formed is of a gravelly or loosenature, it is requisite to carry up in the middle a wall of clay, or some impervious material;the system of puddling is now generally adopted, by which the percolation of the watersis prevented : there are several methods of protecting the face of banks towards the sea;in some parts of Holland, straw is used for this purpose, twisted first into ropes about 2inches in diameter, and then laid on the face of the bank, and pinned down with hookedsticks, the whole surface of the dyke being covered with a regular mat of straw; in thecourse of time vegetation commences under it, blades of grass make way through the strawropes, and the whole becomes a compact mass.

On the banks of the Thames and other large rivers, piling is generally adopted, whichis expensive, maintains the soil very imperfectly, and is far from durable or holding:rows of piles are driven parallel to the river at 2 or 3 feet apart, raking with the slope ofthe bank, one standing in its retreat from the water higher than the other; these aresecured by either working rods or wattling twigs between them, or by spiking horizontalpieces to them : the spaces between the rows are filled with chalk or stone, and brought asnearly as possible to a regular slope; this is called breach piling, and is now nearlyabandoned. Kentish rag or other hard stone is infinitely preferable, and less costly, formaintaining a wall subject to the action of a river on which steam-boats are constantlypassing; this, laid with a gentle slope from a little below the level of the lowest state ofthe tide, up to the summit of the wall, and the interstices run with fine gravel, has beenproved to answer admirably well. The sea walls at Dimchurch and elsewhere, which havea slope of one in thirty or more, appear to have little or no disarrangement throughoutany part of them: fascines and brushwood are used where expense is a matter of consideration,and when laid in the direction of the force, and pegged down with living willows, succeedwell; but with materials of this kind the slope should be brought to one regular inclinedline, and turfed over as far as possible; where a sea-wall is forme'd of sand, the interior ispuddled with clay, and along with the sand Spanish broom or twigs of trees are strewed inlayers, which bind the whole together. Within these walls provision must always be madeto let off the land waters, and as the marshes bordering on the Thames and other greattidal rivers are considerably lower than the level of high water, it is important that theopenings for their discharge should resist the entry of the water when at its higheststate. The simplest form is a clapper or valve hung at the top, and falling over theopening of the pipe or trunk of discharge, which is kept closed, as the tidal water rises andpresses against it.

Sluices are at other times used, which slide up and down in a frame, and the ordinarylock-gate either made to revolve on a pivot or to shut like folding-doors against a fixedframe. The passage through the wall consists of either a mass of masonry or a pipe ofcast-iron ; for culverts the latter is generally preferred.

When the passage of the fresh water is over a low beach or sandy flat, it is necessary to