All banks whatever require to be constantly watch ed in time of floods or spring-tides, in order to re move every object, excepting sand or mud, which may be left by the water. Such objects, put in mo tion by the water, in a short time wear out large holes. These holes, presenting abrupt points to the stream, act as obstructions, soon become much larger, and if not immediately filled up, turfed over, and the turfs pinned down, or the new turfs rendered by some other means not easily softened and raised up by the water, will end in a breach of the bank. A similar effect is produced by a surface formed of un equal degrees of hardness and durability. The banks of this description in Holland, at Cuxhaven, and along the coast of Lincolnshire, are regularly watch ed throughout the year; the surface protection is re paired whenever it goes out of repair, as is the body of the bank in the summer season.
Mound with Puddle-Wall.—it generally happens that the earth of such banks is alluvial, and their foundation of the same description ; but there are some cases where the basis is sand, silt, or gra vel ; or a mud or black earth, as in some parts of Cambridgeshire and Lincolnshire, which does not easily become so compact. Here it is common', be fore beginning the bank, to bring up what is called a puddle ditch, or section of day, in the centre of the highest part of the mound in the direction of its length, and of three or five feet wide, according to the depth of the silt and the intended height of the bank. When the clay of this puddle-ditch is well worked, either by men's feet or clay rammers, the bank will be perfectly impervious to water, and if against a mild stream or shore, need not contain such an accumulation of earth as where the impervious ness of the bank to water depends chiefly on the mass of materials. An important point to attend to in this variety of mound is, to found the section, or wall of clay, so deep as to be in contact with a stra tum either by induration, or its argillaceous nature, impervious to water.
Mounds with reversed slopes, (fig. 4.)—In some cases of embanking rivers, as where they pass through parks, it is desirable to conceal, as much as possible, the appearance of a bank from the protect ed grounds, less able to break the force of waves. Here the mound is simply reversed, the steepest side being placed next the water. It is proper to ob serve, that such banks are not so strong by the dif ference of the weight of the triangliof water which would rest on the prolonged slope, were it placed next the river, and are more liable to be deranged in surface in proportion to the difference of the slopes.
Mound faced with stones.—This is the same spe cies of mound, with a slope next the water of for ty-five or fifty degrees, paved or causewayed with stones or timber. In Holland this pavement or causeway is often formed of planking or bricks ; but in England generally with stones, and the mor tar used is either some cement which will set under water, or, what is better, plants of moss firmly ram med between them. The objection to such banks are their expellee, and their liability to be un dermined invisibly by the admission of the water through crevices, &c. They are, therefore, chiefly
used where there is little room, or where it is desir able to narrow and deepen the course of a river.
Mound protected by a wicker hedge.—This is a Dutch practice, and, where appearance is no ob ject, has the advantage of not requiring watching. Wicker-work, however, subjected to the strain of waves, will be obviously less durable than where it lies flat on the ground, and can only decay chemi cally. This wicker hedge is sometimes a series of hurdles supported by posts and struts ; bat generally in Britain it is a dead hedge or row of stalks wattled or wrought with bushes presenting their spray to the sea or river. Besides placing such a hedge before a bank, others are sometimes placed in parallel rows on its surface; the object of which is to entrap sand, shells, and sea-weeds, to increase the mass of mound, or to collect shells for the purpose of carrying away as manure.
The sea wall (fig. 5.) is an embankment formed to protect abrupt and earthy shores or banks of rivers, and consists of a wall, varying in thickness, and in the inclination of its surface, according to the re quired height and other circumstances. Belidor in his Troia de Hydraulique, has given the exact curve which the section of such a wall ought to have, in order to resist loose earth, and which is somewhat greater than what we have given in the figure re ferred to, where the earth behind the wall is suppos ed to be chiefly firm. Some fine examples of such walls, for other purposes, occur in the CALEDONIAN CANAL, and perhaps the finest in the world are the granite walls which embank the Neva at Pe tersburg, the construction of which may serve as an elnuhple of a river case with a foundation of soft bog earth. A space of the river, say 100 feet long, and 20 wide along shore, being inclosed by a double row ofpiles, and filled in with loam in the usual Way, the water is pumped out, and the ground ex cavated about ten feet deeper than the margin of the bed of the river. Poles are then inserted nearly as close as they can be put in, and driven to their full length. When finished this foundation occupies a breadth of from 12 to 18 feet, generally 15. The tops of the piles are now cut level, and covered with planks, and on this is raised a mass of brick-work for five •r six feet, sloping on both sides as it as tends towards the centre line of the wall. In the course of rising six or seven feet, it is narrowed to five feet, and is within five feet of the bed of the margin of the river. Here the granite facing begins in immense blocks, and is continued at a slope of from the perpendicular till it reaches the surface of the intended pathway. Here the wall from three feet is narrowed to an upright parapet, eighteen inches wide ; and at four feet of height it is finished in a projecting coping of Finnish granite. The voids on each side of the wall are now filled with earth, the pavement on the land side (generally narrow) com pleted, and the piles removed, and another length taken in to repeat the operation.