2. THE EARTHEN MOUND (fig. 3.) iS the most ge neral description of embankment, and as it is exe cuted at considerable expence, is only undertaken by such as have a permanent interest in the soil. This barrier applies to sea lands overflown by every spring tide, and to alluvial plains inundated by every flood. It is set out in a direction parallel to the shore, and to the general turns of the river, but not to its minute windings ; and it is placed farther from or nearer to the latter, according to the quantity of wa ter in time of floods, the rapidity of the current from the declivity of the bed, the straight course of the stream, and the intended height of the bank. The two sides of such a mound are generally formed in different slopes. That towards the land is always the most abrupt, but can never be secure if more so than 450; that towards the water varies from 45° to 150; the power of the bank to resist the weight of the water, as well as to break its force when in motion, being inversely as its steepness. The power of water to lessen the gravity of bodies, or, m other words, to loosen the surfaces over which they flow or stand, is also lessened in a ratio some what similar.
The formation of such a mound consists merely in taking earth from the general surface of the ground to be protected, or from a collateral excavation, distant at least the width of the mound from its base line, and heaping it up in the desired form. The surface is then in general cases covered with turf, well rolled in order to bind it to the loose earth. The earth of such mounds is generally wheeled by barrows; but sometimes it is led by carts placed on a wooden rol ler instead of wheels, which, with the treading of the horses, serves in some degree to consolidate the bank.
The excavation within serves the same purpose as the open drain in the earthen mall, and similarly constructed sluices or valves are introduced on a larger scale. Sometimes also the interior water is drawn off by windmills, and thrown over the mound into the river. This is very common in Hunting donshire, and might be ,greatly improved on by employing steam-engines for entire districts, one of which, of a ten horse power, would do the work of twenty mills, and this in calm weather, when the latter cannot move.
Embankments of this description are the most universal of any, and their sections vary from a scalene triangle of ten feet in base, and three feet in height, as on the Forth near Stirling, and the Thames at Fulham, to a base of 100 feet, and a height of ten feet, as in the great bank of the Ouse near Wisbeach. The neat riven of Germany and Holland are embanked in this way, when so far from the sea as to be out of the reach of the tide; as the Vistula at Marienwerder, the banks of which, near Dannie, are above fifteen feet in height; the Oder the Elbe, &c. All these banks are closely covered in every part with a grassy surface, and sometimes ornamented with rows of trees.
But, near the sea, where such banks are washed by every tide when the course of the wind is towards the shore, and by all land floods and spring-tides, grass is only to be found on and near their summits. The rest of the bank is bare, and to preserve it from the action of waves, currents, and the stones, pieces of wood, and other foreign matters which they carry with them, the surface is covered with gravel, reeds, or straw, kept down by pieces of wood ; faggots, wicker hurdles, nets of straw-ropes, or any other contrivance, according to the situation, to pre vent the washing away of the bank. It is common to attribute to these coverings the power of breaking the force of the waves ; but this power depends, as we have already stated, on the slope of the bank and its smoothness ; and the use of the surface covering, and of the constant attention required to remove all obstacles which may be left on it by floods and tides, is to prevent the loosening power of the water from wearing it into holes. For this purpose, a sheet of canvas or straw netting is as good, whilst it lasts, as a covering of plate-iron or stone pavement.