Although firm clay may for a short time stand at nearly a vertical slope for any height of face not exceeding about 10 to 12 feet, few clays will permanently stand at a less slope than, 1½ to 1, for moderate heights, although they may for a time do so at 1 to 1 in unsubmerged work, but if the slope is covered with water, as in a canal, and subject to wash or wave action, however shallow the depth, it is seldom they repose at a less inclination than 1½ to 1. Should an embankment be more than about 30 feet in height 2 to 1, average. Firm clay from, 1½ to 1 TO 2½ to 1, according to the depth. Plastic-clay, consequent upon the percolation and pressure of water in cuttings, may stand at a steeper slope in a surface protected embankment, and if carefully tipped, it will repose at from 1½ to 1 TO 2 to 1, but from hydrostatic pressure the same clay may require from 2 to 1 TO 3 to 1 slopes in cuttings, and even an inclination of 5 to 1 has been found to be necessary. The varying slope-system is referred to in Chapter VI., as also the range of slopes. It is always well to bear in mind that in the case of clay, loamy or marly soils, or any possessing soluble particles, the quantity of water in them governs the required flatness of the slope, and as it may be variable, a clay that will repose at a certain angle may slip upon receiving an additional amount of water, hence the importance of drainage and protection. The cohesive strength of clay also varies greatly; and as it is likely to be impaired, and, perhaps, destroyed by water and other causes, it should be considered as of fluctuating value, and therefore as generally unreliable. The slopes of repose required in river-banks have a wide range according to the degree of exposure and opposition to the free flow of water, and the protection given to the surface. In canals, and drainage-channels, or ordinary rivers, they range from 1½ to 1, when protected, to 5 to 1; but there are numerous small, shallow, and sluggish streams with almost vertical banks. As the earth of river-banks is frequently of a clayey nature the slopes are referred to under the head of Clay; usually it is mixed in character, and, therefore, the slope of stability cannot be determined from that of any particular and unalloyed soil. Towards the mouth the land often consists of detritus liable to be washed away by a stronger flood than that by which it was deposited; it may also be eroded by constant or increased wave action. Ordinary clayey, loamy, tenacious, or not easily moved, silt and sand river-banks, in a current that will not erode them at moderate depths, if the face is protected, will stand at an inclination of about 1½ to 1, when the channel is freshly cut. If not covered or protected at from 2 to 1 TO 3 to 1, but when in clay soil of a doubtful character and non-homogeneous, or boulder-clay, about 4 to 1.