Short groynes will often protect the toe of an embankment and prevent any longitudinal current undermining it. As a rule, in a tidal estuary the cost of protecting the slopes is considerable, and more above low-water level than below it. In adopting groynes formed of single or double rows of piles, in order to prevent erosion of the toe and a slip and subsidence in an estuary or sea embankment, the littoral currents must be considered before determining their direction and position, the object of their erection being to prevent the waves, and especially the prevailing waves, from scouring the shore, and also to cause a general deposition of shingle, and, therefore, they are usually placed at an angle to the set of the waves, so as to cause the latter to be diffused. The angle will vary; the best guide is to examine the effects of any that may be erected in a similar position to that to be built. An angle of 50° to 70° with the foreshore, in a leeward direction is frequently adopted, and with respect to the distance apart, this depends principally upon the direction of the current and prevailing wind, contour of the shore, degree of exposure, and the length of the groynes. When the whole of an open coast has to be protected, no natural defence existing, the line of the prevailing set of the current and wind on a straight shore can be set off at the end of a groyne, and before the point at which it meets the foreshore another can be erected. They are generally successful when properly placed, and are most frequently straight; if not, they have a concave face to the direction of maximum eroding force; a convex must be avoided, as it will not permanently retain the deposits; their practical effect being that the breach is heaped up or retained upon the prevailing wind, set of current, or wind-wave side to a height of some feet above the leeward side, therefore, a groyne should be so constructed that planks can be added to it as required. They have been proved to prevent the formation of bars when judiciously located, but their success greatly depends upon their proper position and direction, or in loose soils such as sand they may cause a deposit upon the windward and prevailing current side, but an erosion and falling away upon the other; and when placed in front of a reclamation embankment upon shifting sand until the ground at the back of the embankment becomes dry by the tidal waters being excluded, or the surface is impermeably coated, they may be of comparatively little use to prevent a slip or movement of the shore; for as the tide recedes below the level of the toe of the slope, a seaward flow of the tidal and land waters will be created in very porous soil under the seat of an embankment, and may continue until the tide returns and rises to the level of the ground, thus causing the shore to be constantly changing place and permanently established accumulation impossible. Short