34
THE FORESHORE
of safety is lodged, and that its serious depletion amounts to an act of criminal folly, which the State should intervene to prevent.
  The depth at which mud reposes on a sea bed is good evidence as an index of the severity of wave conditions. Airy has shown by calculation that heavy ground-swell scours the sea bed at a depth of 100 fathoms. Lobster pots on the Devonshire coast become filled with sand at a depth of 30 fathoms. Off the west coast of Ireland mud comes to rest in 40 to 60 fathoms water. In the sheltered parts of the east coast loughs it lies at depths of 5 fathoms.
   A wave trap or spending beach is indispensable to the tranquillity of an enclosed harbour or landlocked basin, especially where tidal range is considerable. The run of the sea, after passing the entrance piers of a harbour, assumes the aspect of a miniature bore. Its impetus is cumulative. When the wave thus developed reaches a shelving beach, or a recess leading to slob land, its force is dissipated. If a harbour be so designed that the run entering it encounters a wall or other barrier approximating to the vertical, the recoil of the oncoming wave induces prolonged oscillation. It has been proved experimentally, that in a box 20 feet long, ¿-inch to |-inch waves beat backwards and forwards at least sixty times. Such action causes the accumulation of a transverse rid^fe or bar running centrally across the middle of the area of disturbance, at right angles to the line of the run of the waves.
   A notable instance of this effect is that of the harbour at Torquay (fig. 7). The small inner harbour is of ancient construction, and sufficed for the minor traffic of the district. In order to enclose a larger area of water, the western arm was subsequently built. The opening of the new harbour faces west-south-west. Concurrently a reclamation of foreshore land was carried out, a nearly vertical wall being substituted for the old sloping sea marge, which previously constituted an efficient wave-breaker. The result of these works has been that both the inner and outer harbours have for practical purposes been gutted, as the mud and sand which formerly lay in the harbour have been scoured away and carried to sea. Between the nearly