1S2
EROSION AND ACCRETION
impounded in the angle between the groyne and the barrier of defence behind it. The sea being gorged, its force is concentrated, and the danger of the destruction of the groynes by direct impact increased, as well as the scouring effect. Under these conditions the force of recoil is intensified, with the result that beach is dragged down along the line of the groyne and a deep gutter created, tending to undermine its foundations. Probably, in the majority of instances, a groyne placed nearly at right angles to the maximum force of the wind-waves gives the best results. Its direction may be modified a few degrees to windward or leeward of this line, as observation of the action on a particular stretch of coast-line indicates.
   The tendency of the designers of groynes protecting one particular stretch of coast is to trap all littoral drift. This is in effect to starve the coast-line to leeward. The ideal condition is that of circulation in compartments, the column of drift oscillating under the varying conditions of wind and weather, and leaving, as nearly as practicable, a uniform incline on which the sea forces may spend themselves harmlessly.
   At the outlet of a harbour or river the construction of spur groynes is one of the most efficient devices. It achieves two results:—
   (a)	The arrest of the travel of the circulating medium, thus
         preventing the formation of bars or spits, which distort the contour of the outfall of the harbour or river, cause an impediment to its flow, and set up conditions detrimental to navigation.
   (b)	A well-formed spur groyne forms an embayment, and
         if a series of these is constructed, retardation of travel is brought about, so that on the windward side of a harbour or river an artificial ness is created, sheltering the foreshore. The action of coastwise retardation of shingle or sand may by this expedient be prolonged and a dangerous coast-line effectively defended. The critical portion of a spur groyne is on the outer edge at its junction with the main groyne. It there receives the full stroke of on-shore seas, and the tendency to