COASTAL DEFENCE 635
availability and cost. The length of the groynes is related to the littoral
drift; the smaller it is, the shorter the groynes. The height of the groynes is
related to the depth of deposition required. In the initial stages after the
erection of the groynes, beach material may have to be fed at the down-
drift side of the groynes to avoid undesirable recession of the beach there.
As time progresses, the beach material tops over the groynes to fill succes-
sively the adjoining compartment until the beach encroaches into the sea
(Fig. 15.4).
The length and spacing of the groynes depend on the amount of sedi-
ment to be trapped, the size of the beach material and the angle of wave
attack. Shingle beaches require short groynes, while sand beaches require
long groynes. At the seaward end the groynes should extend to the low-
water level. One American recommendation is that they should end at the
2 m depth contour. The ratio of spacing to the length of groynes varies
between 1 and 4. The Shore Protection Manual(US Army, 1984) recom-
mends 2 to 3. If the wave crests are almost parallel to the beach, larger
ratios may be used. High groynes which reduce the drift significantly are
often not the best solution for stabilizing the beach. The recommended
height of groynes is 0.5–1 m above beach level. Initially, groynes may be
constructed with their tops above the foreshore level and, as accretion
takes place, the groynes are raised to ensure a uniform build-up of the
foreshore. Low groynes reduce local erosion.
Straight groynes are the most common, although zig-zag groynes and
breakwaters parallel to the beach at the end of the groynes have also been
used. If the prevailing angle of wave attack is 30° or more, groynes are
inclined at an angle towards the downdrift side. Groynes may either be
impermeable or permeable. Permeable groynes are intended to allow
the continuation of some littoral drift. They may trap larger sediments
but allow smaller sediments to be transported, thus reducing erosion
downdrift. Experience has shown, however, that on the whole they are no
better than the impermeable ones (Berkeley, Thorn and Roberts, 1981).
Fig. 15.4 Operation of a successful groyne system