Groynes

Coastal Protection: Groynes

A groyne creates and maintains a wide area of beach or sediment on its updrift side, and reduces erosion on the other.
-It is a physical barrier to stop sediment transport in the

direction of long shore transport.
-This causes a build-up, which is often accompanied by

accelerated erosion of the down drift beach, which receives little or no sand from long shore

drift.
-Groynes do not add additional material to a beach, but merely retain some of the

existing sediment on the up drift side of the groynes.
-If a groyne is correctly designed, then

the amount of material it can hold will be limited, and excess sediment will be free to move

on through the system.
However, if a groyne is too large it may trap too much sediment,

which can cause severe beach erosion on the down-drift side.

-Intensity and character of groynes influence on shore behaviour depend on sea

water level, parameters of waves, currents and sediment supply in the surf zone, as

well as a shape and inclination of the cross-shore profile.

-Protection of the shore by use of one groyne only is most often inefficient. Therefore,

shore protection by groynes is designed as a group comprising from a few to tens of

individual structures.
- Besides its positive influence on the shore, it causes numerous

side effects, mainly in the form of coastal erosion on the lee side of the structure. In

the case of a group of groynes, the above effect appears on the lee side of the whole

system. The
- Loss of contact between a groyne and the shore in an unfavourable

effect. In such a case, long shore flows are generated between the shoreline and the

groyne root. These flows are the reason for washing out of the beach.