Groynes

Introduction

A groyne is an active structure extending from shore into sea, most often perpendicularly or slightly obliquely to the shoreline. Adequate supply of sediment and existence of satisfactorily intensive longshore sediment transport are the sine qua non conditions of groynes efficiency. Catching and trapping of a part of sediment moving in a surf zone (mainly in a longshore direction), as well as reduction of the sediment amount transported seawards, are the principle functions of the groyne. As revealed by experiments, during weak and moderate wave conditions, the groynes partly dissipate energy of water motion and lead to sand accumulation in the vicinity of a shore, thus causing its accretion. Under storm waves, mainly approaching the shore perpendicularly, the role of the groynes decreases and a beach is partly washed out.

Effect of groynes on seashore

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. A scheme of a system of interacting groynes is given in Fig. 1. A single groyne, besides its positive influence on the shore, 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 erosion is also observed in direct vicinity of the structures, particularly when waves approaching the shore perpendicularly predominate. Between the groynes, huge mass of water is accumulated which in turn leads to appearance of compensating flows along the structures, causing local erosion of the seabed. With respect to the surf zone width, during severe storms the groynes are “short” structures, with frequently occurring erosion around them, while under weak wave conditions they become “long”, thus helping in sand accumulation and widening of the beach. Loss of contact between a groyne and the shore in an unfavourable effect. In such a case, longshore flows are generated between the shoreline and the groyne root. These flows are the reason for washing out of the beach.

Wooden groynes

subsection

text here

another subsection

subsubsection

more text