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.

Groynes features

Appropriate choice of shapes, dimensions and location of groynes is crucial for effectiveness of shore protection. Groynes length is usually related to mean width of the surf zone and on the other hand to their longshore spacing. An active length of the groyne basically increases together with the growth of wave-to-shoreline angle. While designing groynes, one should remember that they should not trap the whole longshore sediment flux. Numerous investigations and observations suggest that within optimal solutions the groynes spread seawards not further than to 40-50% of the storm surf zone width. Effectiveness of the groynes depends also on their permeability. The groynes which are either structurally permeable or submerged (permanently or during high water levels) allow more sediment to pass alongshore through them, in comparison to impermeable or high groynes. Most often used, pile groynes are usually permeable structures which does not affect their efficiency. The groynes height influences the amount of longshore sediment transport trapped by the groynes. The same groyne can act either as emerged or submerged structure (Fig. 2a), depending on water level which is subject to changes due to astronomical tides (if they exist), as well as storm surges. Generally, the groynes are designed to stick out about hs=0.5-1.0 m above the beach and the mean sea level (MSL). Too high groynes cause wave reflection, resulting in local scours. Considering the shape in plan view, the groynes can be straight, bent or curved, as well as L-shaped, T-shaped or Y-shaped. The most popular shapes and types of groynes are schematically shown in Fig. 2.

Types of groynes

In structural terms, one can distinguish wooden groynes, sheet-pile groynes, concrete groynes and rubble-mound groynes made of concrete blocks or stones, as well as sand-filled bag groynes.

Wooden groynes

The wooden groynes are most often one- or two-row palisade structures. Effects of influence of the T-shape wooden pile groyne on the shore (local erosion on the lee side and accumulation on the other) are illustrated in Fig. 3. One-row wooden groynes are most often partly permeable structures. This results in reduced erosive lee-side effects and prevents from appearance of semi-closed nearshore water circulations. The wooden palisade groynes are cheap but on the other hand they have low durability.

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