Groynes
This article describes possible effects and different types of groynes extending from the shore into the sea. Groynes are examples of hard shoreline protection structures which aim to coastal protection the shoreline from coastal erosion.
Contents
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.
Groynes are frequently used. However, applied as a self-contained shore protection measure it is a very dubious solution. This is because of unfavourable side effects which they can cause locally. Satisfactory supply of sand and existence of longshore sediment transport are fundamental conditions for efficiency of groynes. The groins role distinctly increases if they are applied together with other (soft) shore protection measures, like artificial beach nourishments or detached shore parallel breakwaters.
Effects of groynes on the shoreline
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.
Features of groynes
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 Figure 2.
Types of groynes
In structural terms, one can distinguish between 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 Figure 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.
Steel groynes
Steel groynes are most often constructed of vertical sheet piling, single or double, of various profiles, located perpendicularly to the shoreline. They are impermeable structures. The experiments have shown that the groynes made of single sheet pile walls are not durable. This is due to corrosion of the material and influence (friction) of the moving sand. Besides, ice load is very harmful, causing instability and failure of the steel sheet pilings. Mixed massive structures, constituted of steel and concrete, are much more stable and durable.
Groynes of concrete elements
Groynes built of concrete elements in the form of prefabricated boxes or other reinforced concrete items belong to the most stable and long-lasting coastal structures. Because of considerable unit weight, the elements composing a groyne of this kind require the existence of suitable soil conditions and the appropriate foundation. An example of the groyne constructed of the reinforced concrete elements is depicted in Figure 4.
Rubble-mound and sand-filled bag groynes
Rubble-mound groynes belong to frequently applied coastal protective structures. They are built as either loose mounds of stones or mounds of various armour units, e.g. tetrapods. These groynes are often mixed structures, strengthened inside by the sheet piling. They are relatively massive, durable and impermeable. The rubble-mound groynes are advantageous with respect to the steel, concrete and wooden ones, as they better dissipate energy of waves and currents.
The sand-filled bag groynes as a protection measure should rather be considered as a short-term solution. The bags in a stacked bag groynes can either be sand- or ground-filled. Some additional protection measures are necessary, especially at the groyne head. A special filter cloth should be used under the bags to reduce settlement in soft bottom. Construction of this type of groynes requires larger bags (heavier than 50 kg), even though they are more difficult to handle and require filling on the spot.
Exemplary cross-sections of the rubble-mound and sand-filled bag groynes are shown in Fig. 2.
See also
- Groynes as Shore Protection - a detailed article on the use of groynes as shore protection.
- Coastal protection
- Coastal erosion
References
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