Groynes
This article describes the features, possible effects and different types of groynes that extend from the shore into the sea. Groynes are examples of hard coastal protection structures which aim to protect the shoreline from coastal erosion. A more detailed treatment of the effects of groynes is given in Groynes as shore protection.
Contents
Introduction
A groyne is an active structure extending from the shore into the sea, most often perpendicular or slightly oblique to the shoreline. Adequate supply of sediment and existence of medium-strong longshore sediment transport are major conditions of groynes efficiency.
The main function of a groyne is catching and trapping part of the sediment moving (mainly in a longshore direction) in the surf zone.
As revealed by experiments, groynes partly dissipate energy of water motion during weak and moderate wave conditions, leading to accretion of the updrift shoreline. However, when storm waves approach the shore more or less perpendicularly, the protective role of the groynes decreases and part of the beach is washed away (see Natural causes of coastal erosion).
Although groynes are widely used, it is a dubious solution when applied as sole shore protection measure, because of important lee side erosion. A groyne solution can be efficient if applied in combination with other (soft) shore protection measures, like artificial beach nourishment or shore nourishment.
Effects of groynes on the shoreline
The groyne design (planform, length, height, cross-shore profile, inclination) influences the impact on shore morphology; the impact also depends on sea water level, wave climate and sediment supply in the surf zone.
Protection of the shore by use of a single groyne is most often inefficient. Therefore, shore protection by groynes is generally designed as a group comprising from a few to tens of individual structures (see Groynes as shore protection). A scheme of interacting groynes is shown in Figure 1. Whereas a single groyne produces coastal erosion on the lee side of the structure, erosion in the case of a group of groynes is shifted to the lee side of the whole system. Erosion is also observed in the direct vicinity of the structures, particularly when the dominant wave direction is perpendicular to the shore. Water accumulation between the groynes induces compensating flows along the structures, causing local erosion of the seabed and sand loss to deep water[1]. During severe storms the groynes are “short” compared to the surf zone width, with erosion occurring around them. Under mild wave conditions groynes become “long” (length comparable to the surf zone width), thus favouring updrift sand accumulation and widening of the beach. Loss of contact between a groyne and the shore should be avoided. In such a case, longshore flows are generated between the shoreline and the groyne root. These flows cause washing out of the beach.
Features of groynes
Appropriate choice of shapes, dimensions and location of groynes is crucial for the effectiveness of shore protection. Groyne length is usually related to the mean width of the surf zone and to the longshore spacing in the groyne field. The active length of the groyne increases with increasing wave incidence angle. Groynes are most effective if they do not trap the whole longshore sediment flux. Numerous investigations and observations suggest that the seaward extension of groynes should not exceed 40-50% of the storm surf zone width. The effectiveness of groynes also depends on their permeability. Groynes which are either structurally permeable or submerged (permanently or during high water levels) allow more sediment to pass alongshore, in comparison to impermeable or high groynes[2].
The height of groynes influences the amount of longshore sediment transport trapped by the groynes. The same groyne can act either as emerged or submerged structure (Figure 2a), depending on water level changes due to tides and storm surges. Generally, groynes are designed to stick out about hs=0.5-1.0 m above mean sea level (MSL). Groynes that are too high cause wave reflection, resulting in local scouring. 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, rubble-mound groynes made of concrete blocks or stones and groynes built of sand-filled geobags.
Wooden groynes
Wooden groynes are most often one- or two-row palisade structures. The influence of the T-shape wooden pile groyne on the shore (local erosion on the lee side and accretion on the updrift side) is illustrated in Figure 3. One-row wooden groynes are in general partly permeable structures; permeability reduces lee-side erosion and prevents undesirable nearshore water circulations. Wooden palisade groynes are cheap but their lifetime is rather short.
Steel groynes
Steel groynes most often consist of vertical sheet piles, single or double, with various profiles, located perpendicularly to the shoreline. They are impermeable structures. Experiments have shown that groynes made of single sheet pile walls are not durable, due to corrosion of the material and abrasion by moving sand. Besides, ice loading is very harmful, causing instability and failure of the steel sheet pilings. Mixed massive structures, consisting of steel and concrete, are far more stable and durable.
Groynes of concrete elements
Groynes built of reinforced concrete blocks belong to the most stable and long-lasting coastal structures. Because of their considerable weight, the elements composing such a groyne require the existence of suitable soil conditions and appropriate foundation. An example of a groyne consisting of reinforced concrete elements is shown in Figure 4.
Rubble-mound groynes and groynes built of sand-filled geobags
Rubble-mound groynes are widely applied coastal protection structures. They are built either as loose mounds of stones or as mounds of various armour units, e.g. tetrapods. These groynes are often mixed structures, strengthened inside by a sheet piling. They are massive, durable and impermeable. The rubble-mound groynes are advantageous compared to steel, concrete and wooden groynes, as they better dissipate energy of waves and currents.
Groynes built of stacked sand- or ground-filled bags should be considered as a short-term protection measure. 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 bottoms. This type of groynes requires large bags (heavier than 50 kg), even though large bags are more difficult to handle and require filling on the spot.
Examples of cross-sections of rubble-mound and sand-filled bag groynes are shown in Figure 2.
See also
- Groynes as shore protection
- Deteriorated groynes
- Hard coastal protection structures
- Human causes of coastal erosion
- Natural causes of coastal erosion
- Dealing with coastal erosion
- Accretion and erosion for different coastal types
- Littoral drift and shoreline modelling
- Types and background of coastal erosion
- Port breakwaters and coastal erosion
- Typical examples of structural erosion
- Shoreline management
References
Please note that others may also have edited the contents of this article.
|
