Difference between revisions of "Groynes"

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This article describes the features, possible effects and different types of groynes that extend from the shore into the sea. [[Groyne]]s are examples of [[hard coastal protection structures]] which aim to [[coastal protection|protect]] the shoreline from [[coastal erosion]]. A more detailed treatment of the effects of groynes is given in [[Groynes as shore protection]].
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==Introduction==
 
==Introduction==
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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.
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A groyne is an active structure extending from the shore into the sea, most often perpendicular or slightly oblique to the shoreline. The main function of a groyne is catching and trapping part of the sediment moving in a longshore direction in the surf zone. Adequate supply of sediment and existence of medium-strong longshore sediment transport are major conditions of groyne efficiency.
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However, when storm waves approach the shore more or less perpendicularly, the protective role of the groynes decreases and part of the beach can be washed away (see [[Natural causes of coastal erosion]]).
  
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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.
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Although groynes are widely used, it is a usually not a good solution when applied as sole shore protection measure, because of important lee side erosion.
  
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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.
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==Effects of groynes on the shoreline==
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The groyne design (planform, length, height, [[cross-shore profile]], inclination) influences shore morphology; the impact also depends on sea water level, wave climate and sediment supply in the surf zone.
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[[Image:File1.jpg|350px|thumbnail|right|Figure 1: Scheme of interaction of groynes, waves, currents and shore]]
  
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==Effect of groynes on seashore==
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Groynes are applied in cases of prevailing shore-oblique wave incidence and associated longshore sand transport (littoral drift). The effect of groynes consists essentially of redistributing sand along the shore. Sand is accumulated at the updrift side of the groyne at the expense of the downdrift side where the shoreline retreats. Protection of the shore by use of a single groyne is therefore most often inefficient.
  
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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.
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Shore protection schemes using groynes are 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.  
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[[Image:File1.jpg|350px|thumbnail|right|Fig. 1. Scheme of interaction of groynes, waves, currents and shore]]
 
  
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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.
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Water accumulation between the groynes induces compensating flows along the structures, so-called boundary rips (see [[Rip currents]]). These flows cause local erosion of the seabed and sand loss to deep water<ref>Nordstrom, K.F. 2014. Living with shore protection structures: A review. Estuarine, Coastal and Shelf Science 150: 11-23</ref>. Strong rip flow along the updrift groyne flank has been observed even under relatively small wave significant heights (< 1 m). The groyne length relative to surf zone width strongly controls the offshore extent of boundary rips, with a significant increase in surf zone exits above a relative groyne length (length/surf width) of 1.25. <ref name=S16>Scott, T., Austin, M., Masselink, G. and Russell, P. 2016. Dynamics of rip currents associated with groynes — field measurements, modelling and implications for beach safety. Coastal Engineering 107: 53–69</ref><ref>Castelle, B., Scott, T., Brander, R.W. and McCarroll, R.J. 2016. Rip current types, circulation and hazard. Earth Science Reviews 163: 1–21</ref>
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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.
 
  
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==Groynes features==
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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' (comparable to the surf zone width), thus favouring updrift sand accumulation and local 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.
  
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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.
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==Features of groynes==
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[[Image:File2.jpg|350px|thumb|right|Fig. 2. Types and shapes of groynes]]
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[[Image:File2.jpg|350px|thumb|right|Figure 2 Types and shapes of groynes]]
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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 h<sub>s</sub>=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.
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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<ref>Pilarczyk K. & R.B. Zeidler.(1996): Offshore Breakwaters and Shore Evolution Control. "Balkema", the Netherlands pp560.</ref>.
  
 +
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 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.
  
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==Types of groynes==
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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.
  
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===Wooden groynes===
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[[Image:File3.jpg|350px|thumb|right|Figure 3. Example of two-row pile groyne at Hel Peninsula (the Baltic Sea)]]
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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.
  
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===Steel groynes===
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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.
  
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==Types of groynes==
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===Groynes of concrete elements===
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[[Image:File4.jpg|350px|thumb|right|Figure 4 Concrete groyne, Ukraine (the Black Sea)]]
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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.
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===Rubble-mound groynes and groynes built of sand-filled geobags===
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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.
  
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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.  
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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.
  
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===Wooden groynes===
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Examples of cross-sections of rubble-mound and sand-filled bag groynes are shown in Figure 2.
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[[Image:File3.jpg|350px|thumb|right|Fig. 3. Exemplary two-row pile groyne and adjacent shoreline position, Hel Peninsula (the Baltic Sea]]
 
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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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===Steel groynes===
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==Related articles==
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*[[Groynes as shore protection]]
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*[[Deteriorated groynes]]
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*[[Hard coastal protection structures]]
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*[[Human causes of coastal erosion]]
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*[[Natural causes of coastal erosion]]
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*[[Dealing with coastal erosion]]
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*[[Accretion and erosion for different coastal types]]
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*[[Port breakwaters and coastal erosion]]
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*[[Shoreline management]]
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*[[Stability of rubble mound breakwaters and shore revetments]]
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*[[Sand-filled geosystems in coastal engineering]]
  
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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.
 
  
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===Groynes of concrete elements===
 
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[[Image:File4.jpg|thumb|right|Fig. 4. Concrete groyne, Ukraine (the Black Sea)]]
 
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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 Fig. 4.
 
  
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===Rubble-mound and sand-filled bag groynes===
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==References==
 +
<references/>
  
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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.
 
  
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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.
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{{author
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|AuthorID=12956
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|AuthorFullName= Zbigniew Pruszak
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|AuthorName=Pruszak ZBIGNIEW }}
  
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Exemplary cross-sections of the rubble-mound and sand-filled bag groynes are shown in Fig. 2.
 
  
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==Conclusions==
 
  
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Although quite frequently used, the groynes applied as self-contained shore protection measures are 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 the other shore protection measures, like artificial beach nourishment [[Tina Martens]] or detached breakwaters [[Josep Medina]].
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{{Review
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|name=Job Dronkers
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|AuthorID=120
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}}
  
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==See also==
 
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[[Groynes as Shore Protection]] - a detailed article on the use of groynes as shore protection.
 
  
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[[category:Coastal structures]]
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[[Category:Coastal protection]]
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[[category:Theme 8]]
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[[Category:Hard structures]]

Latest revision as of 17:03, 31 October 2024

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.

Introduction

A groyne is an active structure extending from the shore into the sea, most often perpendicular or slightly oblique to the shoreline. The main function of a groyne is catching and trapping part of the sediment moving in a longshore direction in the surf zone. Adequate supply of sediment and existence of medium-strong longshore sediment transport are major conditions of groyne efficiency. However, when storm waves approach the shore more or less perpendicularly, the protective role of the groynes decreases and part of the beach can be washed away (see Natural causes of coastal erosion).

Although groynes are widely used, it is a usually not a good solution when applied as sole shore protection measure, because of important lee side erosion.

Effects of groynes on the shoreline

The groyne design (planform, length, height, cross-shore profile, inclination) influences shore morphology; the impact also depends on sea water level, wave climate and sediment supply in the surf zone.

Figure 1: Scheme of interaction of groynes, waves, currents and shore

Groynes are applied in cases of prevailing shore-oblique wave incidence and associated longshore sand transport (littoral drift). The effect of groynes consists essentially of redistributing sand along the shore. Sand is accumulated at the updrift side of the groyne at the expense of the downdrift side where the shoreline retreats. Protection of the shore by use of a single groyne is therefore most often inefficient.

Shore protection schemes using groynes are 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.

Water accumulation between the groynes induces compensating flows along the structures, so-called boundary rips (see Rip currents). These flows cause local erosion of the seabed and sand loss to deep water[1]. Strong rip flow along the updrift groyne flank has been observed even under relatively small wave significant heights (< 1 m). The groyne length relative to surf zone width strongly controls the offshore extent of boundary rips, with a significant increase in surf zone exits above a relative groyne length (length/surf width) of 1.25. [2][3]

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' (comparable to the surf zone width), thus favouring updrift sand accumulation and local 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

Figure 2 Types and shapes 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[4].

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 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

Figure 3. Example of two-row pile groyne at Hel Peninsula (the Baltic Sea)

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

Figure 4 Concrete groyne, Ukraine (the Black Sea)

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.

Related articles


References

  1. ↑ Nordstrom, K.F. 2014. Living with shore protection structures: A review. Estuarine, Coastal and Shelf Science 150: 11-23
  2. ↑ Scott, T., Austin, M., Masselink, G. and Russell, P. 2016. Dynamics of rip currents associated with groynes — field measurements, modelling and implications for beach safety. Coastal Engineering 107: 53–69
  3. ↑ Castelle, B., Scott, T., Brander, R.W. and McCarroll, R.J. 2016. Rip current types, circulation and hazard. Earth Science Reviews 163: 1–21
  4. ↑ Pilarczyk K. & R.B. Zeidler.(1996): Offshore Breakwaters and Shore Evolution Control. "Balkema", the Netherlands pp560.


The main author of this article is Zbigniew Pruszak
Please note that others may also have edited the contents of this article.
Citation: Zbigniew Pruszak (2024): Groynes. Available from http://www.coastalwiki.org/wiki/Groynes [accessed on 25-11-2024]




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