Groynes

Fig. 1. Typical saw-tooth shoreline pattern (shoreline advance at the updrift groyne side and retreat at the downdrift side) in a groyne field on the Long Island coast, NY. Image credit USGS EarthExplorer.

A groyne is a shore-connected coastal structure, usually oriented approximately perpendicular or oblique to the shoreline, built across part of the active beach and surf zone to reduce or redirect longshore sediment transport. Groynes interact with obliquely incident waves and modify the wave pattern incident on the beach. The main intended function of a groyne is intercepting part of the sediment moving in a longshore direction in the surf zone.

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. This article describes the features, possible effects and different types of groynes.

Single groyne

Divergence in longshore sediment transport is a common cause of coastal erosion and associated shoreline retreat. Groynes are therefore often constructed where littoral drift (longshore sediment transport) increases along the coast in the drift direction. By (partially) blocking littoral drift, sediment accumulates at the updrift side of the groyne. By sediment continuity, reduced sediment supply to the downdrift coast causes erosion there, although the amount and spatial distribution depend on bypassing, offshore losses and wave climate.

Fig. 2. Boundary rip: seaward-deflected longshore current along a groyne, carrying sediment out of the inner surf zone. Photo credit Dr. Wendy Carey, Delaware Sea Grant. https://www.weather.gov/safety/ripcurrent-photos

The longshore current is deflected seaward along the groyne's updrift sand deposit. Part of the deviated littoral drift passes the groyne and feeds the downstream longshore drift. This will usually be the case if the width of the surf zone is greater than the cross-shore groyne extent. Besides, part of the seaward-deviated littoral drift can carry sediment offshore, especially in situations where obliquely incident waves are highly energetic, see Fig. 2. The net long-term coastal sand budget can therefore be negatively affected by groynes. The effect of groynes on beach morphology depends on the design (planform, length, height, cross-shore profile, inclination). The impact also depends on sea level, wave climate and sediment supply in the surf zone.

Although groynes are widely used, they are usually not an effective solution when applied as the sole shore protection measure. They cause important downdrift erosion and do not create a new sediment source. Where the coastal sediment budget is negative, groyne construction should be supplemented by artificial nourishment or other sediment-management measures.

An updrift jetty that protects a coastal inlet canal from waves, affects the updrift beach in a way similar to a single groyne. Jetties are usually much longer and extend beyond the surf zone to avoid sand bypass into the inlet channel (Fig. 3)

Fig. 3. Beach rip pattern updrift of the inlet jetty at Jones Beach, Long Island (USA). The alongshore rhythmicity of the rip pattern may be due to edge waves reflected from the jetty. From Hapke et al. (2010).

Groyne field

Groyne fields are constructed to limit downdrift beach erosion within the field. 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). 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. Within the groyne field, the shoreline typically exhibits a saw-tooth pattern with shoreline advance at the updrift side of each groyne and shoreline retreat at the downdrift side, as shown in Fig. 1.

Fig. 4. A groyne field on the Nile delta coast (near Burullus) at two consecutive years, showing insufficient protection of the groyne field from beach erosion. Insert: wave rose at Damietta. Upper panel: The typical saw-tooth shoreline pattern for oblique wave incidence. Lower panel: The shoreline has retreated (probably related to storm-driven cross-shore sediment loss and insufficient sediment supply) and left some of the groynes detached from the beach.

If a natural littoral drift is present downstream of the groyne field, coastal erosion downstream of the groyne field will be similar to the erosion of a single groyne. Erosion may even be greater if the groyne field strongly reduces sediment bypassing and the downdrift coast depends on this supply. However, erosion will be less if the littoral drift downstream from the groyne field is smaller than the littoral drift upstream.

In modern practice, groyne fields on eroding coasts are often combined with beach nourishment, either within the compartments and/or downdrift of the field. Whether the groynes increase the residence time of the fill depends on site-specific conditions, including wave climate, sediment size, groyne design, bypassing and rip-current losses. Regular monitoring and maintenance remain necessary because downdrift erosion, offshore losses and structural deterioration can reduce performance.

A groyne field creates a series of small embayed beaches. These beaches adopt an equilibrium saw-tooth planform in the case of a steady wave climate. This planform is the morphodynamic beach response to wave diffraction at the upstream groyne and blocking of the littoral drift at the downstream groyne (see Embayed beaches for further details). In practice, fluctuations in wave conditions will produce an oscillation around this long-term mean beach planform.

Fluctuations of the incident wave field strongly affect the littoral drift. Increase in wave obliqueness will usually increase the littoral drift, unless the wave incidence angle with the breaker line is larger than about 45° at the break point. Increase in incident wave height will enlarge the width of the surf zone. During severe storms the groynes are 'short' compared to the surf zone width, with erosion occurring around them. As most wave breaking occurs seaward of the groyne tips, the longshore drift will not be strongly disturbed by the groyne field. In this case, beach erosion can be more strongly affected by cross-shore sand transport than by a gradient in the littoral drift. A groyne field does not protect the coast from cross-shore erosion in case of strong shore-normal incident waves (Fig. 4). 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.

Under mild wave conditions groynes become 'long' (comparable to the surf zone width). Littoral drift between the groynes follows the shoreline closely and carries sand from the updrift to the downdrift groyne.

Fig. 5. Groyne field with oblique wave incidence and characteristic saw-tooth shoreline pattern. Upper panel: High-energy waves create a wide surf zone where the major part of the littoral drift passes almost undisturbed by the groyne field. Cross-shore transport dominates over longshore transport at the shoreline. Lower panel: The surf zone for moderate waves is situated within the groyne field. The littoral drift carries sand from the updrift groyne to the downdrift groyne where the littoral drift is deflected seaward.

Boundary rips

Fig. 6. Groyne field at Beach Haven, New Jersey (USA), showing boundary rips. From Hapke et al. (2010).

Wave-driven water accumulation between groynes induces compensating offshore-directed flows. These flows arise from wave set-up at the shoreline, alongshore pressure gradients within the groyne compartment and deflection of the longshore current at the downdrift groyne (see Rip currents). They are called boundary rips because they follow closely the downdrift groyne flank, while smaller boundary rips may also occur along the updrift groyne flank. Boundary rip currents can be strong and can erode sediment from the seabed. Part of this sediment may be transported seaward and, under energetic wave conditions, lost from the active coastal zone. Strong rip flow along the updrift groyne flank has been observed even under relatively small wave significant heights (< 1 m), see Fig. 6. Field measurements and modelling indicate that the relative groyne length, defined as groyne length divided by surf-zone width, strongly influences the offshore extent of boundary rips. In the conditions studied by Scott et al. (2016), surf-zone exits increased markedly when this relative length exceeded about 1.25.

Fig. 7. Narrow-spaced groyne field on the Nile delta coast, east of Kitchener drain. Insert: Damietta wave rose. The saw-tooth shoreline pattern is inverted.

If groynes are too closely spaced or too reflective relative to the wave climate and sediment supply, the expected updrift accretion may be replaced by local scour and erosion near the groyne root. In special situations where the geometric wave shadow zone of the groynes is comparable or even wider than the groyne spacing, the littoral drift between the groynes will be small or absent. Incoming oblique waves will be partly reflected on the updrift groyne flank, creating a local hotspot of wave energy with high erosion potential. Not only will there be no sand accumulation at the updrift groyne side, but beach erosion may even prevail. Fig. 7 illustrates beach erosion at the updrift groyne root, detaching the groynes from the beach.

Groyne design

Fig. 8. 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 morphologically active part of a groyne depends on the cross-shore distribution of longshore transport. For more oblique waves, longshore transport is stronger and the sediment deficit or accumulation caused by the groyne can extend over a larger part of the active surf-zone profile. Groynes are most effective if they do not trap the whole longshore sediment flux. The seaward extension of groynes should generally be limited so that a substantial part of the longshore sediment transport can bypass the groyne field, especially during energetic conditions. For sandy beaches this often means that groynes extend only across the inner part of the active surf zone; groynes that intercept too large a part of the storm surf zone can enhance downdrift erosion and offshore losses. More generally, the effect of a groyne field on littoral drift depends strongly on bypassing around the groyne heads and on the cross-shore distribution of longshore sediment transport. The appropriate length is therefore site-specific and should be determined from the cross-shore distribution of longshore transport, beach morphology, wave climate, sediment size and management objectives.

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.

The height of groynes influences the amount of longshore sediment transport trapped by the groynes. Groynes are often 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. The required crest level depends on tidal range, storm water levels, beach levels, wave climate, intended sediment bypassing, safety and maintenance access. A groyne may function as emerged or submerged depending on water level and beach morphology (Fig. 8a). Therefore crest level should be specified relative to the local beach profile and design water levels rather than by a fixed elevation above MSL.

Considering the shape in plan view, groynes can be straight, bent or curved, as well as L-shaped, T-shaped or Y-shaped. The most common shapes and types of groynes are schematically shown in Fig. 8. However, not all shown variants are equally common or equally recommended.

The performance of a groyne field should be assessed at the scale of the sediment cell. Important indicators are shoreline change updrift and downdrift of the field, beach volume within the compartments, active groyne length, structural condition, bypassing around the groyne heads and evidence of offshore sediment loss by rip currents. Because water levels, beach profiles and wave climate vary over time, groyne schemes require monitoring, maintenance and sometimes adaptation, removal or supplementation by beach nourishment.

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

Fig. 9. Example of two-row pile groyne at Hel Peninsula (the Baltic Sea).

Wooden groynes are most often one- or two-row palisade structures. The modest influence of the T-shaped wooden pile groyne on the shore (local erosion on the lee side and accretion on the updrift side) is illustrated in Fig. 9. 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

Fig. 10. 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 Fig. 10.

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 composite structures, strengthened internally by sheet piling. They are massive and durable; their hydraulic and sediment permeability depends on armour grading, core composition and the presence of an internal sheet-pile or filter layer (see Stability of rubble mound breakwaters and shore revetments). 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 Fig. 8.

Related articles

• 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
• Port breakwaters and coastal erosion
• Shoreline management
• Stability of rubble mound breakwaters and shore revetments
• Sand-filled geosystems in coastal engineering

References

The main authors of this article are Zbigniew Pruszak and Job Dronkers Please note that others may also have edited the contents of this article. Citation: Zbigniew Pruszak; Job Dronkers; (2026): Groynes. Available from http://www.coastalwiki.org/wiki/Groynes [accessed on 27-06-2026] For other articles by this author see Category:Articles by Zbigniew Pruszak For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Pruszak ZBIGNIEW For an overview of contributions by this author see Special:Contributions/Dronkers J