Groynes as shore protection
This article explains how groynes influence shoreline evolution when used as coastal protection structures. It complements the article Groynes, which describes the general features and types of groynes. The present article focuses on the morphodynamic functioning of groynes, their effects under different wave-incidence conditions, and their possible adverse impacts on adjacent coasts. It explains how groynes influence littoral drift, shoreline evolution and beach profiles under different wave-incidence and sediment-transport conditions.
The article is not intended as a detailed structural design manual. Its aim is to explain the coastal response to groynes, so that their potential benefits and drawbacks can be judged in relation to sediment supply, wave climate, shoreline orientation and management objectives.
Most of the examples and model simulations are drawn from Mangor et al. (2017[1]).
Contents
Introduction
Groynes are intended to reduce or redirect longshore sediment transport in the surf zone. Their effect is therefore strongly dependent on the magnitude and direction of the littoral drift, the cross-shore distribution of sediment transport, the length and permeability of the structures, the spacing between groynes and the availability of sediment. Groynes do not create sediment; they redistribute sediment along the coast and may also divert part of the sediment offshore. Their use as shore protection should therefore be assessed at the scale of the sediment cell, including possible downdrift effects.
Littoral drift produces erosion only where longshore sediment transport diverges, i.e. where the transport increases in the drift direction. Groynes are sometimes used even when such divergence is absent, with the aim of widening the beach in one place at the expense of another.
The effect of groynes is most easily understood by considering first a single groyne and then a groyne field. A single groyne interrupts part of the longshore sediment transport and generally causes sediment accumulation on the updrift side and erosion on the downdrift side. The severity and extent of this erosion depend on the amount of sediment that bypasses the groyne head, the cross-shore position of the active transport zone and possible offshore losses. A groyne field spreads the effect over a longer coastal section. It can reduce erosion within the protected frontage, but commonly shifts the main sediment deficit to the downdrift end of the field.
The suitability of groynes depends strongly on the wave-incidence regime. On eroding coasts with very little net longshore sediment transport, it is unlikely that littoral drift is the main cause of erosion so it is doubtful that interrupting the littoral drift with groynes will be an effective measure. On eroding coasts with moderate oblique wave incidence, groyne fields can stabilize a beach locally, but downdrift erosion and profile steepening have to be expected unless sediment supply, bypassing or nourishment compensate for the intercepted drift. On eroding coasts with strongly oblique wave incidence and large net littoral drift, groynes can have severe adverse effects, including strong downdrift erosion, offshore sediment loss and unstable shoreline shapes. In such settings groynes require particular caution and are often unsuitable as a stand-alone measure.
The effect of groynes depends on the type of coast, especially the littoral drift. Four types of coast are distinguished (see Classification of sandy coastlines):
- type 1: Perpendicular wave incidence, angle of incidence close to zero, small net littoral drift
- type 2: Nearly perpendicular wave incidence , angle of incidence 1o - 10o, net littoral drift small to moderate
- type 3: Moderate oblique wave incidence , angle of incidence 10o - 50o, large net littoral drift
- type 4: Very oblique wave incidence , angle of incidence 50o - 85o, large net littoral drift
The wave-incidence angle refers here to the angle at the breaker line
The following sections discuss these effects in more detail. They illustrate how groyne length, spacing and active surf-zone width influence sediment bypassing, why groyne fields may behave differently under mild and storm conditions, and how offshore-directed flows near groynes can contribute to sediment loss and beach-safety hazards.
Morphodynamic response to groynes
A groyne modifies shoreline evolution by interrupting part of the longshore sediment transport in the surf zone. The resulting shoreline response depends on the fraction of littoral drift intercepted by the structure. Sediment that is blocked accumulates mainly on the updrift side, while the downdrift side receives less sediment and is therefore prone to erosion. If part of the transport bypasses the groyne head, or if sediment is diverted offshore by currents near the structure, the updrift accumulation and downdrift erosion are reduced or redistributed.
The active effect of a groyne is not fixed but depends on tide and wave climate. During mild wave conditions the surf zone may be narrow and the groyne may extend across a large part of the active transport zone. During storms the surf zone widens and much of the wave breaking and longshore transport may occur seaward of the groyne head. A groyne can therefore be morphologically “long” under mild conditions and “short” under energetic conditions.
Groynes are normally constructed from the coastline, over the beach and some distance into the shoreface (Fig.1). Their effectiveness in trapping sand from the littoral drift depends on their cross-shore extension or, in other words, how big a part of the littoral drift they block. The sand accumulation and downdrift erosion depend on the coastal type and are similar to blocking of the littoral drift by a port, discussed in the article human causes of coastal erosion. However, this comparison is only valid for very long groynes. Groynes are normally designed to cover only part of the surf-zone. For predicting the shoreline response, it is important to know the sediment transport characteristics, as the littoral drift varies greatly over the coastal profile (see Fig.2). Groynes function by intercepting part of the littoral drift. They can widen or stabilize the beach locally by forming an updrift sand accumulation, but this is achieved by reducing sediment supply further downdrift. The degree of protection depends on the stability of this sand sheet under extreme conditions.
It is therefore recommended that the groyne covers the entire beach, so that it is not back-cut during storm surge and high waves. This means that the landward end of the groyne must be constructed on the coastline at the foot of the cliff/dunes and that its height at the landward end is not lower than the top of the backshore. The height of the groyne further seawards can be lower, depending on the requirements for sediment bypass.
Functional characteristics
Groynes on coast type Nearly perpendicular wave approach
A slightly oblique wave attack and a gradient in the littoral drift are assumed, implying shoreline erosion.
A computer simulation was performed for two types of groynes, a long and a short one (compared to the surf zone width), to demonstrate the effects of single groynes and groyne fields on an eroding shoreline, respectively. The effect of spacing between the groynes in the groyne fields, for the same two types of groynes, was also simulated.
The simulations were performed on an E-W oriented shoreline, which was exposed to prevailing waves from the NW and secondary waves from the NE, resulting in a net littoral drift towards the east. There was also an increasing littoral drift towards the east, which means that the shore was eroding. The direction of the normal to the shoreline with zero net littoral drift was calculated at 350o.
The demonstrations were performed using DHI’s LITPACK software. The module for the calculation of the littoral drift, LITDRIFT, and the one-line shoreline evolution module, LITLINE, were applied. The coastal profile and distribution of the littoral drift in the coastal profile are presented in the Fig.2. The width of the surf-zone is approximately 400m.
Case 1: Single groyne, short or long
The first shoreline simulation shows the development of the shoreline without any structures, see Fig.3, upper panel. The shoreline is exposed to a uniform shore erosion over the entire stretch.
The second simulation shows the shoreline response for a single groyne with the same length as the width of the surf-zone. The shoreline responds in the same way as in the case of the port in the figure shoreline development around a port, i.e. accretion of a sand sheet with constant orientation on the updrift side. There is also some initial small and local updrift erosion. As sand bypass has not started in the simulation, severe downdrift erosion has developed and is continuing to develop.
The third simulation shows the shoreline response for a single, but short groyne. The sand sheet accumulation has stopped at the tip of the groyne but is developing slowly along the updrift shoreline tending towards being parallel to the original shoreline. At the present stage of development, the short groyne actually protects a longer section than the long groyne. This is because the long groyne traps most of the sand close to the structure. The downdrift erosion is also large, but the erosion rate is decreasing due to increasing sand bypass.
In both cases, the erosion problem is solved only on the updrift side of the groyne for a length depending on the length of the groyne for the given wave climate. However, downdrift erosion is extensive, unavoidable and permanent. Introduction of the groyne has resulted in a drastic response far from the gentle evenly distributed erosion that existed before the intervention.
Case 2: Groyne field, short or long groynes
Fig.3 shows that a single groyne, long or short, causes downdrift erosion on a shoreline exposed to a slightly oblique wave climate. In order to extend the length of the protected area, and to compensate for the downdrift erosion, it is common practice to construct several groynes along the shoreline, a so-called groyne field. Fig. 4 demonstrates the shoreline development for the following groyne fields:
- three long groynes with a spacing of 600 m, i.e. 1.5 times the length of the groynes.
- three long groynes with spacing equal to 1200 m, i.e. 3 times the length of the groynes.
- three short groynes with a spacing of 600 m
- three short groynes with a spacing of 1200 m
The ability of the groyne field to protect a certain section of a shoreline depends on many parameters discussed in the following.
For the single groyne it was shown that the wave climate and the length of the groyne together determine the length of the section that a single groyne can protect. However, both spacing and time are important parameters for groyne fields, as it takes a relatively long time to fill a groyne field with sand. Until then there will be temporary erosion between the groynes; larger spacing increases the temporary erosion. In the two cases of long groynes, bypass of the first groyne did not start within the simulation time. This means that the only development that takes place between the groynes, is an initial turning of the shoreline to the orientation of zero littoral drift. The erosion downdrift of the groyne field is identical to the erosion caused by the single groyne. However, this will only be the case initially, as bypass has not started in any of the situations. Later, when bypass starts, the erosion will slow down for the single groyne, as shown for the short groyne in Fig.3, whereas major erosion continues in the groyne field, until the two gaps between the groynes have been filled. This means that in the long run a groyne field will give higher downdrift erosion than a single groyne.
In the case of the two short groynes, the initial development in the gaps is very similar to the development seen for the long groynes, i.e. a turning of the local shorelines to the direction of zero littoral drift. However, the influence of the bypass can be seen in the first gap, which is gradually being filled with the bypassed material. The downdrift erosion for the groyne field is larger than for the single groyne because bypass of the groyne field did not start before the end of the simulation period.
Because of these disadvantages, groynes are used more cautiously today than in the past. If, for one reason or another, they are used in new protection schemes, artificial sand filling into the groyne system will be a normal part of the project, in order to avoid temporary erosion. Fig.5. shows how one of the groyne fields tested above was filled initially with sand and how this influences the shoreline development. When comparing with shoreline development for the non-filled field (Fig.4 lower panel), it appears that temporary erosion is avoided and that lee-side erosion is slightly smaller.
After the compartments between groynes have been filled, the erosion rate depends strongly on the cross-shore profile of the sand fill. An oversteepened fill profile is rapidly reworked by waves, causing shoreline retreat, whereas an understeepened profile may erode more slowly or even produce temporary shoreline accretion. The performance of a groyne field with nourishment therefore depends on both the groyne layout and the initial beach-fill profile. The longer term performance is discussed in the section #Long-term erosion in the coastal profile.
Other wave climates
The influence of different wave incidence angles has not been demonstrated in the above simulations, but this influence is demonstrated in the following for a shoreline with zero net littoral drift and for a shoreline with a very oblique wave climate, respectively.
Groynes on coast type Perpendicular wave approach
A slightly curved shoreline is assumed, with a perpendicular wave approach at the centre. The natural development of a slightly curved shoreline, with zero net littoral drift in the middle part and with small gradients in the littoral drift away from the middle section, is shown in the upper part of Fig.6. Although the net littoral drift is zero at the centre, this point can still be erosional if sediment transport diverges away from it in both directions. It is then a negative nodal point: the local net transport is zero, but the gradients on either side remove sediment from the central section.
The middle panel of Fig.6. shows the shoreline development in a situation, where the middle section has been isolated by the introduction of two long groynes. The two groynes practically secure the middle section of the shoreline against erosion by preventing loss of sand to the adjacent sections. In this situation erosion a small distance away from the groynes is slightly larger than the erosion in the situation without groynes; the reason is that the two groynes trap sand and prevent loss from the middle section. This trapped and non-eroded sand is consequently missing along the adjacent sections.
The lower panel of Fig.6 shows the shoreline development for one long groyne located at the point of zero net littoral drift. The groyne causes local accumulation on both sides close to the groyne and erosion at adjacent sections, which is a little larger than the erosion without a groyne.
Groynes on coast type Moderate to very oblique wave approach
An eroding shoreline is assumed, with moderate oblique to very oblique wave incidence. The upper part of Fig.7 demonstrates the natural shoreline development for a shoreline exposed to oblique wave incidence, with increasing littoral drift in the direction of the net drift. As a result, the shore is exposed to uniform erosion along the entire section.
The long groyne develops at first only a relatively short updrift sand sheet, and there is also an updrift erosion area, which is just slowly filled in. A relatively long stretch of downdrift coastline is exposed to severe erosion. When sand bypass starts, sand will be deposited in a shoal east of the groyne head, which means that it will not contribute to the littoral drift budget for the downdrift shoreline. This assumption will be correct for the first (many) years, see the discussion in the article on Groynes and similar structures perpendicular to the shore. The same phenomenon is shown in the article Accretion and erosion for different coastal types, where the influence of a port in a similar wave climate was discussed. In the situation discussed here the only positive influence of the groyne is a short sand sheet updrift of the groyne over a length comparable to the length of the groyne itself, whereas the erosion downdrift has increased considerably compared to the situation without protection. It appears that the protected section is considerably shorter than the protected section for the slightly oblique wave climate as presented in Fig. 3.
Discussion of 2- and 3-D effects and long-term profile changes
The above shoreline developments for various groyne schemes are somewhat simplified as they do not include 2-D or 3-D effects or long-term profile changes. Such effects do, however, occur in connection with groyne schemes.
2-D effect
Offshore-directed flows near groynes, including boundary rips, are discussed in Groynes. For shore protection, their main significance is that they can remove sediment from the groyne compartments or from the groyne heads and transport it seaward. This process is not represented in the one-line shoreline simulations shown above, which mainly illustrate alongshore redistribution of sediment. Offshore losses, lobate deposition near groyne heads and local scour can therefore reduce the effective sediment retention of a groyne field and increase maintenance needs.
A long groyne approximately the same length as the width of the surf-zone, will generate a strong jet as the entire longshore current is blocked. A shorter groyne will generate a smaller jet more or less parallel to the shore due to the interference with the outer uninterrupted part of the longshore current.
These offshore-directed flows can erode the seabed near the groyne and transport sediment seaward. Under energetic wave conditions, part of this sediment may be carried beyond the active littoral zone or deposited in lobate shoals downdrift of the groyne head. Offshore directed sediment forms lobate deposition zones downdrift of the tips of the structures[2]. The contraction of the current near the head of the groyne will also cause local seabed erosion updrift and off the groyne head, which may damage the head.
In the lee zone of the groyne, wave diffraction and the decrease in wave set-up on the foreshore towards the structure (for an explanation see Shallow-water wave theory), will generate an eddy with the outward-directed current running along the downdrift of the structure. This eddy adds to the offshore sand loss and local seabed erosion and it is also dangerous for swimmers during rough weather.
3-D effect
Near the groyne head, contraction and separation of the current can generate three-dimensional vortices and local scour. Scour is especially pronounced around abrupt, impermeable structures and where strong longshore currents are deflected seaward.
Long-term erosion in the coastal profile
The one-line simulations shown above assume that shoreline change is controlled mainly by gradients in longshore sediment transport. On an eroding coast this assumption may be insufficient if the active profile extends seaward of the groyne tips. In that case, littoral drift divergence may continue outside the groyne field and erode the outer part of the coastal profile. This is especially important during energetic conditions, when the surf zone is wider than the groyne length and much of the longshore and cross-shore sediment transport occurs seaward of the groyne heads. The coastal profile will then gradually become steeper. A sand fill placed between the groynes may not retain a stable equilibrium shape. Shoreline retreat can therefore continue even after the compartments have initially been filled. Continued profile steepening and scour near the groyne heads can undermine the structures, so reinforcement, adaptation or maintenance may be required.
Applicability
Although groynes may protect some shoreline sections, it is obvious from the above that groynes have many disadvantages. Furthermore, it appears that the effectiveness of groynes depends on the type of coast. The applicability of groynes on different types of coasts is discussed below, where advantages and disadvantages are highlighted.
Coast type Perpendicular wave approach
Groyne fields can be used on type 1 curved coasts in order to prevent loss of sand into adjacent sections. As there is no net littoral drift in this situation, only local sand is trapped close to the groyne due to small fluctuations in littoral drift. Minor additional erosion does occur in neighbouring sections due to the lack of the supply of sand from the protected section.
Coast type Nearly perpendicular wave approach
Groynes are applicable on coasts (or on coastal sections) with a small angle of wave incidence. The groynes accumulate sand on their updrift side at the expense of downdrift erosion on the downdrift side. Groyne fields will be exposed to initial erosion inside the protected area if they are not nourished as part of the construction scheme. Local erosion and scour will occur near the groyne heads, and the outer part of the coastal profile will continue to erode. Progressive scour and profile steepening near the groyne heads can endanger structural stability unless the heads are adequately founded, protected or maintained.
A combined solution of groynes and nourishment may mitigate erosion within the protected frontage, but its effectiveness depends on the beach-fill profile, sediment supply, bypassing, boundary-rip losses and maintenance.
The negative effects of the groyne fields, downdrift erosion and profile steepening, can be mitigated by regular nourishment. The advantage of the combined solution of groyne fields and nourishment is that well-designed groynes prevent or mitigate erosion of the beach. However, groynes are less effective during extreme events; in such situations they can slow down or accelerate the rate of coastal erosion caused by offshore movement of the sand depending on the strength of boundary rip currents. So, groynes and nourishment are not sufficient coastal protection in themselves, especially if buildings are constructed near the dune foot or the cliff edge.
Coast type Moderate - very oblique wave approach
On coasts with moderate oblique wave incidence and large net littoral drift, groynes may create only limited local benefit while causing substantial downdrift erosion unless combined with nourishment or bypassing. On coasts with very oblique wave incidence, the risks are greater: shoreline instabilities, flying spits, offshore sediment loss and lagoon formation may occur. In these settings groynes are generally unsuitable as stand-alone protection.
General comments
- Groynes tend to trap seaweed and floating debris on the updrift side as well as on the lee side.
- Groynes obstruct passage along the beach.
- Groynes are often dangerous to walk on; however, if they are built to allow passage on the top, they are popular for promenade and fishing.
- The lee zone eddy as well as the updrift rip current can be dangerous for bathers.
- Groynes constitute a foreign element in the coastal landscape due to their unnatural shape being perpendicular to the shoreline.
Conclusion
Groynes can be useful shore-protection measures where the management objective is to reduce longshore redistribution of beach sediment over a limited frontage. Their effectiveness depends on the magnitude and direction of littoral drift, sediment supply, beach type, surf-zone width, groyne layout and the possibility of sediment bypassing. Groynes are least problematic where net littoral drift is small, but in such cases they are only useful if erosion is caused by local divergence of longshore transport or by leakage of sediment from the protected section. Where shoreline retreat is mainly due to cross-shore sediment loss, sea-level rise, sediment deficit over the whole active profile, or dune/cliff erosion, groynes alone are unlikely to be effective. Groynes are most risky where oblique wave incidence produces a large persistent longshore sediment transport. In the latter case, groynes may cause severe downdrift erosion or offshore sediment loss and are rarely appropriate as a stand-alone measure.
A groyne scheme should therefore be assessed at sediment-cell scale. Important questions are how much sediment is intercepted, how much bypasses the field, whether sediment is lost offshore by boundary rips or storm-driven transport, and how the downdrift coast will be affected. Monitoring, maintenance and, where needed, nourishment or sediment bypassing are essential parts of responsible groyne-field management.
Related articles
- Groynes
- Deteriorated groynes
- Human causes of coastal erosion
- Natural causes of coastal erosion
- Accretion and erosion for different coastal types
- Shallow-water wave theory
- Classification of sandy coastlines
- Littoral drift and shoreline modelling
- Coastal Hydrodynamics And Transport Processes
- Hard coastal protection structures
- Port breakwaters and coastal erosion
- Dealing with coastal erosion
- Stability of rubble mound breakwaters and shore revetments
References
- ↑ Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
- ↑ Nordstrom, K.F. 2014. Living with shore protection structures: A review. Estuarine, Coastal and Shelf Science 150: 11-23
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