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Revision as of 13:13, 3 December 2008
The following section on groynes is fairly comprehensive. The reason for this is not a special preference for groyne solutions but rather because groynes are well suited to explain various basic morphological responses including lee side erosion effects. The different types of coasts mentioned in this article are explained in the article Classification of coastlines.
Contents
- 1 Method
- 2 Functional characteristics
- 2.1 Coastal type 2: Slightly oblique wave attack and gradient in the transport, which means an eroding shoreline
- 2.2 Case 1: Type 2 coast, protection by single groynes, long and short
- 2.3 Case 2: Type 2 coast, protection using groyne fields, short and long groynes
- 2.4 Other wave climates
- 2.5 Coastal Type 1: Perpendicular wave approach in a nodal area, eroding shoreline
- 2.6 Coastal Type 3/4: Moderate to very oblique wave approach, eroding shoreline
- 2.7 Discussion of 2- and 3-D effects and long-term profile changes
- 2.8 2-D effect
- 2.9 3-D effect
- 2.10 Long-term erosion in the coastal profile
- 3 Applicability
- 4 See also
- 5 Further reading
Method
Groynes are normally constructed from the coastline, over the beach and some distance into the shoreface. Their effectiveness in trapping sand from the littoral drift depends on their extension or, in other words, how big a part of the littoral drift they block. The sand accumulation and lee side erosion depend on the coastal type and are very similar to what was discussed in the article on human causes of coastal erosion in relation to a port. However, this comparison is only valid for very long groynes. Groynes are normally designed to cover only part of the surf-zone. As the littoral drift varies greatly over the coastal profile, see figure further in article, on it is important to know the transport characteristics so as to be able to predict the shoreline response. A groyne function by trapping sand on the upstream side, and in this way the coast behind the sand filet is protected. Precisely how protected depends on the stability of this sand filet under extreme conditions. The groyne must, therefore, cover 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 bypass, etc.
Functional characteristics
Coastal type 2: Slightly oblique wave attack and gradient in the transport, which means an eroding shoreline
Simulating the shoreline impact of two types of groynes; a long and a short one demonstrate the function of single groynes and groyne fields on an eroding shoreline, respectively. The function of spacing between the groynes in the groyne fields, for the same two types of groynes, is also demonstrated.
This demonstration covers the influence of the following parameters:
- The length of the groyne relative to the width of the surf-zone
- The spacing of the groynes relative to the length of the groynes
The demonstrations are performed on an E-W oriented shoreline, which is exposed to prevailing waves from the NW and secondary waves from the NE, resulting in a net littoral drift towards the east. There is also increasing transport towards the east, which means that the shore is eroding. The direction of the normal to the shoreline with zero net littoral drift is calculated at 350o.
The demonstrations are performed using DHI’s LITPACK software. The module for the calculation of the littoral drift, LITDRIFT, and the one-line shoreline evaluation module, LITLINE, are applied.
The coastal profile and distribution of the littoral drift in the coastal profile are presented in the figure below.
The width of the surf-zone is approximately 400m.
Case 1: Type 2 coast, protection by single groynes, long and short
The first shoreline simulation shows the development of the shoreline without any structures, see figure below, upper part. 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 image:Shoreline development schematici.e. a sand filet with constant orientation accumulates on the upstream side. There is also some initial small and local upstream erosion. As bypass has not started, severe lee side erosion has developed and is continuing to develop.
The third simulation shows the shoreline response for a single, but short groyne. The sand filet accumulation has stopped at the tip of the groyne but is developing slowly along the upstream shoreline tending towards being parallel to the original shoreline. At the present stage of the 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 lee side erosion is also large, but the erosion rate is decreasing due to the increasing bypass.
In both cases, the erosion problem is solved only on the upstream side of the groyne for a length, which depends on the length of the groyne for the given wave climate, but lee side erosion is extensive, unavoidable ever lasting. Introduction of the groyne has resulted in a drastic response far from the gentle evenly distributed erosion, which persisted before the intervention.
Case 2: Type 2 coast, protection using groyne fields, short and long groynes
A single groyne, long or short, on a shoreline exposed to a slightly oblique wave climate causes downstream erosion in Fig. 3. In order to extend the length of the protected area, and to compensate for the lee side erosion, it has been normal practice to construct several groynes along the shoreline, a so-called groyne field. The Fig. 4. below 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.
It was seen for the single groyne that the wave climate and the length of the groyne together determine the length of the section, which a single groyne can protect. However, both the spacing and the time are important parameters for groyne fields, as it takes a relatively long time to fill a groyne field with sand. Until this has been done, there will be temporary erosion between the groynes; longer spacing increases the temporary erosion. In the two cases of the long groynes, bypass of the first groyne did not start within the simulation time. This means that the only development, which takes place between the groynes, is an initial turning of the shoreline to the orientation of zero transport. The erosion downstream 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 seen for the short groyne in the figure above, whereas major erosion will continue 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 lee side 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 transport. However, the influence of the bypass can be seen in the first gap, which is gradually being filled with the bypassed material. The lee side erosion for the groyne field is larger than for the single groyne because bypass of the groyne field did not start at the end of the simulation period.
The design of a groyne field requires great care in order to avoid temporary erosion within the field. It must also be remembered, that the protection provided by a groyne field is always at the expense of lee side erosion.
The obvious disadvantages of groynes mean that they are used less today than previously. If, for one reason or another, they are used in new protection schemes, it will normally be part of the project to fill sand artificially into the groyne system in order to avoid temporary erosion. Fig. 5. below shows how one of the groyne fields tested above was filled initially with sand and how this influences the shoreline development.
It is seen from the shoreline development of the filled groyne field, when comparing this with the similar shoreline development for the non-filled field, see image:Shoreline development for short and long groynes, lower part, that temporary erosion is avoided and that lee-side erosion is slightly smaller.
Other wave climates
The influence of the different wave climate has not been demonstrated in the above simulations, but this influence is demonstrated in the following for a shoreline with zero net transport and for a shoreline with a very oblique wave climate, respectively.
Coastal Type 1: Perpendicular wave approach in a nodal area, eroding shoreline
The natural development of a slightly curved shoreline, with a zero net transport in the middle part and with small gradients in the transport away from the middle section, is shown in the upper part of Fig. 6. below. It is seen that there is erosion in the middle part of the section despite the fact that the net transport here is zero. The reason is that it is a (negative) nodal point, from which sediment escapes in both directions.
The middle section 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 the erosion a small distance away from the groynes is slightly larger than the erosion in the situation without groynes; the reason for this 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 part of Fig. 6. shows the shoreline development for one long groyne installed in the middle of the nodal section. The influence of the groyne is 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.
Coastal Type 3/4: Moderate to very oblique wave approach, eroding shoreline
The upper part of Fig. 7. below demonstrates the natural shoreline development for a shoreline exposed to a moderate to very oblique wave climate, with increasing transport in the direction of the net drift. The result is that the shore is exposed to uniform erosion along the entire section.
The long groyne develops only a relatively short sand filet, and there is an upstream erosion area, which is only filled in slowly. A relatively long stretch of downstream coastline is exposed to severe erosion. When bypass starts, the sand will be deposited in a shoal east of the head of the groyne, which means that it will not contribute to the littoral drift budget for the downstream 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, and also schematic of shoreline development where the influence of a port in a similar position was discussed. Here there is only a positive influence of the groyne in the form of a short sand filet upstream of the groyne over a length comparable to the length of the groyne itself, whereas the erosion downstream has increased considerably compared to the situation without protection. It is seen 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
A groyne redirects the longshore current, as it is constructed perpendicular into the longshore current. The groyne thus directs the current seawards along the structure, whereby an offshore-directed jet is generated. 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. Under all circumstances, the jets will cause loss of sand to deeper water.
The contraction of the current near the head of the groyne will also cause local seabed erosion upstream and off the groyne head, which may damage the head.
In the lee zone of the groyne, the diffraction of the waves, and the decrease in wave set-up on the foreshore towards the structure, will generate an eddy with the outward-directed current running along the lee side 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
When waves and current pass the breakwater head, 3-D current eddies will be formed, which will generate scour in the seabed in addition to the local seabed erosion caused by the 2-D current contraction. The scour is especially pronounced for structures, which cause abrupt current changes.
Long-term erosion in the coastal profile
A groyne field which has been constructed at an eroding shoreline, and which does not cover the entire width of the littoral zone, will only secure the inner part of the coastal profile against erosion. The outer part of the coastal profile will continue to erode and the coastal profile will gradually become steeper. This will destabilise the outer part of the groynes and eventually the groyne heads will collapse unless they are strengthened considerably. A groyne field, which covers the entire littoral zone, will not be exposed to this phenomenon, but groynes are seldom made that long.
Applicability
It is evident from the above that groynes are able to protect sections of shoreline, but it is also evident that groynes have many disadvantages. Furthermore, it has been demonstrated that the function of groynes depends on the type of coast. The applicability of groynes on different types of coasts is discussed in the following, where advantages and disadvantages have been highlighted
Type 1 coasts
Groyne fields can be used on type 1M and 1E coasts in order to prevent loss of sand into adjacent sections. As there is no net transport in this situation, only local sand is trapped close to the groyne, and hardly any lee side erosion occurs. However, minor additional erosion does occur in neighbouring sections due to the lack of the supply of sand from the protected section.
Type 2 and most perpendicular part of type 3 coasts
Groynes are applicable on coasts of types 2M and 2E and on the parts of the 3M and 3E coasts, which have an angle of incidence close to the type 2 coast. The groynes accumulate sand on their upstream side at the expense of lee side erosion on the downstream side.
Groyne fields will be exposed to initial erosion inside the protected area if they are not nourished as part of the construction schedule.
Local erosion and scour will occur near the groyne heads, and the outer part of the coastal profile will continue to erode. Groynes will eventually collapse if their heads are not strengthened.
The negative effects of the groyne fields, lee side 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 secure against or decrease erosion of the beach. However, groynes are no guarantee during extreme events; in that situation they only slow down the rate of coastal erosion caused by offshore movement of the sand. So, groynes and nourishment are not sufficient coastal protection in themselves, especially if buildings are constructed on the edge of the cliff.
Type 3, 4 and 5 coasts
As the sand filet accumulation for these types of coasts is very short, groynes cannot be recommended as a protective measure.
In addition, a groyne on a type 4 or 5 coast may lead to the development of a sand formation off-set from the coastline. This will deprive the downstream shoreline of a supply of sediment. Erosion will follow, and an unwanted lagoon in the shelter of the sand spit may develop. Groynes on these types of shorelines may start an uncontrollable development.
General comments
- Groynes tend to trap seaweed and floating debris on the upstream 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 upstream 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.
See also
Further reading
- Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.
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