Applicability of detached breakwaters
This article describes the applicability of detached breakwaters for different coastlines. This article relates to the article detached breakwaters, which describes different types of detached breakwaters, their effects etcetera.
Contents
Introduction
From the article Detached breakwaters it appears that breakwaters are able to protect sections of shoreline in a more diversified and less harmful way than groynes, but some of the disadvantages of groyne schemes also characterise breakwater schemes. It has been demonstrated that, on some types of coast, breakwaters function differently to groynes. A breakwater can, for example, trap sand on a coastline with a perpendicular wave approach, which is hardly the case for a groyne.
The applicability of detached breakwaters is related to the type of coast. A detailed description of different sedimentary coastlines is provided in the article Classification of sandy coastlines. In summary, this article explains that there are 5 main types of coastlines, each having three subcategories. The 5 main types of sedimentary coasts (which are defined by the angle of incidence of the prevailing waves) are:
- Type 1: Perpendicular wave approach, angle of incidence close to zero;
- Type 2: Nearly perpendicular wave approach, angle of incidence 1o - 10o, net transport small to moderate;
- Type 3:Moderate oblique wave approach, angle of incidence 10o - 50o, large net transport;
- Type 4:Very oblique wave approach, angle of incidence 50o - 85o, large net transport;
- Type 5:Nearly coast-parallel wave approach, angle of incidence >85o, net transport near zero.
This classification has been subdivided according to the wave exposure as follows:
- P: Protected, the “once per year event” having [math]H_{12h/y} \lt 1 m [/math];
- M: Moderately exposed, the “once per year event” having [math]1 m \lt H_{12h/y} \lt 3 m [/math];
- E: Exposed, the “once per year event” having [math]H_{12h/y} \gt 3 m [/math].
[math]H_{12h/y}[/math] is the significant wave height that is exceeded only 12 hours per year.
We further define:
- [math]x[/math] distance to shoreline;
- [math]x_{80}[/math] surf-zone width, approximately 80 % of the littoral transport takes place landwards of this line;
- [math]x^*=x/x_{80}[/math] distance relative to surf-zone width.
Applicability for different coastal types
Type 1 coasts
Breakwaters can be used on 1M and 1E type coasts in a way similar to groynes, namely to prevent the loss of sand into adjacent sections. When groynes are used in this way, they will hardly trap any sand and there will hardly be any lee-side erosion. When breakwaters are used on type 1 coasts, they will trap sand in their lee areas, which causes local erosion on both sides of the breakwaters. This can be avoided if the sheltered areas behind the breakwaters are filled in as part of the project. In addition to this local erosion, adjacent to the breakwaters, a little additional erosion occurs on either side of the protected section, due to the lack of sand from the protected section.
Single breakwaters or segmented breakwaters can both be used as direct protection of specific sections against shore erosion and against coast erosion if they are combined with initial sand fill.
Type 2 and the most perpendicular part of type 3 coasts
Like groynes, breakwaters are applicable on coasts of type 2M and 2E and on the parts of the 3M and 3E coasts, which have an angle of incidence close to that of the type 2 coast. The breakwaters accumulate sand both on their upstream side and in their sheltered zones as a tombolo or as a salient, depending on their characteristics. The upstream accumulation will be very similar to that of a groyne if the breakwater is so long that it forms a tombolo. If this is not the case, the upstream accumulation will be smaller. Both the sand accumulation in the sheltered zone and the upstream accumulation will take place at the expense of lee side erosion on the downstream side of the breakwater scheme. In order to minimise the downstream erosion within the scheme, it is important to include the initial filling of the scheme in the construction project.
Local erosion and scour will occur near the breakwater heads, and the outer part of the coastal profile will continue to erode unless the breakwaters cover the entire littoral zone. Beach breakwaters will eventually collapse if their sea sides are not strengthened, whereas coastal breakwaters constructed at a distance greater than say [math]x^* \gt 1.2[/math] will not be exposed to an eroding seabed.
Lee side erosion and profile steepening effects of segmented breakwater schemes can be mitigated by regular nourishment, but regulating the length of the individual breakwaters in the scheme can also partly mitigate the extent of the lee side erosion. The combined use of segmented breakwaters and nourishment secures against erosion of the beach in the gaps, but also to some extent against coastal erosion in the areas directly protected by the breakwaters. This means that a segmented breakwater scheme combined with nourishment has to be further supplemented with revetment protection in the gaps in order to provide complete coastal protection.
It is recommended to use few large structures instead of many small structures in order to enhance the aesthetic appearance of a segmented breakwater scheme. This is especially true for type 2 coasts, as the individual breakwaters are capable of suspending long upstream sand filets (Fig. 1.).
Type 4 and 5 coasts and the most oblique part of type 3 coasts
A single structure parallel to the coast is not very efficient when the waves are fairly oblique to the shoreline, as the effective length of the structure perpendicular to the wave direction is relatively small is this case. As the sand filet accumulation for these types of coasts is very short, single breakwaters cannot be recommended.
Segmented breakwater schemes with relatively short gaps can be used as a combined shore and coastal protection measure on all types of coasts because the formation of pocket beaches in the gaps does not depend very much on the wave direction when the gaps are small.
General comments on emerged breakwaters
- Breakwaters tend to trap seaweed and floating debris in the bays, which are formed on the upstream side as well as on the lee side.
- A breakwater does not constitute an obstruction for the passage along the beach.
- Breakwaters are dangerous to walk on and should be constructed so that only a salient is formed.
- The lee zone eddies are dangerous for bathers.
- Breakwaters constitute a foreign element in the coastal landscape as they obscure the view of the sea, but if their number is minimised, and they are built relatively far from land, this problem is minimised.
- The disadvantages of breakwaters and groynes can, to some extent, be avoided by optimising the shape of the structures. This is the subject of the next subsection.
See also
Information on coastal protection:
- Dealing with coastal erosion: protection against dune erosion and structural erosion
- Hard coastal protection structures
Information on breakwaters:
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.
Please note that others may also have edited the contents of this article.
|
