Hard shoreline protection structures
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The basis of this article is especially written for the Coastal Wiki by the main author referred to at the bottom of this page.
This article provides an introduction in hard shoreline protection structures. The article provides linkages to more detailed articles and gives a short introduction in 'groynes and breakwaters' and 'seawalls and revetments'. See also Shore protection, coast protection and sea defence methods and seawalls and revetments.
In the article Protection against coastal erosion some basic requirements of 'hard' structures have been given. Also the 'tasks' have been mentioned these structures must fulfil in properly resolving erosion problems. E.g. if one likes for example to resolve a structural erosion problem with the help of a series of groynes, the groynes should interfere in the occurring sediment transports.
How various hard structures 'work' in different possible applications, will be discussed in the present article. Coastal protection is a main issue, but we will see that structures are sometimes also applied in coastal engineering practice where the distinction between coastal 'protection' and coastal 'management' is somewhat vague. More information on hard structures and coastal erosion is provided in the article hard structures and structural erosion.
This section describes the basic principles of two types of 'hard' structures, viz: 'groynes and shore parallel offshore breakwaters' and 'seawalls and revetments'. Besides this, various well-defined types of structure are also discussed in other articles:
- (series) of groynes
- seawalls and revetments
- jetties/(port) breakwaters are described in port breakwaters and coastal erosion
- piers and trestles
- off-shore detached shore parallel breakwaters (see also detached breakwaters)
- floating breakwaters
Groynes and shore parallel offshore breakwaters
Series of groynes or series of shore parallel offshore breakwaters are able to interfere into the sediment transport processes. So in principle they might 'work' as a tool in a protection scheme. Application requires, however, a difficult process of fine-tuning of the design layout. An unavoidable consequence of a well-designed protection scheme in the stretch of coast to be protected, is the lee-side erosion.
Downdrift of the protection scheme (with reduced longshore sediment transports), the original sediment transport rates do occur. This leads to large gradients in the longshore sediment transport (much larger than the original gradients), causing the lee-side erosion as was mentioned.
While protecting the coast in the area of interest, in fact this is achieved at the spent of the coast just outside (at the lee-side of) the area of interest. The lee-side erosion effect can be reduced/mitigated when several terminating groins are built as sediment permeable structures or when they have reduced lengths; the rates of permeability of these terminal groins or their reduced lengths should be determined by numerical modeling. In some cases sudden change of coastal orientation may reduce the lee side erosion to an acceptable level.
While applying series of groynes or series of shore parallel offshore breakwaters, the coastal zone manager has to take these (adverse) consequences fully into account in the decision making process. Often these types of protection measures are too rashly applied.
Seawalls and revetments
From the notion that, when 'hard' structures are applied as a tool to resolve a structural erosion problem due to a gradient in the longshore sediment transport, one has actively to interfere into the occurring transport processes, it is easily to be understood that the application of revetments or seawalls along the face of the mainland, do not, and cannot 'work' properly.
Under ordinary conditions the revetments or seawalls do not interfere into the transport processes. They are outside the reach of the waves and currents at sea which are causing the (gradient in the) longshore sediment transport. Under ordinary conditions the loss of sediments out of the control volume of a cross-shore profile continues.
Only during storm conditions the parts of the cross-shore profiles which are 'protected' by revetments or seawalls, form an integrated part of the entire, then active, cross-shore profile. While the mainland is physically protected from erosion, the erosion of the beaches continues. Due to cross-shore sediment transport processes the beach sediments are transported towards deeper water and next eroded by the gradient in the longshore sediment transport.
After some time all the beaches have disappeared. The attack to the revetment or seawall occurs more frequently and is much heavier than just after the construction of these structures. (At the time of construction still a beach was present in front of the revetment or seawall). Often the level of the foundation is not designed at these conditions after some years, and damage and collapse of the structures will take place. For more information on this topic, see also the article seawalls and revetments.
- Note of the author: While it is quite clear that the application of seawalls or revetments in a structural eroding coastal problem is an inherent 'bad' solution, it is a pity to observe that they are so often (wrongly) applied in practice. This can be considered as a blot on the reputation of our coastal engineering profession.
- Note from editorial board: It should be noted that negative consequences of seawalls or revetments can often be mitigated by regular artificial nourishment of the eroding beach in front of the hard structure.
- US Army Corps of Engineers (2006). Coastal Engineering Manual. Part 5 Chapter 2 Shore Protection Projects
- Dean, R., Dalrymple, R., (2001). Coastal processes with engineering applications, p.387-412
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