Difference between revisions of "Artificial nourishment"
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Revision as of 13:40, 28 August 2008
- 1 Introduction
- 2 Single artificial nourishments
- 3 Regular artificial nourishments
- 4 See Also
- 5 References
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 gives insight in the application and the implications of artificial nourishments. For an additional view on methods, functional characteristics and applicability of artificial nourishments, you are referred to the article shore nourishment.
Artificial nourishments are examples of projects where the coastal system is fed with sediments (sand) from a source (borrow area) at a certain distance from the project area. Other examples of projects where the coastal system is fed, are large scale land reclamation projects and the construction of an artificial island in open sea. In these cases often huge volumes of sand are required; up to several hundreds of millions m3 per project.
In this article on artificial nourishments applications with moderate volumes per project are discussed (order of magnitude up to 10 million m3 per project).
But why should one apply artificial beach nourishments? Artificial nourishments can be applied in various cases; e.g.:
- to broaden the beach (recreation purposes);
- to create entirely new beaches (recreation purposes);
- to enhance the safety of the mainland or to enhance the safety of properties built rather close to the edge of the dunes;
- to compensate losses because of structural erosion.
The first three cases are to be considered as applications to improve an existing undesirable situation. With a mainly single action a ‘better’ situation is achieved. In principle a single action indeed, although often some relatively small maintenance nourishments are necessary in order to keep the situation as required according to the design conditions.
In the fourth case artificial nourishments are applied as a tool in coastal protection projects. Because of structural erosion sediments are more or less continuously lost out of a stretch of coast; (frequent) artificial nourishments are required to compensate on average the occurring losses.
So depending on the actual aim of the nourishment project, the nourishment can be carried out as a mainly single action or the nourishment has to be repeated on a regular basis.
To carry out an artificial nourishment project, various methods can be applied; see e.g. xxxx)
Single artificial nourishments
Figure 2 shows schematically cross-section A-A as was indicated in Figure 1. With an artificial nourishment as also is indicated in Figure 2 the aim, broadening the beach, is simply achieved. (It is first assumed that the 'borrow' sediment is similar to the 'native' material.) However, assuming that the shape of the cross-shore profile in the pre-nourishment period was more or less in equilibrium, it is quite clear that the shape right after finishing the project, is not in equilibrium anymore. Natural coastal morphology processes (in this case: cross-shore sediment transports) will flatten out the 'disturbance' from the equilibrium profile; the so-called equilibration process. Some maintenance will be required in order to keep the desired beach width. The inevitable process of re-establishing the shape of the equilibrium profile, starts with ‘using’ the sediments from the beach nourishment meant to widen the beach. This is often interpreted as unexpected erosion by a non-specialised observer. This phenomenon, however, is a well-known coastal feature; one cannot ‘unpunished’ create a disturbance in the coastal zone. At the end of the day, not only the upper part of the cross-shore profile is widened, but also the deeper part of the cross-shore profile is shifted in seaward direction (say over the entire so-called active part of the cross-shore profile). See Figure 3 for a sketch.
If the borrow material is quite different from the native sediment, the processes as indicated above, will be slightly different. If the borrow material is the same as the native sediment, then essentially the same shape of the cross-shore profile will be achieved as the pre-nourishment equilibrium shape after the equilibration process. However, if the borrow sand is much finer than the native sand the post-nourishment equilibrium shape of the cross-shore profile will be more gentle than before the nourishment. And if the borrow sand is coarser than the native sand, the equilibrium profile will be steeper.With the aim of the project in mind (beach widening) an obvious method is to nourish the beach directly. However, by nourishing the shallow foreshore, ultimately also the aim can be achieved. By onshore directed cross-shore sediment transport processes, the beach will be sooner or later fed at a natural manner.
Nourishing the shallow foreshore has some advantages over nourishing the beach directly. Sometimes it might be cheaper (e.g. so-called rain bowing with small dredges) and/or less hindrance occurs for the beach which is used for recreation purposes in the summer season. A direct nourishment of the beach often hampers recreation on the beach, because the sand has to be brought up to the beach by pipelines and noisy shovels are needed to distribute the sand properly over the beach.
Since it was assumed that the nourishment project to broaden the beach occupies only a restricted piece of coast in alongshore direction (say: 2 km), some losses of sediment out of the actual project area in longshore direction have to be expected; see Figure 4. Also in this case one cannot ‘unpunished’ alter only a part of the coast. Losses do occur. If the required widening of the beach must be maintained over the entire project area, maintenance is necessary.
Creation of new beaches
In many places (e.g. along the Mediterranean coast) the available stretch of coast is small and rocky; hardly any ‘normal’ sandy beaches are present. For recreational purposes it is often desired to create a broad and sandy beach. This can be achieved by applying artificial nourishment. In principle the occurring processes are the same as in the previous case of beach widening. There are, however, two main differences. The first point is that since there was no (sandy) equilibrium profile before the nourishment, the entire profile has to be nourished. The second point is that the longshore spreading as described in the previous case will erode the created beach seriously. This erosion is often much more seriously compared to the previous case, because in the original situation in this case hardly any ‘normal’ sandy beaches were present and then, consequently, rather high waves might be expected close to the shore. Rather large longshore sediment transport capacities occur; causing the large losses in longshore direction.
Some precautions might be taken to keep the sediments of the artificial beach at the desired place. To confine the artificial beach, groynes or breakwaters might be applied. If applying groynes, these have to extend long enough in seaward direction so that the nourished sand cannot easily escape. Perhaps even an additional (submerged) breakwater built parallel to the beach as an extra measure to keep the sand properly confined. For more information on these applications of groynes and breakwaters the reader is referred to the articles groynes and detached shore parallel breakwaters.
Safety of mainland and properties on top of the dunes
During a severe storm surge dunes and mainland will erode in a short period of time. Two main problems are to be discerned:
- In case of low-laying land behind the dunes: chance of break through of the dunes.
- In case of properties built rather close to the edge of the dunes: loss of properties.
The best (from a coastal morphology point of view) and most straightforward method is to reinforce the dunes at the landward side(see Figure 5). Such reinforcement does not affect the day to day morphological processes. Sometimes this way of reinforcement is not easily possible, because of infrastructure or objects present right behind the dunes. Another way of placing the nourishment is then at the seaside of the dunes. In a cross-shore profile which is assumed to be in equilibrium also in this case one cannot 'unpunished' widen only the dunes in seaward direction. Because of the disturbance in the shape of the profile, the cross-shore profile will develop towards a new equilibrium profile. Also losses in longshore direction are to be expected.
Regular artificial nourishments
Artificial nourishments and structural erosion
When a beach suffers from structural erosion artificial nourishments can be applied as a soft remedy. The occurring losses out of a stretch of coast are replenished from time to time. Applying nourishments with borrow material which has the same size as the native material on a regular basis will, to a first approach, not interfere in the occurring longshore sediment transports and so do not change the occurring losses. So the erosion does not stop; this means that after a certain period the nourishment has to be repeated indeed.If some kind of a minimum beach volume has been agreed as a standard, reaching or surpassing the minimum beach volume consequently means that a renourishment project has to be carried out; at moment t1 in Figure 6; red line).
When the borrow sand is the same as the native, and to a first approach it is assumed that the occurring morphological processes do not change, the eroding tendency with time after the nourishment, is equal to the tendency before the nourishment; red line in Figure 6 with the same slopes before and after the nourishment.
If the borrow sediment size is different from the size of the native material, some additional processes are to be expected. Apart from the equilibration process, the occurring sediment transport processes along the coast will change and consequently the sediment transport gradients and so the losses out of the stretch of coast.
When the grain size of the borrow sand is larger than the native sand the longshore transport rate is expected to be smaller than before. The time in which the beach volume will decrease to the minimum volume will be larger. The lifetime of the nourishment becomes longer. See the black line in Figure 6.
When the grain size of the borrow sand is smaller than the native sand the longshore transport will increase in the nourished area Therefore it is to be expected that the lifetime of the nourishment will be smaller than the lifetime of a nourishment with the native grain size. See the blue line in Figure 2.
Artificial nourishments to replenish the apparent sediment losses because of structural erosion out of a stretch of coast might be placed at various positions in a cross-shore profile. As well beach nourishments as shoreface (under water) nourishments will do the job.
Please note that others may also have edited the contents of this article.
- Shore nourishment
- Structural erosion
- Soft shoreline protection solutions
- Experiences with beach nourishments in Portugal
- Light revetments built-in into artificial dunes
- Beach drainage
Dean, Robert G., (2002) Beach Nourishment Theory and practice.
- Dean's book discusses and explains many topics related to artificial nourishments.