Seawalls and revetments
Definition of Seawalls and revetments:
Seawalls and revetments are shore parallel structures at the transition between the low-lying (sandy) beach and the (higher) mainland or dune.
The height of a seawall fills often the total height difference between beach and surface level of the mainland. In many cases adjacent at the crest of a seawall a horizontal stone covered part is present (e.g. boulevard; road; or parking places). At the initial time of construction a seawall is situated close to the position of the dune foot. In the present discussion with a seawall an almost vertical structure is meant. The seaward side of the seawall is thought to be rather smooth. A revetment is similar to a seawall, but often sloping.This is the common definition for Seawalls and revetments, other definitions can be discussed in the article
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Contents
Seawalls and revetments
Seawalls and revetments are both shore parallel structures. Main differences between a seawall and a revetment are that a revetment has a distinct slope (e.g. 1:2 or 1:4), while a seawall is often almost vertical, the surface of a revetment might be either smooth or rough (a seawall is mostly smooth) and that the height of a revetment does not necessarily fill the total height difference between beach and mainland (a seawall often covers the total height difference.
In the article seawall a slightly different definition is given.
In the present article we will start the discussion with a well-defined real life coastal engineering 'problem'. Next we will explain how and why the application of a seawall or revetment might be used to resolve the 'problem'.
Problem: clear transition beach - mainland
Especially in sandy coastal areas with a lot of human (recreational) activities, a clear and fixed distinction between beach and mainland is desirable. A seawall will serve that aim. At the sea side of the seawall a more or less normal beach is assumed to be present; at the land side a road or a boulevard is present. Staircases facilitate the access to the beach. The coast is assumed to be stable. The beaches in front of the seawall do not erode, or in case of a structural eroding coast an essentially (time-averaged) stable situation has been achieved with e.g. regular artificial beach nourishments. So a normal beach is assumed to be present in front of the seawall (and can be used for recreational purposes). See Figure 1.
While in a situation without a seawall even a moderate storm (surge) will attack and erode the mainland, in the situation with a seawall this is prevented. Some scour in front of the seawall during a storm (surge) must be taken into account in the design. (A part of) the 'denied' erosion volume from the mainland, is now eroded just in front of the seawall. The scour hole might undermine the seawall.
(With e.g. the DUROSTA computation model an estimate of expected scour depths can be made.)[1].
The design conditions for the seawall have to be properly chosen. The heavier the design conditions, the heavier the seawall must be constructed and especially the 'safe' foundation depth will increase accordingly. To build a seawall which will be safe under 'all' conditions might be an unrealistic option.
Although achieving a clear transition between beach and mainland was the primary goal in the discussion sofar, automatically some protection of the (infrastructure at the) mainland is achieved. The design conditions as selected, determine the rate of provided protection.
The crest height of a seawall determines (together with the boundary conditions at sea) to a great extent the rate of overtopping (water reaching the mainland by wave run-up and breaking waves and splash water transported by landward directed wind). With an additional wall and/or a slightly curved front, rates of overtopping might be reduced; see Figure 2.
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Problem: decrease risks of valuable infrastructure / buildings
Infrastructure and buildings situated close to the edge of mainland or dunes have a chance to be destroyed during a severe storm surge. The risk (risk = chance x consequence; see Example Fout! Verwijzingsbron niet gevonden.) is felt to be too large. E.g. by extension and improvements of an existing hotel the 'consequence' has been increased and so the risk. By reducing the 'chance', the 'risk' will reduce as well. With a robust seawall the required aim can be achieved. Aspects like proper design conditions and scour holes are in this case similar to the discussion in the previous case.
Let us consider a given a stretch of sandy coast. A very severe storm surge will cause a rate of mainland erosion of say 40 m in case the stretch of coast is unprotected. With a seawall which is able to withstand these conditions the erosion of the mainland will be zero. (In front of the seawall a deep scour hole will be formed.) When the entire seawall keeps its integrity; no further problems arise. (The scour hole will be re-filled again after some time with ordinary boundary conditions.) If, however, the seawall partly collapses and locally a gap in the seawall is formed during the severe storm surge, a rather dangerous situation will occur. Large volumes of sediment from the mainland are able to disappear through the gap and will flow along the sections of the seawall which are still in good condition in both longshore directions, filling the scour hole. It is expected that the ultimate rate of erosion of the mainland behind the gap will be larger than the 40 m as mentioned for the unprotected case. Similar phenomena will occur at the two transitions between seawall and the adjacent, unprotected parts of the coast. Especially just adjacent to an abrupt end of a seawall, relatively much erosion is expected during a severe storm surge.
Seawall is no solution for a structural erosion problem
In the present chapter we generally start our discussion with a type of structure and next some actual problems or a goals to be achieved are mentioned which might be resolved or achieved with the help that structure. It makes then no sense to mention various problems which cannot be resolved with that type of structure. An exception to this rule will be made for the 'solution' of a structural erosion problem with the help of seawalls (or revetments). This is because in coastal engineering practice too often this principally 'wrong' combination is applied. Many bad examples can be found all over the world. Structural erosion caused by a gradient in the longshore sediment transport, means that volumes of sediment are lost out of the control volume area. This loss process takes mainly place under ordinary conditions; the contribution of storm conditions to this loss process is often rather small. The initial losses of sediments out of a cross-shore profile take place where water and waves are; where actual longshore sediment transports do occur; so in the 'wet' part of a cross-shore profile. The 'dry' parts of a cross-shore profile are not involved in the longshore sediment transports; it looks like that the 'dry' parts do not form an integral part of the cross-shore profile. During high tides and/or modest storms all parts of a cross-shore profile participate in the coastal processes. By offshore directed cross-shore sediment transports, sediments from the higher parts of the profile ('dry' beach; even mainland under the more severe conditions) are transported to deeper water, filling the 'gap' that has been developed because of the gradient in the longshore sediment transport (see Fig.Fout! Verwijzingsbron niet gevonden. ). This sequence of processes causes a permanent loss of material out of the upper parts of a cross-shore profile.
By 'protecting' the mainland in this case with a seawall, one indeed prevents that sediments from the mainland are transported in seaward direction (less filling of the 'gap'). The losses, however, continue; the 'dry' beach disappears; it becomes deeper and deeper in front of the seawall. Initially, right after the construction of the seawall, still a more or less normal beach was present. The beach did 'protect' the seawall to some extent; only moderate storms could reach the seawall. When the beach had disappeared, much more frequent wave attacks directly to the seawall will occur. (Most likely in the design of the seawall this was not taken into account.) Damage occurs; reinforcements have to take place. A somewhat confusing element is the time-scale of the developments as have been discussed so far. Local people (their houses are at stake) have noticed in the past that every storm surge has taken some square metres of their gardens. The edge of the mainland is coming closer and closer to their houses. Not seldom the responsible coastal zone manager is 'forced' by the local people 'to do something'. Building locally a seawall (e.g. in front of the properties which are situated closest to the sea) indeed seems to resolve the problem. During the next storm surge, the just 'protected' parts of the coast do not show any further erosion; in the un-protected parts the erosion of the mainland continued. Local people believe that this solution 'works' (own experience). The coastal zone manager is forced to build seawalls along the other parts of the coast. However, when time elapses, it will be quite clear that a quite wrong solution has been chosen. Only with huge costs the situation can be redressed. Problem: existing row of dunes does not meet safety requirements Row of dunes is apparently too weak (too slender) to guarantee the safety requirements. Under design conditions a break-through is expected; the low-lying hinterland behind the slender row of dunes will be flooded. A seawall might be chosen as a solution, provided that the seawall will keep its integrity during the design conditions. A risky alternative would be that the seawall is 'allowed' to collapse in a latter stage of the storm surge. The time left to the end of the storm surge (with lower water levels) is then too short to cause yet a break-through.
References
- ↑ Steetzel, H.J. (1993). Cross-shore Transport during Storm Surges. Ph.D. Thesis Delft University of Technology.
Please note that others may also have edited the contents of this article.
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