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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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.
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.
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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