CLASSICAL, INNOVATIVE AND UNCONVENTIONAL COASTLINE PROTECTION METHODS

摘要

The purpose of this paper is to give a state of the art overview of a number of recent developments in structures for shoreline protection. In other presentations the need for coastal protection structures is discussed, as well as solutions including "soft" technology like beach nourishment (see the papers of R. Dean and J. Pope). Regarding loose armour, new developments can be reported on the stability of rock in shallow water conditions. This topic will be elaborated in more detail in the presentation of M.Van Gent. In case the required rock size becomes too large, usually concrete elements will be used. Many elements exist, a recent PIANC working group has identified 217 different elements, but most of them are applied only seldom. Recent developments in concrete elements are the Core-Loc (developed by the US Army Corps of Engineers) and the Xbloc (developed by Delta Marine Consultants in the Netherlands). Both blocks have specific advantages in making a stable slope protection; the drawback of such blocks is the complicated shape, which makes them more costly. One of the advantages of block like Core-Loc and Xbloc is that they can be applied as a single layer. Recent research has shown that simple concrete cubes can also be applied in a single layer; however, special attention must be paid to the placing density as well as to the rock size in the secondary layer. Another development is the use of extreme heavy aggregates; by using magnetite specific densities up to 4000 kg/m3 can be achieved. The main advantage of using heavier densities is that the weight of the individual block may reduce by a factor of 5. Consequently thinner layers are possible, and also lighter construction equipment can be used. An alternative for rock and concrete structures is through the use of geofabrics. In fact there are many kinds of variations using sandbags. Geotextile technology makes it possible nowadays to create geofabrics with wide ranges of required strengths and filter properties. Although there is still some debate on the durability of geofabrics, in general it is accepted that by use of the material under water, durability is no longer a problem. Basically three types of geotextile structures are relevant in shoreline protection. Of course the classical sandbag is applied, but in large sizes. Large open bags of 1 m3 are often used for the creation of temporary dams, while closed bags are often placed as an underwater revetment or as a hidden protection only to become active in case of calamities. Another recent development in geofabrics is the application of Geocontainers and Geotubes. Geocontainers are huge bags (order of 250 m3), placed in situ using a split hopper barge. With Geocontainers relatively steep underwater structures can be constructed with mainly sand. Geotubes are long tubes, with a diameter in the order of 5m and a length in the order of more than 100m. The Geotubes are filled in situ. Both types are mainly applied under conditions where sand is available, but rock has to be imported, and in consequently rather expensive. A last development to be discussed is the tendency to go to more simple constructions. Especially for small sclae structures it is sometimes attractive not to make an advanced and precisedesign, but to make it more simple by over dimensioning some elements and accepting damage. On the long run this may be a less expensive solution.