par- 10 janvier 2008
Mardi 15 janvier 13h00
Cementation of coastal se- diments (Beachrock forma- tion), characteristics, forma- tion mechanisms, impacts
Michalis Vousdoukas, chercheur post-doctoral au laboratoire , présentera un séminaire le mardi 15 janvier de 13h00 à 14h00 en salle de réunion du COM à Luminy :
Figure : Beachrocks : (a) Baracoa, Cuba) ; (b) Salvador, Bahia, Brazil ; (c) Barcelona, Spain ; (d) Athitos, N. Greece ; (e) Mykonos Island, Greece ; (f) Morrocoy, Venezuela ; (g) Sifnos Island, Greece (the white arrow shows a corridor excavated into the beachrock outcrop to provide access to the sea, whereas the black arrows show the 3 different zones of colonising bio-communities (an inshore black, an inner yellow and an outer yellow zones, see Section 5.2) ; and (h) Attica, Greece (courtesy of I. Issaris).
The presentation will focus on naturally cemented sediments (i.e. beachrocks), their characteristics and their impacts on coastal morphodynamics, ecology and tourism and other beach uses. Beachrocks are hard coastal sedimentary formations consisting of various beach sediments, lithified through the precipitation of carbonate cements (mainly High Magnesian Calcite (HMC) and Aragonite (Ar)). Cement precipitation in the coastal environment is fast and can be controlled by : (i) the physicochemical conditions ; (ii) the presence of organic compounds and microbes ; (iii) the magnitude and distribution of the wave energy along the coast ; and (iv) the textural characteristics of the constituent sediments. Various theories have been proposed to explain beachrock formation itself, linking the phenomenon to either physicochemical or biological processes. These theories, however, do not seem to be of universal validity and acceptance, as each is able to explain only some of the reported occurrences.
The results/conclusions that will be presented come from collecting/collating all available information from the literature, as well as from a detailed study consisting of a) numerical modelling (beach profile modelling, 2D hydrodynamic modelling), b) field data (ecological, sedimentological, topographic and hydrodynamic) and c) a questionnaire survey to investigate how tourists perceive beachrocks.
The presence of beachrocks appears to affect beach morphodynamics by : (i) ‘locking’ the beach profile ; (ii) modifying the nearshore hydrodynamics ; (iii) changing the porous character of the beach and, thus, its response to wave forcing ; and (iv) differential bed erosion at the margins of the beachrock outcrops that can alter significantly the long- and, particularly, the cross-shore sediment transport. Therefore, although relict submerged beachrock outcrops may provide some coastal protection by reducing the wave energy impinging to the coastline, modern beachrocks may promote offshore loss of unconsolidated beach sediments and buried beachrock outcropping. Moreover, the presence of beachrocks may have also significant ecological impacts, as the indigenous (mobile substrate) fauna and flora of the beach is replaced by hard substrate benthic assemblages, which are commonly arranged in hydrodynamically-controlled zones. Finally, beachrock formation can act as a threat for tourism and economical exploitation of beaches, since it can significantly reduce beach amenity value.