Dissertation committee:
Νίκη Ευελπίδου, Καθηγήτρια, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Θεόδωρος Γκουρνέλος, Καθηγητής, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Satoru Kawasaki, Professor, Faculty of Engineering, Hokkaido University, Japan.
Αριάδνη Αργυράκη, Καθηγήτρια, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Ασημίνα Αντωναράκου, Καθηγήτρια, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Εμμανουήλ Βασιλάκης, Αναπληρωτής Καθηγητής, Τμήμα Γεωλογίας και Γεωπεριβάλλοντος, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Ιωάννης Ηλιόπουλος, Αναπληρωτής Καθηγητής, Τμήμα Γεωλογίας, Πανεπιστήμιο Πατρών
Summary:
The purpose of this thesis was to study the geomorphology and micro-morphology of natural beachrocks and reveal their formation mechanism in order to mimic and accelerate the process to create artificial beachrocks. The ultimate goal is the protection of the vulnerable sandy beaches from coastal erosion using a new soft engineering solution by understanding in depth the formation mechanism of natural beachrocks. This PhD focuses on the beachrock formation mechanism through the comparison of cement characteristics, mineral chemistry and granulometry of beachrock occurrences from different geological and geographical localities from Greece, Cyprus and Okinawa, Japan. In order to investigate a potential soft engineering method to protect coasts from erosion, artificial beachrock samples were created in vitro using sand samples and ureolytic bacteria from both areas under accelerating conditions.
Geomorphological observations during our field activities had already shown that beachrocks in many cases play the role of the protective mechanism against coastal erosion. At north Naxos Island, a windy day gave the idea to follow the example of nature, as high waves couldn’t reach the coast as the beachrocks were playing the role of natural breakwater. Thus the idea of imitating nature and find a solution against coastal hazards was born. Although there are many studies on beachrocks, with different backgrounds, methodologies, and objectives, no homogenous-specific methodology that could contribute to understanding the mechanisms of beachrock formation or their diagenetic environment existed. For this reason, a multiproxy analysis took place throughout the study of beachrocks at 13 study areas on Greece, Cyprus and Okinawa (Japan), which have different geological, geomorphological and physico-geographical characteristics, as well as different sediment supply and different granulometry.
Detailed field surveys took place on the shore and underwater. Beachrock samples were retrieved for mineralogical and geochemical analysis to understand the structure of beachrocks, cement micro-morphology order to identify the type of cement and determine the spatial relationship between the past shoreline and beachrock formation zone. Selected samples were collected for dating using OSL dating of quartz.
From selected areas, sand and water samples were retrieved for microbiological analysis to identify ureolytic bacteria, which were essential for the application of the Microbially Induced Carbonate Precipitation (MICP) to create corresponding artificial beachrocks in-vitro. The artificial beachrock samples were analyzed for their mechanical, mineralogical and geochemical properties.
The geomorphological, mineralogical and geochemical analysis of beachrocks showed that the beachrocks of Diolkos, Porto Rafti, Edipsos, Naxos and Paros were formed in the upper/shallow to middle intertidal zone and they consist of detrital material originating from the local bedrock. Okinawa beachrocks were also formed in the upper/shallow to middle intertidal zone and consist of sand, coral sand and foraminifera fragments. Beachrocks that were formed in the middle intertidal zone were the ones located in Skyros, Psatha, Katakolo, Salamina and Rhodes. The beachrocks of Lefkada and Cyprus were formed in the subtidal to middle intertidal zone. The subtidal- middle intertidal beachrocks were formed in areas with high Mg and Cal content lithologies, i.e. Cyprus (volcanic rocks) and Lefkada (limestone). All beaches were part of a well-developed drainage system and fresh water was available.
Geomorphological observations showed that in areas with multiple beachrock slabs, beachrock had more discontinuities, which were mostly submerged or on sea level. The broken beachrocks indicate that they absorb the wave energy before it reaches the sandy beach. Conversely, uplifted beachrocks were found more preserved. Beachrock development stops when sediment supply is disturbed due to human activity or other causes and the beach geomorphological processes affect the beachrock development (e.g. Diolkos). In addition, it was found that longer saturation period and Magnesium Calcite supply may result in thicker beachrock development. When gravels are incorporated in beachrocks more pore space occurs helping the formation of matrix cement which results in better consolidation (e.g. Salamina, Psatha). The beach morphology and type (protected/ open) does not play an important role in the development of beachrock rather than physicochemical factors. At areas where beachrock is destroyed (e.g. Salamina, Paros, Edipsos) the beach morphology indicates beach retreat.
The artificial beachrock experiments contributed to the realization of the concept that Artificial beachrock is a possible natural based methodology for coastal protection. They are controlled by the bacteria metabolism type. It was found that the artificial beachrocks were more favorable in poorly shorting sediments (pore space). Slow circulation of nutrients may affect better artificial beachrock creation. The Micrococcus Yunnanensis sp bacteria case grow better crystals and leads to better consolidation than those of Virgibacillus sp. The Native bacteria performed better with native sediment as they created samples with UCS is equal to 4-5 MPa. The analysis resulted in an optimum combination of parameters to create artificial beachrocks, native bacteria, native sediment, bad shorting sediment, 0.5 M CaCl2 and Urea.
Overall, this work has shown that beachrocks should be studied with a multiproxy approach, using geomorphological, geochemical, and mineralogical methodologies. In this way, confusion with other lithologies (such as sandstone) is avoided, their formation environment is accurately determined, and they can be used as geomorphological indicators of past sea levels. Regarding artificial beachrocks, this work has shown that artificial beachrocks may lead to a new era of soft engineering methods, through which we can imitate the natural beachrocks as breakwaters.
Keywords:
coastal geomorphology, coastal erosion, adaptation and mitigation methods, paleogeography, GIS
