Torab final Book

Natural hazards mapping of mega sea waves on the NW coast of Egypt 1 Magdy TORAB2* & Noura DALAL3 Abstract: Some boulder fields were deposited by the sea waves during winter storms or by paleo-tsunami mega waves and most of these boulders were uprooted from the marine platform and distributed within 90 m of the shoreline, are found up to 4 m above present mean sea level. The objective of this work is to define the systematic characteristics of the high-energy depositional contexts working by storms or paleo-tsunami deposits and to reconstruct the history of mega block deposition along the study area depends on extensive field survey and geomorphic mapping using GIS and GPS techniques. Statistical analysis of the boulders is also performed to determine both extreme events using the significant wave height and period of maximum observed storms and historical tsunamis along the study area, as well as geomorphic hazard mapping and samples dating. The results reveal that such boulders could be the result of both storm and tsunami waves. This proposition is also affirmed at Alexandria by the archaeological excavations and historical sources as well. Tsunami waves and storms caused the displacement of huge boulders from sea bottom and submersible marine terraces (platforms) to the beach due to its major power and ability of carving and graving. These waves are also capable of pulling other boulders from the land and redeposit them along the beach or coastline. 1. Introduction The study area forms a belt about 20 km deep, which extends for about 500 km of the NW coast of Egypt on The Mediterranean Sea between Alexandria City and El Sallum town near the Egyptian-Libyan border “Fig.1”. The objective of this work is to define the systematic characteristics of the high-energy depositional contexts working both on the type of storm or paleo-tsunami deposits and the different geomorphological contexts, and to reconstruct the history of mega block deposition along the study area, using chronostratigraphy methodology. Therefore, the study will aid in evaluating the risk of submersion in an area that is affected by storms and tsunamis. Consequences on the occupations along the coastline are of a great important since it led to the destruction of Alexandria’s ancient lighthouse. As well as dating of mega blocks resulting from high-energy events (storms or tsunamis) using fixed marine bioconstructions is 1 This article has been published in : Journal of African Earth Sciences Volume 112, Part A, December 2015, Pages 353-357 2 Department of Geography, Damanhour University, Egypt. *Corresponding author: [email protected] 3 Department of Geography, Menoufia University, Egypt. 1 Figure 1. Location map of the study area 2. Setting 2.1. Geology The coastal plain of the study area is consists of three Pleistocene calcareous ridges parallel to the coast and separated by flat-bottomed depressions. The ridge’s sediments are composed of wellsorted medium grained aragonic ooids sands. The cliffs of the Middle Miocene plateau run parallel to the coast. A discontinuous series of coastal dunes developed at a distance varying from the coast to 2 km deep. There are some saline depressions and sabkhas in the lower part of the plain, some of which connect to the sea through natural outlets. The escarpment of the plateau is deeply cut by wadis. 2.2. Geomorphology The previous geomorphologic studies of the Northwest coastal plain of Marsa Matruh area as a part of the Northwest coastal plain of Egypt show that the origin of the extended calcareous ridges could be grouped under three environmental conditions as follows: • Continental environment (Hilmy, 1951). • Marine environment (Anwar et al., 1981). • Maine/ continental environment (El-Shazly et al., 1964; Selim, 1974 & Torab, 1984). However, sea waves were able to erode the first calcareous ridge in some parts of the study area and therefore the second performe to evaluate sedimentological impacts and natural hazards associated with these events (e.g., submersion, coastal mobility, erosion, high-energy impacts). 2.3. Climate Average annual wind directions in all climate data sources indicates that most wind blow toward the NW coast of Egypt from NW & NNW directions. Offshore wind speed at 50 m a.g.l., the NW coast of Egypt lies in the most offshore wind speed in the Mediterranean region since its speed range between 6-7 m/s (determined by meso-scale modelling, Wind atlas of Egypt, by Mortensen, et al., 2006). 2 3. Methods This paper depends upon detailed geomorphological field survey, 578 boulders are measured in eight selected sites of the study area. Boulder measurements are selected on 11 elongated sectors as shown in table (1) and figure (2). Table 1. Location of selected field work site Site# Sector# Location 1 El Fyrouz Beach Andalusia Beach 31°22`08" 27°17`49" Alam El Rum Beach 31°22`18" 27°19`22" 4 1.A 2.A 2.B 3.A 3.B 3.C 4.A Coordinates Lat. (N) Long. (E) 31°22`01" 27°16`12" Mina Hasheesh Beach 5 5.A Ras El-Hekma west 31°22`22" 31°13`43" 27°19`46" 27°51`49" 6 7 8 6.A 7.A 8.A Ras El-Hekma east Ras Hossan El-Dabaa 31°13`39" 31°05`37" 31°04`33" 27°52`27" 28°06`32" 28°28`24" 2 3 Each sector started from the coastline to the end of boulder field. First, site location was defined accurately using differential GPS. Second, we measured the three axes of each boulder. Third, distance of each boulder from the coastline and its height above the mean sea level were also measured. Finally, boulder's volume and weight were calculated using volumetric method as 2.2g/cm3. Figure 2. Location of selected field work sites 3 4.Results 4.1. Distribution and dimensions of accumulated mega boulders: The measured dimensions of accumulated boulders “Tab.2” and the field observation reveal that most boulders are rectangular, with sharp broken edges. Most of the boulders consist of limestone and sandstone fragments up to 14 m3 in volume and 43 ton in weight, some of these blocks were observed by local people to have moved after strong winter storms. Based on field observations the maximum mean size of the accumulated boulders appears on site # 3c (Volume 2.25 m3), and the maximum mean weight is 2.57 ton at the same site and also the maximum number of boulders is found in the same site # 3C (90 boulders), but the maximum mean distance between the coastline and the end of the boulder field is (54.93 m) in site # 1A. Table 2. Average dimensions of accumulated boulders S# Bn Average dimensions of boulders a(m) b(m) c(m) D L V W 1.A 57 1.51 1.14 0.51 54.93 0.96 1.02 2.08 2.A 2.B 3.A 3.B 3.C 4.A 5.A 6.A 7.A 8.A Oa 13 85 90 23 38 81 38 55 51 47 52.54 1.82 1.07 1.34 1.42 1.63 1.22 1.45 1.36 0.85 0.87 1.31 1.61 0.79 0.94 1.01 1.21 0.94 0.97 0.97 0.59 0.58 0.98 0.65 0.36 0.42 1.7 .47 0.41 0.40 0.33 0.26 0.17 0.51 26.95 39.69 29.92 28.2 21.63 9.49 10.67 14.46 7.04 19.1 23.81 1 3.41 3.43 0 3.60 1.2 0.95 .37 1.33 1 2.02 2.15 0.41 1.01 0.65 2.25 0.56 0.74 0.51 0.16 0.13 1.56 4.58 0.17 2.57 1.04 7.96 0.89 1.46 0.84 0.18 0.19 2.11 (After: Dalal, 2013) S#: Sector # Bn: Boulders number D: Distance (m) L: Level (m) V: Volume (m3) W: Weight (t) Oa: Overall average 4.2. Topographic profiles and geomorphological maps: Eight topographic profiles have been surveyed and geomorphological maps on the selected sites. They clarify morphology of the accumulated boulders along the beache and coastal platforms as follows: it seems as random shape deposition at Alam El Rum and semi parallel to the coastline “Figs.3&4”. 4 Figure3. Example of topographic beach profiles for site # 1.A Figure.4: Geomorphological map for site #3 at Alam El Rum Beach 4.3. Boulder accumulation positions on the beaches Boulder accumulations can be classified based on their positions on the beaches as follows: 5 Figure 5. Horizontally boulder deposits on site # 3 at Alam El-Rum Figure 6. Crammed boulders inside high coastal notches on site # 3 4.4. Estimation of storms and tsunami wave’s heights Some equations are used to estimate storms and tsunami wave heights depending on boulder's dimensions, volume, weight, and moving distance on the platform (Williams & Halls, 2004) & (Pignatelli, et al., 2009) (Tab.3). The first equation shows that the minimum storms wave height is more than 10 m on sector 2.A and 6 m on sector 3B&C, and tsunami wave height ranges between 2.7m for sector2.A and 1.6 for sector 1.A.,3. B&C. 6 Table 3. Estimated height of storms and tsunami waves. Sector # Boulder # Equation Wave type 1.A 57 2.A 13 2.B 85 3.A 90 3.B 23 3.C 38 4.A 81 5A 38 6.A 55 7.A 51 8.A 47 OA 52.54 Estimated minimum wave height (m) Williams & Halls, 2004 Pignatelli, et al., 2009 HS(m) HT(m) HS(m) HS(m) 6.292 1.573 2.696 0.614 10.786 2.696 7.191 1.797 4.943 1.234 2.696 2.696 5.393 1.348 2.696 0.647 6.292 1.573 3.595 0.898 6.292 1.573 3.595 0.898 2.696 0.674 2.696 0.674 5.842 1.460 2.696 0.674 5.393 1.348 2.696 0.667 4.044 1.011 2.39 0.449 3.146 0.786 1.779 0.440 5.556 1.388 3.156 0. ;952 (After: Dalal, 2013) HS: Storm wave height Ht: Tsunami wave height OA: Overall average 4.3. Boulder accumulation positions on the beaches Boulder accumulations can be classified based on their positions on the beaches as follows: 4.3.1. Horizontally accumulated boulders on the coastal platforms by strong waves, and they always advance horizontally near the coastline « Fig.5 ». 4.3.2. Vertically accumulated boulders on the beach or settling vertically upon earlier boulders. 4.3.3. Buried boulders beneath aeolian sands on the back shore. 4.3.4. Crammed boulders inside high coastal notches (2-3 meters above sea level), as a result of high wave energy « Fig.6 ». 4.3.5. Sequential deposited rock debris by powerful consecutive waves. 4.5. Boulders shells dating Two samples of seashells were collected for dating purpose from the accumulated boulders on site # 3B & 3C at Alam El-Rum. The results show that the first sample was displaced from 60 years ago, as a result of an earthquake centred at the bottom of Mediterranean sea near the southern coasts of Cyprus Island near Limassol city, this earthquake took place on 10-9 1953 and caused tsunami waves that damaged about 135village and killed 40 persons and 1000 others were became homeless (Guidoboni, et al., 1994). The second sample back to 960 ±35 BP (1018 to 1088 AD) an evidence of other tsunami occurred in the eastern portion of The Mediterranean Sea “Tab.4”4. 4 Dating analysis by C14- AMS method in Beta analytic, USA. 7 Table 4. Dating of Seashells were collected from site #3 at Alam El-RUm Dimensions of boulders Sh# S# 1 2 3B 3B A (m) 1.45 2.15 B (m) 0.7 1 C (m) 0.45 0.50 Dating BP D V W 32.70 27.20 0.46 1.08 0.66 2.31 60 ± 4 960 ±35 Sh#: Shell Sample number S#: Sector number D: Distance between boulder & coastline (m) V: Volume (m3) W : Weight (ton) 4.6. Geomorphological Hazard map The geomorphological hazard map « Fig.7 » is produced for the study area using hazard index depending on the following factors: A. B. C. D. E. F. Shape of the coastline. Angle of approaching waves to the coastline. Slope of beach profile. Hardness of beach rocks. Depth of sea water. Estimation of mega boulder's accumulated by storms and tsunami. Figure 7. Geomorphological hazard map of the study area. 5. Conclusion The results show that both possible processes (storm and tsunami waves) can deposit mega boulders as the the NW coast of Egypt in specific has witnessed number of seismic or tsunami events during the Holocene (tsunamis of 23 AD, 365 AD, 746 AD, 881 AD, 1202 AD, 1303 AD, 1870 AD and 1908 AD attested at Alexandria for example by the archaeological excavations and historical sources. 8 Tsunami waves and storms cause the displacement of huge boulders from sea bottom and submersible marine terraces (platforms) to the beach due to its major power and ability of carving and graving. They are also capable of pulling other boulders from the land and re-deposit it along the beach or coastline. The geomorphological hazard map of the study area shows that Alam El Rum and the western coast of Ras El-Hekma, then El Dabaa and El Fyrouz areas east of Mersa Matruh City of about 10, 65, 135 and 2 km respectively, are the most affected and prone sectors to hazards by estimated tsunami catastrophic in the NW coast of Egypt depending on the results of the results of this study. 6. Acknowledgment This study is financially supported by the Egyptian-Franco research program IMHOTEP (research project No. EGY/FR7-016) Titled: Effects of tsunami on the Egyptian Mediterranean coasts. Fieldwork was supported by The Egyptian Society of Environmental Changes. My thanks also go to my colleague Dr. Moawad Badawy Moawad (Professor, Ain Shams University, Egypt) for editing the article. 7. References • • • • • • • • • • • Anwar,Y.M., M.A. El Askary, and S.M. Nasr (1981), Petrography and origin of the oolitic carbonate sediments of Arab’ Bay, western part of the continental shelf of Egypt, N. JB.Geol. 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Pignatelli,C., Sansò,P ., Mastronuzzi.G.,(2009), Evaluation of tsunami flooding using geomorphologic evidence, Marine Geology 260 (2009) 6–18. Selim, A.A., (1974), Origin and lithification of the Pleistocene carbonates of the Salum area, western Coastal plain of Egypt, Jour. Sed. Petrology, 44: 70-78. 10.Torab, M. (1984), Geomorphology of Um El Rakham aea, west of Marsa Matruh City, MA. Degree Thesis, Alexandria University, Egypt. William,D,M., Hall,M,A., (2004), Cliff-top megaclast deposits of Ireland, a record of extreme waves in the North Atlantic—storms or tsunamis, Marine Geology 206 (2004) 101–117. 9