JKAU: Mar. Sci., Vol. 29 No. 1 pp: 21-36 (1441 A.H. 2019 /A.D ) DOI :10.4197/Mar.29-1.2 Characteristics of the Relative Sea Level in Abu-Qir Bay, Alexandria, Egypt Mohammed Ahmed Said, Tariq EL-Gesiry, Zainab Morsi National Institute of Oceanography and Fisheries Abstract. hourly water level variations at Abu-Qir Bay were studied over 21 years. The levels were highly seasonal with a mean of 0.47m, and a rising rate of 6mm/yr. Tides are of semidiurnal type and of low amplitudes. No extreme sea level year was detected. Statistical analysis of surge heights was obtained. The estimation of flooding risks was determined. The design risk factor and return values of important water levels were calculated. An empirical relationship for the different water levels frequency of occurrence stated that the high water level of 3m might be exceeded once in 100 years. Keywords: Relative sea level, Abu-Qir Bay, Astronomical tides, Surges, Return periods, Flooding risk Introduction environmental pressures. Egypt’s The sea level variations along the Egyptian Mediterranean coast and the Nile Delta have Mediterranean coast have been the core of been identified as highly vulnerable to climate many researches, e.g. Moursy, 1989; Moursy, change induced SLR (UNDP, 2014). Dasgupta 1992; Moursy, 1993; Moursy, 1994; Moursy, et al. (2009) ranked Egypt in the top ten most 1998; Tayel, 2008; Hussein et al., 2010; Saad impacted countries (out of 84 developing et al., 2011; El-Geziry and Radwan, 2012; coastal countries considered world-wide) for Maiyza and El-Geziry, 2012; Said et al., 2012; population potentially displaced due to a 1m Radwan and El-Geziry, 2013; El-Geziry, SLR. Alexandria was ranked 11th in terms of 2013; El-Geziry and Said, 2014. All concluded population exposed to coastal flooding in that the tidal variations along the Egyptian 2070s (El-Deberky and Hunicke, 2015). Mediterranean coast are in order of a few The linear trend of increase in the sea level centimeters, and are of the semidiurnal type. along the Egyptian Mediterranean was The coastal zone of the Egyptian previously calculated to range between 0.15 Mediterranean Sea is exposed to multiple cm/yr to 0.30 cm/yr (El-Fishawi and Fanos, 21 22 Mohammed Ahmed Said, Tariq EL-Gesiry, Zainab Morsi 1989; Frihy, 1992;; Frihy, 2003; Said et al., Alexandria, and spots light on the main 2012; Shaker, et al., 2011 and Maiyza and El- statistical characteristics of the sea level; Geziry, 2012). representing the risk factors and the regional Radwan and El-Geziry (2013) investigated the flood factor of the Bay. statistical properties of surge off Alexandria Data and Methods of Analysis over one decade (1996-2005). Their results Abu-Qir Bay (Fig. 1) is a semi-circular basin revealed that the 51-60 cm surge heights are lying 35 km east of Alexandria City between the most frequent to occur over the latitudes 31° 16' and 31° 28' N and longitudes investigated period, while the 81-90 cm are the 30° 03' and 30° 22' E. The bay is a shallow lowest. On seasonal basis, the calculations basin with an average depth of less than 10 m showed that for both the highest and the lowest and most of the bay has a gentle slope with a 10% of occurrence, the summer season maximum depth of about 22 m. The coastline recorded the highest and lowest events, with of Abu Qir Bay extends for about 50 km 163 and 0, respectively. The winter season between the headlands of Abu Qir in the showed only 55 high events and 184 low ones. southwest and Rosetta in the northeast Both spring and autumn surge heights show (Abdallah et al., 2007; Zakaria, 2007). The moderate occurrence for both the highest and surface area of the bay is about 360 km2 and the lowest occurrence percentage. its water volume is 4.3 km3 (Said et al., 1995). Said et al. (2012) calculated the probable The Bay is in contact with Lake Edku through maximum expected water level condition Boughaz Meadeya. The Bay is considered one (extreme return period) off Alexandria to reach of the very sensitive zones within the eastern 165 cm once in 100 years and 181 cm once in Mediterranean Sea to the relative SLR. Wave 500 years, and accordingly they calculated the movement, circulation pattern and sea level design life time of coastal structure and the variations in Abu-Qir Bay have been risk for the 80 cm level off Alexandria. A previously investigated, e.g. El-Gindy, 1988; severe climate event was demonstrated by a El-Gindy et al., 1988; Said et al., 1995; strong storm that hit the Mediterranean coast Abdallah et al., 2007; Said et al., 2012; El- of Egypt on 11 December 2010, associated Geziry, 2013; El-Geziry and Said, 2014. with peak wind speed of more than 60 km/hr The present data set of hourly sea level is (Said et al., 2012). This storm caused coastal taken from an automatic tide gauge fixed in flooding in Alexandria with a surge exceeding Abu-Qir Bay. The position of the instrument is 1 m. This value is much higher than the typical 31°19.54’ N; 30°04.5’ E (Fig. 1). The data are values of storm-surge reported offshore the referred to the zero level of the instrument, and Nile delta coast (40 to 50 cm). Fortunately, the cover 21 years from January 1990 to storm only caused partial damage of some December 2010, which means a total of coastal structures and modest flooding of 184080 hours. However, there are gaps in the lowlands within Abu-Quir bay, east of recorded data and the actual data in hands are Alexandria. El-Deberky and Hunicke (2015) only for 174119 hours (94.58%). The missed succeeded to determine the four most data, on the other hand, represent 5.42 %. vulnerable areas to the risk of the relative SLR Statistically speaking, this missed set does not and inundation along the Egyptian affect the data quality to proceed for the Mediterranean Coast, among which is Abu-Qir proposed investigation. bay. The present paper investigates the relative sea level variations in Abu-Qir Bay, east of Characteristics of the Relative Sea Level in Abu-Qir Bay, Alexandria, Egypt 23 Figure (1) Abu Qir Bay with the location of the automatic tide gauge In the present work, the sea level components φn is the nodal phase of harmonic component; (astronomical tide and surge elevations) are and produced using the WORLD TIDES® package kn is the phase of nth constituent for the time working under the MATLAB® environment origin in use (Boon, 2004). The package is used for the The method used by WORLD TIDES® to analysis and prediction of water levels. It analyze a water level time series is commonly permits quick separation of a time series of known as Harmonic Analysis Method of Least water level measurements into its tidal and Squares (HAMELS). This is a simple but non-tidal components using a selective least powerful means of obtaining tidal constituent squares harmonic reduction employing up to amplitude (An) and phase (kn), needed for tidal 35 tidal constituents. WORLD TIDES® is predictions using Equation (1). principally based on the concept of the ability The five main tidal components (O1, K1, N2, of expressing the tidal amplitudes at any M2 and S2) are the default tidal constituents location as the sum of all the harmonic produced by the software. These are produced components, as in the following equation: for the present work. 2 Using the output from this harmonic tidal (t) ZO fn An cos t n kn (1) analysis, the type of the tidal cycle in Abu-Qir T n n Bay is determined using the following :(Where، constituent factor (Pugh, 2004 η(t) is the vertical displacement of the sea FF = (HO1 + HK1) / (HM2 + HS2) (2) ،surface as a function of time (m); Where Zo is the Mean Sea level (MSL); HO1 is the tidal height of the principal lunar ؛(fn is lunar node factor for nth constituent; diurnal constituent (m An is the amplitude of a harmonic component HK1 is the tidal height of the luni-solar diurnal ؛(m); constituent (m) Tn is the period of a harmonic component (s); HM2 is the tidal height of the principal lunar semidiurnal constituent (m); and 24 Mohammed Ahmed Said, Tariq EL-Gesiry, Zainab Morsi HS2 is the tidal height of the principal solar with which sea levels and risk may be semidiurnal constituent (m) expected to occur in the future. The general characteristics of water levels and Results astronomical tide for Abu-Qir Bay are Characteristics of Relative Sea Level at obtained with the principal features of the Abu-Qir Bay residuals. The hourly sea level varied between Since the knowledge of the expected sea state 0.01 m and 1.0 m above the zero level of the is required for the design and for the safety of instrument (Fig. 2), i.e. 0.99 m tidal range, life at sea, the aim of this study is to get a with a MSL of 0.47 m. better evaluation of forecasting and knowing the frequency and the probability distribution, Figure (2) Hourly Sea Level Recorded in Abu-Qir Bay from January 1990 to December 2010 Figure (3) shows the frequency of occurrence respectively. The probability of occurrence of of the recorded sea level over the period of different water levels is presented and the most investigation. This varied between 6 hourly pronounced frequency was concentrated in the events (0.003%) and 30577 hourly events level of 0.5 m and diminished rapidly in the (16.61%) for the 1 m and 0.5 m water levels, levels less than 0.05 m and more than 0.85 m.