Sea level characteristics and extremes along Alexandria coastal zone

Abstract

Three decades (1974-2006) of hourly sea level records at Alexandria were used to calculate both surge elevations and astronomical tide. The tidal harmonic constituents were constructed using the WORLD TIDES program. Over the study period, the mean sea level (MSL) at Alexandria has increased by 8.09 cm giving a sea level rise (SLR) rate of 2.45 mm/year. The M2 constituent (principal lunar semidiurnal constituent) has the largest amplitude among other astronomical components, being 6.5 cm. Although the sea level rises, the rate of increase in high water (5.8 mm/year) is only 1.6 times the rise in the corresponding low water (3.5 mm/year). Results revealed that the extreme high water levels (coastal flooding) will occur with increasing frequency (i.e., with reducing return period) as a result of the mean SLR. Two extreme sea level years, 1987 and 1992, appeared during the 33 years of investigation. The annual maxima, also known as regional factor, α100 is 0.9 for positive surge along the Alexandria coast, which means that coastal structures along the coast of the City are in considerable risk with the return periods of positive surges.