1 Sea - level changes : the emergence of a

The sea is rarely level. Its surface is perturbed in many ways over contrasting spatial and temporal scales. The wind ruffles the surface of the water generating waves, which travel across the oceans as swell. Groups of waves propagating from one storm interact with wind waves and swell from other directions generated by other storms, and account for the irregular surf where they meet the shoreline. Superimposed on these most obvious of oscillations are longer-term variations in water level. Tides are the most regular of these variations, represented by a rise and fall of the surface of the sea as a consequence of the gravitational forces imposed by the Moon and the Sun. Tidal range is generally small in the open ocean; it is negligible at nodal points (called amphidromic points), but increases with distance from them. It is amplified in shallow coastal regions to reach more than 15m during spring tides in a few embayments around the world. The tidal stage can be predicted with considerable accuracy because the orbital relationships between Sun, Earth, and Moon, and the harmonics which give rise to tidal variations, are known. However, calculation of mean sea level, the average level of the sea without wave or tide, requires 18.6 years of observations to account for the lunar nodal cycle (Pugh, 1987). The sea surface is subject to other perturbations; it responds to atmospheric conditions, such as barometric pressure, regional wind field, and dynamic oceanographic factors that vary with ocean currents. Of particular concern are unusually high water levels, such as storm surges which inundate coastal settlements, and tsunami, which are long wavelength waves caused by earthquakes, submarine landslides involving large sediment masses, or extraterrestrial impacts from space (Bryant, 2001; Owen et al., 2007). The Aceh tsunami of 26 December 2004, which resulted in the death of more