Coastal risks are induced by vertical motions of the ocean surface near the shoreline, typically known as waves. In nature, we observe different types of phenomena that cause these waves. Depending on the region, we can observe that the effects of a specific wave type is more noticeable than others. In this article, we will provide an overview of the types of waves that are present in the oceans and some examples of their effects on coastal regions (1) (2).
Commonly, ocean waves are classified according to their wave period or their wavelength. The wave period of a wave is the time it takes for two successive wave crests to reach a fixed point. The wavelength is the distance between two successive crests or troughs. In Figure 1, the different types of waves present in the ocean are shown, as a function of their energy and their period. In this article, we will focus on the types of waves that are commonly associated to coastal risks: tides, surges, tsunamis and wind waves.
Tides are generated due to the combined effect of the Earth’s rotation and the gravitational attraction from the Moon and the Sun. The period of tides is between 12 and 24 hours, and their wavelength is in the order of hundred to thousand kilometers.
The tidal range, defined as the height difference between a high tide and a low tide, is larger in open-sea regions rather than in enclosed basins. For example, in the Mount Saint Michel (French Atlantic coast), tidal ranges of more than 10 meters can be observed, especially during spring tides (Figure 2). Spring tides occur during full or new moon, when the sun and the moon are aligned, and their force of attraction reaches a maximum. High tides can suppose a risk to coastal regions, particularly when coinciding with storm surges and wind waves.
Storm surges are slightly shorter waves than tides, with periods of 1 or 2 days and wavelengths of a few hundred kilometers. These are generated by large-scale atmospheric systems or storms, characterized by low pressures and strong sustained winds. When a storm approaches the coast, the water piles up and may cause severe flooding (Figure 3).
An exceptional storm surge occurred during the Hurricane Katrina in August 2005, which hit specially the states of Mississippi and Louisiana in the United States, provoking more than USD 100 billion in damages (4) and more than 1800 casualties (5). A maximum storm surge of 8.2 meters was recorded on the central Mississippi coast, reaching inland locations up to 10 miles from the shore.
The tsunamis are waves generated by sudden tectonic changes in the sea bed or landslides that are usually consequences of earthquakes and submarine volcanic activity. Their wave period is between 1 and 20 minutes, and their wavelength is between a few to hundreds of kilometers. Tsunamis have very small amplitude in deep oceans (rarely exceeding an amplitude of 1 meter) but they shoal when reaching shallow waters, considerably increasing their amplitude which may cause substantial overland flood. A paradigmatic example of this type of waves is the tsunami that followed the Great East Japan Earthquake in 2011 (magnitude 9.1 in Richter scale)(Figure 4). Maximum wave heights of 38.9 meters were estimated at Miyako city, according to the national newspaper Yomiuri Shimbun (7).
Wind waves and swells
Wind-generated waves is the wave type with periods lower than 20 seconds. The wind-generated waves with periods larger than 0.25 seconds are known as surface gravitywaves, which are the waves we observe when we go to the beach. When generated by local winds, they are irregular and short-crested and are known as wind sea (Figure 5, left). Away from the generation wind system (such as a storm),we can recognize long crested and regular waves, known as swell (Figure 5, right).
Very high wind waves are observed during storm events, such as tropical cyclones. When breaking, waves can contribute to total water levels in approximately 10% to 14% of the deep water significant wave height (average of 1/3 of the maximum waves in a specific time period) (11), aggravating the overland flood when combined with storm surge and astronomical tides.
Waves and climate change
The simultaneous occurrence of some of the waves described (such as the combination of storm surge, wind waves and tides) can cause extreme sea levels near the shoreline, leading to catastrophic damage due to floods or interruption of the operation of facilities located close to the shoreline. According to the Intergovernmental Panel for Climate Change (12), due to sea level rise as consequence of climate change the extreme sea level events that are currently rare (e.g. with an average return period of 100 years) will occur annually or more frequently at widespread locations around the globe by the end of century (with high confidence), under RCP 8.5 scenario. For some locations, these extreme events will be seen by mid-century for RCP 8.5 and by 2100 for all emission scenarios.
In (14), it is stated that in 2100, for RCP 8.5 and assumming no coastal defences or adaptation measures, it is expected that the world’s land area affected by a 100-year return period extreme sea level will increase from 0.5% at present to 0.7%. The population affected by these events will rise from a current 2.5% to 4.1%, and the value of the assets affected by the flood will increase from 12% to 20% of the global GDP.
These perspectives, for the mid and long term, highlight the importance of reviewing the current designs of coastal protections, as well as the development of further hard (e.g. dikes and floodwalls, storm surge barriers) and soft flood mitigation measures (e.g. nature based measures,spatial planning). The involvement of all the stakeholders in the decision-making for the implementation of adaptation measures in coastal regions is key to buildup an optimal flood resilience strategy.