A8 Extreme Coastal Events

Topic

Extreme coastal events arise from the convergence—within a single coastal zone—of several risk factors typically studied in isolation: sea-level rise (currently advancing at a global average rate of nearly 3.7 millimeters per year and continuing to accelerate); increased frequency and intensity of both tropical and extratropical cyclones; accelerated coastal erosion; and saltwater intrusion into coastal aquifers. When high astronomical tides, storm surges, and extreme wave action coincide, coastal water levels can rise five to ten meters above the usual average, flooding vast areas currently considered safe.

Saltwater intrusion into coastal aquifers is one such risk factor; it occurs when sea levels rise while freshwater extraction reduces the hydraulic head of the inland aquifer, allowing saltwater to penetrate further inland. The subsurface boundary between inland freshwater and seawater roughly follows the Ghyben-Herzberg relationship: for every meter the water table rises above sea level, that boundary extends approximately forty meters below sea level—a ratio reflecting, above all, the density difference between fresh and salt water.

Coastal erosion—another of these risk factors—accelerates when rising sea levels cause the beach's equilibrium profile (the shape the beach maintains on average under typical wave conditions) to shift inland; this relationship between sea-level rise and shoreline retreat is described by the Bruun Rule. Climate change also alters coastal upwelling, albeit in a far less direct manner than it affects other risk factors. Bakun’s theory predicts that the increased temperature contrast between land and ocean will intensify coastal trade winds as warming progresses; these stronger winds, in turn, drive intensified upwelling in eastern boundary current systems. However, this intensification may be offset by increased ocean stratification—which requires more energy to overcome in order to bring deep water to the surface—meaning the net effect on coastal upwelling (whether it intensifies or weakens) depends on which of these two factors ultimately dominates in a given region.

What makes these extreme coastal events so dangerous is rarely a single factor acting in isolation, but rather the simultaneous occurrence of several of them. These are known as compound events—which are becoming increasingly frequent—where phenomena such as sea-level rise, storm surges, saltwater intrusion, and coastal erosion converge on the same coastline, producing a combined impact far greater than the mere sum of their individual effects. Thus, ranging from the annual 3.7-millimeter rise in sea level to the loss of meters of shoreline as a beach retreats, the severity of extreme coastal events is increasingly determined not by a single factor, but by how many of them coincide at the same location.

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