Storm surge
An abnormal rise in sea level that a storm or tropical cyclone drives onto the coast through wind and low pressure, often the most destructive part of a hurricane on a low-lying shore.
Storm surge is the abnormal rise in sea level that a storm or tropical cyclone drives onto the coast, as wind piles up water and low pressure lifts it. When the water overtops defenses, French practice calls it submersion marine. August 29, 2005: Katrina made landfall in Louisiana and then Mississippi, and the National Hurricane Center (NHC) report records surge of roughly 8 meters along the western Mississippi coast. In New Orleans, levees and canal floodwalls failed. Splitting wind from water then became the main fight in the litigation. US homeowners policies exclude flood, which is covered by the federal National Flood Insurance Program (NFIP) at low limits, and thousands of claimants had to prove the roof went before the water arrived. Surge height depends on the size of the system as much as its strength. A shallow continental shelf, a funnel-shaped bay and landfall at high tide all amplify it. That is one reason the Saffir-Simpson scale dropped surge from its criteria in 2010. Sandy, on October 29, 2012, was no longer a major hurricane at landfall; its size was enough to flood the New York subway. The city's transit authority, the MTA, placed the MetroCat Re cat bond in 2013, which triggers on water heights measured at tide gauges. In Europe, Xynthia, overnight from February 27 to 28, 2010, swamped La Faute-sur-Mer and L'Aiguillon-sur-Mer in the Vendée, where single-story houses had been built behind a dike. Models compute surge through hydrodynamic simulation on fine bathymetry, then push it inland over a digital elevation model. A practitioner's caveat: a few tens of centimeters decide whether a dike holds or fails, and exposure geocoding resolution then weighs on the loss more than the hazard model.
Textbook case, made-up figures: 400 houses in a coastal town, behind a dike with a crest at 4.5 meters. The model gives a 1-in-100 surge level of 4.3 meters at spring high tide: no overtopping, close to zero loss. Add 30 centimeters, a level the same model puts at a 150-year return period, and the water goes over, the dike erodes and a meter of water gets into the houses. At 60,000 euros of damage per house, the loss jumps from almost nothing to 24 million euros between two neighboring points on the curve.
storm surge, storm tide, coastal surge, surge flooding
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