3. Why do storm surges occur?

Two distinct types of meteorological disturbances contribute to major storm surges that cause erosion, flooding and associated drownings. These are tropical and extra tropical cyclones. Both are rapid onset natural hazards that can last for days.

a) Tropical cyclones are large low pressure systems (<100 to several hundred km across) that intensify between latitudes circa 5º and 25º. Sea surface temperatures must be high (>26 ºC) with high relative humidity and evaporation, weak vertical wind shear and atmospheric instability (Henderson-Sellers and Lewis, 1998; Landsea et al, 1999). At its centre is an ‘eye’ an area of calm clear sky, surrounded by an eyewall which hosts the strongest winds around which spiral rain bands of high winds and rain, all rotating in a counter clockwise movement in the northern hemispheres, vice versa in the southern hemisphere. Tropical cyclones are small but powerful.

b) Extra-tropical cyclones cause the bulk of flooding and beach erosion in temperate latitudes. The wind speeds and surge heights are much lower than in tropical cyclones but they have a much larger extent, i.e. >1000 km) and invariably extend over several tidal cycles. The USA storms (nor’easters) have been classed again into 5 categories based on the significant wave height (H1/3 = average of 1/3 highest waves), duration (D) and Power (P) where P = H21/3.

Once a tropical cyclone hits the coast, the severity of storm surge impact is influenced by a number of aspects:

  • offshore sea depth.
  • coastline configuration.
  • cyclone orientation as it lands.
  • wind characteristics (speed, intensity).
  • cyclone radius.

Storm surges are pronounced:

  • over shallow water bodies
  • persistent strong winds
  • persistent low atmospheric pressure.

Resulting in coastal flooding that may impact many kilometers inland.

Storm surges are particularly pronounced:

  • in areas having a large tidal range
  • when occurring at high tide

Under such circumstances, the resulting surge is called a storm tide that includes the accumulative sea level elevation arising from the:

  • storm
  • tide
  • wave run-up
  • freshwater flooding.