Natural Hazards

Hurricanes and Storm Surges

A hurricane is a system of violent thunderstorms with high winds circulating about a central low-pressure area, called the eye. Air pressure flows from higher pressure towards lower pressure, although not in a straight line because the Earth's surface spins at different speeds (faster at the equator, slower near the poles); in fact, it spirals inwards, generating the typical cyclonic storm shape associated with a hurricane.

A storm surge is ocean water levels that have been elevated above that expected from astronomical tides and which are caused by a passing storm.

A reduction of atmospheric pressure causes sea water to rise by circa 1cm for each 1 mb. pressure drop (the inverse barometer effect) together with wind stress which pushes water towards the coast and the drag/stress on the sea surface by wind being measured as horizontal force per unit area. This is a function of wind speed and air density. Strength and direction play a large part in elevating the sea surface, as winds blowing towards a coast produce a much greater sea level rise than off shore winds. Additionally, wind effects increase inversely with water depth, so shallow sea areas such as Bangladesh have amplified surges. Surges are major causes of coastal erosion and death by drowning via flooding. Storm surges are affected by a number of variables that include winds, coastline shape, water depth near the coastline, and storm size and structure.

There is a dire need for countries to assess these parameters, particularly for storm surge vulnerable coasts.

 

Prepared by ICoD - Euro-Mediterranean Centre on Insular Coastal Dynamics (Valletta, Malta) & the Editorial Board