The water will slosh back and forth at a period determined by the size and shape of the basin. Surges are caused by sudden changes in atmospheric pressure and by the wind stress accompanying moving storm systems. Storm systems are significant because of their frequency and potential for causing abnormal water levels at coastlines. In many coastal regions, maximum storm surges are produced by severe tropical hurricanes. Determination of water elevations during storms is a complex problem involving interaction between wind and water, and differences in atmospheric pressure.
Winds are responsible for the largest changes in water level due to surge.
A wind blowing over a body of water exerts a horizontal force on the water surface and induces a surface current in the general direction of the wind. These currents are impeded in shallow water areas, causing the water level to rise downwind while at the windward side of the basin the water level falls. A fall of water below the normal level at the upwind side of a basin is called setdown . Along the coasts of Canada, severe storms can produce surges of up to 2 metres, but in some areas of the world, notably Bangladesh, severe storms can produce surges in excess of 8 metres. Generally, setups in lakes and reservoirs are less.