Calculate Flood Return Period using Tide Gauge Data

Sea Defences, Humber Estuary, UK

GeoTide calculates (from tide gauge data) the flood-return period (in terms of years) as a function of sea-defence height.

The combined effect of the normal regular tide and weather induced residual can result in tidal flooding, depending upon the height of sea-defences. Weather induced variations in the tide are caused by atmospheric pressure and wind and its effect upon the ocean. When these effects are large, such fluctuations are known as tidal surges. Surges can be surprisingly frequent. For example, around the East coast of the UK, tidal surges of one or even two meters occur several times per year in some locations. Being caused by weather, tidal surges are difficult to predict and unlike the normal tide cannot be tabulated in advance.

 

Surges can occur at any time, and so it is more likely they will happen during neap tides or spring low tides, in which case overtopping almost certainly will not occur. However, when surges happen at the same instant as a spring high tide, overtopping of sea defences is much more likely.

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The statistics of flooding is therefore the result of the combined effect of the statistics of the astronomical tide and the statistics of the residual tide, this is known as the joint probability method or JPM. Using this method, GeoTide provides statistical estimates of the flood-return probability and flood-frequency for a given height of sea-defence.

 

More on flood probablility