The most obvious examples of flooding happening in the Borough are incidents related to high tides - hence known as tidal flooding. These will occur when tidal predictions are for higher than average tides, and may be linked to specific weather conditions on the day.

 

Otherwise, flooding can (theoretically) occur almost anywhere across the Borough as a result of heavy rain - that is, when rain falls at a greater rate than is allowed for in design of the drainage system. Think of your bath at home - when you pull out the plug, the water takes time to drain away. It's the same for drainage systems - if a cubic metre of water falls on some ground in an hour and the drain can only take away three quarters of a cubic metre an hour, then a quarter of a cubic metre of flooding will occur during that hour. This is known as pluvial flooding. When the rainfall causes river, streams or ditches to break their banks and cause flooding, this is known as fluvial flooding.

 

In the older parts of the Borough the sewerage and drainage system will have been built by 'rule of thumb' to cater for an average rainfall rate of between 25 mm (1 inch) and 50 mm (2 inches) per hour.

 

In more modern developments, more sophisticated calculations take account of higher rainfall rates in small areas and lesser rates over larger areas depending on the length of time it takes water to flow from the furthest point in the catchment to the length of pipe being designed. These rainfall rates are expressed in terms of the probability that they will occur in any one year.

 

In a small area such as a small part of a housing estate or a garage compound the drainage system will be designed to cope with a storm that may occur in any year with a probability of 1:1 or 100%.

 

Larger areas will have systems capable of taking the flow from a bigger storm that will have a 1:5 (20%) probability of occurring in any one year (also expressed as a probability of one occurrence in five years).

 

More significant drainage channels and rivers are designed to dealt with a very large design storm which is likely to occur only once in a hundered years (1% probability) or less.