Breach Analysis

On some occasions a development site is defended by man-made flood defences such as concrete walls or earth banks to protect land from flooding in storm events. These defences can be either privately owned and maintained or owned and maintained by the relevant environmental body.

Flood defences are designed to certain criteria dependent upon the type, and the extent of development they protect. Defences offer varying standards of protection and can protect land and existing development from flood events that have between a 1 in 20 and 1 in 1000 annual probability.

If a potential development is in an area defined as being protected by defences, then on some occasions it can be necessary to assess the level of risk to a site if the defences were to breach.

A breach in a defence is where the defence fails and collapses leaving a hole for water to flow through, this can occur in earth embankments and also concrete defences. A breach analysis assesses the velocity, rate of inundation and depth of flood water anticipated which can determine if a development is safe or not. Generally earth embankment defences are more prone to breach due to their structure but the level of maintenance can make a significant difference to the potential risk of such a failure.

A second assessment can also be required which is for overtopping of defences which is essentially when, for example, a defence designed to withstand a 1 in 100 year storm event is subjected to water levels from say a 1 in 150 year event and water levels exceed the defence levels. This is usually a lower risk to a development than a breach as there is usually less volume of water overtopping a defence than if there is a breach. This is obviously dependent upon the proximity of the site to a defence and generally the closer the site is to a defence the greater the impact overtopping could have.