Breach design for managed realignment sites

In low-lying areas of the coast and particularly around
estuaries, the option to realign some of the seawalls is
becoming increasingly attractive. Although the wholesale
removal of existing walls is possible in some cases,
frequently this is not possible for a variety of reasons and it is necessary to consider simply breaching the existing wall over a limited length. In order to undertake this in a controlled manner the size of the breach needs to be determined in advance. Current guidance on breach
design was derived from a regression analysis of data from
sites on the east coast that have breached during storms.
This provides a simple relationship between the tidal
prism of the site and the breach width. The basis of this
approach is re-examined and shown to have limited
explanatory power. An alternative method is presented,
which uses the basic flow conditions, a limiting threshold of erosion and a description of the site hypsometry (variation in area with level) to estimate the breach profile. The proposed method is tested using the east coast data for seawalls that have breached and shown to provide better estimates. As the basis of the method is quite general, it can be used in widely varying situations, allowing rough estimates to be made with the minimum of data, or more refined estimates when more site-specific
information is available