Wave overtopping at vertical and steep seawalls

Wave overtopping is the critical response of most sea defence structures and one of the more important responses for many coast defences around the UK and other developed shorelines. Sea defences in rural areas are commonly provided by embankment seawalls. Steep or vertical seawalls are more commonly used in urban areas to protect against erosion, flooding and local overtopping hazards, and to protect the base of eroding cliffs in urban or rural areas. Vertically faced breakwaters have been common around many European countries, and caisson-type breakwaters are heavily used in Japan. For simple slopes or embankments, overtopping performance can be predicted by simple monotonic empirical formulae. Overtopping of vertical or slightly battered walls is however rather more complicated, with substantial differences in overtopping volumes and velocities depending on the form of wave interaction at or close to the wall. The present paper draws together results from a number of UK and European research projects over the last 10 years. Their results improve and validate prediction methods for wave overtopping discharges and velocities for steep battered, composite and vertical seawalls/breakwaters. The methods presented herein support, extend and qualify guidance given in the UK Environment Agency overtopping manual. The present paper shows how the use of these methods can now explain why overtopping of some seawalls/breakwaters can be greatest at mid-water level, rather than at the highest water levels.