Toe scour at seawalls: Monitoring, prediction and mitigation

This paper summarises the main results from the Defra/EA R&D project Understanding the Lowering of Beaches in front of Coastal Defence Structures, Stage 2 (FD1927). Evidence has been presented showing beach lowering and recovery happening over a range of timescales. In particular, beach levels have been shown to drop and recover close to their original levels within a tidal cycle. This behaviour cannot be detected from beach profiles alone. A large set of toe scour data from medium- to large-scale laboratory and field experiments has been analysed to produce a new equation for the maximum scour depth. Beach profiles have been analysed to show that the annual rate of beach lowering in front of a coastal structure varies considerably from one decade to the next. The residual beach levels about the long-term trend were found to have a probability distribution close to Gaussian, providing there were no significant changes in beach management. A list of monitoring methods has been compiled and an indication of the suitable timescale for deployment has been given. The use of remote sensing is becoming more common and is likely to lead to the greatest changes to the type and quantity of data available for monitoring. A review has also been carried out of the mitigation methods that have been, or could be used to delay or to reduce the severity of problems arising from the lowering of beach levels in front of a seawall or a similar coastal structure. These measures include ones that reduce the hazard without removing the lowering. Mitigation methods include filling in scour holes with rock, encouraging deposition of sediment using, for example, groynes, beach recharge and the underpinning of seawalls so that they remain structurally sound when beach levels drop