Development of guidance for the management of the toe of coastal defence structures

Beach lowering and the effects of scour in front of coastal defences and erosion protection structures are recognised as a principle cause of their failure including collapse/breaching and washing out of fill materials. Both localised scour and more widespread beach lowering can lead to undermining of the Toe (see Figure 1 for definition). This can promote structural instability and result in partial or total collapse with subsequent reduction in, or loss of performance for flood defence or erosion protection. Lowering of the ‘ground levels’ in front of seawalls, revetments or other coastal structures is a common phenomenon not only in the U.K. but also around the world. Toe scour is a serious and costly problem – moreover, it is one which is not limited to any particular coastal environment or to any particular type of defence structure. The prevention of, design for, and management of beach lowering and scour at the Toe of coastal structures is an important factor in the management of those structures. Understanding, designing for and managing toe scour at coastal structures therefore is a key issue for coastal managers, designers and engineers. To date there has been no guidance available tailored specifically to meet these needs. Recognising this, the Defra/ Environment Agency Flood and Coastal Risk Management programme commissioned the development of guidance on the management of the Toe of coastal structures in 2008 (Toe Scour Guide). The guide draws together recent key research and development work on beach lowering and scour including ‘Understanding the Lowering of Beaches in Front of Coastal Defence Structures’ (Sutherland et al, 2006, 2008). It is also integrating work on performance and reliability of coastal structures developed through the Performance-based Asset Management Project (PAMS) including performance analysis and asset inspection methods. This paper reflects on and highlights some of the developments made in the preparation of the guide