Coastal structure toe management Undermining of Wartime Coastal Structures at Kilnsey, Holderness (courtesy HR Wallingford) Andy Bradbury
Defence structure Visible toe (or sediment / structure interface) Beach The toe of the structure (courtesy HR Wallingford)
Chapter 1 - Introduction Chapter 2 Physical Processes Definition and description of localised scour, beach lowering and liquefaction Chapter 3 Asset Management Description of and specific guidance on asset management activities for Toe Structures Chapter 4 Applications, structure types & materials Description and illustration of the various types of materials and structures used to mitigate against the affects of scour, beach lowering and liquefaction processes Chapter 5 Toe Design Illustration and description of typical designs for toe protection structures and particular considerations in the design process Chapter 6 Maintenance Information on maintenance, repair, reconstruction or adaptation, and on emergency interventions options Appendices Case studies of toe scour and foreshore down-wearing. Information on monitoring equipment. Science summary on scour in cohesive sediments
Avoidance of undermining most common cause of seawall failure (courtesy HR Wallingford)
Seawall failures Courtesy Steve McFarland
Drawdown during a storm Courtesy Steve McFarland
Sloping revetments Avoidance of loss of core Avoidance of undermining
Simplified Forces on a Gravity Wall (courtesy HR Wallingford)
Erosion and undermining Beach levels decreasing with time seawall beach Courtesy Steve McFarland
Erosion and undermining Courtesy Steve McFarland
Post storm surveys
Erosion and undermining Courtesy Steve McFarland
(courtesy HR Wallingford)
4.5 Beach level (modn) 4 3.5 3 2.5 2 1.5 1992 1990 1988 1986 1984 1982 1980 1978 1976 1974 1972 1970 1968 1966 1964 1962 1960 1958 Level Trend Year since 1900 (courtesy HR Wallingford)
Scour monitors in operation on a shingle beach (courtesy HR Wallingford)
Tide, beach levels (mcd) H s (m) 3.5 3 2.5 2 1.5 1 0.5 Recovery Lowering Recovery Lowering Tide Height Scour monitor Hs 0 24/05/2005 00:00 24/05/2005 12:00 25/05/2005 00:00 25/05/2005 12:00 Date and Time GMT (courtesy HR Wallingford)
(courtesy HR Wallingford)
Erosion and undermining- defining alarm conditions Purely beach loss no storm component Analysis of beach trends Determine beach position relative to seawall toe with time Basic structural analysis to determine likelihood of overturning or sliding in absence of storms Real life calibration. Courtesy Steve McFarland
Drawdown during a storm During storms draw down may allow direct wave action on the seawalls. Profile analysis for various storms Beach position against seawall including scour Seawall stability analysis Repeat for future years taking account of beach trends Most complicated failure type simplifying assumptions normally required Courtesy Steve McFarland
Eg. Van Der Meer, Powell. Drawdown during a storm S.W.L. Wave Height, Period, Angle Crest Beach Berm Eg. Van Der Meer, Powell. S.W.L. Wave Height, Period, Angle Courtesy Steve McFarland
Defining intervention levels Use the data collected as part of the regional monitoring to provide a risk based assessment of coastal defence vulnerability Long data sets best, requires wall construction details and any modifications Quite labour intensive but the basics only need to be done once Calculate expected levels during storm Relate to expected failure Requires engineering experience and local knowledge Will provide clear triggers for action eg beach recharge / recycling Often not calculated due to lack of structure data Courtesy Steve McFarland
Extreme example of beach depletion, foreshore erosion and downcutting note the level of the base of the access steps in relation to the level of the beach
Beach lowering at access steps (courtesy HR Wallingford)
(courtesy HR Wallingford)
Failure of a concrete toe beam at St Ouens Bay, Jersey Concrete toe beams (courtesy HR Wallingford)
Abraded concrete toe and onset of undermining (courtesy HR Wallingford)
Exposed reinforcing mesh due to steel oxidation and resultant cracking and spalling of concrete cover layer
Erosion of a cohesive shore platform at the Toe of a seawall
Undermining and fill washout of stepped revetment and wave return wall Defence Washout of fill Beach (courtesy HR Wallingford)
21/08/08
23/08/08
As built detail required Regular performance monitoring desirable Significance of beach performance critical
Grouted Stone / Masonry (courtesy HR Wallingford)
Scour apron added - Jersey (courtesy HR Wallingford)
Rock infill of scour trough, Le Dicq, Jersey (courtesy HR Wallingford)
Toe modifications - Paignton (courtesy HR Wallingford)
Extended scour apron, masonry steps and armour, St Ouens Bay, Jersey (courtesy HR Wallingford)
Timber bulkhead with rock toe protection at Lepe, Hampshire
Cribwork with rubble- Norfolk (courtesy HR Wallingford)
Scour mattresses - gabion (courtesy HR Wallingford)
Concrete stepped revetment. Crosby, UK Courtesy Sefton Council)
Underpinning or encasement
Thrust Block and Piled Toe (courtesy HR Wallingford)
Concrete toe beams