Coastal structure toe management

Transcription

1 Coastal structure toe management Undermining of Wartime Coastal Structures at Kilnsey, Holderness (courtesy HR Wallingford) Andy Bradbury

2 Defence structure Visible toe (or sediment / structure interface) Beach The toe of the structure (courtesy HR Wallingford)

3 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

4 Avoidance of undermining most common cause of seawall failure (courtesy HR Wallingford)

5 Seawall failures Courtesy Steve McFarland

6 Drawdown during a storm Courtesy Steve McFarland

7 Sloping revetments Avoidance of loss of core Avoidance of undermining

8 Simplified Forces on a Gravity Wall (courtesy HR Wallingford)

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10 Erosion and undermining Beach levels decreasing with time seawall beach Courtesy Steve McFarland

11 Erosion and undermining Courtesy Steve McFarland

12 Post storm surveys

13 Erosion and undermining Courtesy Steve McFarland

14 (courtesy HR Wallingford)

15 4.5 Beach level (modn) Level Trend Year since 1900 (courtesy HR Wallingford)

16 Scour monitors in operation on a shingle beach (courtesy HR Wallingford)

17 Tide, beach levels (mcd) H s (m) Recovery Lowering Recovery Lowering Tide Height Scour monitor Hs 0 24/05/ :00 24/05/ :00 25/05/ :00 25/05/ :00 Date and Time GMT (courtesy HR Wallingford)

18 (courtesy HR Wallingford)

19 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

20 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

21 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

22 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

23 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

24 Beach lowering at access steps (courtesy HR Wallingford)

25 (courtesy HR Wallingford)

26 Failure of a concrete toe beam at St Ouens Bay, Jersey Concrete toe beams (courtesy HR Wallingford)

27 Abraded concrete toe and onset of undermining (courtesy HR Wallingford)

28 Exposed reinforcing mesh due to steel oxidation and resultant cracking and spalling of concrete cover layer

29 Erosion of a cohesive shore platform at the Toe of a seawall

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32 Undermining and fill washout of stepped revetment and wave return wall Defence Washout of fill Beach (courtesy HR Wallingford)

33 21/08/08

34 23/08/08

35 As built detail required Regular performance monitoring desirable Significance of beach performance critical

36 Grouted Stone / Masonry (courtesy HR Wallingford)

37 Scour apron added - Jersey (courtesy HR Wallingford)

38 Rock infill of scour trough, Le Dicq, Jersey (courtesy HR Wallingford)

39 Toe modifications - Paignton (courtesy HR Wallingford)

40 Extended scour apron, masonry steps and armour, St Ouens Bay, Jersey (courtesy HR Wallingford)

41 Timber bulkhead with rock toe protection at Lepe, Hampshire

42 Cribwork with rubble- Norfolk (courtesy HR Wallingford)

43 Scour mattresses - gabion (courtesy HR Wallingford)

44 Concrete stepped revetment. Crosby, UK Courtesy Sefton Council)

45 Underpinning or encasement

46 Thrust Block and Piled Toe (courtesy HR Wallingford)

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48 Concrete toe beams

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