coastal structure toe management
TRANSCRIPT
www. Channelcoast. org
Coastal structure toe management
Undermining of Wartime Coastal Structures at Kilnsey, Holderness
Andy Bradbury
•(courtesy HR Wallingford)
www. Channelcoast. org
Visible toe (or sediment / structure interface)
The toe of the structure
Defence structure
Beach
•(courtesy HR Wallingford)
www. Channelcoast. org
Chapter 4 – Applications, structure types & materialsDescription 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 DesignIllustration and description of typical designs for toe protection structures and particular considerations in the design process
Chapter 3 – Asset ManagementDescription of and specific guidance on asset management
activities for Toe Structures
Chapter 6 – MaintenanceInformation 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
Chapter 1 - Introduction
Chapter 2 – Physical ProcessesDefinition and description of localised scour, beach lowering and
liquefaction
www. Channelcoast. org
•(courtesy HR Wallingford)
• Avoidance of undermining – most common cause of seawall failure
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Beach levels decreasing with time
beach
seawall
Erosion and undermining
Courtesy Steve McFarland
www. Channelcoast. org
1.5
2
2.5
3
3.5
4
4.5
19581960
19621964
19661968
19701972
19741976
19781980
19821984
19861988
19901992
Year since 1900
Beac
h le
vel (
mO
DN)
Level Trend
•(courtesy HR Wallingford)
www. Channelcoast. org
0
0.5
1
1.5
2
2.5
3
3.5
24/05/2005 00:00 24/05/2005 12:00 25/05/2005 00:00 25/05/2005 12:00Date and Time GMT
Tide
, bea
ch le
vels
(mC
D) H
s (m
)
Tide Height Scour monitor Hs
Lowering
Lowering
Recovery
Recovery
•(courtesy HR Wallingford)
www. Channelcoast. org
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
www. Channelcoast. org
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
www. Channelcoast. org
Drawdown during a storm
Wave Height,Period, Angle
S.W.L.
Berm
Beach
Crest
Eg. Van Der Meer, Powell.
Wave Height,Period, Angle
S.W.L.
Eg. Van Der Meer, Powell.
Courtesy Steve McFarland
www. Channelcoast. org
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
www. Channelcoast. org
Extreme example of beach depletion, foreshore erosion and down-cutting – note the level of the base of the access steps in relation to the level of the beach
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Failure of a concrete toe beam at St Ouens Bay,
Jersey • Concrete toe beams
•(courtesy HR Wallingford)
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Exposed reinforcing mesh due to steel oxidation and resultant cracking and spalling of concrete cover layer
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Undermining and fill washout of stepped revetment and wave return wall
Beach
Defence
Washout of fill
•(courtesy HR Wallingford)
www. Channelcoast. org
• As built detail required • Regular performance monitoring desirable • Significance of beach performance critical
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Extended scour apron, masonry steps and armour, St
Ouens Bay, Jersey
•(courtesy HR Wallingford)