Coastal structure toe management

www. Channelcoast. org

Coastal structure toe management

Undermining of Wartime Coastal Structures at Kilnsey, Holderness

Andy Bradbury

•(courtesy HR Wallingford)


Visible toe (or sediment / structure interface)

The toe of the structure

Defence structure

Beach



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



• Avoidance of undermining – most common cause of seawall failure


Seawall failures

Courtesy Steve McFarland


Drawdown during a storm



• Avoidance of loss of core

Sloping revetments

• Avoidance of undermining


• Simplified Forces on a Gravity Wall



Beach levels decreasing with time

beach

seawall

Erosion and undermining



Post storm surveys


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



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


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



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.




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




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.



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 down-cutting – note the level of the base of the access steps in relation to the level of the beach


• Beach lowering at access steps



Failure of a concrete toe beam at St Ouens Bay,

Jersey • Concrete toe beams



• Abraded concrete toe and onset of undermining



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

Beach

Defence

Washout of fill



21/08/08


23/08/08


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


Grouted Stone / Masonry



Scour apron added - Jersey



Rock infill of scour trough, Le Dicq, Jersey



Toe modifications - Paignton



Extended scour apron, masonry steps and armour, St

Ouens Bay, Jersey



Timber bulkhead with rock toe protection at Lepe,

Hampshire


Cribwork with rubble- Norfolk



Scour mattresses - gabion



Concrete stepped revetment. Crosby, UK

Courtesy Sefton Council)


Underpinning or encasement


Thrust Block and Piled Toe



Concrete toe beams

Coastal structure toe management

Documents

Transcript of Coastal structure toe management