recent coastal evolution of a tropical island and relation ... · modern beach deposits soil...
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Recent coastal evolution of a tropical island and
relation to beach ridge formation: the case of Anegada, British Virgin
Islands
Anna Lisa Cescon PhD student, School of Environmental Sciences
Supervision: Prof Andrew Cooper, Prof Derek Jackson
Palaeotempestology
2
Tropical cyclones 1985-2005 Image source NOAA
First hurricane records with Columbus trips (500 years
ago).
First cyclone records in Australia during the 18th
century
Typhoon historical records can be tracked to 2000 years BC.
Bibliography reference : Fan and Liu, 2008
Anna Lisa Cescon
Palaeotempestology
3
Tropical cyclones 1985-2005 Image source NOAA
First hurricane records with Columbus trips (500 years
ago).
First cyclone records in Australia during the 18th
century
Typhoon historical records can be tracked to 2000 years BC.
Bibliography reference : Fan and Liu, 2008
Anna Lisa Cescon
Beach ridge modes of formation
4
5 main
hypotheses
Beach ridge terminology and genesis are widely discussed in the literature (Hesp, 2004; Nott 2010; Otvos, 2000; Tamura, 2012; Taylor & Stone, 1996).
Break-point bar development
Longshore drift bar
development
Storm swash run up
Redistribution of storm sediments
Combination of all
Three types of extreme wave event have been linked to the creation of beach ridges.
Swell Storm Tsunami
Anna Lisa Cescon
5
Study site: Anegada Island
Horseshoe reef
Anegada Island
Anegada Passage Source: Google Earth, 2013
Wind Rose Wave Rose
Source: Shannon Gore, PhD Dissertation, University of Ulster, 2012.
Anna Lisa Cescon
Study site: Anegada Island
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Source: Walker Mangum, Wikipedia 2013
West End
Pomato Point
Cow Wreck Bay
Ruffling Point
Windlass Bight
Keel Point
Bones Bight
Walkover Set Bay
Anna Lisa Cescon
Historical coastal changes
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Storm and hurricane
events
Legend Sale 1:2
Shoreline mapping
Arc Map 10.1
Digital Shoreline Analysis
System 4.3
Historical coastal changes
Satellite Image
GeoEye 1 2009
Aerial Photo Scans
Rectification ER Mapper
7.2
GPS and DGPS data collection
No tsunami event
recorded for the 1953-
2012 period.
Anna Lisa Cescon
Shoreline change analysis
Net shoreline behaviour of West Anegada between 1953 and 2009.
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Erosion Accretion
Anna Lisa Cescon
Shoreline change analysis
9
Longshore variability in net shoreline behaviour on the North shore of West Anegada between 1953 and 2009.
Sand deposition for Cow Wreck Bay and Walkover Set Point
Strong erosion for Keel Point
Rubble and sand deposition for Windlass Bight
500 m
Windlass Bay Max= 83.18 m ; Min = -38.97 m Rate 1953-2009= 0.17 m/year
Keel Point Max= 49.42 m ; Min = -51.13 m Rate 1953-2009= -0.47 m/year
Walkover Set bay Max= 62.9 m ; Min = 8.9 m
Rate 1953-2009= 0.49 m/year
Anna Lisa Cescon
1953 1969
2002 2009
Shoreline change analysis
10
Beach Ridge plains: Conformity between storm events and beach ridge topography? Looking for Hurricane Donna (1960) signature.
2002 1966 1953
2009
2002 1966 1953 2009
Aeolian post-processes dominant? Beach ridge plain construction
Anna Lisa Cescon
Shoreline change analysis
11
Strong variability in net shoreline behaviour on the South shore of West Anegada between 1953 and 2009.
1953
1959
1966
1992
1969
2002
2009
Anna Lisa Cescon
Modern beach deposits
Soil formation
1 m Anegada beach ridge section
Fieldwork and Sedimentology
A soil level have been identified on Anegada Beach ridges. The limitation of the soil extent and the relative absence of animal and roots reworking activity has led to good preservation of primary sedimentary structures.
12
1 m
The microscopic observation of sand samples collected along the modern beaches and in the beach ridges sections tells us that the main fraction is composed by broken carbonate clasts of diverse origin. Anna Lisa Cescon
Fieldwork and Sedimentology
13
By the description of the modern coastal depositional environment, we want to do a comparative analysis of the beach ridge sections.
Windlass Bay
1 m
Modern deposits Beach Ridge sections (60 m inland)
Anna Lisa Cescon
Conclusions
14
Longshore drift along the North coast
Little to no change along this section of the South coast
Beach ridge plain construction in the embayments
Beach ridge plains
construction and erosion
Sediment export from this section
of the South coast
1. The historical shoreline change analysis shows that extreme events along the Anegada coast produce a spatially variable sedimentary record.
Anna Lisa Cescon
Conclusions
15
2. The way how extreme events affect Anegada should be taken into account in the discussion. To understand the way how the Hurricanes modify Anegada shorelines we need to identify what kind of event has more effects on Anegada.
3. The biogenic production of sediments seems to be a constraining factor for Anegada beach ridges: if there is no sediment available for transport and deposition at the time of an extreme event, there is a possibility that the extreme wave event will be not recorded.
Donna 1960
Luis 1995
Noel 2007
Anna Lisa Cescon
Palaeotempestology
16 Image source NOAA
Questions?
Anna Lisa Cescon