Rossbeigh
Barrier Beach
Investigating
the hydrodynamics
of a
breached barrier beach
Michael O’Shea
Hydraulics and Maritime Research Centre
U.C.C.
EGU 2012
Study Area
•
Dingle Bay, Co Kerry, Ireland
•
Bay enclosed by rocky
headlands
•
A 3.2m Spring Tidal Range
•
Waves direction in a narrow
band: 250-270 deg
•
Self Contained Coastal Cell
Cromane
Inch
Rossbeigh
Breaching of Rossbeigh
Breach
in winter
2008
Why study
Rossbeigh?
Scientific Value – Naturally dynamic coastal system
Civic Concern – Flooding, Economic Value, Habitat
Similar Morphology Studies on Breaching Barriers
Nauset Breach, MA, USA
Giese Et al (2009) conducted a re- analysis of the
morphological cycle after previous model failed to predict
breaching in 2007
Slapton Sands & Loe Bar, South West UK
(NUPSIG Project)
Shingle barrier beach monitoring
Numerical Monitoring and extensive measurement system
Established a 2 Phase conceptual model based on
analysis of maps and images from 1800’s to present,
regular GPS surveys and water level recording.
Pre and Post Storm surveys
Focus on swash dynamics
Monitoring of Rossbeigh
•Regular Topographic surveys using RTK GPS
•GIS and Remote sensing :
Aerial photography
Historical maps
Satellite imagery
Identified Centennial morphodynamic cycle
5 million tonnes eroded
Breach widened to 800m
Growth of ebb tidal bars
Realignment of beach
Monitoring of Rossbeigh
2005
2007
2011
Numerical Modelling
• Based on low resolution bathymetry and offshore wave data
• Indication of circulation patterns & wave climate
• Useful for planning field data campaign
Field Work - Objectives
• Identify drivers of variations in erosion patterns along
Rossbeigh
• Acquire field data to validate a combined Wave, Tidal,
Sediment transport and Morphodynamic model
Field Work - Wave and Tidal Current Data Collection
•
Vane guided propeller current meters
•
Seabed mounted pressure gauges
Results - Tidal
Mid Flood
Mid Ebb
Field Work - Wave
•
•
5 onshore locations each for 5 days
1 located 4km offshore continuous for 30 days
Results - Wave
Drift Aligned at High Tide
0.35
Hs and Tidal Signal
1
3
0.3
0.9
0.7
0.6
Height (m)
0.5
0.4
1
0.3
0.2
Offshore
Onshore
0.15
0.1
0.2
0.5
0.1
0.05
0
0
9:36:00 PM
7:12:00 PM
4:48:00 PM
Time
2:24:00 PM
12:00:00 PM
9:36:00 AM
7:12:00 AM
4:48:00 AM
2:24:00 AM
12:00:00 AM
•
0.25
Spectral Energy
2.5 Hs
Onshore
2 Hs
Offshore
tidal
1.5
height
0.8
High tide coincides with largest
Hs
0
0
•
0.1
0.2
0.3
Frequency
0.4
0.5
0.6
Convergence of high frequency
energy in onshore as waves pass
over intertidal bar
Results – Wave Directionality at High Tide
Drift Aligned
Swash Aligned
Energy
Energy
0.4
0.009
0.35
0.01
0.4
0.01
0.009
0.35
0.008
0.008
0.3
0.3
0.007
0.007
0.25
0.006
0.2
0.005
0.004
0.15
0.003
0.1
Frequency (Hz)
Frequency (Hz)
0.25
0.006
0.005
0.2
0.004
0.15
0.003
0.1
0.002
0.05
0
0.001
50
100
150
200
Direction (degrees)
250
300
350
0
0.002
0.05
0
0.001
50
100
150
200
Direction (degrees)
250
300
350
0
Results – Tidal Cycle of Rossbeigh
Low Tide
•
•
Funnelling of waves
at drift shore as
waves diffract
around intertidal bar
Sediment in intertidal
zone moved towards
tidal inlet
Mid Flood
•
Tidal current moves
sediment offshore on
drift aligned into tidal
inlet creating a
sediment sink
High Tide
•
•
Waves become
multidirectional on drift
aligned as water level
rises above bar
Drift zone grows as
hinge point moves
south
Mid Ebb
•
•
Tidal inlet transports
sediment to offshore
bars
Funnelled waves
moves sediment
towards end of
Rossbeigh and tidal
inlet
Conclusions
Based on limited field data:
•
Erosion rates will continue to be high in the drift aligned zone.
•
The hinge point between zones will continue to move in the direction of the
swash aligned zone, increasing the area susceptible to erosive processes
described.
•
The risk of coastal flooding and inundation in the back barrier area is increasing
Conclusions
•
Present monitoring techniques alone are insufficient
conclusions and predictions on future evolution of system
•
Need for novel monitoring techniques focussing on system as a whole rather
than single point measurements
to provide definitive
In 2012-13 HMRC will be trialling :
HF wave radar to measure circulation system in Inner Dingle bay
Remote operated Helicopter Photogrammetry to map sand bar change
Conduct regular shallow water bathymetry surveys
Thank You
Questions?