investigating the hydrodynamics of a breached barrier beach
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
Rossbeigh Barrier BeachInvestigating 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 Rossbeigh
Inch
Cromane
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 BarriersNauset Breach, MA, USA
Giese Et al (2009) conducted a re- analysis of the morphological cycle after previous model failed to predict breaching in 2007
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.
Slapton Sands & Loe Bar, South West UK (NUPSIG Project)
Shingle barrier beach monitoring
Numerical Monitoring and extensive measurement system
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 mapsSatellite imagery
Identified Centennial morphodynamic cycle 5 million tonnes erodedBreach widened to 800m Growth of ebb tidal barsRealignment 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
• High tide coincides with largest Hs
• Convergence of high frequency energy in onshore as waves pass over intertidal bar
50 100 150 200 250 300 3500
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Direction (degrees)
Results – Wave Directionality at High Tide
EnergyEnergy
Direction (degrees)
Swash AlignedDrift Aligned
50 100 150 200 250 300 3500
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
Results – Tidal Cycle of Rossbeigh
• Funnelling of waves at drift shore as waves diffract around intertidal bar
• Sediment in intertidal zone moved towards tidal inlet
• Tidal current moves sediment offshore on drift aligned into tidal inlet creating a sediment sink
• Waves become multidirectional on drift aligned as water level rises above bar
• Drift zone grows as hinge point moves south
• Tidal inlet transports sediment to offshore bars
• Funnelled waves moves sediment towards end of Rossbeigh and tidal inlet
Low Tide Mid Flood High Tide Mid Ebb
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 to provide definitive conclusions and predictions on future evolution of system
• Need for novel monitoring techniques focussing on system as a whole rather than single point measurements
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?