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?