Coupled hydrodynamic-sediment transport modelling and
habitat modelling in Galway Bay, West of Ireland
Siddhi Joshi*, Garret Duffy, Colin Brown and Anthony Grehan, Biogeosciences Group, Earth and Ocean
Science, National University of Ireland, Galway, University Road, Galway, Ireland (*siddhi.joshi@nuigalway.ie).
Introduction
Coupled models provide an opportunity to model the combined effect of currents and waves on the seabed. Modelling the sediment transport pathways in the coastal
zone remains of importance to a range of industries, such as coastal engineering, renewable energy, habitat conservation and water quality. The INFOMAR seabed
mapping programme has acquired high-resolution multibeam echosounder data from Galway Bay, a large bay located in the west of Ireland. Biological and
geological habitats here include coralline algae (maërl beds) and Carboniferous limestone outcrops. Terrigeneous sediment input into the system originates from
the River Corrib and the bay is dominated by fine sands with shell hash.
This study makes use of the DHI’s MIKE 21/3 suite of modelling tools to model the physical environment of Galway Bay and in turn increase understanding of the
sediment dynamics. Additionally, maërl habitats in Galway Bay are of great conservation significance with two species, Lithothamnium coralloides and
Phymatholithon calcareum, found in the EC Habitats Directive. Few studies have modelled the influence of oceanographic forcing factors on the distribution of
mobile maërl sediments. This poster describes ongoing and planned work as part of a PhD project funded by the Griffith Geoscience Research Award.
The Model Domain:
An unstructured
flexible mesh (FM),
with high resolution
nests adjacent to the
coast.
It is hypothesized that wave-driven currents under storm conditions are an important driver for sediment transport in waters below 30m and hence the hydrodynamic
model is coupled to a wave model. The model domain covers a region from Loop Head in the south, with the island of Inish Turk to the north and the western boundary
found at 10.6oW longitude. The modelled time period covers one month in which the INFOMAR survey of Galway Bay took place (July to August 2007).
Model Domain
Location Maps
Hydrodynamic Modelling
River Corrib discharge input: Global
Runoff Data Centre
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A regional hydrodynamic model has been set up to model current
Current Speeds
in the ebbing
spring tide
Water level boundary conditions:
TOPEX/ POSEIDON global tidal
constituents
speeds and water level.
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Mike 21 HD FM solves the incompressible Reynolds averaged
Navier-Stokes equations using the finite volume method.
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The flexible mesh has a resolution of ~1.25km in the bay, with high
resolution nests of ~50m adjacent to the coast.
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A combination of INFOMAR multibeam bathymetry and ETOPO1
relief are used, with manual edits to ensure model stability.
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Validation is using the Inishmór and Galway tide gauges (surface
elevation) and Marine Institute’s ADCP data from Spiddal (currents).
Spectral Wave Modelling
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Mike 21 SW FM is a third-generation, phase-averaged spectral
Significant Wave
Height
+ ETOPO1 relief
Sediment
Analyses
wave model, which models the average wave parameters.
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Water levels are coupled from the hydrodynamic model.
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Boundary conditions are extracted from the UK Met Office second
generation wave model.
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Validation is using Marine Institute’s waverider buoy at Spiddal.
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Radiation stresses are coupled back into the hydrodynamic model,
which is rerun to model wave- driven currents.
Bed Shear
Stress of Maërl
Non-Cohesive Sediment Transport Modelling
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Experiments to measure the
critical bed shear stress of maërl
are taking place in a flume.
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The current velocity profile is
being measured using an acoustic
velocimeter.
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The law of the wall and turbulent
kinetic energy will be used to work
out the critical bed shear stress and
the hydrodynamic roughness of
maërl.
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Mike 21 ST combined wave-current sediment transport model is
Erosion-Deposition
Patterns
Carraroe
Coral Strand
Finevarra
-Muckinish
used to model the erosion and deposition patterns and sediment
transport rate.
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This uses the Engelund and Fredsoe, 1976 formulation with the
integrated momentum approach to model wave-current boundary
layer (Fredsoe, 1984).
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The one dimensional LITSTP module takes into account the inertial
forces when modelling shingle transport and will be used for detailed
studies of intrawave sediment transport of maërl.
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A series of grab sampling surveys
took place aboard the Celtic
Explorer and Celtic Voyager to
augment the existing database.
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Particle size analyses were
carried out using the Malvern
Mastersizer 2000 and dry sieving in
the NUIG Zoology department.
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Statistics for ~120 samples were
calculated in Gradistat and
interpolated (Blott and Pye, 2001).
Aran
Islands
Storminess in Galway Bay
Sediment Mobility
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Sediment mobility is defined as the percentage of time
grains of a particular size are mobile in a tidal cycle and is
considered to be an indicator of the disturbance of a habitat.
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This will be modelled by estimating the % of time the critical
Shields parameter is exceeded during a tidal cycle.
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The residual currents, residual sediment transport and
zones of divergence/convergence will be derived, to help
identify the sediment transport pathways .
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The west of Ireland is a high energy coast,
Maërl beds in Carraroe’s Trá
an Doilín (“Coral Strand”)
which experiences intense storm conditions.
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Data from the Mace Head Atmospheric
Research Station will be used in conjunction
with historical records as part of the storminesserosion scenarios.
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Insitu observations using the benthic lander
will be analysed, to monitor the impact of recent
storm events.
Wind Speeds from Mace Head in 2007 (10m)
Future Work: Habitat Modelling of Maërl Beds
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Maërl beds have been found to harbour a high diversity of associated organisms in
comparison with surrounding habitats.
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An ecological niche model to predict the distribution of maërl beds will be developed
using the BIOMOD platform in the R computing environment (Thuiller et. al. 2009).
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Species occurrence data will be used in combination with environmental layers from the
coupled modelling.
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The distribution of maërl has been linked to current speed, light intensity, water
temperature and grain size.
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Multibeam backscatter has also been used to discriminate between maërl habitats and
softer sediments using seabed classification.
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The sediment mobility will also be used as a layer in the model, to help integrate sediment
dynamics into habitat modelling approaches.
References
BLOTT, S. J. & PYE, K. (2001) GRADISTAT: a grain size distribution and statistics package for the
analysis of unconsolidated sediments. Earth Surface Processes and Landforms, 26, 1237-1248.
ENGELUND, F. & FREDSOE, J. (1976) Sediment transport model for straight alluvial channels.
Nordic Hydrology, 7, 293-306.
FREDSOE, J. (1984) Turbulent Boundary Layer in Wave-current Motion. Journal of Hydraulic
Engineering, 110, 1103-1120.
THUILLER, W., LAFOURCADE, B., ENGLER, R. & ARAÚJO, M. B. (2009) BIOMOD – a platform
for ensemble forecasting of species distributions. Ecography, 32, 369-373.
Acknowledgements and Disclaimer
This research is funded by the Griffith Geoscience Research Award, administered by
the Geological Survey of Ireland (GSI). Based on research grant-aided by the
Department of Communications, Energy and Natural Resources under the National
Geoscience Programme 2007-2013 INFOMAR data have been provided under the
memorandum of understanding with GSI.The views expressed in this study are the
author's own and do not necessarily reflect the views and opinions of the Minister for
Communications, Energy and Natural Resources.
Maërl - Calcareous