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

Hydrodynamic Modelling

Spectral Wave Modelling

Non-Cohesive Sediment Transport Modelling

Sediment Mobility Storminess in Galway Bay

Future Work: Habitat Modelling of Maërl Beds

Bed Shear Stress of Maërl

ReferencesBLOTT, 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.

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.

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

ofound at 10.6 W longitude. The modelled time period covers one month in which the INFOMAR survey of Galway Bay took place (July to August 2007).

?A regional hydrodynamic model has been set up to model current speeds and water level. ?Mike 21 HD FM solves the incompressible Reynolds averaged Navier-Stokes equations using the finite volume method. ?The flexible mesh has a resolution of ~1.25km in the bay, with high resolution nests of ~50m adjacent to the coast.?A combination of INFOMAR multibeam bathymetry and ETOPO1 relief are used, with manual edits to ensure model stability. ?Validation is using the Inishmór and Galway tide gauges (surface elevation) and Marine Institute’s ADCP data from Spiddal (currents).

? Maërl beds have been found to harbour a high diversity of associated organisms in comparison with surrounding habitats. ? 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). ? Species occurrence data will be used in combination with environmental layers from the coupled modelling. ? The distribution of maërl has been linked to current speed, light intensity, water temperature and grain size. ? Multibeam backscatter has also been used to discriminate between maërl habitats and softer sediments using seabed classification. ? The sediment mobility will also be used as a layer in the model, to help integrate sediment dynamics into habitat modelling approaches.

?A series of grab sampling surveys took place aboard the Celtic Explorer and Celtic Voyager to augment the existing database. ?Particle size analyses were carried out using the Malvern Mastersizer 2000 and dry sieving in the NUIG Zoology department.? Statistics for ~120 samples were calculated in Gradistat and interpolated (Blott and Pye, 2001).

?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. ?This will be modelled by estimating the % of time the critical Shields parameter is exceeded during a tidal cycle. ?The residual currents, residual sediment transport and zones of divergence/convergence will be derived, to help identify the sediment transport pathways .

?The west of Ireland is a high energy coast, which experiences intense storm conditions.?Data from the Mace Head Atmospheric Research Station will be used in conjunction with historical records as part of the storminess-erosion scenarios.? Insitu observations using the benthic lander will be analysed, to monitor the impact of recent storm events.

?Mike 21 ST combined wave-current sediment transport model is used to model the erosion and deposition patterns and sediment transport rate. ?This uses the Engelund and Fredsoe, 1976 formulation with the integrated momentum approach to model wave-current boundary layer (Fredsoe, 1984). ?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.

Maërl - Calcareous

?Mike 21 SW FM is a third-generation, phase-averaged spectral wave model, which models the average wave parameters. ?Water levels are coupled from the hydrodynamic model. ?Boundary conditions are extracted from the UK Met Office second generation wave model. ?Validation is using Marine Institute’s waverider buoy at Spiddal. ?Radiation stresses are coupled back into the hydrodynamic model, which is rerun to model wave- driven currents.

Sediment Analyses

+ ETOPO1 relief

The Model Domain: An uns t ruc tu red flexible mesh (FM), with high resolution nests adjacent to the coast.

?Experiments to measure the critical bed shear stress of maërl are taking place in a flume. ?The current velocity profile is being measured using an acoustic velocimeter.?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.

Water level boundary conditions: TOPEX/ POSEIDON global tidal

constituents

River Corrib discharge input: Global Runoff Data Centre

Significant Wave Height

Maërl beds in Carraroe’s Trá an Doilín (“Coral Strand”)

Erosion-DepositionPatterns

Model Domain Location Maps

Current Speeds in the ebbing

spring tide

Wind Speeds from Mace Head in 2007 (10m)

Carraroe Finevarra AranCoral Strand -Muckinish Islands

Carraroe Finevarra AranCoral Strand -Muckinish Islands