MODELLING MAЁRL HABITAT DYNAMICS IN RESPONSE TO INCREASED STORMINESS

Geological Survey Ireland Geoscience Research Programme Short Call 2017-2018

Figure 1. A. Biogenic gravel composed entirely of dead maёrl (debris) at Trá an Doilin (Carraroe) beach, County Galway. B. Individual clasts of maёrl debris on beach (photo length: 0.05m). C. Living maёrl beds in approximately 5m bathymetry (photo length: c.0.60m). D & E. Concentric patterns at Trá an Doilin suggests maёrl has a higher mobility than quartz sediments.

Field Design in early 2018 A week long multi-disciplinary field experiment will be conducted at Trá an Doilin (Carraroe) beach, County Galway (Figure 3).

WP.1 Beach morphology 1.1. Beach elevation changes in the swash zone Changes in bed elevation and swash depths will be measured with-in the upper-swash zone using a cross-shore array of 9 ultrasonic distance sensors. The sensors record high frequency (1 Hz) verti-cal changes in bed elevation at a resolution of 0.001m. 1.2. Digital Surface Models High resolution DSMs of the dry beach will be generated using UAV, GPS, and Structure-from-Motion (SfM) technologies. Surveys will be conducted twice a day during low tide. A DJI Matrice 600 Pro will be used (weather conditions allowing). Photoscan Agisoft soft-ware will be used to generate DSMs using SfM. WP.2 Hydrodynamic model 2.1. Hydrodynamics A Nortek AWAC wavemeter will be deployed at 5-8m depth sea-ward of the field site for a period of 8 weeks during winter (storm conditions). The results will provide wave height, wave directions, current speeds, current directions and water levels (tidal range) for the water column (averaged through 0.50m cells). The data will provide validation for a nested SWAN model for Greatman’s Bay and the boundary conditions for developing an XBeach model. 2.2. Wave run up An automated camera monitoring system (Plotwatcher) will collect daylight images every 10 minutes to observe the variability in shoreline position and wave runup over a period of 6-8 months. WP.3 Groundwater dynamics 3.1. A temporary well with autonomous data loggers (CTD and baro divers) will record fluctuations in water levels every 15 minutes over a period of 3-6 months. The well will be installed mid beach. The Divers will record water pressure at hourly intervals (the Baro is re-cording air pressure at the same interval), which can be converted to water elevation. WP. 4 Sediment characteristics 4.1. The sediment characteristics will be characterized using the previous work by lead author (Joshi et al. , 2014; 2017b) and co-PI (Farrell and Sherman, 2015) to ascertain the optimal method to represent the hydraulic characteristics of maёrl in shoreline change models. WP.5. Integrated process-response model & MFI - EDI models 5.1. The WPs 1-4 will be integrated using the conceptual morpho-dynamic model by Buscombe and Masselink (2006) (Figure 3) to il-lustrate the reciprocal relationships between the expected exchang-es on maёrl beaches. The feedback mechanisms between the hy-drodynamics, sediment transport, and morphological changes should not be treated independently. The results will be used to de-velop and test a Mobilization Frequency Index and Ecological Dis-turbance Index for maёrl (see next section).

Siddhi Joshi1, Eugene Farrell2  (1) Geography, Na onal University of Ireland Galway (siddhi.joshi@nuigalway.ie) 

(2) Geography, Na onal University of Ireland Galway (eugene.farrell@nuigalway.ie) 

Introduction & Research Objectives The purpose of this project is to model the drivers of short- and long-term variability in the behaviour of maёrl beaches and to determine the mobility of maёrl beds located in a shallow bay in the west of Ireland. Where dead maёrl (hereafter called debris) is washed up on beaches it is often mistaken for coral, as it has a superficially similar appearance. Maёrl debris beaches are some of the most unique, rarest, and least understood environ-ments in the world.

Primary Objectives:

A) Multi-disciplinary field experiment to measure the response of a maёrl beach to Atlantic storms.

B) Use results from (A) to develop and validate a morphody-namic model of the maёrl beach using SWAN and XBeach.

C) Develop and validate high resolution sediment mobility indi-ces for maёrl beds in Greatman’s Bay.

Motivation for Study Our scientific understanding of the behaviour of maёrl beaches and beds is very limited, at best (Figure 1). Furthermore, the current suite of morphodynamic models used to predict shore-line change have been developed and tested on beaches com-posed dominantly of lithogenic surface sediments that have very different mineral compositions, particle shapes and porosi-ty characteristics to calcareous maёrl deposits. It is unknown how these differences are manifested in the hydraulic proper-ties of the sediment and, subsequently, how they impact the dy-namics of maёrl debris beaches and offshore maёrl deposits. The primary research objectives are to develop, test, and validate the first ever process-response morphodynamic model for maёrl beaches and beds.

Study Site Where? Greatman’s Bay, Carraroe, Co. Galway (Figure 2)

Why? 1) Biogenic maёrl gravel beach is situated here

2) Site is very susceptible to Atlantic storms

Figure 2: Location of Study Site - Greatman’s Bay, Carraroe, County Galway with inset map.

Figure 3. Morphodynamic model to examine the morpho-sedimentary evolution of maёrl beaches over timescales of seconds to months using detailed measurements of nearshore hydrodynamics, morphological and sedimentary change and nearshore sediment transport (flowchart from Buscombe and Masselink, 2006).

Modelling: Mobility Indices Benthic habitats are affected by abiotic factors and the oceanography of the benthic boundary layer is of signifi-cance to the sediment mobility. Maёrl has been found to have a significantly higher mobility than quartz grains of equivalent sieve diameter. Based on previous flume measurements to determine the critical bed shear stress and the mobility number (Joshi et al. 2017b), results show that maёrl falls below the Shields curve (not shown) and with the exception of intertidal maёrl beds, also the mobility curve (Figure 4).

With the changing climate and projected increased stormi-ness, modelling tools provide a unique opportunity to model impacts of episodic natural disturbance events (storms) on maёrl as biogenic sediment. A key study by Brodie et al. (2014) on the impact of a high CO2 world on benthic flora suggests increased storminess is one of the factors which lead to a biogeographical shift in the range of maёrl beds. Joshi et al. (2017a) found that combined wave-current induced residual current dis-tribution during storm conditions and sediment mo-bility indices, such as the MFI, are the most signifi-cant hydrodynamic variables governing maёrl distri-bution (Figure 5). This part of the research will focus on determining: (1) the Mobilization Frequency Index with-in Greatman’s Bay using a nested modelling approach based on previous findings, and (2) extend the research to quantify the Ecological Disturbance Index (EDI) to generate spatial representations of physical patch-clearing ecological disturbance processes - a decision support tool for marine conservation managers and policy makers (Harris and Hughes, 2012).

Figure 5. Combined wave-current residual currents in Galway Bay 2013 storm peri-od, with maёrl polygons (white) (modified from Joshi et.al 2017a).

Impacts of research Integrating validated hydro– and morphodynamic models of maёrl mobility with maёrl distribution maps will allow us to develop a conceptual model of transport pathways of maёrl. This can provide insight into the expected re-sponse of maёrl beds and beaches to increased Atlantic storminess. The integration of the relevant physical pa-rameters controlling maёrl mobility and disturbance pat-terns will provide the platform to simulate climate change scenarios to determine if there are genuine concerns for maёrl conservation in Ireland and other regions.

Acknowledgements: Reference List: This research is funded by the Geological Survey Ireland Short Call 2017-SC-043. The views and recommendations con-tained in this study reflect the views of the authors and do not necessarily reflect the views and opinions of the Minister for Communications, Climate Action and Environment.

Brodie, J., Williamson, C.J., Smale, D.A., Kamenos, N.A., Mieszkowska, N., Santos, R., Cunliffe, M., Steinke, M., Yesson, C., Anderson, K.M., Asnaghi, V., Brownlee, C., Burdett, H.L., Burrows, M.T., Collins, S., Donohue, P.J.C., Harvey, B., Foggo, A., Noisette, F., Nunes, J., Ragazzola, F., Raven, J.A., Schmidt, D.N., Suggett, D., Teichberg, M., Hall-Spencer, J.M. (2014) The future of the northeast Atlantic benthic flora in a high CO2 world. Ecology and Evolution 4, 2787-2798.

Buscombe, D., and Masselink, G. (2006) Concepts in gravel beach dynamics. Earth-Science Reviews, 79 (1–2) (2006) 33-52.

Farrell, E.J., and Sherman, D.J. (2015) The fall velocity of sand grains in air: a review. Progress In Physical Geography, 1-27.

Harris, P.T., Hughes, M.G. (2012) Predicted benthic disturbance regimes on the Australian continental shelf: a modelling approach. Marine Ecology Progress Series 449, 13-25.

Joshi, S., Duffy, G., & Brown, C. (2014). Settling Velocity and Grain Shape of Maёrl Biogenic Gravel, Journal of Sedimentary Research, 84 (8), 718-727

Joshi, S., Duffy, G., & Brown, C. (2017a) Mobility of maёrl-siliciclastic mixtures: Impact of waves, currents and storm events, Estuarine, Coastal and Shelf Science, Volume 189, Pages 173-188.

Joshi, S., Duffy, G., & Brown, C. (2017b) Critical bed shear stress and threshold of motion of maёrl biogenic gravel, Estuarine, Coastal and Shelf Science, Volume 194, Pages 128-142.

Figure 4: Mobility curve comparing the mobility threshold of quartz with maёrl from three different environments (modified from Joshi et al. 2017b). A : rotating annular flume, B: Intertidal maёrl C: Beach debris D: Subtidal maёrl.

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