Critical bed shear stress of maerl experiment

Maerl Beach
Concentric patterns at Maerl Beach, Trá an Doilín in Carraroe, County Galway

Just by going to the beach, I had been fascinated by how maerl was freely moving, carried, mobilised and transported by almost every wave. The beach, composed almost entirely of “coral” is actually made of branched free-living coralline algal gravels (maerl). I was intrigued to see these concentric patterns, almost like “beach cusps,” observed at Trá an Doilín maerl beach in Carraroe, County Galway.  Furthermore, large maerl megaripples (or sub-aqueous dunes) had been observed subtidally, such as those in Northern Ireland (video). The flow strength required for initiation of motion is a classical problem in fluid dynamics and we found very little work had been done on maerl and the conditions under which it is mobilised and transported.



Our new study entitled “Critical bed shear stress and threshold of motion of maerl biogenic gravel” has just been published in Estuarine, Coastal and Shelf Science (in press). The critical bed shear stress is a fundamental sediment dynamics quantity – a measure of the threshold of motion of sediment. When we began our study on modelling the sediment mobility of maerl in Galway Bay, we found that this quantity for maerl coralline alga was an unknown which had largely been overlooked in classical sediment transport experiments. Its knowledge was a prerequisite for quantifying maerl mobility, rate of erosion and deposition in conservation management. Through as series of lab (flume) experiments on biogenic free-living maerl beds, our study determines the critical Shields parameter for maerl in three contrasting environments (open marine, intertidal and beach) in Galway Bay, west of Ireland.

The bed shear stress was determined using two methods, Law of the Wall and Turbulent Kinetic Energy, in a rotating annular flume and in a linear flume. The velocity profile of flowing water above a bed of natural maerl grains was measured in four runs of progressively increasing flow velocity until the flow exceeded the critical shear stress of grains on the bed (from Abstract, Joshi 2017b).

The critical Shields parameter and the mobility number are estimated and compared with the equivalent curves for natural quartz sand. The critical Shields parameters for the maerl particles from all three environments fall below the Shields curve. Along with a previously reported correlation between maerl grain shape and settling velocity, these results suggest that the highly irregular shapes also allow maerl grains to be mobilised more easily than quartz grains with the same sieve diameter (from Abstract, Joshi 2017b).

Live maerl thalli (Lithothamnion glaciale, Image credit to Jason Hall-Spencer, University of Plymouth)

The intertidal beds with the roughest particles exhibit the greatest critical shear stress because the particle thalli interlock and resist entrainment. In samples with a high percentage of maerl and low percentage of siliciclastic sand, the lower density, lower settling velocity and lower critical bed shear stress of maerl results in its preferential transport over the siliciclastic sediment. At velocities ∼10 cm s−1 higher than the threshold velocity of grain motion, rarely-documented subaqueous maerl dunes formed in the annular flume (from Abstract, Joshi 2017b).

The full research paper can be found here, as well as the related papers in the full study below.


Joshi, S., Duffy, G., & Brown, C. (2014). Settling Velocity and Grain Shape of Maerl Biogenic Gravel Journal of Sedimentary Research, 84 (8), 718-727 DOI:   (Paper 1)

Joshi, S., Duffy, G., & Brown, C. (2017a). Mobility of maerl-siliciclastic mixtures: Impact of waves, currents and storm events Estuarine, Coastal and Shelf Science DOI:    (Paper 3)

Joshi, S., Duffy, G., & Brown, C. (2017b), Critical bed shear stress and threshold of motion of maerl biogenic gravel, Estuarine, Coastal and Shelf Science,   (Paper 2)

Sediment Mobility of Maerl Modelling Study

Maerl biogenic gravel beach at Carraroe, County Galway

ResearchBlogging.orgOur new study on “Mobility of maerl-siliciclastic mixtures: impact of waves, currents and storm events,” has just been published (in press) in Estuarine, Coastal and Shelf Science. This is the final part of my PhD in maerl sediment dynamics. Sediment mobility in its simplest form is the percentage of time grains of a particular size are mobile during  a tidal cycle (Idier, 2010). This study focuses on the sediment mobility of maerl in particular, utilising coupled hydrodynamic-wave-sediment transport models to model the oceanography during calm and storm conditions and the resulting sediment transport. Sediment mobility models are another way of quantifying the disturbance of the seafloor as a result of currents, waves and combined wave-currents. This study calculates two sediment mobility indices, the Mobilization Frequency Index (MFI) and the Sediment Mobility Index (SMI), related to the magnitude and frequency of disturbance events (Li, 2015). The residual currents, which are the part of the current remaining after removing the oscillatory tidal component, show that maerl prefers intermediate mobility environments and is often found at the periphery of the residual current gyres. Sediment mobility maps can be used to inform marine spatial planning for the management of both live and dead (fossil) maerl beds, as a result of climate change or anthropogenic activity.

The full research paper, Joshi 2017, can be found here.



Idier, D., Romieu, E., Pedreros, R., & Oliveros, C. (2010). A simple method to analyse non-cohesive sediment mobility in coastal environment Continental Shelf Research, 30(3-4), 365-377 DOI: 10.1016/j.csr.2009.12.006

Joshi, S., Duffy, G., & Brown, C. (2017). Mobility of maerl-siliciclastic mixtures: Impact of waves, currents and storm events Estuarine, Coastal and Shelf Science DOI: 10.1016/j.ecss.2017.03.018

Li, M., Hannah, C., Perrie, W., Tang, C., Prescott, R., Greenberg, D., & Rygel, M. (2015). Modelling seabed shear stress, sediment mobility, and sediment transport in the Bay of Fundy Canadian Journal of Earth Sciences, 52 (9), 757-775 DOI:10.1139/cjes-2014-0211

GeoHab Conference, Winchester


This year the annual GeoHab international marine habitat mapping conference took place in Winchester, UK. This is an annual conference with over 160 people attending from 24 countries, for the first time in England! Organised by my former department of study at the National Oceanography Centre, it had been a long time since I had been back in the South of England. The Monday started with a workshop on Object Based Image Analysis (OBIA). Seabed classification methods can be based on classifying pixels, whereas these newer OBIA methods are based on classifying a group of similar pixels or “objects” on the seafloor. The conference began on the Tuesday with the key note speaker Dr. Larry Mayer of Center for Coastal & Ocean Mapping/Joint Hydrographic Center. The first session was on “Technological Advances in Habitat Mapping” with presentations on how new hydrographic surveying techniques can be used for habitat mapping. A poster session took place where one minute oral snapshots of posters were given. The following session on “Coastal and shallow water habitats” discussed environments such as tidal inlets, seagrass beds and mangroves and then “Shelf and deep-sea habitats” had rhodolith beds, shelf breaks, deep sea corals, submarine canyons, mud volcanoes and cold seeps. “National mapping programs” session then discussed important issues regarding the seabed mapping programs internationally and within the UK. The following session on “Anthropogenic and natural disturbance effects” then looked at man’s and nature’s impacts on benthic habitat and “Role of oceanography in habitat mapping” looked at the physical processes driving habitat distribution. This was for me one of the most interesting sessions. Following this, was very pleased to introduce a special lunchtime screening of my full one hour documentary- “Maerl:A Rare Seabed Habitat.” Being a firm believer in science communication, marine science documentaries can serve to educate, inform and transform the science and are useful tools for stakeholder management. It was great to be able to share our team’s work with scientists and educators internationally and even had one request to translate into Swedish! The final session was on “Development of standards for classification, confidence and assessment of habitat maps“- an important session to conclude on new methods to quantifying the uncertainty of the habitat map.  The conference concluded with thanking the organising committee and preparing for Geohab next year in Halifax, Canada. Overall, it was my first GeoHab in ten years and I was so inspired by the dedication of the GeoHab community – at home and abroad.

Admiral Nelson at the Naval Museum, Portsmouth Historic Dockyard

Book Review: “Ocean of Life” by Callum Roberts

“Ocean of Life – How our seas are changing” by Callum Roberts was shortlisted for the Royal Society Winton Prize for Science Books. In an insightful prologue, Callum Roberts, Professor of Marine Conservation, outlines his motivations for writing this book and how from his own personal experience, he has seen the seas changing in the past 30 years. Motivated by challenging problems and realising the need for a more multidisciplinary dialogue to take place between scientists, this book aims to take the reader through both the problems and the solutions to the changing seas of the 21st century. The book begins with a history of life on Earth and how through out geological time, the conditions necessary for life evolved. Throughout the first chapter we begin to realise the transient nature of life on Earth with respect to geological processes and time.

Discussing human origins and man’s relationship with the sea; for food and how fishing methods have evolved over time through to the present day.  A shocking but revealing study (Thurstan et. al. 2010) of the landings from bottom trawlers explains the steep decline in fish stocks especially of the larger fish. Then changes due to greenhouse gas emissions on the thermohaline circulation and the consequences of low oxygen zones on the life of the sea are discussed; climate change is already underway. The impacts of sea level rise; the human cost of climate change; ocean acidification to rivers of the world. Oil spills and the threat of pollution; PCBs to plastic pollution to underwater noise, “Mare Incognitum” or “unknown seas” of the future are inevitable, especially with exploitation. This book is a comprehensive and thought provoking summary of the current state of the oceans and in some ways serves to be a much needed warning for the future. But hope is not lost; the following part of the book deals with solutions for the great clean-up. A new deal for the ocean is proposed, solutions are possible for the next hundred years if we make the steer right now. It is also a book which drives one to change. For the ocean activist there are the appendices with a collection of conservation charities to protect the oceans. A persuasive read!

Further Links

Thurstan, R.H., Brockington, S. and Roberts, C.M (2010). The effects of 118 years of industrial fishing on UK bottom trawl fisheries. Nature Communications 1 (15): 1-6.

Lecture by Callum Roberts in English with German

Review by the Guardian


Book Review: “Mapping the Deep” by Robert Kunzig

“Mapping the Deep” is a book which draws you in to the realm of the deep sea – far from shore. Imagine looking out of the window into a sky filled with the sea, we are taken into the world of the abyss. The author describes the book as a portrait of our understanding of the sea- with its distinct focus on the distant and the unfamiliar making it such a pleasure to discover. Laurence Madin of WHOI describes this popular science book as “The best account of discovery in oceanography I’ve ever read” and the chapters which follow are just as inspiring to read.

mappingthedeep Space and the Ocean narrates the meeting of oxygen to hydrogen and the story of Albert Cheung, the graduate student who first observed water in space. The birth of the solar system and the possible role comets played in bringing water and organic matter onto the Earth are discussed to explain the origins the earth and its ocean basins. Then we start of on a wonderful narrative about the history of early bathymetric mapping and charting in Sounding the depths – from Charles Bonneycastle to the Challenger Expeditions to Atlantis. Introducing marine geologists Marie Tharp and Bruce Heezen’s discovery of the V-shaped notch at the top of The Rift in the Atlantic and Hess’s concept of seafloor spreading; the chapter leads to a paradigm shift in earth science to the theory of plate tectonics. Their first bathymetric map of the ocean and its geological features are described in A Map of the World. Turbidity currents and their extraordinary impact on the abyss and the early remote sensing satellites prior to the advent of sonar lead to  Sandwell and Smith’s famous map of the oceans.The Seafloor at Birth goes on to discuss the history of multibeam sonar and the mapping of the US Exclusive Economic Zone (EEZ) and the quest to understand the birth of the seafloor. Continue reading “Book Review: “Mapping the Deep” by Robert Kunzig”

Maerl Documentary – A short trailer


Finally I can bring to you the trailer for the maerl documentary! This trailer gives you a small taster of the final hour long documentary film. As a PhD student studying maerl I encountered many researchers with diverse and in-depth knowledge about maerl beds in Ireland and worldwide and felt quite compelled to make this documentary. It includes interviews about marine botany, zoology, ecology, geology and marine geophysics, as well as the threat of anthropogenic impacts on maerl, climate change and possible solutions. Having been busy editing to sew together nine interviews, breathtaking scenery and diving footage. I am now consulting with my team and friends for suggestions of how to improve the near-final cut. Please tell your friends about this film and we hope it will help the next generation of scientists, educators and policy makers to conserve, protect and manage this vulnerable benthic habitat.