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 et.al 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 et.al 2017b).

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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 et.al 2017b).

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

References

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: https://doi.org/10.2110/jsr.2014.51   (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: https://doi.org/10.1016/j.ecss.2017.03.018    (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, https://doi.org/10.1016/j.ecss.2017.06.010   (Paper 2)

Sediment Mobility of Maerl Modelling Study

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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 et.al., 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 et.al, 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 et.al. 2017, can be found here.

IMG_2884

References

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

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.

Settling Velocity and Grain Shape of Maerl

ResearchBlogging.orgOur recent study on maerl sediment dynamics has found that the settling velocity of maerl is primarily governed by the grain shape properties of maerl. A grain shape parameter known as the convexity has been linked to the settling velocity via the Ferguson and Church model (Ferguson and Church, 2004). Due to the grain shape of maerl and roughness, it experiences a greater drag than the natural quartz grain. Detailed measurements of maërl grain shape using microscopic image analysis confirm this link.

Maërl tends to form beach deposits with a low percentage of sand and it is hypothesised that the lower settling velocity of maerl results in this preferential transport of biogenic maerl sediments compared to quartz sands and gravels. Maërl samples found in open marine, intertidal, and beach environments show a different linear relationship between roughness and grain size, due to different degrees of abrasion. A combination of different wave climates and transport histories result in this increased spatial variability of grain textures.

The paper and study then goes on to discuss to what extent a general equation for maërl settling velocity is possible or not and to whether the sediment mobility of maerl can be predicted using the settling velocity as an input parameter.

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The apparatus used to determine the settling velocity of maerl
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Microscopic image analysis of the maerl grain
Lithophyllum fasciculatum
Maerl grains were found to be more convex, with a high grain roughness
IMG_2884
Carraroe Maerl Beach in County Galway shows a higher maerl to sand ratio, with a high percentage maerl- an occurance explained here to be due to the lower settling velocity of maerl.

Ferguson, R., & Church, M. (2004). A Simple Universal Equation for Grain Settling Velocity Journal of Sedimentary Research, 74 (6), 933-937 DOI: 10.1306/051204740933

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: 10.2110/jsr.2014.51

International Rhodolith Workshop

The IV International Rhodolith Workshop took place in Granada, Spain in September. Meeting every three years, delegates were from Brazil, Spain, United Kingdom, USA, Ireland, Mexico, New Zealand and Australia, as well as other countries. Rhodolith is a term largely used interchangeably to “maerl,” as free living non-geniculate coralline algae. Researchers came to share the latest research about one of the big four macrophyte dominated benthic communities (others being kelp beds, seagrass meadows and biogenic reefs) (Foster, 2001). Topics included taxonomy, ecology,  management and conservation biology, genetics, geochemistry, evolution, palaeoecology, climate change studies and sediment dynamics.

Two excursion took place; the Granada coast to look at living rhodoliths and a two-day excursion to Almería-Cabo de Gata to observe both fossil and living rhodolith beds. The first excursion involved diving off the Granada coast or shorkelling to explore the small sea-caves along the coast. The second excursion involved exploring the processes responsible for deposition of rhodolith debris as cliff-deposits and how they have been preserved across geological time.

Further information can be found on the conference website. My poster presented to the conference can be found on the Griffith NUIG Biogeosciences website

References

Foster M, 2001, Rhodoliths: Between rocks and soft places, Journal of Phycology, Vol 37 Issue 5, 659-667