Tag: sediment dynamics

Science, What's New

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).

cropped-litgla.jpg
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)

Science

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
20121128_155357
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

Welcome

Porosity- the Philosophy of Life

Oranges

A philosophy professor stood before his class with items on the table in front of him. When the class began, he picked up a very large and empty jar and proceeded to fill it with rocks. He then asked the students if the jar was full. They agreed that it was.

So the professor then picked up some pebbles and poured them into the jar. He shook the jar lightly. The pebbles of course rolled in to the open areas between the rocks. He then asked the students again if the jar was full. They agreed it was.

The professor picked up some sand and poured it into the jar. Of course, the sand filled up everything else. He then asked once more if the jar was full. The students responded with a unanimous “yes.”

The professor then produced two cans of beer from under the table and proceeded to pour their entire contents into the jar- effectively filling the empty space between the sand. The students laughed. “Now,” said the professor as the laughter subsided, “I want you to recognise that this jar represents your life. The rocks are the important things; Your life, Your health, Your family, things that if everything else was lost and only they remained, your life would still be full. The pebbles are the other things that matter- like your job, your home, your career. The sand is everything else- the small stuff which seems important but is not really that necessary. If you put the sand into the jar first, there is no room for the pebbles or the rocks. The same goes for your life. If you spend all your time on the small stuff, you will never have room for the things that are important to you. Pay attention to things that are critical to your happiness. But if you put the rocks in first; the things that really matter in your life, than your life will be a lot more fulfilling and happier. Set your priorities; the rest is just sand.”

One of the students raised their hand and enquired what the beer represented? The professor smiled, “I am glad you asked. It goes to show, no matter how full your life may seem, there is always room for a couple of beers.”

(Author unknown)

Porosity: The science

Porosity (p) in sediment dynamics is the ratio of the volume of voids, to the total volume. For non-cohesive sediments such as sand, p= 0.3 to 0.4. Well-graded and poorly sorted sediments have a large standard deviation of the mean, with p= 0.3. Poorly graded and well sorted sediments have a small standard deviation of the mean with large porosity, with p=0.4. Coastal engineers often take p=0.35 when the actual value is unknown.

This is a story I heard from David Basco, Old Dominion University, Virginia, USA, during his lectures in Coastal Hydrodynamics and Sediment Transport. In Plymouth, I had the opportunity to meet him and his wife in person, and share with him this photograph of our outreach activity at the National University of Ireland, Galway, a jar full of oranges, marbles and maerl.

What's New

Griffith Research Jam Talk

maerl

The Griffith Geoscience Programme is administered by the Geological Survey of Ireland (GSI) as part of Ireland’s National Geoscience Strategy and was established by the Department of Communications, Energy and Natural Resources in 2007. The scheme honours the memory of Richard Griffith (1784-1878), the celebrated geologist and engineer. The project uses data obtained as part of the INFOMAR programme, with the use of Ireland’s National research vessels, the Celtic Explorer and the Celtic Voyager. Galway Bay is one of INFOMAR’s 26 selected priority bays, as the INFOMAR project now focuses on mapping Ireland’s shallow water regions.

As part of my PhD research, last week I was invited to speak at the Geological Survey of Ireland in Dublin for the “Griffith Research Jam.” This was an event where researchers and postgraduates funded by the Griffith Geoscience Research Award had the opportunity to present their research and share findings. Researchers came from all four provinces around Ireland including from National University of Ireland, Galway, Queens University Belfast, University College Dublin, University College Cork, Trinity College Dublin and some industry representatives. Here is my presentation entitled “Sediment Mobility Modelling and Benthic Disturbance of Maerl Habitats,” (Siddhi Joshi, Garret Duffy et. al. 2012) The web version of the slides can be accessed below.

 

References

Siddhi Joshi, Garret Duffy, Martin White and Colin Brown, 2012, Sediment Mobility Modelling and Benthic Disturbance of Maerl Habitats, Oral Presentation, Griffith Research Jam, Geological Survey of Ireland, Dublin.