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)