Critical bed shear stress and threshold of motion of maërl biogenic gravel

A determination of the critical bed shear stress of maerl is a prerequisite for quantifying its mobility, rate of erosion and deposition in conservation management. The critical bed shear stress for incipient motion has been determined for the first time for samples from biogenic free-living maerl beds 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. 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. 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.

确定藻砂（maerl）的临界床面剪切应力（critical bed shear stress），是量化其在保护管理中的迁移能力、侵蚀与沉积速率的前提条件。本研究首次针对爱尔兰西部戈尔韦湾三类差异显著的生境（开阔海洋、潮间带与海滩）中的生源自由生长藻砂床样本，完成了起动临界床面剪切应力的测定。床面剪切应力的测定采用壁面定律（Law of the Wall）与湍流动能（Turbulent Kinetic Energy）两种方法，实验分别在旋转环形水槽与直线水槽中开展。研究对天然藻砂颗粒床上方的水流流速剖面进行了四次逐级递增流速的测量，直至水流超过床面颗粒的临界剪切应力。本研究估算了临界希尔德斯参数（critical Shields parameter）与迁移数（mobility number），并将其与天然石英砂的对应曲线进行对比。三类生境中藻砂颗粒的临界希尔德斯参数均低于希尔德斯曲线；结合此前已报道的藻砂颗粒形状与沉降速度的相关性，本研究结果表明，相较于具有相同筛孔直径（sieve diameter）的石英颗粒，形态高度不规则的藻砂颗粒更易被启动迁移。颗粒最为粗糙的潮间带藻砂床表现出最高的临界床面剪切应力，这是因为颗粒的藻体（thalli）相互咬合，可抵抗起动夹带（entrainment）。在藻砂占比高、硅质碎屑砂（siliciclastic sand）占比低的样本中，藻砂较低的密度、沉降速度与临界床面剪切应力，使其相较于硅质碎屑沉积物更易被优先输运。在较颗粒起动临界流速（threshold velocity）高出约10 cm·s⁻¹的水流条件下，旋转环形水槽中形成了鲜有文献记载的水下藻砂沙丘（subaqueous maerl dunes）。