Hydraulic conductivity and experiments of sediment beds

In marine environments, sediments from different sources are stirred and dispersed, generating beds that are composed of mixed and layered sediments of differing grain sizes. Traditional engineering formulations used to predict erosion thresholds are however, generally for unimodal sediment distributions, and so may be inadequate for commonly occurring coastal sediments. We tested the transport behavior of deposited and mixed sediment beds consisting of a simplified two-grain fraction (silt (D50 = 55 µm) and sand (D50 = 300 µm)) in a laboratory-based annular flume with the objective of investigating the parameters controlling the stability of a sediment bed. To mimic recent deposition of particles following large storm events and the longer-term result of the incorporation of fines in coarse sediment, we designed two suites of experiments: (1) "the layering experiment": in which a sandy bed was covered by a thin layer of silt of varying thickness (0.2 - 3 mm; 0.5 - 3.7 wt %, dry weight in a layer 10 cm deep); and (2) "the mixing experiment" where the bed was composed of sand homogeneously mixed with small amounts of silt (0.07 - 0.7 wt %, dry weight). To initiate erosion and to detect a possible stabilizing effect in both settings, we increased the flow speeds in increments up to 0.30 m/s. Results showed that the sediment bed (or the underlying sand bed in the case of the layering experiment) stabilized with increasing silt composition. The increasing sediment stability was defined by a shift of the initial threshold conditions towards higher flow speeds, combined with, in the case of the mixed bed, decreasing erosion rates. Our results show that even extremely low concentrations of silt play a stabilizing role (1.4% silt (wt %) on a layered sediment bed of 10 cm thickness). In the case of a mixed sediment bed, 0.18% silt (wt %, in a sample of 10 cm depth) stabilized the bed. Both cases show that the depositional history of the sediment fractions can change the erosion characteristics of the seabed. These observations are summarized in a conceptual model that suggests that, in addition to the effect on surface roughness, silt stabilizes the sand bed by pore-space plugging and reducing the inflow in the bed, and hence increases the bed stability. Measurements of hydraulic conductivity on similar bed assemblages qualitatively supported this conclusion by showing that silt could decrease the permeability by up to 22% in the case of a layered bed and by up to 70% in the case of a mixed bed.

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

Critical bed shear stress for incipient motion has been determined for biogenic free-living coralline algae known as maërl. Maërl from three different sedimentary environments (beach, intertidal, and open marine) in Galway Bay, west of Ireland have been analysed in a rotating annular flume and linear flume. Velocity profile measurements of the benthic boundary layer, using an Acoustic Doppler Velocimeter, have been obtained in four different velocity experiments. The bed shear stress has been determined using three methods: Law of the Wall, Turbulent Kinetic Energy and Reynolds Stress. The critical Shields parameter has been estimated as a non-dimensional mobility number and the results have been compared with the Shields curve for natural sand. Maërl particles fall below this curve because its greater angularity allows grains to be mobilised easier than hydraulically equivalent particles. From previous work, the relationship between grain shape and the settling velocity of maërl suggests that the roughness is greatest for intertidal maërl particles. During critical shear stress determinations, beds of such rough particles exhibited the greatest critical shear stress probably because the particle thalli interlocked and resisted entrainment. The Turbulent Kinetic Energy methodology gives the most consistent results, agreeing with previous comparative studies. Rarely-documented maërl megaripples were observed in the rotating annular flume and are hypothesised to form at velocities ~10 cm s-1 higher than the critical threshold velocity, where tidal currents, oscillatory flow or combined-wave current interaction results in the preferential transport of maërl. A determination of the critical bed shear stress of maërl allows its mobility and rate of erosion and deposition to be evaluated spatially in subsequent applications to biological conservation management.