Current- and Wave-Generated Bedforms on Mixed Sand–Clay Intertidal Flats: A New Bedform Phase Diagram and Implications for Bed Roughness and Preservation Potential

Discontinuity in Equilibrium Wave‐Current Ripple Size and Shape and Deep Cleaning Associated With Cohesive Sand‐Clay Beds

“…( 2018 ) and Baas et al. ( 2021 ), D 50 = 0.227 mm, 0 < h ≤ 3.5 m, 0 < U c ≤ 0.6 ms −1 , 0 < U o ≤ 0.45 ms −1 , and 0 < C 0 ≤ 14%. During a storm event (tidal inundations two to six of Lichtman et al.…”

“…( 2018 ) and Baas et al. ( 2021 )), the peak τ max , calculated in the same way as above, varied between 0.55 and 1.69 Pa and the clean‐sand Shields parameter threshold, θ 0 , according to Soulsby ( 1997 ) was 0.05 or 0.15 ≤ θ max ≤ 0.47 and 3 ≤ θ max / θ 0 ≤ 9. For the present experiments, the values of h , U c , U o , and C 0 were all within these ranges (Table 1 ), and since θ max = 0.19, θ 0 = 0.03, and θ max / θ 0 ≈ 7, the shear stress was also within the field range.…”

“…The nondimensional Shields parameter is defined by θ = τ/(ρ s -ρ)gD 50 , where ρ s is the sediment density (=2650 kg/m 3 ), ρ is the water density (=1,000 kg/m 3 for freshwater and 1,027 kg/m 3 for saltwater), and g is the acceleration due to gravity (=9.81 m/s 2 ). During the intertidal Dee Estuary (UK) field deployment of Lichtman et al (2018) and Baas et al (2021), D 50 = 0.227 mm, 0 < h ≤ 3.5 m, 0 < U c ≤ 0.6 ms −1 , 0 < U o ≤ 0.45 ms −1 , and 0 < C 0 ≤ 14%. During a storm event (tidal inundations two to six of Lichtman et al (2018) and Baas et al (2021)), the peak τ max , calculated in the same way as above, varied between 0.55 and 1.69 Pa and the clean-sand Shields parameter threshold, θ 0 , according to Soulsby (1997) was 0.05 or 0.15 ≤ θ max ≤ 0.47 and 3 ≤ θ max /θ 0 ≤ 9.…”

“…However, both thresholds will be affected by the additional presence of EPS-induced biological cohesion in sediment in the field (Baas et al, 2019), which has a stronger capacity to resist erosion compared to physical cohesion; small proportions of EPS, of the order of 0.1%, are highly effective in hindering bedform evolution (Malarkey et al, 2015;Parsons et al, 2016). Indeed, Baas et al (2021) observed a reduction in current ripple height on an intertidal flat during neap tides from c. 20 to c. 10 mm for relatively low clay content between 2% and 5% combined with EPS content between 0.05% and 0.08%. Nonetheless, the findings in this paper should act as a stimulus for further studies toward fully quantifying changes in ripple morphology as a function of hydrodynamic forcing and bed cohesion.…”

“…During the intertidal Dee Estuary (UK) field deployment of Lichtman et al (2018) and Baas et al (2021), D 50 = 0.227 mm, 0 < h ≤ 3.5 m, 0 < U c ≤ 0.6 ms −1 , 0 < U o ≤ 0.45 ms −1 , and 0 < C 0 ≤ 14%. During a storm event (tidal inundations two to six of Lichtman et al (2018) and Baas et al (2021)), the peak τ max , calculated in the same way as above, varied between 0.55 and 1.69 Pa and the clean-sand Shields parameter threshold, θ 0 , according to Soulsby (1997) was 0.05 or 0.15 ≤ θ max ≤ 0.47 and 3 ≤ θ max /θ 0 ≤ 9. For the present experiments, the values of h, U c , U o , and C 0 were all within these ranges (Table 1), and since θ max = 0.19, θ 0 = 0.03, and θ max /θ 0 ≈ 7, the shear stress was also within the field range.…”