Bed load transport on the shoreface by currents and waves

Kleinhans, Maarten G.; Grasmeijer, B.T.

doi:10.1016/j.coastaleng.2006.06.009

Cited by 36 publications

(25 citation statements)

References 26 publications

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“…For consistency with the methods of numerical models, the amplitude of near‐bed orbital motion

u_{w}

was calculated according to linear theory with

π H_{s} false/ T_{m - 10} \sinh false(k h false)

, in which

k

was approximated using the method of Guo () with

T_{m - 10}

and

h

. Wave‐ and current‐related Shields parameters were calculated according to Kleinhans and Grasmeijer ():

θ_{x} = \frac{τ_{x}}{false(ρ_{s} - ρ_{w} false) g D 50}

…”

Section: Methodsmentioning

confidence: 99%

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Brakenhoff

Kleinhans

Ruessink

et al. 2020

Earth Surf Processes Landf

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Ebb‐tidal deltas are highly dynamic environments affected by both waves and currents that approach the coast under various angles. Among other bedforms of various scales, these hydrodynamics create small‐scale bedforms (ripples), which increase the bed roughness and will therefore affect hydrodynamics and sediment transport. In morphodynamic models, sediment transport predictions depend on the roughness height, but the accuracy of these predictors has not been tested for field conditions with strongly mixed (wave–current dominated) forcing. In this study, small‐scale bedforms were observed in the field with a 3D Profiling Sonar at five locations on the Ameland ebb‐tidal delta, the Netherlands. Hydrodynamic conditions ranged from wave dominated to current dominated, but were mixed most of the time. Small‐scale ripples were found on all studied parts of the delta, superimposed on megaripples. Even though a large range of hydrodynamic conditions was encountered, the spatio‐temporal variations in small‐scale ripple dimensions were relatively small (height η≈ 0.015 m, length λ≈ 0.11 m). Also, the ripples were always highly three‐dimensional. These small dimensions are probably caused by the fact that the bed consists of relatively fine sediment. Five bedform height predictors were tested, but they all overestimated the ripple heights, partly because they were not created for small grain sizes. Furthermore, the predictors all have a strong dependence on wave‐ and current‐related velocities, whereas the ripple heights measured here were only related to the near‐bed orbital velocity. Therefore, ripple heights and lengths in wave–current‐dominated, fine‐grained coastal areas ( D50<0.25 mm) may be best estimated by constant values rather than values dependent on the hydrodynamics. In the case of the Ameland ebb‐tidal delta, these values were found to be η=0.015 m and λ=0.11 m. ©2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

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“…For consistency with the methods of numerical models, the amplitude of near‐bed orbital motion

u_{w}

was calculated according to linear theory with

π H_{s} false/ T_{m - 10} \sinh false(k h false)

, in which

k

was approximated using the method of Guo () with

T_{m - 10}

and

h

. Wave‐ and current‐related Shields parameters were calculated according to Kleinhans and Grasmeijer ():

θ_{x} = \frac{τ_{x}}{false(ρ_{s} - ρ_{w} false) g D 50}

…”

Section: Methodsmentioning

confidence: 99%

Spatio‐temporal characteristics of small‐scale wave–current ripples on the Ameland ebb‐tidal delta

Brakenhoff

Kleinhans

Ruessink

et al. 2020

Earth Surf Processes Landf

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“…[42], [43]):where ρ is the water density, f2.5 is the grain roughness friction factor calculated as 2.5d50. The wave height related orbital velocity at the bed (U w ) was estimated using [44]:in which g is the gravitational acceleration, and h is the mean water depth.…”

Section: Methodsmentioning

confidence: 99%

Low-Canopy Seagrass Beds Still Provide Important Coastal Protection Services

et al. 2013

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One of the most frequently quoted ecosystem services of seagrass meadows is their value for coastal protection. Many studies emphasize the role of above-ground shoots in attenuating waves, enhancing sedimentation and preventing erosion. This raises the question if short-leaved, low density (grazed) seagrass meadows with most of their biomass in belowground tissues can also stabilize sediments. We examined this by combining manipulative field experiments and wave measurements along a typical tropical reef flat where green turtles intensively graze upon the seagrass canopy. We experimentally manipulated wave energy and grazing intensity along a transect perpendicular to the beach, and compared sediment bed level change between vegetated and experimentally created bare plots at three distances from the beach. Our experiments showed that i) even the short-leaved, low-biomass and heavily-grazed seagrass vegetation reduced wave-induced sediment erosion up to threefold, and ii) that erosion was a function of location along the vegetated reef flat. Where other studies stress the importance of the seagrass canopy for shoreline protection, our study on open, low-biomass and heavily grazed seagrass beds strongly suggests that belowground biomass also has a major effect on the immobilization of sediment. These results imply that, compared to shallow unvegetated nearshore reef flats, the presence of a short, low-biomass seagrass meadow maintains a higher bed level, attenuating waves before reaching the beach and hence lowering beach erosion rates. We propose that the sole use of aboveground biomass as a proxy for valuing coastal protection services should be reconsidered.

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“…These definitions, which strongly emphasize the importance of waves and wave-induced fluid processes on shoreface dynamics, illustrate how the shoreface is commonly considered as a strictly wave-dominated environment. Although waves generally play a major role on shoreface sediment dynamics (Niedoroda et al, 1984;Vincent et al, 1991Vincent et al, , 1998Cowell et al, 1999;Green and Black, 1999;Ruessink et al, 1999;Hanes et al, 2001), the existence of fluid processes driven by forcing mechanisms other than waves has been recognized in a number of studies, including tidal currents (Vincent et al, 1998;Kleinhans and Grasmeijer, 2006), wind-forced compensatory flows such as downwelling return currents Wright et al, Marine Geology 249 (2008) 226 -242 www.elsevier.com/locate/margeo 1991; Héquette and Hill, 1993;Xu and Wright, 1998;Héquette et al, 2001), alongshore coastal jets generated during storm surges (Niedoroda and Swift, 1981;Swift et al, 1985) and mega-rip currents that may exceptionally extend far beyond the surf zone (Short, 1985), not to mention possible densitydriven currents associated with riverine freshwater outflows (De Ruijter et al, 1997). Therefore, the shoreface should better be considered as a shallow friction-dominated zone where the response of the seabed to hydro-metorological forcings results from the complex interactions between wind, waves and currents, the respective importance of the different forcing mechanisms depending on the local to regional wind, wave and tidal regimes.…”

Section: Introductionmentioning

confidence: 99%