Sheet flow and large wave ripples under combined waves and currents: field observations, model predictions and effects on boundary layer dynamics
Abstract
Large wave ripples and upper-plane bed sheet flow were observed under combined waves and currents during storms at 39 m water depth on Scotian Shelf. These data show that the critical wave mobility number and Shields parameter for sheet flow are not constant, but rather their values decrease with an increase in grain size. The critical sheet-flow Shields parameter under combined flows is found to be about 50% smaller than that for pure waves. Though the bed shear stress based on bedload roughness or wave parameterization using the one-tenth largest waves both offer adequate resolution to this difference, a definitive explanation does not exist at present. The large wave ripples (LWR) observed under combined flows are generally symmetrical, rounded and 3-dimensional, with small current ripples superimposed. The wavelengths of these LWR are approximately half of the wave orbital diameter and their average steepness is about 0.1. Analyses of storm processes on continental shelves show that as a storm builds up, ripples generally change directly into upper-plane beds. LWR generally form following the peaks of storms due to sediment fall out from suspension and moulding of the seabed by the long wave oscillation. If the spin up of a storm is gradual and strongly wave dominant, however, LWR can also develop from small ripples before sheet-flow conditions are reached. We interpret the three-dimensional, mound-like, low-relief LWR as combined-flow hummocky megaripples. This suggests that hummocky megaripples and hummocky cross-stratification (HCS) on continental shelves are formed under wave-dominant combined flows, and that they can form in medium sand as well as in silt and fine sand. The application of the combined-flow boundary layer model of Grant and Madsen (1986) and a modified Rouse suspension equation shows that the incorrect prediction of the presence of ripples in place of upper-plane bed causes over-estimation of shear velocities and suspension concentration and a 300% over-prediction of the suspended sediment transport rate. By contrast, failing to predict correctly the formation of LWR causes an under-estimation of shear velocities and sand resuspension, and will result in under-prediction of the suspended sediment transport rate by nearly one order of magnitude.
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Intensity and frequency of seabed shear stress and sediment mobilization on the Canadian Atlantic Shelf — A modelling study
2024, Continental Shelf ResearchOcean surface waves and currents can interact to produce strong seabed shear stress and sediments mobilization affecting infrastructure safety, benthic habitat distribution and decision for sustainable development of the oceans. Modelled waves, tidal current and circulation current data for a 3-year period were used in a combined-flow sediment transport model to simulate the seabed shear stresses and the mobilization of observed sediments on the Canadian Atlantic Shelf. The modelling results are presented and analyzed to update the framework of seabed disturbance and sediment mobility on the Atlantic Shelf. The Atlantic Shelf is affected by strong waves and tidal currents. Maximum mean significant wave height over the modelled period can reach 3.5 m and mean tidal currents reach up to 1.2 m s−1. Our modeling results indicate that the mean wave and tidal current shear velocities both reach the maximum values of ∼4 cm s−1. Observed sediments on the Atlantic Shelf can be mobilized by tidal currents at least once during the modelled 3-year period over 30 % of the shelf area while storms can mobilize sediments over 35 % of the shelf area suggesting a slightly stronger sediment mobilization by storms. Furthermore, waves and currents interact to cause enhanced combined wave-current shear velocities >5 cm s−1 that is capable of mobilizing sediments over 63 % of the shelf area, double that due to tides or waves alone. The spatial variation of the relative importance of waves, tidal current and circulation current in mobilizing sediments was used to classify the Atlantic Shelf into six disturbance types. Wave-dominant and tide-dominant disturbance types are equally important and both occupy ∼25 % of the shelf area. Mixed disturbance is insignificant and accounts for only 3 % of the shelf area. Universal Seabed Disturbance Index (SDI) and Sediment Mobility Index (SMI) were applied to better quantify the exposure of the seabed to oceanographic processes and sediment mobilization, incorporating both the magnitude and frequency of these processes. The values of SDI and SMI on the Canadian Atlantic Shelf are found to be comparable to those on the Australian shelves and the Irish Sea shelf. These indices, together with the seabed disturbance type classification scheme, potentially can be used as standard parameters to quantify seabed disturbance and sediment mobility on other shelves of the world.
Provenance and sedimentary processes on Pleistocene storm deposits in Harhoura (Northern Coastal Atlantic, Morocco): New constraints from a source to sink perspective
2023, Marine GeologyThe present work studies the Harhoura coastal deposits located immediately southwest of Rabat, Morocco. The base of this sequence has been previously dated by 14C as 9900 ± 150 BP. These deposits exhibit a thick record potentially related to the footprint of a high-energy storm. They are primarily characterized by coarse-grained dark shelly/gravelly units bearing highly fragmented shells inherited from shallow water platform. A high amount of organic matter was also observed.
The provenance signature using modal analysis of detrital grains, and major and trace elements indicates two main phases. The first sedimentation is a marine within the shallow platform, followed by a coastal phase belonging to the littoral zone.
The first source to sink pathway indicate that platform was supplied by sediments from felsic magmatic source, low-grade and medium metamorphic rocks belonging to the fold and thrust belt of the Sehoul Block, with a minor contribution from mafic source rocks. The second pathway indicates their subsequent remobilization and deposition on the calcarenite coastal plain by mean of high-energy storm(s). The provenance relationship of the latter phase was corroborated by the geochemical signature using major, trace and rare earth elements, assigning their latest source to a coastal-marine origin.
Evolution of an estuarine valley to barrier-lagoon system promoted by tidal prism change during a transgressive event (Lower Permian, Paraná Basin)
2021, Journal of South American Earth SciencesIn modern marine marginal environments, the coastal morphology directly influences the tidal and wave processes. In this paper we present an example of an ancient coastal zone where the change in coastal morphology controlled the tidal range. This study focuses on the Permian Rio Bonito Formation in the southern Paraná Basin, where the major coal deposits of South America are located. Sedimentological and stratigraphical approaches were applied to cores drilled in the Candiota paleovalley area. We describe the stratigraphic architecture of a transgressive event of the Lower Permian (Rio Bonito Formation). Two third order depositional sequences were identified, which show differences in the paleoenvironmental record. In the lower sequence, associated with the infilling of an incised valley, facies related to fluvial and tidal currents are dominant. In a micro-tidal inner sea context, tidal range is amplified in a funnel-shaped valley. In the upper sequence, already evolving in linear coastal conditions, evidence of tidal depositional processes almost disappear, and the sedimentation becomes dominated by wave processes, thus a coastal barrier system associated with lagoons and swamps is established. Our study, besides detailing the paleoenvironmental evolution of the sedimentary infilling of an incised valley, also provides for a better understanding of the origin of southern Brazil coal.
Dynamics of ripples superimposed on a sand ridge on a tideless shoreface
2020, Estuarine, Coastal and Shelf ScienceThe presence of ripples superimposed on an active sand ridge is the most usual situation under low-to moderate-energy conditions in the wave-dominated, tideless Ebro Delta coast. Four types of small-scale bedforms were identified, from the critical threshold of sediment motion to wash-out conditions: (i) small undulations with η < 0.5 cm and λ ~8 cm, formed as a precursor of wave ripples when the Shields parameter was just below the theoretical critical; (ii) 2D wave ripples with η = 1.2 cm and λ = 7–10 cm; (iii) mixed 2D/3D current-dominated ripples with a maximum of η ~1.5 cm and λ = 10–15 cm; and (iv) 3D wave-current ripples with a maximum of η ~2.2 cm and λ = 7–20 cm. Ripple degradation was observed to occur when the energy of the hydrodynamic regime increased (wash-out conditions) and under low-energy hydrodynamic conditions, when ripples progressively decayed mainly as a consequence of biological activity. The wave ripples were static, while the current-dominated ripples migrated towards the SE at a rate of ~10 cm/h.
The potential role of ripple migration as an additional long-term mechanism of sediment transport is addressed. On the Ebro Delta shoreface, 3D ripple migration follows the direction of sand ridge migration towards the SE. The respective migration rates and their differences in size support the hypothesis that a subordinate part of sand ridge migration can result from the contribution of ripple migration under low to moderate regimes, suggesting that in specific environments the dynamics of small-scale bedforms can play a subordinate but not negligible role in the evolution of larger bedforms.
Field investigation of bedform morphodynamics under combined flow
2019, GeomorphologyFieldwork was conducted at the Red Cliff sandbar located in the upper Humber Estuary in order to investigate bedform dynamics under different hydrodynamic conditions related to combinations in tidal and wave-generated currents. A fixed mooring was deployed to obtain current flow and wave properties across the sand bar site at both spring and neap tidal flows and during conditions of high wind generated waves. A terrestrial laser scanner was used to scan the sandbar during low water across the various forcing conditions, acquiring detailed information of ripple geometries. Under spring tide and calm wave conditions, two-dimensional asymmetrical ripples with straight crest lines were generated on the sandbar. Surveys were also conducted during strong winds. Prior to high-wind-waves, two-dimensional symmetrical and washed-out ripples were observed, during spring and neap tidal conditions respectively. The lengths of these ripples were almost the same as those generated under current only conditions, but their heights were relatively smaller. After the strong wind periods, 2D current-induced ripples were replaced by flatbed conditions and 2D symmetrical wash-out ripples, which indicates wave-induced bed shear stress is enhanced by the presence of a tidal current. Most pertinently our study reveals discrepancies between field observations and existing predictions of bed configurations that are largely based on laboratory investigations. This is likely due to a larger grain size distribution in field conditions and due to turbulence dampening by high concentrations of suspended clay particles. The study thus highlights a need to extend a range of field investigations that explore the current deficiencies in our abilities to predict bedforms and bedform dynamics in estuarine systems.
Wave-current interactions in the continental shelf bottom boundary layer of the Australian North West Shelf during tropical cyclone conditions
2018, Continental Shelf ResearchCitation Excerpt :Other studies have described more complicated three-dimensional ripple patterns due to combined wave- and current-generated ripples (e.g. Marten, 2010) or due to rapid changes in the wave conditions (e.g. Li and Amos, 1998; Traykovski et al., 1999). Li and Amos (1999) observed bed form dynamics under combined wave-current conditions during storms (Hs,max = 4 m) on the Scotian Shelf, Canada. They observed different types of current-generated, wave-generated or combined wave-current generated ripples over the seabed; whereas during the most energetic storm conditions sheet flow conditions were established that eliminated (flattened) all bed forms.
The distant passage of an intense and large Category 3 Tropical Cyclone (TC) generated highly energetic surface waves in a large region across the Australian North West Shelf (NWS). Two sites on the continental shelf experienced maximum significant wave heights of up to 10 m with near-bed wave orbital velocities up to 0.7 m s−1 at 40 and 74 m depth. Concurrent current profiles were measured between 0.5 and 8.5 m above the seabed at these sites and therefore the observations presented in this study allowed a detailed analysis of the wave-current interactions in the continental shelf bottom boundary layer during highly energetic tropical cyclone conditions. These observations revealed a strong modification of the current profiles during the TC with an increase of the apparent bottom roughness by up to 2 orders of magnitude compared to the typical tide-dominated current conditions. This modification was predominantly due to wave-current interactions; however, the results also suggest that changes in bed forms during the TC conditions likely acted as a secondary mechanism contributing to the enhanced bottom roughness experienced by the mean currents. A comparison of the total mean bed shear stresses as a function of the waves and currents showed a somewhat larger enhancement due to the nonlinear wave-current interactions than predicted by common semi-empirical models. Overall, the enhancement of the mean bed shear stresses during the TC significantly reduced the mean current velocities in the bottom boundary layer, highlighting the importance of including wave-current interactions within ocean circulation models to accurately predict shelf currents during TC conditions.