Contribution of hydrodynamic conditions during shallow water stages to the sediment balance on a tidal flat: Mont-Saint-Michel Bay, Normandy, France (original) (raw)
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Relationships Between Wave-Induced Currents and Sediment Grain Size on a Sandy Tidal-Flat
Journal of Sedimentary Research, 2001
Studies of sedimentation on sandy tidal-flats have long acknowledged that tidal flows and waves are the most influential hydrodynamic forcing factors operating on surficial sediments. Tidal flows influence the long-term evolution of tidal-flats because of the asymmetry of tidal regimes (flood and ebb). Tidal current activity is mainly confined to channels. Outside the channels, mainly on the upper tidalflats, tidal current velocities decrease and sediment entrainment is frequently ascribed to wave action. The role of wave-induced processes on intertidal-flat sediments is frequently stated but has seldom been investigated, apparently because of methodological constraints. In this paper we report on investigations into the role of wave processes in determining surficial sediment distribution patterns on a sandy tidal-flat at Newtownards, at the northern end of Strangford Lough, Northern Ireland (spring tidal range 3.5 m). The main aim of this paper is to investigate the potential relationship between waveinduced processes and surficial sediments without attempting to achieve a comprehensive analysis of other sedimentation. The methodological approach involves numerically modeling wave propagation and comparing this with sediment distribution on a controlled field site. The method enables the role of waves on tidal-flats to be more fully assessed. Waves were modeled at a variety of tidal levels and wave data were extracted from the model grid at positions co-incident with sediment sampling locations. Multiple correlation of sediment mean grain size with simulated wave-induced orbital velocity showed a distinctive variation in correlation coefficients according to water level. The best (and statistically significant) correlations occur when water levels occupy a vertical range between Ϫ0.15 and 1.0 m local Ordnance Level (OD). In the absence of strong tidal currents, sediment distribution appears to be best explained by waves acting at these water levels. It is inferred that at lower-than-optimum water levels, wave energy is less influential on sediment texture since (1) part of the tidal-flat is still exposed and (2) the fetch over which the waves are generated is reduced. At higher-than-optimum water levels, wave energy may be sufficient to transport sediment, but the wave penetration in the column of water is not sufficient to permit wave energy dissipation at the sediment surface. Tidal currents are dominant at low tidal elevations so outside channels wave-induced currents exert most influence on the tidal-flats.
Marine Ecology Progress Series, 2004
This article synthesises a series of studies concerned with physical, chemical and biological processes involved in sediment dynamics (sedimentation, erosion and mixing) of the Molenplaat tidal flat in the Westerschelde (SW Netherlands). Total sediment accretion rate on the flat (sand to muddy sand) was estimated to be ~2 cm yr -1 , based on 210 Pb and 137 Cs profiles. 7 Be showed maximum activity in the surface sediments during summer, reflecting accretion of fine silt at this time of year, and total vertical mixing of sediment to be in the order of 50 cm 2 yr -1 . The extent to which different physical and biological processes (tidal currents, air exposure, bio-stabilisation, biodeposition and bioturbation) contributed towards sediment dynamics was estimated. A sediment transport model based on physical factors estimated sedimentation rates of 1.2 cm yr -1 , but did not account for tidal or seasonal variation in suspended particulate matter (SPM), wind or effects of spring-neap tidal cycles. When the model was run with an increased critical bed shear stress due to the microphytobenthos, net sedimentation rates increased 2-fold. These higher rates were in closer agreement with the rates derived from the depth profiles of radionuclides for the central region of the tidal flat (2.0 to 2.4 cm yr -1 ). Therefore a significant part of the sedimentation rate (~50%) may be explained by spatial-temporal changes in biological processes, including 'bio-stabilisation' by microphytobenthos, together with the enhanced biodeposition of silt by suspension feeders, and offset by processes of 'bio-destabilisation' by grazers and bioturbators. In the centre of the tidal flat there was a shift from high sediment stability in spring-summer 1996 to low sediment stability in autumn 1997, quantified by a significant reduction in critical erosion velocity of 0.12 to 0.15 m s -1 , and accompanied by a 30-to 50-fold increase in sediment erosion rate. The change was associated with a shift from a tidal flat dominated by benthic diatoms and a low biomass of bioturbating clams (Macoma balthica), to a more erodable sediment with a lower microphytobenthos density and a higher biomass of M. balthica. Vertical mixing of sediment and organic matter, studied using a variety of tracers, was rapid and enhanced by advective water flow at sandy sites and by burrowing polychaetes and bivalves at silty sites. Resale or republication not permitted without written consent of the publisher 42 Fig. 1. Location of Molenplaat tidal flat and 5 sampling sites. Isolines represent low-tide level (2 m) and mid-tide level (0 m)
Marine Geology, 2019
Sediment dynamics in tidal flats, ranging from daily to seasonal timescales, are particularly relevant as they control key ecological and geomorphic processes that ultimately contribute to the long-term evolution of coastal and estuarine landscapes. Yet, insights into bed level changes, including the full range of relevant timescales from intra-tidal to daily and seasonal scales, are currently limited due to a lack of efficient methods to record high-resolution (< 1 day) data over the long-term (> 1 year). Accordingly, this contribution intends to improve our understanding on spatio-temporal patterns of long-term (> 1 year) high-resolution (daily) bed level dynamics in tidal flats in relation to the dominant hydrodynamic driving forces, namely tides and waves. Specifically, this study was conducted along two 200 m long cross-shore transects on an intertidal flat located in the macrotidal (> 5 m tidal range) Scheldt estuary, Belgium. Results showed that daily bed level changes at the low tidal flat (i.e. 4.10 m below mean high tide level) were dominated by tidal currents, with a strong fortnightly (or spring-neap) signature, whereas wave activity was of secondary importance. Conversely, bed level changes in the high tidal flat (i.e. 0.65 m below mean high tide level) were almost exclusively dominated by wave activity. Additionally, seasonal deposition-erosion cycles that superimposed on the daily bed level changes were associated with the seasonality of wind wave activity and benthic biology. Analysis of wave and current-induced bed shear stresses at the respective locations confirmed this spatial variability of tidal-dominated sediment dynamics at the low tidal flat versus wave-dominance at the high tidal flat, and comparison with local critical bed shear stresses for sediment motion also revealed differences in morphological impacts of the hydrodynamics between the two transects. These distinctive responses of bed level dynamics across the tidal flat can be partly explained by the spatially varying sediment properties across the tidal flat and may be further mediated, on a seasonal time scale, by the growth of the algal mat with its effect on stabilizing the sediment bed. In view of the large spatial and temporal variability of internal and external forcing revealed in this study, comprehensive and detailed field measurements are even more necessary to understand and predict long-term bed level dynamics and related ecological implications in tidal flats.
The Role of Surges During Periods of Very Shallow Water on Sediment Transport Over Tidal Flats
Frontiers in Marine Science, 2021
Periods of very shallow water (water depth in the order of 10 cm) occur daily on tidal flats because of the propagation of tides over very gently sloping beds, leading to distinct morphodynamical phenomena. To improve the understanding of the characteristics of velocity and suspended sediment concentration (SSC) surges and their contribution to sediment transport and local bed changes during periods of very shallow water, measurements of near-bed flow, and SSC were carried out at two cross-shore locations on an intertidal flat along the Jiangsu coast, China. Furthermore, the role of surges in local resuspension and morphological change was explored. Results indicate that flow and SSC surges occurred at both stations during very shallow water periods. On the lower intertidal flat, flood surges were erosive, while weaker surges on the middle intertidal flat were not. Surges on lower intertidal flats resulted in local resuspension and strong turbidity, contributing up to 25% of the ons...
Journal of Marine Science and Engineering
Sediment transport is a key element in intertidal beach morphodynamics, but measurements of sediment transport are often unreliable. The aim of this study is to quantify and investigate cross-shore sediment transport and the resulting topographic changes for a tide-dominated, sandy beach. Two fortnight-long field experiments were carried out during which hydrodynamics and sediment dynamics were measured with optical and acoustic sensors, while the beach topography was surveyed with a permanent terrestrial laser scanner. Suspended sediment was generally well-mixed and currents were largest at approximately 1.5 m above the bed, which resulted in a peak in sediment transport at 1/3 of the high tide level. The mean transport direction was onshore during calm conditions (wave height <0.6 m) thanks to tidal currents and offshore during energetic conditions due to undertow. Oscillatory transport was always onshore because of wave asymmetry but it was subordinate to mean transport. The i...
Continental Shelf Research, 2005
Tidal inlet characteristics are controlled by wave energy, tidal range, tidal prism, sediment supply and direction and rates of sand delivered to the inlet. This paper deals with the relations between inlet and lagoon evolutions, linked by the tidal prism. Our study is focused on the Maumusson Inlet and the Marennes-Ole´ron Bay (first oyster farming area in Europe), located on the western coast of France. The tidal range (2-6 m) and wave climate (mean height: 1.5 m) place this tidal inlet system in the mixed energy (tide, waves), tide-dominated category. The availability of high-resolution bathymetric data since 1824 permits to characterise and quantify accurately morphological changes of both the inlet and the tidal bay. Since 1824, sediment filling of the tidal bay has led to a 20% decrease in its water volume, and a 35% reduction of the inlet throat section. Furthermore, the bay is subjected to a very high anthropic pressure, mainly related to oyster farming. Thus, both natural and human-related processes seem relevant to explain high sedimentation rates. Current measurements, hydrodynamic modelling and cross-sectional area of the inlet throat are used in order to quantify tidal prism changes since 1824. Both flood and ebb tidal prism decreased by 35%. Decrease in the Marennes-Ole´ron Bay water volume is inferred to be responsible for a part of tidal prism decrease at the inlet. Tidal prisms decrease may also be explained by an increase in frictional resistance to tidal wave propagation, due to a general shoaling and oyster farms in the bay. A conceptual model is proposed, taking into account natural and human-related sedimentation processes, and explaining tidal inlet response to tidal bay evolutions. r
Tidally-induced sediment transport patterns in the upper Bay of Fundy: A numerical study
Continental Shelf Research, 2011
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Characterization of bottom sediment resuspension events observed in a micro-tidal bay
Ocean Science Discussions
In this contribution we investigate the origin of the variability in near-bottom turbidity observations in the Alfacs Bay (NW Mediterranean Sea). This bay is characterized by a micro-tidal environment and a relevant seiching activity which may lead to flow velocities of more than 50 cm•s-1. A set of current meters and optical sensors mounted near the sea bottom were used to acquire synchronous hydrodynamic and optical information of the water column. The time-series observations 15 showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node/anti-node location for the fundamental and first resonance periods of the bay. The implementation of a coupled wave-current numerical model shows a strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible of the resuspension of fine sediment within the bay. This is confirmed using retrieval of suspended sediment concentration from Sentinel-2 data. We suggest that the sequence of resuspension events plays a 20 relevant role in SSC, in such a way that previous sediment resuspension events may influence the increase of suspended sediment in subsequents events. The suspended sediment events likely affect the ecological status of the Bay and the sedimentary process at long-term period.
Earth Surface Processes and Landforms, 2010
Tidal channels are ubiquitous in muddy coastlines and play a critical role in the redistribution of sediments, thus dictating the general evolution of intertidal landforms. In muddy coastlines, the morphology of tidal channels and adjacent marshes strongly depends on the supply of fi ne sediments from the shelf and on the resuspension of sediments by wind waves. To investigate the processes that regulate sediment fl uxes in muddy coastlines, we measured tidal velocity and sediment concentration in Little Constance Bayou, a tidal channel in the Rockefeller State Wildlife Refuge, Louisiana, USA. The tidal measurements were integrated with measurements of wave activity in the bay at the mouth of the channel, thus allowing the quantifi cation of feedbacks between waves and sediment fl uxes. Results indicate that the sediment concentration in the channel is directly related to the wave height in the adjacent bay during fl ood and high slack water, whereas the concentration during ebb depends on local channel velocity. Moreover, the sediment fl ux during ebb is of the same order of magnitude as the sediment fl ux during the previous fl ood, indicating that only a small fraction of transported sediments are stored in the marsh during a tidal cycle. Finally, very low tides, characterized by high ebb velocities, export large volumes of sediment to the ocean.