P. Bassoullet

Characterization of intertidal flat hydrodynamics

Abstract The paper reviews the different physical forcings that control tidal flat hydrodynamics. Tidal propagation and cross-shore or long-shore currents, tidal asymmetry, wind-induced circulation, wave propagation and drainage processes are successively considered. Some simple methods are described for estimating cross-shore currents and wave-induced bottom shear stresses, and the results obtained are compared to field measurements on three contrasted sites in Europe. In particular the cross-shore current is shown uniform in the lower part of the flat, and decreasing towards the shore. Bottom friction-induced wave attenuation is simply formulated on gently sloping beds, leading to a maximum wave height that a flat can experience; it is proportional to the water height according to the ratio between the slope and the wave friction factor. The maximum related shear stress occurs at high water and is also proportional to the water depth. Maximum tidal velocities are very similar in the three sites where bottom sediment is muddy, suggesting a relationship between physical stresses and sediment characteristics. The consequences of physical forcings on sediment transport are listed. The bottom shear stress is suggested as the relevant parameter for comparing tidal and wave effects. In general, tide induces onshore sediment transport, whereas waves and drainage favour offshore transport. The processes leading to a possible tidal equilibrium profile are analysed: they involve the intrinsic asymmetry that favours net deposition at high water, and an ebb dominance generated by the resulting bottom profile convexity. Eroding waves are likely to upset such a balance; this equilibrium then reduces to a trend for the system.

The influence of bedforms on flow and sediment transport over intertidal mudflats

Abstract A range of bedforms of different types are found at a range of scales on intertidal mudflats. The different types include (a) channels, creeks and gullies, (b) ridge–runnel systems, (c) ripples and other micro-topography, and (d) cliffs. The main features of these bedform types are illustrated with examples found at a number of NW European mudflat sites studied during the EC INTRMUD project. Some comments are made on their environment of formation and observations of bed sediment properties and flow and suspended sediment processes are presented. Finally, the influence of bedforms on the hydrodynamics, shallow water tidal flow and wave propagation, and sediment processes are discussed.

Development of ALTUS, a high frequency acoustic submersible recording altimeter to accurately monitor bed elevation and quantify deposition or erosion of sediments

In order to accurately measure the level variations of the bed in muddy dominant environments, a selfcontained device has been specifically designed to be used in shallow waters. The submersible developed system is of small size and can be easily deployed by hand on intertidal areas, and eventually be placed on a submerged frame in water. It acts as an echo sounder located at a desired place while emitting 2 MHz acoustic waves towards the bottom. Altitude data corresponding to various detection thresholds are recorded. This device has been designed to precisely survey and quantify short or long-term changes of bed elevation at locations where deposition of (fine) sediments or erosion measurements are of greatest interest for general hydrosedimentological studies and investigation of physical processes, in order to validate relevant numerical models and particularly sediment transport ones. This system has been deployed in association with a multiparameter probe on intertidal mudflats in bay and estuary.

Chapter 11 Erodibility of natural sediments: experiments on sand/mud mixtures from laboratory and field erosion tests

Abstract Natural sediments are often mixtures of cohesive and non-cohesive sediment, and there are few data on the specific behaviour of these sediments, in particular, their erodibility. After a brief synthesis of recent works on the erosion threshold and the erosion rate of non-cohesive, cohesive and mixed sediments, the erodimetre, a new erosion device developed by IFREMER, is described. The instrument is portable and enables the separate quantification of the erosion of mud and sand fractions. Four data sets are presented, consisting of erosion tests on pure sands, laboratory mixtures with two types of mud and well-sorted sands, and natural mixed sediments. A clear relationship between the critical shear stress for erosion and the mud volume fraction (over the whole range) is shown. The correlation is not so good with the clay fraction. When the sand is fine (140 μm), the relationship is linear. When the sand size increases (280 μm), a sharp transition from non-cohesive to cohesive behaviour appears when the mud fraction exceeds 35–40%. It is suggested that the ratio between the grain sizes of the sand and the fine fractions, and more generally the whole size spectrum of the sediment, should be considered to characterise the sediment erodibility. This first interpretation extends the conceptual framework for the erosion behaviour described by Van Ledden et al. (2004).

Chapter 29 Sedimentary processes in a shellfish farming environment, Mont Saint Michel Bay, France

Abstract Mont Saint Michel Bay is a 30 km wide bay located on the French side of the English Channel. The tidal range reaches 14 m during spring tides, which leads, along with the gentle slope of the bathymetry, to a tidal flat up to 11 km wide. The sedimentary coverage exhibits strong longshore and cross-shore gradients, with purely muddy environments to the west of the domain, and pure sands to the east, where the natural channels of three incoming rivers induce rapid morphological changes. The contributions of tides and waves to sedimentary processes are analysed by means of numerical modelling and results of field observations. Maximum tidal bottom shear stresses are shown to account for the sediment distribution throughout the bay, while the longshore gradient in wave intensity seems to drive the amount of suspended sediment concentration. The bay has been a priviledged ground for shellfish farming for over a century. Farming structures (oyster tables, mussel posts and wooden fences used as permanent fishing nets) have significantly hindered natural flow patterns, creating quiescent areas which significantly increase fine sediments deposits in farming areas. The paper focuses on introducing into a numerical model the effects of mussel farms on flow circulation and sediment dynamics.

CONSOLIDATION OF SOFT MARINE SOILS: UNIFYING THEORIES, NUMERICAL MODELLING AND IN SITU EXPERIMENTS

The first part of this article gives an overview of the development works that have been carried out to date along the French coast, covering types, operating principles and impacts. A philosophy is identified with regard to the schemes implemented so far and those recommended for the future: rather than static structures that resist the action of the sea, it would often be preferable to substitute dynamic structures and schemes that work in harmony with it. Much research is being carried out on designs of this type. An analysis and summary of this work is given in the second part of this article. Most such work is currently at the experimental stage and no miracle solution has been found so far. Local conditions must be examined carefully before deciding to use any of the new alternatives being proposed.

The "SAMPLE system", a new concept of benthic station

In order to measure hydrodynamical forcings and sediment transport in muddy dominant environments, a self-contained device has been specifically designed to be used in shallow waters. The developed system is of small weight and size enough for easy handy deployments on intertidal areas, but it can also be placed in a frame for profiling the water column. Sensors are exchangeable. A high-performance data logger assumes independent programming on nine channels, a sampling rate up to 5 Hz and a large data storage. Attention has been paid to reduce electric consumption. The device has been applied to investigate the vertical structure of fluid mud in an estuary as well as the sediment resuspension on intertidal mudflats under wave and current forcings.<<ETX>>