Hervé Jestin

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.

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>>