Sedimentation on intertidal mudflats in the lower part of macrotidal estuaries: Sedimentation rhythms and their preservation

Abstract

International audienceThe objective of this study is to propose an original approach to the analysis of the formation of intertidal rhythmites, their preservation, and the evaluation of sedimentation rates on estuarine mudflats. Three mudflats, one from each of three estuaries, were analysed using a combination of long-term (> a year), high-frequency (1 burst/10 or 20 min), and high-resolution (0.2 cm) altimeter datasets and X-ray images of sediment cores collected during topographic surveys. The results highlight the roles played by sediment supply, hydrodynamics, and morphology of the lower estuaries on the sedimentation rhythms. While the sediment-starved Medway estuary (Kent, UK) remains stable at different time-scales, the annual sedimentation rates on the sheltered Authie mudflat (Pas-de-Calais, France) and the open Seine mudflat (Normandy, France) are relatively high at 18 and 15 cm yr− 1, respectively. On the Authie mudflat, sedimentation rhythms correspond to the semi-lunar cycle, with a good correlation between tidal range and deposit thickness. Sedimentation occurs at the beginning of the recovery (mean value of 0.25 cm per semi-diurnal cycle), and is not disturbed by wind-induced waves. In the lower Seine estuary, semi-diurnal mechanisms of deposition occur mainly when the mudflat is covered by a minimal water height (tidal range threshold value = 7.1 m). Sedimentation rhythms are discontinuous and deposition occurs only during the highest spring tides. Mean deposit thickness is 0.6 cm per semi-diurnal cycle, controlled by the turbidity maximum and the long high tide slack (2–3 h). The fluid mud is sensitive to wind-waves in this open funnel-shaped estuary, which undergoes wind-induced erosion (0.2 to 2 cm) about 10 times per year.Comparison of altimeter datasets and lithology of the sediments cored at the same points provided improved understanding of the sediment record rhythms and the sedimentation events. On the sediment-starved Medway mudflat, the result of sediment processes is a single superficial lamina. The elementary deposit in the Authie bay corresponds to a semi-lunar-linked layer, typical of sheltered environments. On the lower Seine mudflat, sedimentation rhythms are linked to the highest spring tides (i.e., the lunar cycle), resulting from increased sediment availability related to the high suspended matter concentration in the turbidity maximum. These results underline the complex response of intertidal mudflats to hydrodynamics and sediment supply conditions, from the semi-diurnal to the annual scale

Potential of UAVs for Monitoring Mudflat Morphodynamics (Application to the Seine Estuary, France)

“…The Seine river flow ranged from 100 m 3¨s´1 to 2000 m 3¨s´1 over the last 20 years, with a mean annual discharge around 500 m 3¨s´1 . Previous studies show that deposition occurs in bursts during periods of low fluvial discharge, and is closely related to the highest spring tides and the presence of the turbidity maximum zone in the estuary mouth [2]. These deposits are dominated by fine sediments (<50 µm), which represent 70%-90% of the material.…”

Sedimentation rhythms and hydrodynamics in two engineered environments in an open coast managed realignment site

“…Waves originating over the former agricultural field on the opposite bank have been observed by the authors, which refract through the pre-existing channel and travel up the cut channel during periods of inundation. The effect of wind induced erosion is related not only to the properties of the waves, but also to the cohesion of the surface sediment (Deloffre et al, 2007). Despite sedimentary rhythmites at this site being more typical of a natural intertidal mudflat, compared to Site 5, the sediment remains heavily waterlogged due to the high levels of subsurface sediment compaction generated during site construction.…”

“…At Site 3, which consists of waterlogged sediment in a sheltered central site area, sedimentation corresponded to a semi-lunar rhythm typical of a sheltered natural mudflat (Deloffre et al, 2007) with a superimposed rhythm of deposition during the flood tide and resuspension during the ebb. Deposition of sediment matched a decrease in near-bed SSC and following erosion the SSC increased.…”

“…Deloffre et al, 2007). This paper presents an investigation of sedimentary rhythmites and ETDC processes in a recently breached and inundated estuarine environment: the Medmerry Managed Realignment site, West Sussex, UK.…”

Functional trait responses to sediment deposition reduce macrofauna-mediated ecosystem functioning in an estuarine mudflat

“…Per tidal cycle, different elevation changes have been observed, e.g. from decreases of 3.3 mm in the Yangtze estuary (China) to increases of 6 mm in the estuary of the Seine (France) (Deloffre et al, 2007;Shi et al, 2012). Our study was performed on a mudflat in the estuary of the river Scheldt (Belgium, the Netherlands), which is characterized by its meso-to macrotidal regime and well-mixed water column.…”