Long-term and high-resolution measurements of bed level changes in a temperate, microtidal coastal lagoon

Abstract

This study presents the results of a long-term monitoring program of bed level changes measured during 8 yr at an intertidal mudflat in a microtidal, temperate coastal lagoon. Additionally, bed level measurements obtained at a 10-min temporal resolution at the same tidal flat and at the bed of a nearby tidal channel are presented. Short-term changes in bed level are one or two orders of magnitude larger than the annual net-deposition rate, which shows that the environment is highly dynamic with respect to erosion, transport and deposition of fine-grained sediment. Some seasonality in the bed level changes was observed and there is a tendency for mudflat deposition in spring, summer and early autumn and erosion during the rest of the year, but interannual variations are large and different parts of the mudflat show different seasonal signals. A close coupling between sub-and intertidal deposition and erosion was observed. The time-series showed that some of the material eroded from the mudflat was not exported to the open sea, but instead temporarily deposited in a nearby shallow tidal channel and later returned to the mudflat during calmer weather conditions. These findings support previously published hypothesis and results of modelling studies. Based on the observed abundance of fine-grained sediment at the study sites and the high accretion rates generally found on fine-grained tidal flats in the Danish Wadden Sea area, it is argued that these fine-grained tidal flats are not seriously threatened by the expected sea level rise in the 21st century. D

Wadden Sea tidal basins and the mediating role of the North Sea in ecological processes: scaling up of management?

“…This transport of fresh organic matter and SPM can be assumed to be most intense during spring and early summer, in line with enhanced sedimentation during spring and summer (e.g. Cadée, 1980;Andersen et al, 2006). The SPM transport towards the Wadden Sea will fade when autumn cooling starts and denser Wadden Sea water carries SPM away from the coast , in line with a decline in sediment accumulation as observed in the List tidal basin (Andersen et al, 2006).…”

Temporal changes of accretion rates on an estuarine salt marsh during the late Holocene — Reflection of local sea level changes? The Wadden Sea, Denmark

“…The high accretion rates which have been found at the Kongsmark mudflat (Andersen et al, 2006) indicate that the availability of fine-grained sediment in the sedimentary system is high also today. Whether the supply will be able to keep up with the proposed acceleration in sea level rise is not known but the present data-set and most other studies of fine-grained deposits in the Danish Wadden Sea area suggest that the stability of the coastal lagoons is not significantly threatened by at least a modest increase in sea level.…”

“…Much less is known about mudflat behaviour, partly due to the less accessible nature of this environment. However, a recent example of high mud deposition rates has been observed along a transect adjacent to Rømø (Kongsmark) in the southern part of the Danish Wadden Sea, where average accretion rates of 12 to 17 mm a − 1 over the last 8 yr have been observed (Andersen et al, 2006). These high rates show that sedimentation may also take place rapidly on mudflats if accommodation space and sediment are available.…”

Macroflocs, fine-grained sediment transports, and their longitudinal variations in the Ems Estuary

“…Additionally, the erosion properties of the bed vary over the cross-section, where muddy deposits are mainly found at the edges of certain tidal flats, bordering the deeper channels. This is discussed in more detail in Van Leussen (1991) and Andersen et al (2006), where it is concluded that the exchange of fine-grained material between the tidal flats and the deeper channels is essential in the total fine-sediment transport. It is the impression that a slide of the muddy channel banks also contributed to higher suspended sediment concentrations in the second half of the ebb period.…”