Morten Pejrup

A comparison of in situ techniques for estuarine floc settling velocity measurements

An Intercomparison Experiment carried out in the turbidity maximum of the Elbe estuary aimed to determine the relative performance of a number of methods of measuring the settling velocity of estuarine flocs. These comprised several Owen Tubes, with different sampling protocols, side withdrawal tubes, a settling box, and two in situ video systems. There were significant differences between the results which may partly relate to small-scale spatial and temporal patchiness in the turbidity field. Owen Tubes generally give settling velocities an order of magnitude smaller than the direct video measurements, which may indicate that the tubes disrupt flocs on sampling. Between Owen Tubes different methods of calculating the results may lead to different median settling velocities, especially at low concentrations. However, a well-controlled sampling protocol with the settling tubes gave consistent results. The direct video methods appear to give comparable results, and need to be evaluated further.

Flocculated suspended sediment in a micro-tidal environment

Abstract Silt and clay particles are unstable when suspended in saline waters. They coagulate into sediment flocs having settling velocities orders of magnitude higher than the individual particles constituting them. In the Danish Wadden Sea in-situ analyses were carried out to obtain the natural textural composition of the suspended sediment. A settling tube, Braystoke SK 110, was used; and the analysis revealed that the suspended sediment at the investigation sites was composed of an unflocculated population coarser than about 4.5 φ and of a flocculated population finer than this grain size. Median settling velocities from 10 −4 to 10 −3 m s −1 were determined within the concentration range 50–1000 mg 1 −1 . It is concluded that suspended sediment concentration is a major determinant for the natural textural composition of flocculated sediment. The variation in concentration can account for about 80% of the variance in median settling velocity. Turbulent mixing within the water column is found to be the limiting factor for the growth of sediment flocs.

Deposition and mixing depths on some European intertidal mudflats based on and activities

Abstract Vertical profiles of 210 Pb and 137 Cs have been collected at micro-, meso- and macrotidal mudflats in order to calculate present accumulation rates. Sediment cores were taken at the mudflat at Kongsmark in the microtidal Lister Dyb tidal area (Denmark), at the BOA bridge in the mesotidal Dollard estuary (Netherlands), and at the Skeffling mudflat in the macrotidal Humber estuary (UK). Except for the Kongsmark site and site A in the Humber estuary, no accurate calculation of accumulation rates was possible for the investigated sites. The accumulation rate obtained for the Kongsmark site is confirmed by other independent data and shows that the accumulation is not supply limited at this site at present. For site A in the Humber estuary a modified CIC method was applied to the 210 Pb -profile and the result agrees with results from the nearby Welwick Marsh. The salt marsh at Skeffling is advancing out onto the fringing mudflat with accumulation at the innermost part of the mudflat but with erosion taking place further offshore. The reasons for the generally poor dating results seems to be either erosion (two Humber sites) or postdepositional reworking of the deposited material (bioturbation and continued resuspension) and the data set suggests that the mixing depth may increase with tidal range and thereby hydrodynamic forcing.

Suspended sediment transport across a tidal flat

Abstract Many investigations have shown that most of the suspended sediment supplied to the Wadden Sea originates from the North Sea. Based on measurements of current velocity, depths and suspended sediment concentrations in the Danish part of the Wadden Sea, computations indicate, however, that suspended sediment is in fact exported to the North Sea. The reason for this discrepancy is twofold: Firstly, measurements have been carried out mainly in tidal channels where the largest amounts of water and suspended sediment are in the main transported during ebb periods. Secondly, computation of net transport in an estuarine environment such as the Wadden Sea always suffers from the difficulty that net transport within each single tidal period is of the same order as the uncertainty of the transport rates during ebb and flood periods respectively. The first problem can be dealt with if measurements are carried out in both tidal channels and over the tidal flats. The second problem can be solved by collecting time series instead of discrete measurements because accumulation of the transport rates found for successive tidal periods will neutralise the relatively large random error. In this paper, time series of suspended sediment flux across a tidal flat covering a full spring neap tidal cycle are presented. They showed net transport of suspended sediment in a landward direction which is mainly caused by flocculation of fine-grained sediment during periods of high concentration level. During such periods the field settling velocities of suspended sediment are so large that the settling- and scour-lag processes can be effective.

Enhanced erodibility of fine-grained marine sediments by Hydrobia ulvae

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a shortterm laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10000 and 50000 ind m 2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10000 to 50000 ind m 2 . The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions. D 2002 Elsevier Science B.V. All rights reserved.

Parameters affecting fine-grained suspended sediment concentrations in a shallow micro-tidal estuary, Ho Bugt, Denmark

Abstract Many shallow estuarine environments contain considerable quantities of fine-grained sediments. It is difficult, however, to determine theoretically the concentration and transport of fine-grained sediments in such environments, because the fine particles flocculate and because the suspended sediment concentration is dependent on a number of different parameters. The most important of which are: (1) bottom shear stress (current velocity), (2) wind speed and direction, and (3) salinity. In this study the relative importance of these parameters is evaluated in relation to a shallow micro-tidal environment, Ho Bugt, Denmark. Furthermore, a mathematical model explaining the variation in the concentration of fine-grained suspended sediments at a station within this area was created and calibrated to measurements of wind speed and direction, salinity and tidal current velocity. The model describes about 80% of the variance of the suspended sediment concentration at the site of observation.

A fine-grained sediment budget for the Sylt-Rømø tidal basin

A budget for net accumulation of fine-grained sediment (<63 μm) has been set up for the Sylt-Rømø tidal basin. Net accumulation within the basin was computed from210Pb core dating and mapping of the intertidal and supratidal surface sediments. It was found that a yearly mean value of 58·103 tons of sediment was deposited in the tidal basin. The largest sediment source for the net input of fine-grained sediment is the North Sea contributing about 64% to the net budget; the fluvial input and primary production contribute 14% and 15%, respectively. Local salt marsh erosion accounts for about 5% of the budget and atmospheric deposition for only 2%. The total amount of sediment deposited in the investigated area was low compared with earlier investigations in the Wadden Sea. This is explained partly by the intensive diking of the natural salt marshes fringing the area in the past, and partly by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water exchanged between the North Sea and the Sylt-Rømø tidal area is defined as the ratio between yearly net sediment input from the North Sea and yearly exchanged water volume between the tidal basin and the sea. This index shows that in the Sylt-Rømø tidal basin, fine-grained suspended sediment “filters” out of the exchanged sea water at a rate that is 12 times lower than in the Gr»dyb tidal basin. It is concluded that the net deposition of fine-grained sediment in a tidal basin is mainly a function of physiographical and hydrodynamical parameters and to a lesser degree of sediment availability

Major coastal impact induced by a 1000-year storm event

Extreme storms and storm surges may induce major changes along sandy barrier coastlines, potentially causing substantial environmental and economic damage. We show that the most destructive storm (the 1634 AD storm) documented for the northern Wadden Sea within the last thousand years both caused permanent barrier breaching and initiated accumulation of up to several metres of marine sand. An aggradational storm shoal and a prograding shoreface sand unit having thicknesses of up to 8 m and 5 m respectively were deposited as a result of the storm and during the subsequent 30 to 40 years long healing phase, on the eroded shoreface. Our results demonstrate that millennial-scale storms can induce large-scale and long-term changes on barrier coastlines and shorefaces, and that coastal changes assumed to take place over centuries or even millennia may occur in association with and be triggered by a single extreme storm event.

The influence of ice on sediment transport, deposition and reworking in a temperate mudflat area, the Danish Wadden Sea

Abstract The impact of ice formation on the transport, deposition and reworking of sediment at temperate intertidal mudflats has only been little studied. Investigations at the Kongsmark tidal flat in the Danish Wadden Sea reveal that sediment transported incorporated in drifting ice floes and deposited as melt out sediment may significantly influence both the sediment balance and the morphology of intertidal mudflats and supra tidal salt marshes. It was found that sediment deposited as melt out sediment from ice floes may account for a major part of the net accumulation on the fringing salt marshes, and it is suggested that sediment in the intertidal is transported from the lower part of the mudflat towards the higher lying parts thus influencing both the micro relief and the slope of the mudflat significantly. The transport distance was estimated to be at least in the order of 2 km, and the transport from the intertidal onto the salt marsh was found to be a one way transport process because vegetation quite rapidly grows through the melt out sediment, thus preventing resuspension. The results suggest that the ice transport process can be defined as a secondary process because it almost exclusively redistributes sediment already imported to the estuary. This is contrasting the primary estuarine processes of settling–scour-lag, tidal pumping and estuarine circulation which continuously concentrate cohesive sediment in the estuarine waters.

Vertical fluxes of particulate matter in Aarhus Bight, Denmark

Abstract Sediment traps, transmissometer readings and sediment core-dating have been used to describe sediment dynamics within a shallow micro-tidal coastal environment, characterized by intense resuspension. A method to separate the downward fluxes of primary settling matter and resuspended sediments is proposed. It is shown that these fluxes can be separated during periods when a marked pycnocline exists in the water column, preventing resuspended sediment from reaching the sediment traps above the pycnocline. At such times it is possible to make quantitative calculations of the flux of primary settling matter at the sediment-water interface. Good agreement between particulate fluxes calculated on the basis of sediment trap measurements and accumulation rates determined from Pb-210 core dating have been obtained in this study, supporting the idea of using sediment trap measurements to calculate sediment accumulation at the bottom.