Alexander Bartholomä

Response of the wadden sea to a rising sea level: a Predictive empirical model

A clue to the stratigraphic evolution in the case of a barrier island migrating up against a landward obstruction has recently been obtained from a detailed analysis of sediment patterns in the mesotidal backbarrier environment of the Wadden Sea (southern North Sea). Here the landward boundary is man-made and comprises a continuous dike line constructed in the wake of extensive land reclamation over the past 1000 years. Sea-level rise currently amounts to 25 cm/century. The system is further characterized by a lack of external sediment supply and a pronounced cross-shore energy gradient resulting in progressive landward fining of the backbarrier sediments. Adjacent to the dike the depositional system today is dominated by mixed flats with mud contents rarely exceeding 30%. Land reclamation has thus truncated the natural succession of sediment facies by eliminating mud flats and salt marshes. Contrary to expectations, the re-establishment of a complete but compressed facies sequence incorporating mud flats and salt marshes has not materialized. This would suggest that energy levels at the foot of the dike today are considerably higher than was previously the case along the undiked shoreline. A quantitative relative measure of backbarrier energy levels is the mean settling velocity of the sediment. In extreme cases the system has reached the stage where the finest sands with settling velocities below 0.3 cm s-1 (mean grain sizes < 0.1 mm) are being eliminated. It is postulated that, in the wake of continued sea-level rise and associated landward migration of the barrier island system, the sediment elimination process adjacent to the dike will continue and involve progressively coarser sediments. In the case of the Wadden Sea, the settling-lag and scour-lag mechanism by which fine-grained sediments are carried shoreward must be modified to incorporate an export loop for those particle sizes which have settling velocities below the local elimination threshold.

Seasonal Dynamics of Fine-Grained Sediments in a Back-Barrier Tidal Basin of the German Wadden Sea (Southern North Sea)

Abstract In the Wadden Sea of the southern North Sea, severe depletion of fine-grained sediments has been observed since dike construction due to increased energy levels. Of particular interest is the seasonal turnover of the fine sediment fractions. To better understand the seasonal response of muddy sediments, surface sediments, box-core sedimentary facies, and accumulation rates were investigated in a back-barrier basin of the East Frisian Wadden Sea, Germany. Measurements of accumulation rates indicate that the tidal flats are currently erosional. In summer, fine-grained sediments drape large areas of the nearshore tidal flats, whereas in winter the same tidal flats are dominated by sand. The distribution pattern of grain-size modes shows that in summer the sediment is mostly unimodal, whereas in winter the sediment is bimodal. This pattern is explained by the mixing and unmixing of different source populations in response to seasonally changing energy conditions. Size analyses of the mud fractions have revealed a general dearth of particles around 7 phi. The mud is thus composed of at least two different subpopulations: a better sorted coarse population (<7 phi) and an unsorted fine population (>7 phi). This suggests that the finer particles deposited in summer are incorporated in larger flocs and aggregates, which are hydraulically equivalent to codeposited sand particles; the flocs and aggregates subsequently are broken down and eliminated in winter. The apparent seasonal coarsening/fining is therefore regarded to be an artefact of the mechanical procedure of grain size analysis, in the course of which flocs and aggregates are broken down into their constituent particles. Box core peels confirm that most surface sediments are draped by mud in summer, whereas the sediments display wave-generated sedimentary structures in winter.

Bedform dynamics and net sediment transport paths over a flood-ebb tidal cycle in the Grådyb channel (Denmark), determined by high-resolution multi-beam echosounding

Abstract High-resolution bathymetric surveys were carried out by means of a high-frequency (455 kHz) multibeam-echosounder (MBES) to determine the total volumetric sediment transport over a tidal period in a tidal channel at the Danish west coast. With the high repetition rate and high accuracy of the MBES, a simple comparison of morphological changes recorded at short time intervals allows an accurate calculation of dune heights and migration rates, the main parameters required for realistic approximations of volume budgets in tidal channels. The net sediment volume of 0.061 m3 per m2 over one tidal period is ebb-orientated, which means an annual net volume export of 44.5 m3 m−2 year−1. The general ebb-oriented export trend recorded for the large, compound dune system is contrasted by a net flood-oriented transport in the ‘active zone’ involving smaller superimposed dunes, which amounts to 0.6 m2 per flow phase on top of the large dunes, and 0.15 m2 per flow phase on the slopes and in the troughs. This gives a mean volumetric sediment transport rate of 0.7697×105 m2 s−1 for the ebb flow, and 1.059×105 m2 s−1 for the flood flow, strongly deviating from earlier calculated transport rates.

Wissenschaftliche Konzepte für ein Monitoring des ökologischen Zustands des deutschen Küstenmeeres

Aussagen zum Zustand und der Entwicklung von Küstenmeeren können nur durch langfristige Beobachtungen und Modellansätze erfolgen. Im Verbundprojekt WIMO (Wissenschaftliche Monitoringkonzepte für die Deutsche Bucht) werden Konzepte und Mess ver fahren entwickelt und bewertet, die sowohl grundsätzliche wissenschaftliche Frage stel lungen behandeln, als auch den Anforderungen des behördlichen Monitorings im Rahmen der europäischen Gesetzgebung genügen.

Sedimentological and Rheological Properties of the Water–Solid Bed Interface in the Weser and Ems Estuaries, North Sea, Germany: Implications for Fluid Mud Classification

ABSTRACT Papenmeier, S.; Schrottke, K.; Bartholomä, A. and Flemming, B.W., 2013. Sedimentological and rheological properties of the water–solid bed interface in the Weser and Ems estuaries, North Sea, Germany: Implications for fluid mud classification. Fine, cohesive sediment suspensions are a common feature of estuarine environments. Generally, multilayer models are used to describe the vertical distribution of such sediments. Such conceptional models normally distinguish at least high suspended sediment concentrations (SSCs) as a topmost layer and a consolidated bed layer, often including an intermediate, fluid mud layer. Rheological, and in particular sedimentological properties are rarely included in these models. New data from two different estuaries provide new insights that can contribute toward the classification of near-bed cohesive sediments. The water–solid bed interfaces within the turbidity maximum zones of the Weser and Ems estuaries were sampled with 2–4-m-long cores. At 10-cm intervals, values of SSC, viscosity, particulate organic matter, mud ∶ sand ratio, temperature, salinity, and grain-size distributions were determined. By normalizing these parameters to SSC and performing a cluster analysis, sediment suspensions of <20 g/L SSC, fluid mud with up to 500 g/L SSC, and an underlying cohesive/consolidated bed can each clearly be distinguished. However, changes in flow behaviour and sedimentological characteristics represented by a shift in the cluster grouping support a subdivision of the fluid mud into a low-viscosity (I) (20–200 g/L SSC) and a high-viscosity (II) (200–500 g/L SSC) layer. Furthermore, by normalizing SSC measurements, site-specific differences were observed in the rheological behaviour of the fluid mud which might be caused by differences in grain-size composition. This suggests that the widely accepted 3-layer model of vertical SSC profiles should be extended by two layers of fluid mud identified in this study.

Grouping dynamics of suspended matter in tidal channels

[1] Small-scale suspended sediment dynamics involve transport modes where grouping of suspended particulate matter (SPM) forms distinct patterns in the water. Field data on suspended matter dynamics at a particular location of a tidal channel show the frequent occurrence of turbidity clouds. These appear at spatial scales of tens of meters and temporal orders of minutes. In situ grain-size analysis reveals that the turbidity clouds are mainly composed of aggregates around 0.35 mm in size. Although the SPM signal reveals similar oscillations as the transporting current, no direct correlation was observed between the occurrence of clouds and local hydrodynamic characteristics. To test how the formation of turbidity clouds can be related to the hydrodynamic environment and particle motion, we developed a numerical model for the simulation of particle grouping. The simulations show stable and weakly stable modes of particle clustering, depending on the environmental conditions. A prominent dependence of group stability on the amplitude of current velocity oscillations is revealed. As the local dynamics can be considered to be near the derived threshold of stable and weakly stable grouping, the temporal occurrence of turbidity clouds is explained by the temporal exceedance of a critical oscillation amplitude and the availability of suspended matter as the main limiting factors.

On the formation of current ripples

For grain sizes finer than coarse sand, the first flow-transverse bedforms to develop are current ripples. Although numerous studies have analysed different aspects of bedform morphodynamics, to date no comprehensive physical explanation for the formation of ripples has been given. We offer such an explanation based on a virtual boundary layer concept, and present a model predicting ripple height on the basis of grain size, current velocity and water depth. The model contradicts the conventional view of current ripples as bedforms not scaling with flow depth. Furthermore, it confirms the dependence of ripple dimensions on grain size, and their relative insensitivity to flow strength.

Abrupt emergence of a large pockmark field in the German Bight, southeastern North Sea

A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. Covering an area of around 915 km2, up to 1,200 pockmarks per square kilometer have been identified. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. The gas source and the trigger for the simultaneous outbreak remain speculative. Subseafloor structures and high methane concentrations of up to 30 μmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of postglacial deposits in a paleo river valley. Storm waves are suggested as the final trigger for the eruption of the gas. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping.

Tools to evaluate seafloor integrity: comparison of multi-device acoustic seafloor classifications for benthic macrofauna-driven patterns in the German Bight, southern North Sea

To determine the spatial resolution of sediment properties and benthic macrofauna communities in acoustic backscatter, the suitability of four acoustic seafloor classification devices (single-beam echosounder with RoxAnn and QTC 5.5 seafloor classification system, sidescan sonar with QTC Swathview seafloor classification, and multi-beam echosounder with QTC Swathview seafloor classification) was compared in a study area of approx. 6 km2 northwest of the island of Helgoland in the German Bight, southern North Sea. This was based on a simple similarity index between simultaneous sidescan sonar, single-beam echosounder and multi-beam echosounder profiling spanning the period 2011–2014. The results show a high similarity between seafloor classifications based on sidescan sonar and RoxAnn single-beam systems, in turn associated with a lower similarity for the multi-beam echosounder system. Analyses of surface sediment samples at 39 locations along four transects (0.1 m2 Van Veen grab) revealed the presence of sandy mud (southern and western parts), coarse sand, gravel and cobbles. Rock outcrops were identified in the north-eastern and eastern parts. A typical Nucula nitidosa–Abra alba community was found in sandy muds to muddy sands in the northern part, whereas the southern part is characterised by widespread occurrence of the ophiuroid brittle star Amphiura filiformis. A transitional N. nitidosa–A. filiformis community was detected in the central part. Moreover, the southern part is characterised by a high abundance of A. filiformis and its commensal bivalve Kurtiella bidentata. The high number of A. filiformis feeding arms (up to ca. 6,800 per m2) can largely explain the gentle change of backscatter intensity along the tracks, because sediment composition and/or seafloor structures showed no significant variability.

Seafloor monitoring west of Helgoland (German Bight, North Sea) using the acoustic ground discrimination system RoxAnn

Marine habitats of shelf seas are in constant dynamic change and therefore need regular assessment particularly in areas of special interest. In this study, the single-beam acoustic ground discrimination system RoxAnn served to assess seafloor hardness and roughness, and combine these parameters into one variable expressed as RGB (red green blue) color code followed by k-means fuzzy cluster analysis (FCA). The data were collected at a monitoring site west of the island of Helgoland (German Bight, SE North Sea) in the course of four surveys between September 2011 and November 2014. The study area has complex characteristics varying from outcropping bedrock to sandy and muddy sectors with mostly gradual transitions. RoxAnn data enabled to discriminate all seafloor types that were suggested by ground-truth information (seafloor samples, video). The area appears to be quite stable overall; sediment import (including fluid mud) was detected only from the NW. Although hard substrates (boulders, bedrock) are clearly identified, the signal can be modified by inclination and biocover. Manually, six RoxAnn zones were identified; for the FCA, only three classes are suggested. The latter classification based on ‘hard’ boundaries would suffice for stakeholder issues, but the former classification based on ‘soft’ boundaries is preferred to meet state-of-the-art scientific objectives.