Eigenschaften und Dynamik von Schwebstofffrachten und bodennahen kohäsiven Sedimentsuspensionen in Ästuaren. Beispiele aus dem Weser, Ems und Elbe Ästuar (Deutschland)

Estuaries are often used as transport ways to cities and harbours in the hinterland and have emerged as an important focus in coastal research. Mankind aspires to understand and control the complex hydro- and sediment dynamics in order to optimize the system due to social-economic demands. In this process, river regulations (e.g. dredging activities) change the natural dynamics of the environment sustainably. This study provides new knowledge about near bed cohesive sediment dynamics as well as of fine cohesive sediment dynamics not only under ‘natural’ tidal flow but also under the influence of Water Injection Dredging (WID). In-Situ Particle Size Distributions (ISPSDs) and Primary Particle Size Distributions (PPSDs) measured in the German Elbe and Weser estuaries indicate that the organic and inorganic Suspended Particulate Matter (SPM) is in a flocculated state. The substrate for the organic matter, which is needed for flocculation processes, is mainly transported from the seaside into the estuaries. Regional differences in PPSD have been observed in winter when the freshwater discharge is high and the extension of the Turbidity Maximum Zone (TMZ) is large. Individual sorting between the seaward and landward section as well as in the TMZ has not been observed in summer when the TMZ extension is small. Regional differences in the PPSD have no influence on the ISPSD. The latter is controlled primarily by the particle collision frequency powered by tidal forces and increased Suspended Sediment Concentrations (SSCs). Although, flocs break-up due to shear stress with progressing tidal current they do not change their PPSD. Knowledge about floc size and composition is important to estimate settling velocities of the SPM. Increased particle settling can lead to enhanced near bed fine cohesive sediment concentrations. High resolution vertical sampling of near bed SPM in the Weser and Ems estuaries indicates that the widely accepted 3-layer models, often used to describe vertical, cohesive sediment distribution is evidently incomplete. Sedimentological and rheological parameters, statistically proven by a cluster analysis, have shown that the intermediate fluid mud layer has to be subdivided in a low-viscosity fluid mud layer (I) and high-viscosity fluid mud layer (II). On the basis of a multi-parameter analysis it was possible to define the exact SSC-limits of both fluid mud types. The upper boundary of the fluid mud (I) is characterised by a strong SSC-gradient (lutocline) which is detected with the low frequency channel of a parametric sediment echo sounder. The amplitude of the acoustic interface correlates with the SSC-gradient sampled at the acoustic interface which had not been quantified before in literature. Decreasing SSC-gradients with progressing tidal currents indicate an interfacial mixing but significant changes do not occur in areas of smooth bed morphology until one hour after slack water. Fluid mud (II) layers are suggested to represent recurrent, cohesive sediment accumulations which frequently have to be dredged in harbours and navigation channels. Over the last few decades the hydraulic WID technique has gained increased interest for the removal of mud shoals and subaqueous sand dunes in tidal controlled environments. Extensive hydroacoustical, optical and ground-truthing data collected during WID in the brackish- and freshwater reach of the Weser estuary shows that the crests of subaqueous sand dunes were exactly removed at the demanded height. Potential dredging effects are restricted to the approximate dredging site. Destruction of the internal sediment structure is limited to the upper decimetres and mobilized sandy sediments are accumulated on the dune slopes or in the adjacent troughs. Significant variations in the SSC or floc size are neither observed in the brackwater nor in the freshwater reach although acoustic interferences suggest increased turbulences over a distance of some hundreds of metres at the current lee-side of the dredging device.Tidedominierte Flussmündungen, sogenannte Ästuare, werden häufig als Transportwege zu Häfen und Städten im Hinterland genutzt. Um wirtschaftlichen und sozialen Interessen gerecht zu werden ist der Mensch bestrebt die komplexe Hydro- und Sedimentdynamik des Systems zu verstehen und zu kontrollieren. Bedingte Maßnahmen zur Flussregulierungen (z.B. Baggeraktivitäten) führen meist zu nachhaltigen Veränderungen der natürlichen Dynamik des Ästuars. Diese Arbeit untersucht die Dynamik von kohäsiven Schwebstoffen in der Wassersäule und im bodennahen Bereich, sowohl unter natürlichen Tidebedingungen als auch unter dem Einfluss von Wasserinjektions (WI)-Baggerung. Die Korngrößenverteilung von in-situ Partikeln und Primärpartikeln im Weser und Elbe Ästuar zeigen, dass die organischen und anorganischen Schwebstoffe sich in einem aggregierten Zustand befinden und sogenannte Flocken bilden. Das Trägermaterial für organische Substanzen, die für das Zusammenhaften der einzelnen Partikel benötigt wird, wird von der Seeseite in die Ästuare transportiert. Im Winter, unter hohem Oberwasserabfluss und einer ausgedehnten Trübungszone, wurden unterschiedlich große Primärpartikel in der Trübungszone sowie im see- als auch im landwärtigen Bereich gemessen. Entsprechende Verteilungsmuster existieren im Sommer unter geringem Oberwasserabfluss und einer kurzen Trübungszone nicht. Die regionalen Unterschiede in der Primärpartikelgröße haben keine Auswirkungen auf das Größenspektrum der in-situ Partikel. Die in-situ Größe wird maßgeblich durch die Kollisionsrate der Partikel bestimmt die wiederum vom Tidestrom und der Schwebstoffkonzentration abhängig ist. Obwohl die in-situ Partikel im Laufe eines Tidenzykluses unter der Wirkung von Schubspannung zerfallen und sich unter ruhigeren Strömungsbedingungen wieder neu aufbauen, verändert sich deren Primärpartikel-Zusammensetzung nicht. Erkenntnisse über in-situ Partikelgrößen und Zusammensetzung sind wichtig um das Sinkverhalten von Schwebstoffen abzuschätzen zu können. Hohe Sinkgeschwindigkeiten können zu erhöhten Schwebstoffkonzentrationen in Bodennähe führen. Vertikal hoch auflösende Beprobungen der bodennahen Schwebstoffkonzentrationen und kohäsiven Ablagerungen zeigen, dass allgemeingültige 3-Schichten Modelle für vertikale Schwebstoffverteilungen unvollständig sind. Sedimentologische und rheologische Parameter beweisen, dass die mittlere Schicht der Modelle – die Fluid Mud Lage – in eine gering-viskose Fluid Mud (I) Lage und in einen hoch-viskose Fluid Mud (II) Lage unterteilt werden muss. Anhand von einer Clusteranalyse konnten die Grenzen der Schwebstoffkonzentration beider Fluid Mud Typen genau bestimmt werden. Die obere Grenze der Fluid Mud (I) Lage ist geprägt durch einen abrupten Anstieg in der Schwebstoffkonzentration, die als akustischer Reflektor mit der niedrigen Frequenz eines parametrischen Sedimentecholots detektiert wurde. Gegenüber früheren Studien kann zum ersten Mal die Beziehung zwischen der Amplitude des akustischen Reflektors und dem Schwebstoffgradienten am Reflektor quantifiziert werden. Die Abnahme des Gradienten mit zunehmender Strömungs-geschwindigkeit belegt einen Schwebstoffaustausch an der Grenzschicht. In Bereichen der Weser wo die Morphologie sehr plan ist, findet der Austausch an der Grenzschicht allerdings frühestens eine Stunde nach Stauwasser statt. Kohäsive Sedimente, vorzugsweise des Typs Fluid Mud (II), die mit der Tideströmung nicht wieder in Schwebe gebracht werden, führen zu Ablagerungen, die regelmäßige Baggeraktivitäten erfordern. Um Untiefen in tidedominierten Fahrwasserrinnen und Häfen zu beseitigen, hat das hydraulische WI-Verfahren in den letzten Jahrzehnten an Bedeutung gewonnen. Umfassende hydroakustische und optische Messungen, gekoppelt mit Sediment- und Wasserprobenentnahmen fanden begleitend zu WI-Maßnahmen im Brack- und Frischwasserbereich der Weser statt. Die Daten zeigen, dass die Kuppen von sandigen Unterwasserdünen exakt auf die angeforderte Höhe abgetragen wurden. Hydroakustische Messungen zeigen, dass sich der Einfluss der Baggeraktivitäten lediglich auf die direkte Baggerumgebung beschränkt. Die internen Sedimentstrukturen werden nur in den obersten Dezimetern zerstört und die mobilisierten Sedimente akkumulieren auf den angrenzenden Dünenflanken oder im nächsten Dünental. Signifikante Änderungen im Schwebstoffgehalt und in der in-situ Partikelgröße in der Wassersäule wurden nicht beobachtet obwohl akustische Interferenzen, assoziiert mit Turbulenzen, über mehrere hundert Meter auf der strömungsabgewandten Seite des Baggers zu beobachten waren

RV Littorina 02/12 Cruise Report [L12-02] Sagasbank (Mecklenburg Bay, Baltic Sea) 27th February – 2nd March 2012

This cruise with the RV LITTORINA was the first of three planed legs to be carried out in the area of the Sagasbank (Mecklenburg Bay). Sagas-Bank is an elevation east of the Wagrian peninsula with several elevations of up to 8 meters below sea level (Figure 1). The center of Sagasbank is marked by the 10 m contour line. Sagasbank and the adjacent submarine areas (in total 3.238 km²) are declared as FFH-site (flora-fauna-habitat). Residual sediments (boulders, blocks, sand and gravel) of the last glacial period are ideal habitat for submarine flora and benthic organisms. Here, 115 macro-zoobenthic species (with at least 20 red list species) and 17 algae species (with 6 red list species) are living on Sagasbank. The shallow water area is also habitat for porpoises and one of the most important bird resting places in the Baltic Sea. The habitat is exposed to fishing industry, military and sporting and leisure activities. This cruise is part of cooperation between the Institute of Geosciences at the University of Kiel and the local authority ‘Landesamt für Landwirtschaft, Umwelt und ländliche Räume’ (LLUR). The aim of this cruise (and the following two) is a full coverage, hydroacoustic mapping of Sagasbank and the surrounding area. The hydroacoustic data are calibrated by grab sampling and under water videos. Of special interest are the regions covered with hard substrate providing habitat for macro-zoobenthos and fishes

RV Littorina 04/12 (1) Cruise Report [L12-04/1] Sagasbank (Mecklenburg Bay, Baltic Sea) 2nd – 5th April 2012

This cruise with the RV LITTORINA was the first of three planed legs to be carried out in the area of the Sagasbank (Mecklenburg Bay). Sagas-Bank is an elevation east of the Wagrian peninsula with several elevations of up to 8 meters below sea level (Figure 1). The center of Sagasbank is marked by the 10 m contour line. Sagasbank and the adjacent submarine areas (in total 3.238 km²) are declared as FFH-site (flora-fauna-habitat). Residual sediments (boulders, blocks, sand and gravel) of the last glacial period are ideal habitat for submarine flora and benthic organisms. Here, 115 macro-zoobenthic species (with at least 20 red list species) and 17 algae species (with 6 red list species) are living on Sagasbank. The shallow water area is also habitat for porpoises and one of the most important bird resting places in the Baltic Sea. The habitat is exposed to fishing industry, military and sporting and leisure activities. This cruise is part of cooperation between the Institute of Geosciences at the University of Kiel and the local authority ‘Landesamt für Landwirtschaft, Umwelt und ländliche Räume’ (LLUR). The aim of this cruise (and the following two) is a full coverage, hydroacoustic mapping of Sagasbank and the surrounding area. The hydroacoustic data are calibrated by grab sampling and under water videos. Of special interest are the regions covered with hard substrate providing habitat for macro-zoobenthos and fishes

Short-term quantitative analysis of submarine bedform movement in the Sylt Outer Reef, North Sea

A new approach on optimizing and rectifying backscatter images from side-scan sonar were applied in the seafloor images of a marine protected area in the German part of the North Sea, to monitor bedform change. Hydro-acoustic data from 2017 to 2018 of side-scan and multibeam echosounder were evaluated using the Digital Shoreline Analysis System (DSAS) to quantify the movement pattern of submarine bedforms. Results were verified with grain-size analyses and underwater video footages. The new method provides the rate of change of bedform movement, net bedform movement, and linear regression rate, which can assist in the environmental monitoring of the marine protected area

RV Littorina 04/12 (2) [L12-04/2] Cruise Report Sagasbank (Mecklenburg Bay, Baltic Sea) 16th – 20th April 2012

This cruise with the RV LITTORINA was the first of three planed legs to be carried out in the area of the Sagasbank (Mecklenburg Bay). Sagas-Bank is an elevation east of the Wagrian peninsula with several elevations of up to 8 meters below sea level (Figure 1). The center of Sagasbank is marked by the 10 m contour line. Sagasbank and the adjacent submarine areas (in total 3.238 km²) are declared as FFH-site (flora-fauna-habitat). Residual sediments (boulders, blocks, sand and gravel) of the last glacial period are ideal habitat for submarine flora and benthic organisms. Here, 115 macro-zoobenthic species (with at least 20 red list species) and 17 algae species (with 6 red list species) are living on Sagasbank. The shallow water area is also habitat for porpoises and one of the most important bird resting places in the Baltic Sea. The habitat is exposed to fishing industry, military and sporting and leisure activities. This cruise is part of cooperation between the Institute of Geosciences at the University of Kiel and the local authority ‘Landesamt für Landwirtschaft, Umwelt und ländliche Räume’ (LLUR). The aim of this cruise (and the following two) is a full coverage, hydroacoustic mapping of Sagasbank and the surrounding area. The hydroacoustic data are calibrated by grab sampling and under water videos. Of special interest are the regions covered with hard substrate providing habitat for macro-zoobenthos and fishes

Hydroacoustic Mapping of Geogenic Hard Substrates: Challenges and Review of German Approaches

Subtidal hard substrate habitats are unique habitats in the marine environment. They provide crucial ecosystem services that are socially relevant, such as water clearance or as nursery space for fishes. With increasing marine usage and changing environmental conditions, pressure on reefs is increasing. All relevant directives and conventions around Europe include sublittoral hard substrate habitats in any manner. However, detailed specifications and specific advices about acquisition or delineation of these habitats are internationally rare although the demand for single object detection for e.g., ensuring safe navigation or to understand ecosystem functioning is increasing. To figure out the needs for area wide hard substrate mapping supported by automatic detection routines this paper reviews existing delineation rules and definitions relevant for hard substrate mapping. We focus on progress reached in German approval process resulting in first hydroacoustic mapping advices. In detail, we summarize present knowledge of hard substrate occurrence in the German North Sea and Baltic Sea, describes the development of hard substrate investigations and state of the art mapping techniques as well as automated analysis routines

Impact of Sparse Benthic Life on Seafloor Roughness and High-Frequency Acoustic Scatter

Quantitative acoustic marine habitat mapping needs to consider the impact of macrobenthic organisms on backscatter data. However, the sensitivity of hydroacoustic systems to epibenthic life is poorly constrained. This study explores the impact of a benthic community with sparse abundance on seafloor microroughness and acoustic backscatter at a sandy seafloor in the German North Sea. A multibeam echo sounder survey was ground-truthed by lander measurements combining a laser line scanner with sub-mm resolution and broad-band acoustic transducers. Biotic and abiotic features and spatial roughness parameters were determined by the laser line scanner. At the same locations, acoustic backscatter was measured and compared with an acoustic scatter model utilizing the small-roughness perturbation approximation. Results of the lander experiments show that a coverage with epibenthic features of 1.6% increases seafloor roughness at spatial wavelengths between 0.005–0.03 m, increasing both spectral slope and intercept. Despite the fact that a strong impact on backscatter was predicted by the acoustic model based on measured roughness parameters, only a minor (1.1 dB) change of backscatter was actually observed during both the lander experiments and the ship-based acoustic survey. The results of this study indicate that benthic coverage of less than 1.6% is insufficient to be detected by current acoustic remote sensing