Since the Wadden Sea area belongs to the World Nature Heritage the public pressure of habit protection grows distinctly. To assess the recent stage of the different habitats and to monitor their ...changes in the near future standard mapping and quality tools are requested. To map the sub-tidal part of the Wadden Sea area optical approaches are mostly less successful. To develop standard procedures for sub-aquatic monitoring, three hydroacoustic devices were tested their resolution, their redundancy and their value of benefit detecting habitats. In order to find out which are the system-specific limitation, foot-print sizes and coverage in different water depth we surveyed several test areas in water depth of 5 to 15 m simultaneously with singlebeam echosounder (200 kHz), multibeam echosounder (455 kHz) and sidescan sonar (380 kHz). Two surveys (in 2007 and 2008) are described here. By means of acoustic seabed classification tools the different characteristics of seafloor roughness were analysed. Four major acoustic classes were distinguished which cover sea surface sediments of sand, broken shells, gravel, less mud and infrequently some peat. All acoustic devices show similar complex spatial pattern with higher variability in 2007. Class 1 and Class 3 are the most redundant classes which are characterized by medium sand and varying amounts of gravel and shell fragments. Stronger differences in the acoustic classes were observed in the singlebeam which is stronger influenced by its lower frequency. The feedback of footprint and sediment matrix is not sufficiently explained nowadays.
The breaking of internal waves (IWs) is one of the most important factors for the vertical mixing of suspended sediments and nutrients in estuaries. Information on the generation and evolution of IWs ...in such environments is therefore required to better understand their physics and related processes of sediment transport. New data on highly resolved IWs recorded in the turbidity maximum zone of the Ems estuary (Germany) have provided new insights into the dynamics of estuarine IWs.
To investigate the generation and evolution of IWs as well as their contribution to vertical mixing, a variety of hydro-acoustic devices was deployed in combination with a vertical sampling of suspended particular matter. Wave parameters such as significant wave height, wave frequency, wave length and steepness were computed from these data.
The hydro-acoustic data reveal the formation of a prominent lutocline during slack water, at which IWs begin to be generated with the onset of tidal forcing. The two water bodies, which are characterised by markedly different suspended sediment concentrations, show clear differences in flow behaviour. As a consequence of current shear along the lutocline, Kelvin–Helmholtz instabilities are generated, which then produce IWs.
The IWs break when high shear stresses between the two layers are coupled with great wave steepness, and the breaking causes vertical mixing of the sediment. Most IW breaking events occur during the decelerating ebb phase and thereby promote downstream sediment transport.
► Internal waves are generated by shear instabilities along the lutocline. ► Internal waves are generated during the onset of ebb current. ► Internal wave breaking occurs mainly during ebb tide. ► The fluid mud and the water layer show different responses to tidal forcing. ► Three different hydro-acoustic devices revealed the same phenomena.
Pebbles, cobbles, and boulders are commonly found on abrasion platforms of retreating soft rock cliff-coasts, which are widespread along the southwestern Baltic Sea coast. So far, little has been ...known about the behavior of these residual sediments under the influence of wave impact. Thus, detailed field measurements were carried out at three different coastal stretches of the southwestern Baltic Sea between 1997 and 1999. Underwater photo stations were established and marked pebbles and cobbles (tracer) were placed on the seafloor. During an 18-month period, these stations were revisited up to 10 times, taking reproducible photos of the seafloor and measuring any tracer movement. Local wave data were collected by wave buoys. In water depths of up to 5.6 m and distances from the shoreline of up to 300 m pebbles, cobbles and even boulders were moved by wave action. Displacements were in the range of up to tens of meters, but highly dependent on water depth and wave energy input. Nearly all marked pebbles moved onshore, coinciding with the direction of the main wave propagation. It can be assumed that residual sediments are an important sediment source for the development of beach ridges in the adjacent coastal lowlands. There is evidence that the whole residual sediment layer, even of several centimeters to decimeters in thickness, has been reworked repeatedly. Former assumptions, that residual sediments covering the solid seafloor might protect it from abrasion, seem no longer valid.
We have investigated high-performance GaAs/AlAs terahertz (THz) quantum-cascade lasers (QCLs) with respect to the long-term stability of their operating parameters. The output power of lasers that ...contain an additional, thick AlAs refractive-index contrast layer underneath the cascade structure decreases after three months by about 35%. The deterioration of these lasers is attributed to the oxidation processes in this contrast layer starting from the facets. However, GaAs/AlAs THz QCLs with an Al0.9Ga0.1As refractive-index contrast layer exhibit long-term stability of the operating parameters over many years even when they are exposed to atmospheric conditions. Therefore, these lasers are promising high-power radiation sources in the terahertz spectral region for commercial applications.
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