Modelling hyperconcentrated flow in the Yellow River

A large amount of the total sediment load in the Chinese Yellow River is transported during hyperconcentrated floods. These floods are characterized by very high suspended sediment concentrations and rapid morphological changes with alternating sedimentation and erosion in the main channel, and persistent sedimentation on the floodplain. However, the physical mechanisms driving these hyperconcentrated floods are still poorly understood. Numerical modelling experiments of these floods reveal that sedimentation is largely caused by large vertical concentration gradients, both in the channel during the rising stage of the flood, as well as on the floodplains, during a later stage of the flood. These vertical concentration gradients are large because the turbulent mixing rates are reduced by the increased sediment‐induced density gradients, resulting in a positive feedback mechanism that produces high deposition rates. Erosion prevails when the sediment is largely held in suspension due to hindered settling, and is strengthened by the reduced wetted cross‐section caused by massive sedimentation on the floodplain. Observed patterns of erosion and sedimentation during these floods can be qualitatively reproduced with a numerical model in which sediment‐induced density effects and hindered settling are included. Copyright © 2009 John Wiley & Sons, Ltd.     

The use of low frequencies in a full‐waveform inversion and impedance inversion land seismic case study

Velocity model building and impedance inversion generally suffer from a lack of intermediate wavenumber content in seismic data. Intermediate wavenumbers may be retrieved directly from seismic data sets if enough low frequencies are recorded. Over the past years, improvements in acquisition have allowed us to obtain seismic data with a broader frequency spectrum. To illustrate the benefits of broadband acquisition, notably the recording of low frequencies, we discuss the inversion of land seismic data acquired in Inner Mongolia, China. This data set contains frequencies from 1.5–80 Hz. We show that the velocity estimate based on an acoustic full‐waveform inversion approach is superior to one obtained from reflection traveltime inversion because after full‐waveform inversion the background velocity conforms to geology. We also illustrate the added value of low frequencies in an impedance estimate.     

Improving hydrodynamic modeling of an estuary in a mixed tidal regime by grid refining and aligning

Water levels and flows in the Singapore coastal waters are driven by the complex interactions of the Indian and Pacific Ocean tides, seasonal monsoon-driven contributions and also forced by local winds. The Singapore Regional Model was developed to simulate hydrodynamics in the Strait of Singapore which produces representative sea level variation in this region. However, resolution and alignment of the grid system of the model with respect to depth contours in some of its subregions, i.e., the Johor Estuary area require further improvement. For this, the grid system of the model was modified and compared the simulated results with field measurements. The computed flow velocities agreed better with field observations when the grid resolution was increased. However, improving the alignment of the grid with the channel boundary (with a much lower increase in grid resolution) provided a substantially larger improvement of the model performance. The grid modification greatly influenced the computed salinity in the estuary, while water levels are slightly affected. Further analysis of model results showed a pronounced ebb tidal asymmetry generated by the O1–K1–M2 tidal constituents in the estuary.     

Qualitätsziele für die Nordsee?- Pro und Kontra-Pro

Zusammenfassung Bereits im Oktober 1973 wurde von der EG ein Aktionsprogramm für den Umweltschutz verabschiedet, in dem die Definition von Qualitätszielen gefordert wird. Danach sollen Qualitätsziele für den Schutz des Menschen gegen Umweltverschmutzung, aber auch für den Schutz der Umwelt an sich formuliert werden. Die Vorstellungen bezogen das Meerwasser zwar ausdrücklich mit ein, waren jedoch stark funktionsbezogen (Badewasser, Fischzucht). 1986 fanden Qualitätsziele in die Leitlinien Umweltvorsorge Eingang. Die Dritte Internationale Nordseeschutzkonferenz 1991 forderte, Qualitätsziele international zu entwickeln. Im selben Jahr erhielt das Nationalparkamt vom Schleswig-Holsteinischen Umweltministerium den Auftrag, ein Diskussionspapier zu erarbeiten. Dieses Papier liegt inzwischen vor. Es beschreibt ökologische Schäden und schlägt auch Parameter für Qualitätsziele vor.In der aktuellen Diskussion wird z. T. bestritten, daß die Beschreibung ökologischer Ziele von Nutzen sein könnte. Sinnvolles Handeln zum Schutz der Umwelt ist aber nur möglich, wenn Klarheit über die Ziele besteht. Auch in der Umweltpolitik reichen vage Formulierungen nicht aus, zumal dann allzu leicht eine breite, aber folglose Zustimmung erfolgt. Präzise definierte Ziele ermöglichen dagegen eine Kontrolle über den Erfolg von eingeleiteten Maßnahmen.Aus diesen Gründen müssen ökologische Qualitätsziele für den Natur- und Umweltschutz definiert werden. Die dafür benötigten Begriffe sind eindeutig festzulegen und zu verwenden (z. B. Qualitätsstandards, Grenzwerte, Richtwerte). Erst dann ist es möglich, die Ziele von den anzuwendenden Methoden zu unterscheiden, die in der bisherigen Diskussion ständig verwechselt werden. Ohne Beteiligung der Naturwissenschaftler können Qualitätsziele nicht verständlich und präzise formuliert werden.

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For and against ecological quality objectives for the North Sea-for

Summary In October 1973 the EC approved environmental protection measures in which a demand was made for a definition of ecological quality objectives. These were to be formulated to ensure that humans were protected against environmental pollution, but aims to protect the environment itself were also to be drawn up. Sea water was included in the programme, but suggestions were strongly oriented towards its various uses (bathing, fish farming). Quality aims were included in the 1986 Leitlinien Umweltvorsorge (Guidelines for environmental precautions). The 3rd International North Sea conference in 1991 requested that ecological quality objectives should be developed internationally. Also in 1991, the Ministry of the Environment in Schleswig-Holstein instructed the Nationalpark agency to draw up a discussion document which has now been presented. It describes ecolgical damage and suggests parameters for ecological quality objectives.In the current discussion, it is partly disputed that describing ecological aims could be useful. But sensible action to protect the environment is only possible if there is no confusion about the aims. In ecological policy, too, vague formulations are not sufficient, particularly if what follows is broad but ineffective agreement. Precisely defined aims, on the other hand, mean that the success of measures taken can be checked.It is for these reasons that ecological quality objectives must be defined for conservation purposes. The terms to be used must be fixed and used unambiguously (e. g. quality standards, limit values, guide values). Only then it will be possible to distinguish between aims and the methods which in previous discussions have always been confused. Ecological quality objectives cannot be formulated intelligibly and precisely without the pariticipation of scientists.

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Veröffentlichung Nr. 32 des Projektes Ökosystemforschung Wattenmeer     

SPM response to tide and river flow in the hyper-turbid Ems River

In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit down-estuary of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured as well by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPM dynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.     

Sedimentation processes in silt-rich sediment systems

Sediment found in China’s Yangtze and Yellow River systems is characterized by large silt fractions. In contrast to sand and clay, sedimentation and erosion behaviour of silt and silt–clay–sand mixtures is relatively unknown. Therefore, settling and consolidation behaviour of silt-rich sediment from these river systems is analysed under laboratory conditions in specially designed settling columns. Results show that a transition in consolidation behaviour occurs around clay contents of about 10 %, which is in analogy with the transition from non-cohesive to cohesive erosion behaviour. Above this threshold, sediment mixtures consolidate in a cohesive way, whereas for smaller clay percentages only weak cohesive behaviour occurs. The settling behaviour of silt-rich sediment is found to be in analogy with granular material at concentration below 150 g/l. Above 150–200 g/l, the material settles in a hindered settling regime where segregation is limited or even prevented. The results indicate that for modelling purposes, multiple sediment fractions need to be assessed in order to produce accurate modelling results.     

Opening a new window to other worlds with spectropolarimetry

A high level of diversity has already been observed among the planets of our own Solar System. As such, one expects extrasolar planets to present a wide range of distinctive features, therefore the characterisation of Earth- and super Earth-like planets is becoming of key importance in scientific research. The Search (Spectropolarimetric Exoplanet AtmospheRe CHaracerisation) mission proposal of this paper represents one possible approach to realising these objectives. The mission goals of Search include the detailed characterisation of a wide variety of exoplanets, ranging from terrestrial planets to gas giants. More specifically, Search will determine atmospheric properties such as cloud coverage, surface pressure and atmospheric composition, and may also be capable of identifying basic surface features. To resolve a planet with a semi major axis of down to 1.4 AU and 30 pc distant Search will have a mirror system consisting of two segments, with elliptical rim, cut out of a parabolic mirror. This will yield an effective diameter of 9 m along one axis. A phase mask coronagraph along with an integral spectrograph will be used to overcome the contrast ratio of star to planet light. Such a mission would provide invaluable data on the diversity present in extrasolar planetary systems and much more could be learned from the similarities and differences compared to our own Solar System. This would allow our theories of planetary formation, atmospheric accretion and evolution to be tested, and our understanding of regions such as the outer limit of the Habitable Zone to be further improved.     

Reservoirs as sentinels of catchments: the Rappbode Reservoir Observatory (Harz Mountains, Germany)

Reservoirs can be viewed as sentinels of their catchments and a detailed monitoring of reservoir systems informs about biogeochemical and hydrological processes at the catchment scale. We developed a comprehensive online monitoring system at Rappbode reservoir, the largest drinking water reservoir in Germany, and its inflows. The Rappbode Reservoir Observatory comprises of a set of online-sensors for the measurement of physical, chemical, and biological variables and is complemented by a biweekly limnological sampling schedule. Measurement stations are deployed at the four major inflows into the system, at the outlets of all pre-reservoirs, as well as in the main reservoir. The newly installed monitoring system serves both scientific monitoring and process studies, as well as reservoir management. Particular emphasis is paid to the monitoring of short-term dynamics and many variables are measured at high temporal resolution. As an example, we quantitatively documented a flood event which mobilised high loads of dissolved organic carbon and changed the characteristics of the receiving reservoir from eutrophic to dystrophic within a few days. This event could have been completely missed by conventional biweekly sampling programs, but is relevant for biogeochemical fluxes at the catchment scale. We also show that the high frequency data provide a deeper insight into ecosystem dynamics and lake metabolism. The Rappbode Reservoir Observatory; moreover, offers a unique study site to apply, validate, and develop state of the art lake models to improve their predictive capabilities.