Long-term flow field monitoring at the Upper Rhine floodplains

River hydrodynamicsOverbank flows and vegetatio

Hydrodynamic simulation of the effects of stable in-channel large wood on the flood hydrographs of a low mountain range creek, Ore Mountains, Germany

Large wood (LW) can alter the hydromorphological and hydraulic characteristics of rivers and streams and may act positively on a river's ecology by i.e. leading to increased habitat availability. On the contrary, floating as well as stable LW is a potential threat for anthropogenic goods and infrastructure during flood events. Concerning the contradiction of potential risks and positive ecological impacts, addressing the physical effects of stable large wood is highly important. Hydrodynamic models offer the possibility of investigating the hydraulic effects of anchored large wood. However, the work and time involved varies between approaches that incorporate large wood in hydrodynamic models. In this study, a two-dimensional hydraulic model is set up for a mountain creek to simulate the hydraulic effects of stable LW and to compare multiple methods of accounting for LW-induced roughness. LW is implemented by changing in-channel roughness coefficients and by adding topographic elements to the model; this is carried out in order to determine which method most accurately simulates observed hydrographs and to provide guidance for future hydrodynamic modelling of stable large wood with two-dimensional models. The study area comprises a 282 m long reach of the Ullersdorfer Teichbächel, a creek in the Ore Mountains (south-eastern Germany). Discharge time series from field experiments allow for a validation of the model outputs with field observations with and without stable LW. We iterate in-channel roughness coefficients to best fit the mean simulated and observed flood hydrographs with and without LW at the downstream reach outlet. As an alternative approach for modelling LW-induced effects, we use simplified discrete topographic elements representing individual LW elements in the channel. In general, the simulations reveal a high goodness of fit between the observed flood hydrographs and the model results without and with stable in-channel LW. The best fit of the simulation and mean observed hydrograph with in-channel LW can be obtained when increasing in-channel roughness coefficients throughout the reach instead of an increase at LW positions only. The best fit in terms of the hydrograph's general shape can be achieved by integrating discrete elements into the calculation mesh. The results illustrate that the mean observed hydrograph can be satisfactorily modelled using an adjustment of roughness coefficients. In conclusion, a time-consuming and work-intensive mesh manipulation is suitable for analysing the more detailed effects of stable LW on a small spatio-temporal scale where high precision is required. In contrast, the reach-wise adjustment of in-channel roughness coefficients seems to provide similarly accurate results on the reach scale and, thus, could be helpful for practical applications of model-based impact assessments of stable LW on flood hydrographs of small streams and rivers

Implications of hydraulic anisotropy in periglacial cover beds for flood simulation in low mountain ranges (Ore Mountains, Germany)

The simulation of floods with conceptual rainfall-runoff models is a frequently used method for various applications in flood risk management. In mountain areas, the identification of the optimum model parameters during the calibration is often difficult because of the complexity and variability of catchment properties and hydrological processes. Central European mountain ranges are typically covered by Pleistocene periglacial slope deposits. The hydraulic conductivity of the cover beds shows a high degree of anisotropy, so it is important to understand the role of this effect in flood models of mesoscale mountain watersheds. Based on previous field work, the study analyses the sensitivity of the NASIM modeling system to a variation of vertical and lateral hydraulic conductivity for the Upper Flöha watershed (Ore Mountains, Germany). Depending on the objective function (Nash-Sutcliffe coefficient, peak discharge), two diametric parameter sets were identified both resulting in a high goodness-of-fit for total discharge of the flood events, but only one reflects the hydrological process knowledge. In a second step, the knowledge of the spatial distribution of the cover beds is used to investigate the potential for a simplification of the model parameterisation. The soil types commonly used for the spatial discretisation of rainfall-runoff models were aggregated to one main class (periglacial cover beds only). With such a simplified model, the total flood discharge and the runoff components were simulated with the same goodness of fit as with the original model. In general, the results point out that the anisotropy in the unsaturated zone, which is intensified by periglacial cover beds, is an important element of flood models. First, a parameter set corresponding to the hydraulic anisotropy in the cover beds is essential for the optimum reproduction of the flood dynamics. Second, a discretisation of soil types is not necessarily required for flood modeling in Central European mountain areas

Water and sediment pollution of intensively used surface waters during a drought period — a case study in Central Northern Namibia

Semiarid regions are often affected by water scarcity and poor water quality. Seasonal changes in precipitation and drought events increase the pressure of use on water bodies and their pollution. In Central Northern Namibia, a high seasonal intra- and inter-annual variability of precipitation caused a 5-year lasting drought period. In the semiarid region, ephemeral channels and water pans represent the main water source, besides the institutionalized water supply. No systematic analysis of its quality has been conducted so far. The states of the surface waters at the end of the dry season in 2017 and the end of the rainy seasons in 2018 and 2019 were characterized by the analysis of physical–chemical parameters, focusing on usability. The first results show coarse contamination of the waters, which results in high turbidity values. Salt concentrations, such as Ca2+ and Na+, greatly increased due to evaporation. Al is present in high concentrations in solid and liquid phases, which indicates direct anthropogenic pollution. Spatial differences are evident in the study area and based on the precipitation gradient, land use, and population density. The waters cannot be used as drinking water without prior treatment

Endbericht des Forschungsprojektes RuBuS

Der vorliegende Bericht behandelt die Ergebnisse des Forschungsprojektes RUBUS, welches im Zeitraum November 2013 bis November 2015 von der Freien Universität Berlin durchgeführt wurde. Gegenstand des Projektes ist die Untersuchung der aktuellen Belastungssituation und der Dynamik der Sedimente in der Rummelsburger Bucht als Teil der urbanen Spree in Berlin. Das Projekt hat die Klärung der räumlichen Verteilung, Quantifizierung und Bewertung der Schadstoffbelastung in den Sedimenten der Rummelsburger Bucht zum Ziel. Es soll vor diesem Hintergrund die Ausbildung einer unbelasteten Sedimentauflage geprüft werden, um damit auch der Frage nachzugehen, ob es möglicherweise zu einer Remobilisierung von schadstoffbelasteten Sedimenten kommt. Um diese Hypothese zu untersuchen wird ein indirekter Messansatz gewählt, der sowohl die Erfassung der Randbedingungen als auch den qualitativen und quantitativen Nachweis einer Sedimentremobilisierung berücksichtigt. Da die Prozesse als solche nur schwer messbar sind, zielt dieser Ansatz darauf ab, entsprechende Leitindikatoren zu wählen, anhand derer Mobilisierungsmechanismen und Ursachen aufgezeigt werden können. Die Erfassung der Schadstoffe (Schwermetalle, organische Verbindungen) erfolgt systematisch an 5 Schichten der obersten 15 cm des Gewässersedimentes und über die Erfassung von akkumuliertem Material in Sedimentfallen. Die Beprobungen und kontinuierlichen Messungen werden an 15 über den gesamten Wasserkörper rasterförmig ausgelegten Bohr- bzw. Messpunkten durchgeführt. Zur Bemessung des potenziellen Risikos wird für die Feststoffphase der Consensus 2 - Wert der Sedimentqualitätsrichtlinien nach de DECKERE et al. (2011) zugrunde gelegt und durch Elutionsversuche sowie ökotoxikologische Tests ergänzt. Die Laborergebnisse zeigen, dass an allen Standorten und in annähernd allen erfassten Sedimentschichten weitestgehend sämtliche untersuchten Elemente und Verbindungen in zum Teil erheblichen Konzentrationen nachgewiesen werden können. Die Konzentrationen variieren sowohl in der Fläche als auch mit der Tiefe der Sedimentschichten. Die Analysen und statistischen Tests weisen das nordwestliche Untersuchungsgebiet als am stärksten belastet aus. Ungeachtet dieser räumlichen Variationen zeigen die Ergebnisse, dass in der Rummelsburger Bucht eine durchgängige Belastung der oberen 15 cm Sedimentschicht vorliegt. Eine flächendeckende Belastung kann auch für die beprobten Sedimentfallen nachgewiesen werden. Im Vergleich zum Referenzstandort in der Spree zeigt sich, dass die Rummelsburger Bucht sowohl hinsichtlich der Sedimentkerne als auch hinsichtlich der Sedimentfallen eine durchgängig höhere Belastung aufweist. Effektbasierte SQG werden in den meisten Fällen überschritten. Eine Untersuchung aquatischer Makrozoen durch die Bundesanstalt für Gewässerkunde zeigt zudem, dass die Artenvielfalt gering ist und sich das Vorkommen auf tolerante, euryöke Arten mit geringen Ansprüchen an die Gewässer- und Sedimentqualität beschränkt. Für den hohen gemessenen Sedimentumsatz ist vor allem die Remobilisierung von Sedimenten innerhalb der Bucht verantwortlich. Die stark belasteten, sehr schluffhaltigen Sedimente werden über eine permanente Strömung diffus über die gesamte Seefläche verteilt. Als bisherige Ursache für den hohen Sedimentumsatz können u.a. meteorologische Bedingungen und der Bootsverkehr identifiziert werden. Die mechanische Mobilisierbarkeit von feststoffgebundenen Schwermetallen und zinnorganischen Verbindungen ist unter den vorherrschenden Bedingungen in der Rummelsburger Bucht gering. Die chemisch-physikalischen Randbedingungen des Untersuchungsgebiets sind im Jahreszyklus nur geringen Schwankungen unterworfen, was der weiteren Fixierung der Schwermetalle entgegenkommt. MKW und PAK werden hingegen durch die Elution in höheren Konzentrationen freigesetzt, insbesondere aus den Schwebstoffen. Im Fall der PAK werden in den meisten Proben die ZHK-UQN bzw. die JD-UQN der Europäischen Wasserrahmenrichtlinie für die Wasserphase überschritten. Keiner der biologischen Tests zeigt eine gefährliche Belastung an. Es werden jedoch mit dem Sedimentkontakt- und dem Leuchtbakterientest bei mehreren Proben kritische Belastungen ermittelt. Von den PAK geht offensichtlich keine Gefährdung für die Testorganismen Vibrio fischeri (Leuchtbakterien) und Lemna minor (Wasserlinsen) aus. Die Wasserlinsen reagieren durchgehend wenig sensitiv auf die Inhaltsstoffe der Eluate. Es ist anzunehmen, dass die signifikant höhere MKW-Freisetzung aus den Schwebstoffen (im Gegensatz zu den Sedimenten) die toxische Wirkung im Leuchtbakterientest verursacht. Der Sedimentkontakttest weist bei einer den Schadstoffgehalten nach relativ unbelasteten Probe einen toxischen Effekt nach. Die hohen Schwermetallgehalte in den Sedimenten haben keine Wirkung auf die Bakterien des Sedimentkontakttests, was darauf hindeutet, dass sie nicht in bioverfügbarer Form vorliegen. Die Schwermetalle scheinen unter stabilen Bedingungen gebunden vorzuliegen. Die PAK und MKW zeigen eine deutlich höhere Dynamik in der Schadstoffmobilität. Die Rummelsburger Bucht stellt selbst im internationalen Kontext eine Besonderheit hinsichtlich der hohen Belastung mit organischen Schadstoffverbindungen und Schwermetallen dar und zeigt, dass die industriellen Fingerabdrücke der Gesellschaft über äußerst lange Zeiträume erhalten bleiben und dies eine Herausforderung für Politik und Planung darstellt. Die Untersuchungsergebnisse zeigen aber auch, dass die Rummelsburger Bucht einer ausreichend großen Dynamik unterliegt, welche zu einer (Re-) Mobilisierung dieser hochbelasteten Sedimente führen kann. Die Sedimente fungieren als langfristige potenzielle Schadstoffquelle für den Wasserkörper und damit auch für die Spree.The present report covers the results of the research project RuBuS, realised by the Freie Universität Berlin between November 2013 and 2015. The investigated water body “Rummelsburger Bucht” is a former anabranch of the Spree River located in the centre of Berlin. Covering an area of more than 45 ha, it is only at the upstream side connected to the main river course. For almost a century, untreated industrial and municipal wastewater was discharged into this river branch. Consequently, the quality of both the water and the sediments decreased dramatically over that period. In order to address this problem, the project “RuBuS” (co-financed by the Berlin State Government and the European Funds for Social and Regional Development) was established for the above mentioned period. To perform any risk assessment, the investigation should provide an improved knowledge and a better understanding about the type and spatio-temporal pattern of sediment contamination and dynamics. To detect the spatial distribution of pollutants in the sediment, over 200 sediment samplings were carried out via drill cores from 16 locations. The upper 15 cm of each drill core was systematically divided into 5 layers (each of 3 cm) for separate examination. The investigation of sedimentation and remobilisation rates was accomplished by installing 18 sediment traps, which collected deposits over more than a year. The presence of selected heavy metals and organic pollutants in the sediments was determined for every position and layer of the drill cores, as well as for all sediment traps. Detection of changes in boundary conditions, influencing the spatial and temporal distribution of sedimentation and resuspension, was accomplished by placing devices within the water body and taking different mobile measurements. At all sampling locations, flow conditions were recorded every week over the whole water column with a vessel-mounted ADCP (Acoustic Doppler Current Profiler). Water quality parameters like oxygen, turbidity, chlorophyll-(a) and temperature were also measured weekly with a mobile multiparameter sensor at all stations in 50 cm steps. In addition, these parameters were continuously recorded every 10 minutes with stationary sensors at different water depths (1, 2 and 3 m) from the beginning of the monitoring. For mobile measurements the probe YSI 6600 V2 was used, whereas permanent recording was done with Cyclops-7 and MiniDot devices from PME. A weather station was set up on the shore side to determine wind conditions, precipitation, temperature and solar radiation, concerning the external impacts on water currents, turbulences and algae-bloom. The scope of the investigation included the determination of the total content of inorganic (Hg, Cd, Cr, Pb, Ni, Cu, Zn) and organic compounds (polycyclic aromatic hydrocarbons (PAH), total petroleum hydrocarbons (TPH), selected nitro-compounds, selected organotin compounds and polychlorinated biphenyls (PCB, AOX and EOX) in the sediment and suspended matter. The relevant particle size for the investigation of samples for heavy metals was < 63 μm and for the organic components < 2 mm. The compounds were detected by ICP-OES, ICP-MS and AAS analyses. Grain size distributions weredetermined using laser diffractometry and sieving. An elemental analyser was used to determine the presence of carbon and nitrogen in the samples. The release of soluble components upon contact with water is considered one of the main mechanisms leading to a potential hazard to the environment. The research into soluble and easily mobile sediment-bounded pollutants is based upon a 24 hour batch test. In addition, certain toxic effects of the sediments were determined by ecotoxicological test methods. Three species were chosen as relevant test organisms: Vibrio fischeri for the luminescent bacteria test, Lemna minor for the Duckweed growth inhibition test and Arthrobacter globiformis for the sediment contact test. In addition, the thresholds of the sediment quality guidelines, published by de Deckere et al. (2011), were used to assess the solid contents. The study demonstrated that so far the concentration of organic and inorganic pollutants in the sediments has remained very high. The total contents of the PAHs and all heavy metals exceeded the effect-based threshold values according to de Deckere et al. (2011). The results were compared to the “consensus 2” thresholds to identify the measured concentrations, which have surpassed the limits and now giving rise to concern regarding toxicity. The only exception among the heavy metals was mercury. With respect to organic pollution the reference point in the Spree River, which is not very far away from the sheet pile wall, showed concentrations below the threshold value. In contrast, the reference point disclosed pollution by heavy metals except for cadmium and mercury concentrations, which were below the threshold. However, the load of heavy metals at the reference point was lower than the one found in the bay. A different spatial distribution of the organic contamination was observed for the sediments of the “Rummelsburger Bucht”. In terms of contamination with PAHs, Cd and Pb, areas with different loads could be defined, this was statistically proven with a significant difference greater than 99%. The less contaminated area was situated between the sheet pile wall and the centre of the bay. Higher loads (sampling sites 10-15) were found from the centre of the water body to the north-western shore. However, the sediments in the traps showed a homogeneous distribution of the measured pollutants. It was also found, that the level of contamination commonly increased with sediment depth, except at the sampling sites right in the centre of the bay, which revealed no clear tendency to increase or decrease depending on the depth. With regard to the level of the European Environmental Quality Standards (EQS), the results of the 24 hour batch test indicated a low mobility of the heavy metals, organotin compounds and Benzo(a)pyrene. By contrast, other PAH compounds exceeded in several eluates the annual average maximum concentration permitted by the EQS. The results of the Luminescent bacteria test demonstrated for the eluates of sediment cores, that all samples are not or harmless polluted (non- or low toxic effect). However, with the exception of two samples, all of the eluates of suspended particles were critically polluted, they showed a moderate or increased toxic effect. The recorded high turnover rates of sediments were most likely caused by a remobilisation of local sources within the “Rummelsburger Bucht”. These highly contaminated and silty sediments can be evenly distributed over the whole area of the water body due to very slow but constant currents with changing directions. Up to now, specific meteorological boundary conditions and motorboat-traffic have been identified as some of the main reasons for the remobilisation and local transport of the sediments, as well as external sediment sources could be neglected. The research project “RuBuS” provides a detailed insight into this water body. It gives a better understanding how changes in boundary conditions affect the spatial and temporal distribution of sedimentation and the resuspension of sediments. The study demonstrates, that so far the concentration of organic and inorganic pollutants in the sediments has remained on an exceptionally high level. The “Rummelsburger Bucht” can be considered as a case study about spatio-temporal pattern of sediment contamination and dynamics of the Spree River and about areas of conflict presented by highly stressed urban water bodies. A high pressure on demands of local residents for recreational and commercial use of the “Rummelsburger Bucht” needs to improve the environmental situation by a targeted and affordable management plan

Microplastics in Namibian river sediments – a first evaluation

The African continent is rarely the focus of microplastics research, although the ubiquity of microplastics in the environment is undisputed and still increasing. Due to the high production and use of plastic products and the partial lack of recycling systems in many parts of the African continent, it can be assumed that microplastic particles are already present in limnic and terrestrial ecosystems. Few studies, mainly from South Africa and the Northern African region, show a contamination with microplastics, especially in marine environments. This study aims to explore the presence and composition of microplastics in fluvial sediments of the major catchments in Namibia with a regional focus on the Iishana system in Northern Namibia, as one of the most densely populated areas in the country. In March 2019 and March 2021, at the end of the rainy seasons, sediments from the Iishana system and of the largest river catchments were sampled. Extraction was performed by density separation using the Microplastic Sediment Separator (MPSS) with the separation solution sodium chloride (density of 1.20 g/cm3). The particle size was determined by filtration and fractionation, and the polymer type by measurement with ATR-FTIR spectroscopy (minimum particle size 0.3 mm). Microplastics were found in the sediments of each river system, most of the particles in the Iishana system (average of 13.2 particles/kg dry weight). The perennial, the ephemeral rivers, and the Iishana system are similar concerning polymer type and particle size. Polyethylene and polypropylene were the dominant polymer types. Most of the particles were found in the size fractions 0.3 – 0.5 mm and 0.5 – 1.0 mm. The particles were found mainly as fragments and films, the majority transparent and brown

GNSS mobile road dam surveying for TanDEM-X correction to improve the database for floodwater modeling in northern Namibia

The aim of this study is the improvement of the TanDEM-X elevation model for future floodwater modeling by implementing surveyed road dams and the use of filter algorithms. Modern satellite systems like TanDEM-X deliver high-resolution images with a high vertical and horizontal accuracy. Nevertheless, regarding special usage they sometimes reach their limits in documenting important features that are smaller than the grid size. Especially in the context of 2D-hydrodynamic flood modelling, the features that influence the runoff processes, e.g. road dams and culverts, have to be included for precise calculations. To fulfil the objective, the main road dams were surveyed, especially those that are blocking the flood water flowing from south Angola to the Etosha Pan in northern Namibia. First, a Leica GS 16 Sensor was installed on the roof of a car recording position data in real time while driving on the road dams in the Cuvelai Basin. In total, 532 km of road dams have been investigated during 4 days while driving at a top speed of 80 km/h. Due to the long driving distances, the daily regular adjustment of the base station would have been necessary but logistically not possible. Moreover, the lack of reference stations made a RTK and Network-RTK solution likewise impossible. For that reasons, the Leica SmartLink function was used. This method is not dependent on classic reference stations next to the GNSS sensor but instead works with geostationary satellites sending correction data in real time. The surveyed road dam elevation data have a vertical accuracy of 4.3 cm up to 10 cm. These precise measurements contribute to rectifying the TanDEM-X elevation data and thus improve the surface runoff network for the future floodwater model and should enhance the floodwater prediction for the Cuvelai Basin