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An enhanced flux of fine silt and clays to areas near the dam was observed and is likely driven by a steady current towards the dam. Occurrence of detrital material in a lateral bay reveals that sediment derived not only from the main inflow but also from surface runoff through non-permanently water bearing stream channels around the reservoir. In addition to the exceptional flood layer, 22 microscopically thin detrital layers were detected in the sediment cores, most of them at the deepest core locations close to the main dam.
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Sedimenteintrag durch das Augusthochwasser in die Talsperre Lehnmuhle [Osterzgebirge] Anhand von 18 Kurzkernen aus der Talsperre Lehnmuhle Inbetriebnahme im Osterzgebirge Deutschland wurden mittels mikrofaziellen und hochauflosenden u-XRF Scanning Verfahren Auswirkungen des extremen Augusthochwassers auf den Sedimenteintrag untersucht. Fast iiber den gesamten Talsperrenboden hinweg wurde eine fur die gesamte Sedimentsequenz einmalig markante detritische Lage detektiert, welche eine Machtigkeit von 5 mm an der Staumauer bis 33 mm nahe dem Zufluss misst.
Die eingetragene Sedimentmenge dieser Lage wird auf ca. Feine Silt- und Tonpartikel wurden dagegen vornehmlich weiter in Richtung Staumauer transportiert, forciert durch eine standige Wasserstromung durch das Staubecken. Eine erhohte Akkumulation von detritischem Material in einer seitlichen Bucht zeigt, dass Sedimente nicht nur durch den Hauptzufluss eingetragen wurden, sondern ebenfalls durch Oberflachenabfluss in nicht standig wasserfiihrenden Rinnen um die Talsperre herum. Neben der markanten Lage des Jahres , wurden 22 weitere, mikroskopisch diinne detritische Lagen in den Sedimentkernen nachgewiesen, die meisten im Profundalbereich nahe der Staumauer.
Eine Chronologie der detritischen Lagen wurde an drei n7 Cs datierten Kernsequenzen erstellt und durch detaillierte Korrelation mittels vier lithologischer Marker auf die iibrigen Kerne iibertragen. Brauer, P. Feger, F. E-Mail: lucask gfz-potsdam. Particularly valuable archives are lakes because they form ideal traps in the landscape, continuously recording land surface processes in the catch- ment including extreme events Brauer , Brauer it Casanova , Thorndycraft et al.
Discrete flood-triggered sediment fluxes of detrital channel, bank and catchment material into lakes result in long chronol- ogies of detrital event layers e. Chapron et al. Comparisons with instrumental hydrological data have revealed that sediment flux is not linearly related to discharge strength and that even strong flood events can miss in the sediment records Lamoureux , Swierczynski et al.
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Sediment in- put to the lake can be minimized, for example due to wash out of river channel material by former floods Schiefer et al. Since sediment records often are obtained from a single location in the lake, missing detrital layers can also be caused by variability in sediment dispersion due to lake internal currents, stratification and basin morphometry Best et al.
Advanc- ing the knowledge about the entire chain of sedimentary processes, leading to generation of detrital layers in lake sediments is crucial for improving their hydrological in- terpretation. Besides natural lakes, reservoirs are suitable research objects, since in-depth monitoring provides a large variety of instrumental data, enabling a verification of sedimen- tological information Shotbolt et al.
Furthermore, flood events are of particular interest for reservoir management due to the consequences of an enhanced sediment delivery and the potential impact on water quality De Cesare et al. Therefore, this study aims to provide basic information about flood related sediment transport and deposition in a reservoir.
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The main objective was to investigate the deposits of the severe summer flood in the Lehnmiihle reservoir, lo- cated in eastern Erzgebirge Germany. It provides an ideal case study to investigate the impact of floods on sedimentation and to attempt to estimate total sediment flux into the reservoir during this single hydrometeorological event. It was a further objec- tive to compare these deposits with the entire sediment record, formed since the operation started in , to test if similar events have occurred in the past decades.
Reservoir surface area It is the upper of a chain of two reservoirs and was built 1 to reduce flood severity downstream and 2 to provide constant water supply for the downstream drinking water reservoir Klingenberg. In result of the latter, the water level of the Lehnmuhle reser- voir has been intensively regulated and underwent strong fluctuations.
Even an almost complete emptying took place in autumn , because of technical reasons Kaulfuss The reservoir is fed by the tributary Wilde Weifieritz, entering the main basin from the south after passing an open dam Fig. The dam was built as traffic link and to trap sediments in a flat, approx. Downstream of the reservoir the stream enters into the Elbe River in the city of Dresden.
The watershed of the upper Wilde Weifieritz Fig. Dominant soils are poorly devel- oped cambisols and podzols, which have developed from periglacial cover beds mainly above gneiss, phyllite and rhyolite Kaulfuss and, thus, are characterized by dominance of siliciclastic minerals, mainly mica, quartz and feldspars.
Annual mean precipitation ranges from approx. Maximum rainfall amounts are recorded in summer and are caused by instantaneous, often intense convective rainfall. A second maximum in winter relates to more per- sistent precipitation, often as snowfall, which is driven by westerlies Bernhofer et al.
The latter is the reason for maximum mean discharges during the snow melt sea- son from March to April, attended by the highest flood fre- quency Fig. Additionally, intense discharges frequently occur in summer due to heavy short term precipitation, like in the case of the August flood Bernhofer et al. These cores were After cutting the cores into two halves, lithological description, digital photographs and magnetic susceptibility scanning on the split core surface Barington MS2E Sensor were car- ried out for each sediment core.
The core sequences were correlated, using distinct lithological markers Fig. In addition, the uppermost 10 to 18 cm have been analyzed from most other cores Tab. Mi- crofacies analyses have been carried out on large format thin sections. For this, overlapping samples 10 cm x 2 cm x 1 cm were taken from the fresh sediment surface of a split core half. Analyses have been carried out un- der magnifications between Thin-section im- ages were obtained with a digital camera Carl Zeiss Axi- ocam and the software Carl Zeiss Axiovision 2.
Water level drawdown has led to exposure of cor- ing locations most recently notified. Exposure: Jungstes Trockenfall- en der Kemposition bei niedrigem Wasserstand. All measure- ments were performed under vacuum on a single scan line with urn spot size, im step width and a counting time of 60 s. The fluorescent radiation emitted from the sample was recorded by an energy dispersive Si Li de- tector and transformed into element information for each measuring point.
Each data point reflects the mean element intensity, expressed in counts per second cps. Arrows mark the litho- logical markers used for core-to-core correlation. Die Pfeile markieren die lithologischen Markerhorizonte fur die Korrelation der Kernprofile. The most prominent detrital layer has been sub-sampled separately for calculating the accumulated sediment mass by multiplying bulk density with layer thickness as measured in thin sections.
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Runoff data of Wilde Weifieritz were used for interpre- tation of investigated sedimentological data and are based on mean daily discharge values, recorded at Ammelsdorf gauging station Fig. The gauging station was destroyed by water masses of the flood in August and data from this time derive from the LTV The State Reservoir Administration of Saxony , calculated to daily means by water level change and withdrawal.
No discharge data are available in the time periods November to October and November to October Daily water level data measured at the main dam of Lehnmuhle reservoir since January l sl were supplied by the LTV. Sediment unit II is characterized by a general- ly higher content of organic matter and finer grain sizes, ranging from clay to coarse silt. The sharp boundary between the two main units I and II Fig. Note the diatom layer DL underlying K Thin section images were taken under crossed polarized light with DL markiert eine Diatomeenlage, unterhalb von K Dunnschliffbilder wurden unter gekreuzt polarisiertem Licht mit This boundary was observed in sediment depths between For those and seven other cores Tab.
Sediment unit Ha is characterized by a light brownish color and consists of a predominantly minerogenic, fine- grained homogenous matrix. Sediment unit lib represents a thin and dark brownish horizon, ranging in thickness from 0. Sediment unit lie is light brownish and macroscopically rather similar to sediment unit Ha.
The total thickness of sediment units Ila-c ranges from A slight increase in diatom abundance within sediment unit lie was observed in thin sections. A sharp boundary between the light brownish sediment unit lie and the uppermost darker unit lid was detected in all cores in depths between 4 cm at the basin center TSLM 5 and 5.
Compared to lower sedi- ment units, unit lid is characterized by higher organic mat- ter contents reflected in an increase of TOC of about 1.
Only in this sediment unit diatom frustules form discrete layers Fig. A characteristic feature of sediment unit lid is an in- tercalated light-colored detrital layer Fig. Thin section images were taken under crossed polar- ized light with Most abundant minerals are mica, quartz, feldspar and clay minerals.
The structure of this layer is characterized by normal grading, shown in decreasing grain sizes upwards within the layer, as measured in thin sections. Grain di- ameters in the basal part range from 40 to 60 um and only occasionally individual grains exceed urn in diameter. Quartz and feldspar minerals are dominating the coarse grained fraction, whereas in the upper part of the layer, more fine grained horizontally arranged mica minerals are abundant Fig.
The mineralogical sorting upwards with- in the layer is illustrated by increasing Al and potassium K counts, reflecting an upward increase of clay mineral and mica contents. In general, the thickness of the top clay layer is increasing towards the dam at the expense of the silt-sized part and is absent in the most proximal sediment cores TSLM 6, 7 and The total sediment input through this discrete detrital- minerogenic layer into the reservoir has been calculated to approx.
A total volume of 4, m 3 was calculated by linear interpolation of layer thickness be- tween sample points and rough extrapolation of the layer up to the rising lateral slopes Fig. In addition to this exceptional layer, microfacies analy- ses enabled to identify 22 further detrital layers Fig. All these additional detrital layers range in thick- ness from 0. In some cases, plant remains are also included. Internal structures like grading have not been observed.
Due to their detrital- minerogenic composition, even thin detrital layers can be distinguished from matrix sediments in the Al scan Fig. The abundance of detrital layers in different cores var- ies depending on their location within the basin Tab. In shallow water locations TSLM 7, , thin detrital layers are absent, likely due to erosion during low water levels HAkanson The profiles from the two deep cores clear- ly exhibit two peaks in Cs activity, related to nuclear weapon tests in and to the Chernobyl fallout in Putyrskaya et al.
These Cs peaks do not appear distinctly in core TSLM 7, likely due to the shallow-water location water depth: Similar observations were made in sediment cores from shallow water locations in Lago Maggiore by Putyrskaya et al. It is assumed that the signal faded out due to sediment mixing during times of lower water levels. Water depth at this location was at maximum 4. In the years , and this location dried up completely.