The variability of European floods since AD 1500

The paper presents a qualitative and quantitative analysis of flood variability and forcing of major European rivers since AD 1500. We compile and investigate flood reconstructions which are based on documentary evidence for twelve Central European rivers and for eight Mediterranean rivers. Flood variability and underlying climatological causes are reconstructed by using hermeneutic approaches including critical source analysis and by applying a semi-quantitative classification scheme. The paper describes the driving climatic causes, seasonality and variability of observed flood events within the different river catchments covering the European mainland. Historical flood data are presented and recent research in the field of historical flood reconstructions is highlighted. Additionally, the character of the different flood series is discussed. A comparison of the historical flood seasonality in relation to modern distribution is given and aspects of the spatial coherence are presented. The comparative analysis points to the fact that the number of flood events is predominately triggered by regional climatic forcing, with at most only minor influence on neighbouring catchments. The only exceptions are extreme, supra-regional climatic events and conditions such as anomalous cold winters, similar to that of 1784, which affected large parts of Europe and triggered flood events in several catchments as a result of ice-break at the beginning of the annual thaw. Four periods of increased occurrence of flooding, mostly affecting Central European Rivers, have been identified; 1540–1600, 1640–1700, 1730–1790, 1790–1840. The reconstruction, compilation and analysis of European-wide flood data over the last five centuries reveal the complexity of the underlying climatological causes and the high variability of flood events in temporal and spatial dimension.     

Seasonal variation of diel vertical migration of zooplankton from ADCP backscatter time series data in the Lazarev Sea,Antarctica

Ten-month time series of mean volume backscattering strength (MVBS) and vertical velocity obtained from three moored acoustic Doppler current profilers (ADCPs) deployed from February until December 2005 at 64°S, 66.5°S and 69°S along the Greenwich Meridian were used to analyse the diel vertical zooplankton migration (DVM) and its seasonality and regional variability in the Lazarev Sea. The estimated MVBS exhibited distinct patterns of DVM at all three mooring sites. Between February and October, the timing of the DVM and the residence time of zooplankton at depth were clearly governed by the day–night rhythm. Mean daily cycles of the ADCP-derived vertical velocity were calculated for successive months and showed maximum ascent and descent velocities of 16 and –15 mm s^?1. However, a change of the MVBS pattern occurred in late spring/early austral summer (October/November), when the zooplankton communities ceased their synchronous vertical migration at all three mooring sites. Elevated MVBS values were then concentrated in the uppermost layers (<50 m) at 66.5°S. This period coincided with the decay of sea ice coverage at 64°S and 66.5°S between early November and mid-December. Elevated chlorophyll concentrations, which were measured at the end of the deployment, extended from 67°S to 65°S and indicated a phytoplankton bloom in the upper 50 m. Thus, we propose that the increased food supply associated with an ice edge bloom caused the zooplankton communities to cease their DVM in favour of feeding.     

Biomarker and stable carbon isotope analyses of sedimentary organic matter from Lake Tswaing: evidence for deglacial wetness and early Holocene drought from South Africa

Comparing the organic matter (OM) composition of modern and past lake sediments contributes to the understanding of changes in lacustrine environments over time. We investigate modern plant and lake-water samples as well as modern and ancient sediment samples from the Tswaing Crater in South Africa using biomarker and stable carbon isotope analyses on bulk OM and specific biomarker compounds. The characteristic molecular markers for higher land plants (predominantly C3-type deciduous angiosperms) in Lake Tswaing are long-chain n-alkanes (n-C₂₇₋₃₃), n-alkanols (n-C₂₈₊₃₀), stigmasterol, β-sitosterol, β-amyrin, α-amyrin and lupeol. The C₁₇ n-alkane, tetrahymanol, gammaceran-3-one and C₂₉ sterols dominate the lipid fraction of autochthonously produced OM. By comparing stable carbon isotope analyses on bulk OM and the characteristic biomarkers, we follow the modern carbon cycle in the crater environment and find indications for methanotrophic activity in the lake from isotopically depleted moretene. A comparative study of core sediments reveals changes in the terrestrial (C3 versus C4) and aquatic bioproductivity and allows insights into the variability of the carbon cycle under the influence of changing climatic conditions for the time from the end of the last glacial (Termination I) to the late Holocene, ca. 14,000–2,000 calibrated years before present (years BP). The most pronounced changes occur in the aquatic realm after ca. 10,000 years BP when our results imply climate swings from more humid to more arid and after 7,500 years BP to gradually more humid conditions again, which can be related to a shift in the position of the Inter-Tropical Convergence Zone or to changes in the tropical atmosphere–ocean interaction. Long-chain alkenones (LCAs) have been identified in ancient lake sediments from Africa for the first time. They occur in samples older than 7,500 years BP and their distribution (dominance of C₃₈ and of tri- over tetra-unsaturated LCAs) is distinctly different from other published records suggesting a to date unknown source organism.     

Determination of hydraulic conductivity of sand-bentonite mixtures for engineering purposes

Pollution of the environment due to leakage from waste repositories is a well-known and wide spread problem. Emphasis has therefore been put on design of liners for such repositories, focusing on hydraulic conductivity and its variation with time, liner composition, water content, compaction etc. The paper addresses the hydraulic conductivity of sand/bentonite mixtures, especially the variation of the hydraulic conductivity as a function of bentonite content, compaction and degree of saturation. In order to better understand the variation of the hydraulic conductivity of a sand–bentonite mixture a new parameter k ₁ has been proposed. The parameter reflects the amount of bentonite per pore volume and can easily be calculated based on the amount of bentonite and the dry density of the soil mixture. Thereby, the hydraulic conductivity can be predicted as a function of different degres of compaction. This method can be used for engineering purposes to predict the hydraulic conductivity at an early stage of a design to get an idea of the required design and hence, cost.     

Trocken- und Niederschlagsperioden und ihre theoretische Behandlung

Zusammenfassung Die beobachtete Anzahl von Trocken- und Niederschlagsperioden einer gewissen Länge und Mindestlänge sowie die mittlere Perioden-Andauer wird mit den Erwartungswerten nach der Theorie vonL. v. Bortkiewicz verglichen und keine befriedigende Übereinstimmung festgestellt; das gleiche ergibt sich bezüglich einer Poissonschen Verteilung. Berücksichtigt man eine eventuelle Verkettung der Merkmale trocken oder Niederschlag aufeinanderfolgender Tage nach der Theorie der Wahrscheinlichkeits-Ansteckung, so stehen Theorie und Beobachtung in weit besserer Übereinstimmung. Zur Erweiterung der Theorie ist zu berücksichtigen, daß die mittlere Andauer der Perioden bei verschiedenen Typen von Wetterlagen verschieden groß ist und daß eine Verkettung über die Größenordnung der Periodendauer hinaus noch dadurch zustande kommt, daß gewisse Wetterlagen gehäuft auftreten. Die Theorie führt zu zusammengesetzten Poisson-Verteilungen. Zahlenwerte über die mittlere Periodendauer bei verschiedenen Typen von Wetterlagen liegen noch nicht vor.

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Summary The observed number of dry and wet periods of a certain length and minimum length as well as the mean duration of the periods compared with the expectation values of the theory ofL. v. Bortkiewicz. As a result no satisfactory agreement was found. The result was the same with regard to a Poisson distribution. If an eventual concatenation of the characteristics dry or wet of successive days is taken into account according to the theory of probability contagion, theory and observation are in much better agreement. To extend the theory it must be considered that the mean duration of the periods varies with different types of weather conditions and that a concatenation, beyond the order of magnitude of the period duration, is caused by a greater frequency of certain weather situations. The theory leads to composite Poisson distributions. Numerical values of the mean period duration with different types of weather situations are not yet available.

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Résumé L'auteur compare le nombre de périodes sèches et humides d'une longueur déterminée ainsi que la durée moyenne des périodes aux valeurs résultant de l'application de la théorie deL. v. Bortkiewicz et ne trouve pas d'accord satisfaisant; la distribution de Poisson ne fournit pas de meilleur résultat. Si l'on considère un enchaînement éventuel entre jours secs et pluvieux du point de vue de la théorie de contagion probable, l'accord se révèle bien meilleur. Si l'on veut développer la théorie, il faut admettre que la longueur moyenne des périodes diffère selon les types de temps et que certaines périodes peuvent être anormalement longues par suite d'une fréquence accure de certaines situations météorologiques. On aboutit alors à des combinaisons de distributions de Poisson. On ne possède pas encore de données relatives aux périodes sèches et humides classées par types de temps.