The role of flood wave superposition in the severity of large floods

Abstract. The severity of floods is shaped not only by event- and catchment-specific characteristics but also depends on the river network configuration. At the confluence of relevant tributaries with the main river, flood event characteristics may change depending on the magnitude and temporal match of flood waves. This superposition of flood waves may potentially increase the flood severity downstream in the main river. However, this aspect has not been analysed for a large set of river confluences to date. To fill this gap, the role of flood wave superposition in the flood severity at downstream gauges is investigated in four large river basins in Germany and Austria (the Elbe, the Danube, the Rhine and the Weser). A novel methodological approach to analyse flood wave superposition is presented and applied to mean daily discharge data from 37 triple points. A triple point consists of three gauges: one in the tributary as well as one upstream and downstream of the confluence with the main river respectively. At the triple points, differences and similarities in flood wave characteristics between the main river and the tributary are analysed in terms of the temporal match and the magnitudes of flood peaks. At many of the confluences analysed, the tributary peaks consistently arrive earlier than the main river peaks, although high variability in the time lag is generally detected. No large differences in temporal matching are detected for floods of different magnitudes. In the majority of cases, the largest floods at the downstream gauge do not occur due to perfect temporal match between the tributary and the main river. In terms of spatial variability, the impact of flood wave superposition is site-specific. Characteristic patterns of flood wave superposition are detected for flood peaks in the Danube River, where peak discharges largely increase due to inflow from alpine tributaries. Overall, we conclude that the superposition of flood waves is not the driving factor behind flood peak severity at the major confluences in Germany; however, a few confluences show the potential for strong flood magnifications if a temporal shift in flood waves was to occur

Structure of the silicon vacancy in 6H-SiC after annealing identified as the carbon vacancy–carbon antisite pair

We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S=1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite–carbon vacancy pair (CSi-VC) in the doubly positive charge state can explain all experimental findings. We propose that the (CSi-VC) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy

Coenzyme A-transferase-independent butyrate re-assimilation in Clostridium acetobutylicum - evidence from a mathematical model

The hetero-dimeric CoA-transferase CtfA/B is believed to be crucial for the metabolic transition from acidogenesis to solventogenesis in Clostridium acetobutylicum as part of the industrial-relevant acetone-butanol-ethanol (ABE) fermentation. Here, the enzyme is assumed to mediate re-assimilation of acetate and butyrate during a pH-induced metabolic shift and to faciliate the first step of acetone formation from acetoacetyl-CoA. However, recent investigations using phosphate-limited continuous cultures have questioned this common dogma. To address the emerging experimental discrepancies, we investigated the mutant strain Cac-ctfA398s::CT using chemostat cultures. As a consequence of this mutation, the cells are unable to express functional ctfA and are thus lacking CoA-transferase activity. A mathematical model of the pH-induced metabolic shift, which was recently developed for the wild type, is used to analyse the observed behaviour of the mutant strain with a focus on re-assimilation activities for the two produced acids. Our theoretical analysis reveals that the ctfA mutant still re-assimilates butyrate, but not acetate. Based upon this finding, we conclude that C. acetobutylicum possesses a CoA-tranferase-independent butyrate uptake mechanism that is activated by decreasing pH levels. Furthermore, we observe that butanol formation is not inhibited under our experimental conditions, as suggested by previous batch culture experiments. In concordance with recent batch experiments, acetone formation is abolished in chemostat cultures using the ctfa mutant

Mathematical modelling of clostridial acetone-butanol-ethanol fermentation

Clostridial acetone-butanol-ethanol (ABE) fermentation features a remarkable shift in the cellular metabolic activity from acid formation, acidogenesis, to the production of industrial-relevant solvents, solventogensis. In recent decades, mathematical models have been employed to elucidate the complex interlinked regulation and conditions that determine these two distinct metabolic states and govern the transition between them. In this review, we discuss these models with a focus on the mechanisms controlling intra- and extracellular changes between acidogenesis and solventogenesis. In particular, we critically evaluate underlying model assumptions and predictions in the light of current experimental knowledge. Towards this end, we briefly introduce key ideas and assumptions applied in the discussed modelling approaches, but waive a comprehensive mathematical presentation. We distinguish between structural and dynamical models, which will be discussed in their chronological order to illustrate how new biological information facilitates the ‘evolution’ of mathematical models. Mathematical models and their analysis have significantly contributed to our knowledge of ABE fermentation and the underlying regulatory network which spans all levels of biological organization. However, the ties between the different levels of cellular regulation are not well understood. Furthermore, contradictory experimental and theoretical results challenge our current notion of ABE metabolic network structure. Thus, clostridial ABE fermentation still poses theoretical as well as experimental challenges which are best approached in close collaboration between modellers and experimentalists

The Anxious and the Climbers: Ambivalent Attitudes Towards Democracy Among South Africa's Middle Class

Beyond the hopes placed in Africa's emergent middle class as an engine of economic growth, some analysts see this group as a bastion of political stability and enduring democratisation across the continent. This paper's approach differs from that of most studies, which treat the middle class as a homogeneous group, through two key contributions. First, using cluster analysis, I propose a novel way of conceptualising social class that broadly draws on the Weberian idea of shared life chances. I apply this method to South Africa and identify five social classes characterised by their members' living standards, overall life satisfaction, and self-perceived upward mobility. Second, the empirical analysis reveals significant discrepancies in attitudes towards democracy between the downwardly and upwardly mobile strata of the middle class, which I term the "anxious" and the "climbers", respectively. On the one hand, the "climbers" show the highest generic support for democracy as a form of government, whereas the "anxious" middle class displays feelings of resignation. On the other hand, I find indicative evidence of a status-quo bias among the "climbers". Rather than assuming a more demanding or critical stance in politics, they allow their political priorities to be at least partly shaped by an interest in securing and expanding attained living standards; being upwardly mobile is even associated with a higher tolerance for government attempts to constrain freedom of information, opinion, or expression

Comparative analysis of scalar upper tail indicators

Different upper tail indicators exist to characterize heavy tail phenomena, but no comparative study has been carried out so far. We evaluate the shape parameter (GEV), obesity index, Gini index and upper tail ratio (UTR) against a novel benchmark of tail heaviness\u2013the surprise factor. Sensitivity analyses to sample size and changes in scale-to-location ratio are carried out in bootstrap experiments. The UTR replicates the surprise factor best but is most uncertain and only comparable between records of similar length. For samples with symmetric Lorenz curves, shape parameter, obesity and Gini indices provide consistent indications. For asymmetric Lorenz curves, however, the first two tend to overestimate, whereas Gini index tends to underestimate tail heaviness. We suggest the use of a combination of shape parameter, obesity and Gini index to characterize tail heaviness. These indicators should be supported with calculation of the Lorenz asymmetry coefficients and interpreted with caution