Jürgen Ihringer

Statistical analysis of the flood situation and assessment of the impact of diking measures along the Elbe (Labe) river

Abstract The subject of the present paper is the flood situation in the German part of the Elbe river. For this, no (international) agreement could be reached till now. Problems occur due to missing, non-plausible or uncertain flow data and due to non-stationary hydrological conditions in the basin caused by climatological variability and human impact (diking and installation of reservoirs). Regarding this complex hydrological situation a systematic procedure consisting of existing hydrological tools was developed. In a harmonised and complementary structure this procedure mainly includes the consistency analysis of flow data, statistical analyses of the flow process, especially flood statistics and their regionalisation, stochastic simulations of the flow process in the basin and flow routing. Therewith a stepwise increasingly reliable, representative, detailed and funded knowledge of the flow process may be achieved. Till now complete longitudinal sections of statistical flood parameters along the German part of the watercourse were determined for a sufficiently long period (1964–1995) best reflecting the present situation. Moreover, approximative flood statistics could be derived for a longer period (1936–1995), which may be regarded as representative for the variable hydrological conditions during the 20th century. In addition to this fundamental study the tools were applied in order to contribute to a decision support with respect to the presently discussed measures of dike shifting. The analyses revealed that a significant impact of these measures—even the impact of all measures together—on flood peaks may be achieved only if control systems at the sites of the retention spaces are used. Finally, the paper considers options of a further utilisation of the hydrological analysis results by the definition of interfaces to other disciplines.

Analyse von Abflußzeitreihen der Elbe

Zur Untersuchung landschaftsokologischer, morphologischer und wasserhaushaltlicher Zusammenhange im Flussystem der Elbe ist die Kenntnis der Abflusverhaltnisse in der Elbe notwendig. Im Rahmen des BMBF-Verbundprojekts ‘Morphodynamik der Elbe‘ (Forderkennz 0339566) werden daher hydrologische Untersuchungen durchgefuhrt. Dieser Beitrag bezieht sich, gemeinsam mit weiteren Beitragen, auf das Biospharenreservat Mittlere Elbe, um exemplarisch die Vorgehensweise im Verbundprojekt zu demonstrieren. Es werden die Ergebnisse des Pegels Aken dargestellt, der aufgrund seiner Nahe als Referenzpegel gelten kann. Hiermit wird ein Uberblick uber hydrologische Verfahren gegeben, deren Ergebnisse bei der weiteren Erforschung des Raums nutzbar sind. Mit den Ergebnissen wird gleichzeitig eine hydrologische Rahmeninformation geliefert, die eine Orientierungsgrundlage darstellt zur detaillierteren Definition von Grenz-, Ziel- und Schwellenwerten, der konkretisierte hydrologische Analysen folgen konnen. Vor Beginn der Auswertungen der verfugbaren Abflusreihen (mittlere tagliche Abflusse und Monatsextrema bezuglich der Mestermine von November 1935 bis Oktober 1996) wurden diese kritisch untersucht. Dabei wurden fur Teilabschnitte der 70er und 80er Jahre Inkonsistenzen festgestellt. Nach der Erarbeitung eines Korrekturvorschlags wurden die Reihen fur die Auswertungen korrigiert (zum Vgl. Abb. 1). Die weiteren Untersuchungen bezogen sich im Hinblick auf Aussagen zu Hochwasserschutz und Auenokologie vor allem auf den Hochwasserbereich. Hierzu wurden die Reihe der mittleren jahrlichen Abflusse, sowie die der Scheitelabflusse der Jahre und der Vegetationszeiten (1.4.–30.9.) ermittelt und neben den Tagesreihen analysiert.

Large scale hill creep in cohesive sediments: A field and model study

A fine-grained slope that exhibits slow movement rates was investigated to understand the mechanisms which lead to a consecutive development of mass movements in the Vorarlberg Alps (Austria). For that purpose intensive hydro-meteorological, hydro geological and geotechnical observations as well as survey of surface movement rates were conducted from 1998 –2001. Subsurface water dynamics at the creeping slope turned out to be dominated by a 3 dimensional pressure system. The pressure reaction is triggered by fast infiltration of surface water and subsequent lateral water flow in the south west part of the hillslope. The related pressure signal was shown to propagate further downhill, causing fast reactions of the piezometric head in 5.5 m depth on a daily time scale. The observed pressure reactions might belong to a temporary hillslope water body that extends further downhill. The related buoyancy forces could be one of the driving forces for the mass movement. A physically based hydrological model was adopted to model simultaneously surface and subsurface water dynamics including evapotranspiration and runoff production. It was possible to reproduce subsurface pressure reactions and observed runoff in principle. However, as soil hydraulic functions were only estimated on pedotransfer functions a quantitative comparison between observed and simulated subsurface dynamics is not feasible. Nevertheless, the results suggest that similar methods of coupling surface and subsurface processes should be employed in coupled models for large mass movement and that it is possible to reconstruct important spatial structures based on sparse observations in the field.