Markus Noack

Uncertainty analysis of settling, consolidation and resuspension of cohesive sediments in the Upper Rhine

ABSTRACT Understanding the dynamics of settling, consolidation and remobilization of cohesive and contaminated sediment is an important requirement to assess the risk of erosion and to manage mud-dominated ecosystems. Here, we present data on the erodibility of cohesive sediments from the impounded Upper Rhine River and develop a modelling concept to understand the suspended sediment dynamics along the impounded Upper Rhine. The conceptual framework includes 10 reservoirs of the Upper Rhine between Basel and Iffezheim, which serve as long-term sinks of cohesive sediment. Each reservoir is represented by a 1D sediment budget model, which is coupled to its upstream and downstream neighbour. In this paper, we focus on the uncertainty associated with the measurement of the erodibility of cohesive sediments and on the implications to model the risk of erosion. The statistical analysis showed a large uncertainty of the estimated critical shear stresses and erosion rates. Root mean square errors are in the order of 50% and 100% of the average critical shear stresses and average erosion rates, respectively. Preliminary model results are in good agreement with bed changes of the Iffezheim reservoir, which were measured using approximately quarterly echo-soundings of the reservoir. The probabilistic approach that is based on Monte-Carlo simulations allows assessing the uncertainty related to our limited knowledge (i) of the dynamics of contaminated sediments and (ii) of the environmental conditions in the reservoir that control the risk of erosion of contaminated sediments.

An Evolution Volume Balance Approach to Determine Relevant Discharge Threshold for Bed Load Transport

The aim of this study is to investigate discharge rates at which a flow hydrograph becomes relevant for bed load transport on the example of the Alpine river Saalach. Two characteristic flow hydrographs of the river Saalach are selected and reduced to discharges above pre-specified values. With each set of reduced hydrographs, a 2D morphological simulation is conducted. Two important morphological parameters, namely total river bed evolution (i) and sorting effects of the active layer (ii) are analyzed and evaluated. Moreover, an additional simulation is conducted with a linearly increasing flow hydrograph as boundary condition to analyze the initiation of bed load transport as a function of discharge (iii). The results are analyzed and evaluated with respect to the total sediment output at the downstream boundary of the model and at some inner control sections to validate the findings for (i) and (ii). The model results show: (i) For regions with fine bed materials, discharge thresholds between 100 and 130 m3/s and for regions with mainly coarse bed materials discharge thresholds in the range between 150 and 180 m3/s can be identified as relevant discharges for bed load transport and may be used for reducing hydrographs without affecting the accuracy of the model results; (ii) The active layer shows no obvious changes in the mean diameter of sediments for all tested hydrographs. (iii) The investigations with a linearly increasing flow hydrograph show that the sediment output from the investigated river reaches increase significantly for discharges exceeding 150 m3/s.

Modellversuch zur Untersuchung des Sandaustrags aus Geröllfängen mittels Druckbelüftung

Zur Vermeidung des Eintrags von grobem Sediment in die mechanische Reinigungsstufe einer Kläranlage ist ein vorgeschalteter Geröllfang mit ächigem Belüftungssystem vorgesehen. Durch den Einsatz mehrerer ächig angeordneter Belüftungselemente soll der feinkörnige Sedimentanteil remobilisiert und von der Grobfraktion abgetrennt werden. Die Ergebnisse eines physikalischen Modellversuchs zeigen, dass die Korngrößenseparation und der Austrag von Feinsedimenten aus dem Geröllfang besser gelingen bei erhöhtem Luftvolumenstrom, einem relativ großen Verhältnis von Geröllfangtiefe zu eingetragener Geröllschicht und großem Zu uss.