Abstract Title
Total Page：16
File Type：pdf, Size：1020Kb
14th Swiss Geoscience Meeting, Geneva 2016 Modelling geomorphic responses to human impacts and extreme floods: Application to the Kander river, Switzerland Jorge Alberto Ramirez1, Andreas Paul Zischg1,2, Stefan Schürmann1,2, Markus Zimmermann1, Rolf Weingartner1,2 & Margreth Keiler1 1Institute of Geography, University of Bern, Hallerstrasse 12, 3012 Bern, CH 2Oeschger Centre for Climate Change Research, Mobiliar Lab for Natural Risks, University of Bern Originally, the Kander river flowed into the Aare river causing massive flooding in the region of Thun and this is why the Kander river was deviated to lake Thun by engineering works (Figure 1a). Besides the reduction of flooding, the goal of this deviation (Kander correction) was to prevent damming of the Aare from large sediment loads delivered by the Kander causing backwater effects and subsequent floodings in Thun upstream of the confluence. Moreover, the flood peaks occuring between Thun and Bern could be reduced. The Kander correction was pioneering, but it had unintended hydrological and geomorphic consequences. As a result of the correction the catchment area of Lake Thun increased by 50% and this additional discharge exeeded the limited storage capacity of the lake during floods. Direct sediment delivery to the Aare from the Kander ceased, but at the same time sediment flux to lake Thun increased forming the Kander delta. Furthermore, the correction shortened the Kander by 8 km and this substantially increased the slope and bed shear of the Kander upstream from the correction. Subsequently within four years the deviation itself quickly began to erode at an unprecedented rate (~30 m) and resulted in massive river bed changes that propagated up the Kander and Simme rivers. Today we may have at our disposal the theoretical and empirical foundations to foresee the consequences of such a human intervention into a natural system. One method to investigate such geomorphic changes are numerical models that estimate the evolution of a river by simulating the movement of water and sediment. Although much progress has been made in the development of these geomorphic models, few models have been tested in circumstances with extreme forcings. As such, it remains uncertain if geomorphic models are useful and stable in extreme situations that include large movements of sediment and water over short periods of time. Resolving this uncertainty is particularly important as there is a need to estimate the geomorphic effects of river restoration efforts and resolve how river channels will respond to extreme floods that are becoming more frequent with climate change. Here, in this study, we use historic maps and documents to develop a detailed geomorphic model (CAESAR-Lisflood (Coulthard et al., 2013)) of the Kander river starting in the year 1714. We use this model to simulate the extreme geomorphic events that proceeded the deviation of the Kander river into Lake Thun. We test our model by replicating the rates of incision within the correction (Figure 1b,c and Figure 2) and gauge the sensitivity of the model with floods of different magnitude and duration. 14th Swiss Geoscience Meeting, Geneva 2016 Figure 1. (a) Study site (map adapted from Wirth et al. (2011)) and modeled channel change within the Kander correction, Simme and Kander rivers after (b) year one, and (c) year four. Figure 2. Comparison of modeled and observed elevation profile of the Kander correction. Modeling period between 1714-1718. Dashed lines represent the beginning and ending of Kander correction. REFERENCES Coulthard, T.J., Neal, J.C., Bates, P.D., Ramirez, J., de Almeida, G.A.M., Hancock, G.R., 2013. Integrating the LISFLOOD-FP 2D hydrodynamic model with the CAESAR model: implications for modelling landscape evolution. Earth Surf. Process. Landforms 38, 1897–1906. doi:10.1002/esp.3478 Wirth, S.B., Girardclos, S., Rellstab, C., Anselmetti, F.S., 2011. The sedimentary response to a pioneer geo-engineering project: Tracking the Kander River deviation in the sediments of Lake Thun (Switzerland). Sedimentology 58, 1737–1761. .