What drives flood trends along the Rhine River: climate or river training?

Abstract

Abstract. The Rhine River catchment was heavily trained over the past decades and faced the construction of the Rhine weir cascade, flood protection dikes and detention basins. For the same time period, several studies detected positive trends in flood flows and faced the challenge of flood trend attribution, i.e. identifying the drivers of observed change. The presented study addresses the question about the responsible drivers for changes in annual maximum daily flows at Rhine gauges starting from Maxau down to Lobith. In particular, the role of river training measures including the Rhine weir cascade and a series of detention basins in enhancing Rhine floods was investigated. By applying homogenisation relationships to the original flow records in the period from 1952 till 2009, the annual maximum series were computed that would have been recorded had river training measures not been in place. Using multiple trend analysis, the relative changes in the homogenised time series were found to be smaller up to about 20% points compared to the original records. This effect is attributable to the river training measures and primarily to the construction of the Rhine weir cascade. The increase in Rhine flood discharges was partly caused by the unfavourable superposition of the Rhine and Neckar flood waves. It resulted from the acceleration of the Rhine waves due to construction of the weir cascade. However, at the same time, the tributary flows across the entire Upper and Lower Rhine, which enhance annual Rhine peaks, showed very strong positive trends. This suggests the dominance of a large-scale driver such as climate variability/change which acted along with river training. In particular, the analysis suggests that the river training measures fell in a period with increasing flood trends driven by factors other than river training of the Rhine main channel.