Reconstructing parameters of the Holocene paleofloods in alluvial lowland river valleys – An example from the Bug valley (East Poland)

Abstract

One of the ways to improve the accuracy of hydrological forecasts is to extend the data strings with the parameters of the maximum flows in the river from the period preceding their instrumental recording. The reconstruction of such parameters is difficult in the case of alluvial river valleys, mainly due to the difficulties in determining the shape of the channel cross-section. The article proposes a method for reconstructing extreme Holocene paleo-flow in the case of large alluvial rivers in the European Plain. It is based on identifying specific features of the geological structure of the valley, as well as elements of its bottom morphology, which can act as high water marks (HWM). The research was carried out in the section of the Bug valley, a large lowland river flowing through Ukraine, Belarus, and Poland.In the valley reach under analysis, the cross-section of the high water channel is determined by: the surface of the resistant to erosion sub-alluvial bedrock protrusion built of glacial till, and traces of flood erosion on the Pleistocene terraces, including traces of dune washouts. The age of the paleo-flow was determined by means of the C14 dating of sediments filling the flood erosion troughs. Due to the limited number of tests, the reconstructed maximum flow values are subject to uncertainty. Yet, even quite uncertain the reconstructed paleofloods may result in a significant improvement in the estimates of high quantiles using the flood frequency analysis, or at least allow to prepare for the floods not recorded by the monitoring system. In consequence, increasing the number of measurement points, and thus improving the correctness of the determination of paleo channels cross-sections, may contribute to a significant reduction in the uncertainty of the obtained paleo-flow values.