Integration of multi-archive datasets for the development of a four-dimensional paleoflood model of alpine catchments

Abstract

Both natural and documentary evidence of severe and catastrophic floods are of tremendous value for completing multidimensional flood calendars, as well as for mapping the most extreme riverine flooding phenomena in a river basin, over centennial and millennial time scales. Here, the integration of multi-archive flood series from the Hasli-Aare, Lütschine, Kander, Simme, Lombach, and Eistlenbach catchments in the Bernese Alps constitutes a unique approach to the reconstruction of flooding events over the last six centuries and to the development of a temporal-spatial model of past flood behavior. Different types of flood archive, be they of natural or anthropogenic origin, record different processes and legacies of these physical phenomena. In this study, paleoflood records obtained from floodplains (four flood series) and lake sediments (four series), together with documentary data (six series), were analyzed and compared with instrumental measurements (four series) and the profiles of lichenometric-dated flood heights (four series) to i) determine common flood pulses, ii) identify events that are out-of-phase, iii) investigate the sensitivity of the different natural archives to flood drivers and forcing, iv) locate past flooding in an alpine region of 2117 km2, and v) simulate atmospheric modes of climate variability during flood-rich periods from 1400 to 2005 CE. Asynchronous flood response across the sites is attributed to differences in their local hydrologic regimes, influenced by (i) their physiographic parameters, including size, altitude, storage capacity and connectivity of basins, and (ii) their climate parameters, including type, spatial distribution, duration, and intensity of precipitation. The most accurate, continuous series, corresponding to the period from 1400 to 2005 CE, were integrated into a synthetic flood master curve that defines ten dominant flood pulses. Six of these correspond to cooler climate pulses (around 1480, 1570, 1760, 1830, 1850 and 1870 CE), three to intermediate temperatures (around 1410, 1650 and 1710 CE), while the most recent corresponds to the current pulse of Global Warming (2005 CE). Furthermore, five coincide with the positive mode of the Summer North Atlantic Oscillation, characterized by a strong blocking anticyclone between the Scandinavia Peninsula and Great Britain. For two of the most catastrophic flood events in the Bernese Alps (those of 1762 and 1831 CE), the location and magnitude of all the flood records compiled were plotted to provide an accurate mapping of the spatial pattern of flooding. This was then compared to the pattern of atmospheric variability. The comprehensive 4-D picture of paleofloods thus achieved should facilitate an in-depth understanding of the floods and flood forcing in mountain catchments.