Frequency, predisposition, and triggers of floods in flysch Carpathians: regional study using dendrogeomorphic methods

Abstract

Dangerous overland flood events in the foothills of the flysch Carpathians often result from a cumulative effect of floods in high-gradient channels. Detailed understanding of the origin of floods in these catchments is only possible if the occurrence of these floods in the past has thoroughly been studied. Yet, no gauging stations can be found in the local catchments. The reconstruction of floods in ungauged catchments has so far been performed using dendrogeomorphic methods. Stems or branches floating in the floodwater can affect stems or roots of living trees and injure them. Trees are able to record these signals in their tree-ring series. Within the flysch Carpathians, the floods have been reconstructed based on the analysis of 446 cross sections from scarred tree roots and 192 increment cores from the stems of affected trees in the studied area of 10 catchments surrounding the highest peak of the Moravskoslezské Beskydy Mts, the Lysá hora Mt. The dating comprised 64 floods (in different catchments) in 28 flood years for the maximal period of 1883–2012. Most catchments (nine out of ten) were affected by floods in the year 1997. Above-average frequency of floods has also been found for the last two decades, namely thanks to numerous samples taken from young tree roots that revealed more flood impacts. By contrast, although tree-ring series enabled the reconstruction of a longer time series, they only recorded the major floods.The most significant factor affecting the frequency of floods is the orientation of catchments toward the prevailing wind direction. Positive influence of catchment gradient on flood frequency and higher occurrence of floods in the period of intensive slope deforestation show that the floods of the 1950s to 1970s could have a character of flash floods. This assumption is also supported by the character of probable triggering precipitation (high magnitude–short duration precipitation) of this period. Generally, the most frequent probable triggering factor is, however, an above-average seven-day precipitation amount, which indicates — along with the positive relation between the catchment extent and flood frequency — that a majority of floods hardly ever occur as an immediate response to one-day precipitation extremes. Fast spring melting of thick snow cover plays an insignificant role in floods in the studied medium–high mountains.