Interpreting historical, botanical, and geological evidence to aid preparations for future floods

River flooding is among the most destructive of natural hazards globally, causing widespread loss of life, damage to infrastructure and economic deprivation. Societies are currently under increasing threat from such floods, predominantly from increasing exposure of people and assets in flood-prone areas, but also as a result of changes in flood magnitude, frequency, and timing. Accurate flood hazard and risk assessment are therefore crucial for the sustainable development of societies worldwide. With a paucity of hydrological measurements, evidence from the field offers the only insight into truly extreme events and their variability in space and time. Historical, botanical, and geological archives have increasingly been recognized as valuable sources of extreme flood event information. These different archives are here reviewed with a particular focus on the recording mechanisms of flood information, the historical development of the methodological approaches and the type of information that those archives can provide. These studies provide a wealthy dataset of hundreds of historical and palaeoflood series, whose analysis reveals a noticeable dominance of records in Europe. After describing the diversity of flood information provided by this dataset, we identify how these records have improved and could further improve flood hazard assessments and, thereby, flood management and mitigation plans

R. S. Sigafoos’s 1961 and 1964 papers on botanical evidence of paleofloods

The interaction between vegetation and fluvial processes leaves many possibilities for research. Since the publication in the early 1960s of the US Geological Survey Professional Papers by R.S. Sigafoos, numerous contributions in paleohydrology and riparian ecology have deepened the interaction between vegetation and fluvial geomorphology. In this article, we briefly review the impact of Sigafoos’ research in past and current scientific developments. We highlight the importance of the botanical evidence described by Sigafoos and the need for further work in this research line

Reconstruction of debris-flow activity in a temperate mountain forest catchment of central Mexico

The use of dendrochronology to study and date geomorphic processes in volcanic environments is still incipient, even more so on the volcanic slopes covered by temperate forests in central Mexico. Mass movements, such as debris flows, often impact forest stands where they cause damage to individual trees, thereby generating growth disturbances (GD) in the tree-ring records. The identification and dating of GD enables reconstruction of the age of trees colonizing bare surfaces after major events, but also allows the assessment of the frequency or spatial distribution of past geomorphic process activity. Here we used increment cores from 65 Pinus leiophylla, Abies religiosa, and Alnus jorullensis trees growing in the Axal gorge, on the southern slopes of La Malinche volcano, to unravel past debris-flow activity both temporally and spatially. Based on the combination of GD records, a weighted tree response index (Wit), field evidence and hydrometeorological records, we reconstructed 23 debris flows since 1933. Interestingly, almost two-thirds of the reconstructed years with debris-flow activity in Axal gorge match with events recorded in Axaltzintle gorge located on the NE slopes of La Malinche. These findings suggest a regional triggering mechanism, most likely related to the occurrence of hurricanes. This research could be useful for disaster risk management of the La Malinche National Park

XRCT images and variograms reveal 3D changes in wood density of riparian trees affected by floods

This paper combines X-ray computed tomography (XRCT) images and variogram analyses to document the response of riparian trees to mechanical damage caused by floods. Changes in wood anatomy and density have been described in the past to occur as a result of severe cambial tissue damage. However, knowledge is still fragmentary insofar as the spatial extent of responses is concerned and in terms of causative factors controlling the magnitude of response. Here, we present a novel approach combining non-destructive XRCT images with geostatistical tools to describe the extension of anatomical changes in 30 specimens of 3 Mediterranean riparian species (Alnus glutinosa (L.) Gaertn., Fraxinus angustifolia Vahl. and Salix atrocinerea Brot.) scarred by floods. We visually assess tree and wound characteristics (i.e., wound size, decayed area, callus length, callus mark) as well as the health state of trees prior to wounding. In parallel, we systematically computed 1D variograms using XRCT images so as to quantify relative tangential changes in wood density after wounding. Based on non-parametric statistical tests and Principal Component Analyses (PCA), we identify possible controls of macroscopic anatomical features on tangential affected area (TAA) and decay processes. Reactions in trees are controlled differently between species, but are driven above all by the health state of the tree prior to wounding. In view of the expected increase in the frequency of hydrogeomorphic processes and/or changes in the availability of sediments in a future greenhouse climate, wounding of trees is expected to occur more frequently in riparian forests, which could have negative effects on the sustainability of riparian vegetation