Unravelling past flash flood activity in a forested mountain catchment of the Spanish Central System

Abstract

Flash floods represent one of the most common natural hazards in mountain catchments, and are fre-quent in Mediterranean environments. As a result of the widespread lack of reliable data on past events, the understanding of their spatio-temporal occurrence and their climatic triggers remains rather limited. Here, we present a dendrogeomorphic reconstruction of past flash flood activity in the Arroyo de los Puentes stream (Sierra de Guadarrama, Spanish Central System). We analyze a total of 287 increment cores from 178 disturbed Scots pine trees (Pinus sylvestris L.) which yielded indications on 212 growth disturbances related to past flash flood impact. In combination with local archives, meteorological data, annual forest management records and highly-resolved terrestrial data (i.e., LiDAR data and aerial imag-ery), the dendrogeomorphic time series allowed dating 25 flash floods over the last three centuries, with a major event leaving an intense geomorphic footprint throughout the catchment in 1936. The analysis of meteorological records suggests that the rainfall thresholds of flash floods vary with the seasonality of events. Dated flash floods in the 20th century were primarily related with synoptic troughs owing to the arrival of air masses from north and west on the Iberian Peninsula during negative indices of the North Atlantic Oscillation. The results of this study contribute considerably to a better understanding of hazards related with hydrogeomorphic processes in central Spain in general and in the Sierra de Guadarrama National Park in particular.

Regional reconstruction of flash flood history in the Guadarrama range (Central System, Spain)

“…(AEMET, 2011). Results from the previous study (Ballesteros-Cánovas et al, 2015b) carried out in the PU and MJB catchments suggest that the rainfall thresholds triggering flash floods vary with the seasonality of events. Therefore, flash floods occurring in spring were typically released during events with lower rainfall totals than those triggered in autumn and winter, therefore suggesting an important role of snowmelt processes in the triggering of flash floods (Ballesteros-Cánovas et al, 2015b).…”

“…A total of seven adjoining catchments have been selected for this study, namely the Quebradas (QUE), Paular (PAU), Altozano (ATZ), Juncional (JUN), Cárcavas Del Valle (CV), Pintadas (PIN) and Valhondillo (VLH) streams. Moreover, we compare data reconstructed for these streams with results from existing dendrogeomorphic analyses of the adjacent Puentes (PU) and Majabarca (MJB) catchments (Ballesteros-Cánovas et al, 2015b).…”

“…Increment cores were then prepared and measured using the standard dendrochronological procedures as described in Stoffel and Corona (2014). From each tree, growth series were measured visually checked for dating errors and then statistically cross-dated using an existing reference chronology from the study area (Ballesteros-Cánovas et al, 2015b;Touchan et al, 2013). From the sampled trees the following characteristics have been considered reactions to flash flood activity and related scarring: callus tissues bordering scars as well as sudden growth suppression resulting from stem and/or root damage.…”

“…Flash flood seasonality has instead been inferred from those samples showing wound borders on the increment rings and from a comparison between damage and meteorological record. Definition of flash flood events was only based on scars and associated growth anomalies, whereas meteorological data has been used to narrow down the range of possible event dates by using the thresholds suggested by Ballesteros-Cánovas et al (2015b); and references therein.…”

“…Based on a first series of data reported in a previous study (Ballesteros-Cánovas et al, 2015b) as well as highly resolved field data (i.e. LiDAR, aerial pictures, meteorological time series and forest management data), we analyze the role of geomorphic features and land use catchment predisposition on flash flood triggering.…”