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

Regional flood occurrence in the culmination zone of medium-high mountain ranges by tree-ring based reconstruction: Frequency, triggers, dynamics

Floods are among the most dangerous geohazards in Central Europe. Their occurrence is often the result of the cumulative contribution of sub-catchments in the culmination zone of a mountain range, which subsequently has a devastating effect in the foreland. However, data on discharges from gauging stations are mostly missing from these sites (high-gradient streams), which are crucial to understanding the origin of floods in low-lying populated areas where they can cause significant damage. Therefore, this study focuses on an extensive reconstruction of flood events in 13 sub-catchments in the culmination zone of the Orlicke hory Mts. Flood events were reconstructed using dendrogeomorphic approaches, currently the most accurate absolute dating method. The analysis revealed 111 floods in all sub-catchments during the 34 to 84 year period by dating 844 growth disturbances in a tree-ring series of 632 trees (Picea abies (L.) Karst.) damaged during the floods. Regional reconstruction across the mountain range revealed events of regional and local significance, with no direct link between event magnitude and areal extent. This is consistent with the two dominant rainfall patterns identified that likely triggered the floods (short-term extreme rainfall and medium-term above-average rainfall). In particular, however, the study revealed several patterns of spatial transformation of flood events from source sub-catchments to their form captured at gauging stations in the foreland. The combination of various lines of evidence (geomorphic mapping, growth disturbance patterns, spatial pattern of flooding) suggests a limited erosional effect of most of the reconstructed events. The findings thus shed new light on the overall dynamics of floods in the mountain massif and their impact on flood discharges in the foreland.

250 years of flood frequency and discharge in an ungauged Corsican mountain catchment: A dendrogeomorphic reconstruction

The primary goal of paleoflood hydrology is to estimate the frequency and magnitude of past floods. Botanical evidence, and particularly scars on trees, has been used repeatedly as paleostage indicators to reconstruct peak discharges and flood height. Yet, these reconstructions depend on the presence of visible scars on tree stems which tend to be masked as trees grow older. Here, we estimated flood magnitude using an alternative approach based on growth disturbances in tree-ring series, tree positions and the minimal discharge necessary to submerge the root collar of a tree as estimated by hydraulic modeling. We tested the reliability of this newly developed approach by using the traditional scar-based reconstruction as a benchmark. To this end, we sampled 60 trees showing evidence of flood damage on their stems along a 787-m long segment of the Asco river (Corsica, France). Based on 440 growth disturbances dated in tree-ring series, we reconstructed 28 floods between 1759 and 2020 and 18 during the 20th century. Using the two-dimensional Iber hydraulic model and detailed topographic data of the study site obtained from UAV imagery, we estimated that peak discharges of the 28 reconstructed events ranged between 10 and 210 m³s^-1, with 200 m³s^-1 being considered as the threshold for extreme floods. Not only do the scar-based and root collar submersion approaches yield similar results, findings are also clearly in line with the sparse information available from historical archives and short gauge station records on past floods. The unprecedented length and depth of the record presented here opens new avenues for climate change and flood impact research.

Occasional but severe: Past debris flows and snow avalanches in the Helmos Mts. (Greece) reconstructed from tree-ring records

The eastern Mediterranean is a hotspot in terms of geomorphic hazards, but the activity of gravitational processes in mountainous areas is largely unexplored. We carried out dendrogeomorphic research in the Helmos Mountains (Northern Peloponnese, Greece) to determine the timing, spatial extent, and hydrometeorological triggers of debris flows and snow avalanches. Specifically, we sampled increment cores from 123 injured Greek firs (Abies cephalonica L.) growing on a debris flow cone and growing along a snow avalanche track. Tree rings were counted and cross-dated with the reference chronology using CooRecorder and CDendro software and the event years were determined on the basis of the location of scars and traumatic resin ducts. We compiled an 118-year chronology (1904-2021) with seven debris flow event years and only one severe debris flow occurring in the 1970/1971 dormant period (W_It = 148.0), followed by spatially limited records for 1986/1987 (W_It = 3.8) and 1993/1994 (W_It = 2.5). Similarly, seven snow avalanche event years were identified in the period 1854-2021, with one major event in 1997/1998 (W_It = 304.5) followed by the 1998/1999 event (W_It = 6.3). Extremely wet conditions in February-March 1971 followed by rain-on-snow precipitation were considered as the most likely trigger of the analysed debris flow event using data from nearby meteorological stations and the ERA5 reanalysis. The snow avalanche event was deciphered in the spring of 1998, when heavy snowfall over three days (62 cm) was followed by rapid snowmelt due to high average temperatures (6-11 °C). We conclude that the abundance of snow is a crucial factor in the geomorphic activity in the study region and that the temperature fluctuations and rain-to-snow transitions are the leading factors for the debris flows or snow avalanches to occur. Furthermore, the dendrogeomorphic approach used can be useful to clearly identify large-scale geomorphic events and excludes potential geomorphic noise caused by other ecological stresses.

Flood signals in tree-ring δ18O and wood anatomical parameters of Lagerstroemia speciosa: Implications for developing flood management strategies in Bangladesh

Bangladesh consists of 80% of the flood plain of the Ganges-Brahmaputra-Meghna river system (GBM), making the country one of the highest flood prone countries of the world. Due to the high rate of discharge of the GBM caused by the summer monsoon and the snowmelt of the Eastern Himalaya and Southern Tibetan Plateau due to climate change, Bangladesh witnessed 16 flood events over 1954-2017. We performed a multiproxy tree-ring analysis to investigate the impact of extreme flood events on tree growth, xylem anatomical parameters and oxygen isotope composition of tree-ring cellulose (δ¹⁸O_tr) in a Bangladeshi moist tropical forest and to establish relationships between water level of the regional rivers and tree-ring parameters. By using pointer year analysis and comparing the pointer years with historical flood records (a cut-off threshold of the country's flooded land area of 33.3%), we identified the three extreme flood events (hereafter called flood years) 1974, 1988, and 1998 in Bangladesh. Superposed epoch analysis revealed significant changes in Tree-ring width (TRW), total vessel area (TVA), vessel density (VD), and δ¹⁸O_tr during flood years. Flood associated hypoxic soil conditions reduced TRW up to 53% and TVA up to 28%, varying with flood events. In contrast, VD increased by 23% as a safety mechanism against flood induced hydraulic failure. Tree-ring δ¹⁸O significantly decreased during the flood years due to the amount effect in regional precipitation. Bootstrapped Pearson correlation analysis showed that wood anatomical variables encoded stronger river level signals than TRW and δ¹⁸O_tr. Among the wood anatomical parameters, VD showed a strong relationship (r = -0.58, p < 0.01) with the water level of the Manu River, a regional river of the north-eastern part of Bangladesh, indicating that VD can be used as a reliable proxy for river level reconstruction. Our analyses suggest that multiproxy tree-ring analysis is a potential tool to study tropical moist forest responses to extreme flood events and to identify suitable proxies for reconstructing hydrological characteristics of South Asian rivers.

Intra-annual dendrogeomorphic dating and climate linkages of flood events in headwaters of central Europe

Unlike large rivers, floods in headwaters have been poorly documented despite the fact that greater discharges are expected in such areas due to ongoing rainfall intensification. The purpose of this study is to carry out intra-annual dating of past floods combined with analysis of their climate linkages which may point on distribution and origin of floods. To this end, we applied dendrogeomorphic dating of impact scars on riparian vegetation to provide flood chronologies in twelve headwaters of eastern Czechia and determined their seasonal occurrences. Furthermore, we analyzed the precipitation indices and evaluated the flood events using reanalysis of pressure fields and the climate oscillation indices, particularly those representing the North Atlantic, Scandinavian, and East Atlantic/Western Russian patterns. Based on 434 dated trees, we identified 22-31 flood event years in each region over the last 70 years. The most frequent floods occurred from May to July. In addition, in ten event years it was possible to identify multiple flood events: for example May and July-August floods in 2010, 2014, and 2016. The monthly precipitation showed the strongest, but still weak, correlation with indices of Scandinavian climate oscillation during summer months (R_s = 0.23-0.42; p < 0.00), suggesting the influence of a blocking anticyclone over Scandinavia and wet air propagation to central Europe. This finding was also confirmed by the most frequent positions of low-pressure centers located east of the study sites except for the region with western orographic enhancement. In addition, timing shift of extreme precipitation to earlier period was identified at two out of three regions. We conclude that intra-annual dating of floods supported by analyses of climate extremes provides new data from sparsely gauged headwaters, thereby supplementing the information on possible changes in flood occurrences during ongoing climate change.

The Impact of Reforestation Induced Land Cover Change (1990–2017) on Flood Peak Discharge Using HEC-HMS Hydrological Model and Satellite Observations: A Study in Two Mountain Basins, China

Understanding the effect of land use and land cover (LULC) type change on watershed hydrological response is essential for adopting applicable measures to control floods. In China, the Grain to Green Program (GTGP) and the Natural Forest Conservation Program (NFCP) have had a substantial impact on LULC. We investigate the effect of these conservation efforts on flood peak discharge in two mountainous catchments. We used a series of Landsat images ranging from 1990 to 2016/2017 to evaluate the LULC changes. Further to this, the hydrological responses at the basin and sub-basin scale were generated by the Hydrologic Modeling System (HEC-HMS) under four LULC scenarios. Between 1990 and 2016/2017, both catchments experienced an increase in forest and urban land by 18% and 2% in Yanhe and by 16% and 8% in Guangyuan, respectively. In contrast, the agricultural land decreased by approximately 30% in Yanhe and 24% in Guangyuan, respectively. The changes in land cover resulted in decrease in flood peak discharge ranging from 14% in Yanhe to 6% in Guangyuan. These findings provide a better understanding on the impact of reforestation induced LULC change on spatial patterns of typical hydrological responses of mountainous catchment and could help to mitigate flash flood hazards in other mountainous regions.

In situ reconstruction of long-term extreme flooding magnitudes and frequencies based on geological archives

Extreme flooding magnitudes and frequencies are essentially related to assessment of risk and reliability in hydrological design. Extreme flooding and its discharge are highly sensitive to regional climate change. Presently, its discharge can be reconstructed by a geological archive or record along the river valley. Two units of typical extreme flooding deposits (EFDs) carrying long-term information preserved in the Holocene loess-palaeosol sequence were found at Xipocun (XPC), which is located in Chengcheng County, China. It is situated in the downstream section of the Beiluohe (hereafter BLH) River. Based on multiple sedimentary proxy indices (grain-size distribution (GSD), magnetic susceptibility (MS), and loss-on-ignition (LOI), etc.), EFDs were interpreted as well-sorted clayey silt in suspension. They were then deposited as a result of riverbank flooding in a stagnant environment during high water level. Through the Optically Stimulated Luminescence (OSL) dating technique and stratigraphic correlations, chronologies of two identified extreme flooding periods were 7600-7400 a B.P. and 3200-3000 a B.P. Two phases of extreme flooding occurrence under climate abnormality scenarios were characterized as having high frequencies of hydrological extremes in river systems. According to simulation and verification using the Slope-Area Method and Hydrologic Engineering Center's River Analysis System (HEC-RAS) model, the extreme flooding discharges at the XPC site were reconstructed between 9625 m³/s and 16,635 m³/s. A new long-term flooding frequency and peak discharge curve, involved gauged flooding, historical flooding at Zhuangtou station and in situ reconstructed extreme flooding events, was established for the downstream BLH River. The results improve the accuracy of low-frequency flooding risk assessment and provide evidence for predicting the response of fluvial systems to climate instability. Thus, this improves the analysis of the BLH River watershed.

Improving Flood Maps in Ungauged Fluvial Basins with Dendrogeomorphological Data. An Example from the Caldera de Taburiente National Park (Canary Islands, Spain)

Flash floods represent one of the more usual natural hazards in mountain basins, and, combined with the lack of reliable flow data and the recreational use of the drainage basin by tourists and hikers, there is a significant risk of catastrophe. Here, we present a dendro-geomorphological reconstruction of a past flash flood event in the Caldera de Taburiente N.P. (Canary Islands, Spain), an ungauged drainage basin in the SW side of the volcanic island of La Palma. We couple two-dimensional hydraulic modelling in a highly-resolved topographic environment (LiDAR data) with (1) peak flow data for various Tyear return periods from an uncalibrated hydrological model and (2) a data set of scars on trees, to investigate the magnitude of a 1997 dated flash-flood. From the results, flood hazards and associated risks would be clearly underestimated by using only the unique available hydrological data (a rainfall gauge station downstream of the study area). Hydraulic models using scars data show a higher flood hazard scenario, improving the flood hazard map by using all available flood evidence. Moreover, all this will allow for better implementation of appropriate adaptation policies by National Park managers, and therefore the mitigation of future disasters.

Tree ring-based chronology of hydro-geomorphic processes as a fundament for identification of hydro-meteorological triggers in the Hrubý Jeseník Mountains (Central Europe)

Hydro-geomorphic processes have significantly influenced the recent development of valley floors, river banks and depositional forms in mountain environments, have caused considerable damage to manmade developments and have disrupted forest management. Trees growing along streams are affected by the transported debris mass and provide valuable records of debris flow/flood histories in their tree-ring series. Dendrogeomorphic approaches are currently the most accurate methods for creating a chronology of the debris flow/flood events in forested catchments without any field-monitoring or a stream-gauging station. Comprehensive studies focusing on the detailed chronology of hydro-geomorphic events and analysis of meteorological triggers and weather circulation patterns are still lacking for the studied area. We provide a spatio-temporal reconstruction of hydro-geomorphic events in four catchments of the Hrubý Jeseník Mountains, Czech Republic, with an analysis of their triggering factors using meteorological data from four nearby rain gauges. Increment cores from 794 coniferous trees (Picea abies [L.] Karst.) allowed the identification of 40 hydro-geomorphic events during the period of 1889-2013. Most of the events can be explained by extreme daily rainfalls (≥50mm) occurring in at least one rain gauge. However, in several cases, there was no record of extreme precipitation at rain gauges during the debris flow/flood event year, suggesting extremely localised rainstorms at the mountain summits. We concluded that the localisation, intensity and duration of rainstorms; antecedent moisture conditions; and amount of available sediments all influenced the initiation, spatial distribution and characteristics of hydro-geomorphic events. The most frequent synoptic situations responsible for the extreme rainfalls (1946-2015) were related to the meridional atmospheric circulation pattern. Our results enhance current knowledge of the occurrences and triggers of debris flows/floods in the Central European mountains in transition between temperate oceanic and continental climatic conditions and may prompt further research of these phenomena in the Eastern Sudetes in general.