Karel Šilhán

Frequency of fast geomorphological processes in high-gradient streams: case study from the Moravskoslezské Beskydy Mts (Czech Republic) using dendrogeomorphic methods

High-gradient streams are localities with the most dynamic activity of geomorphological processes in medium-high mountains. This study of the frequency of floods and debris flows in a selected high-gradient stream in the Moravskoslezské Beskydy Mts was based on a dendrogeomorphic approach. It makes use of the most accurate methods applied in the dating of historic geomorphological processes. Individual events were reconstructed on the basis of the dating of various growth disturbances displayed in 99 samples taken from 56 predominantly broad-leaved trees.As for the studied area, 26 years out of the last 113 years have been identified as years of rapid geomorphological processes. The frequency of the processes has been high above average since the 1970s. A majority of the events can be considered as flash floods. Debris flows, which can only be observed sporadically, originate due to the reactivation of old accumulation material that subsequently ends up re-accumulated on the alluvial fan at the mouth of a stream. A large number of events occur in connection with extreme short-term precipitation in summer months. In addition, they are affected by fast snow melting in spring, which has also been proved by intra-seasonal dating of selected events.

Geomorphic evidence of ancient catastrophic flow type landslides in the mid-mountain ridges of the Western Flysch Carpathian Mountains (Czech Republic)

Abstract Ancient flow type landslides are relatively frequent fossil forms of the relief in mid-mountain conditions of the Czech Carpathian Mountains. Sixty rather distinctive displays of debris flows, rock avalanches, and debris avalanches have been mapped in the uppermost part of the territory. Unlike contemporary sporadic and low volume debris flows, ancient (Pleistocene and Lower Holocene) accumulations are a few orders of magnitude more extensive and were of considerable geomorphologic significance in forming the steep sections of mountain valleys and slopes. This geomorphic pattern does not hold for flow type slides, the source of which is material released as a consequence of numerous deep-seated landslides. Due to deep disruption of slopes, a few high-magnitude flow type landslides (e.g., rock avalanches), quite rare in flysch mid-mountain conditions, also occurred in the Late Holocene.

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.

Recurrent landslides predisposed by fault-induced weathering of flysch in the Western Carpathians

Abstract The interrelationship between slope deformation and fault-induced weathering as a predisposing factor for the development of sliding is analysed through several case studies from the Western Carpathians in the Czech Republic. The study area comprises flysch nappes with alternating sandstone and shale of different permeability. These lithological structures are affected by systems of faults. Recurring slope instability is found associated with zones of deep weathering in tectonically weakened areas. Climatic variability of landslide activity can be identified during the Holocene by means of radiocarbon dating and pollen analysis. Areas affected by recurring landsliding suggest gradual and cyclic landslide frequency.

Dynamics of gully side erosion: a case study using tree roots exposure data

Abstract Erosion is a frequently studied natural process. Unfortunately, detailed analysis mostly requires longterm field monitoring or detailed digital elevation models (DEM) which are still absent for many areas. For these reasons we used the dendrogeomorphic method, a valuable tool for erosion analysis. The slopes of a ~220 m long gully close to the Kozárovce village (Slovakia) were the object of our study. We analyzed 53 tree root samples coming from 16 broad-leaved trees of different species. 23 erosion event years were explored during the reconstruction period AD 1972–2013. The mean erosion rate ER was 1.00 cm/year. We were able to create four erosion patterns of slope erosion based on subsequent exposure of different parts of root. These erosion patterns serve as a base for a model of gully cross profile development. The highest values of erosion rate were reconstructed in the lowest parts of gully slopes. The gully sides formed by volcanic rocks are affected by lateral retreat here. Upward erosion dominates in the middle parts of slopes in loess, and downward erosion in the highest parts of the slopes.

Spatio‐temporal specifics of hydro‐geomorphic processes in headwater parts of mid‐mountains reconstructed based on tree‐ring data: a case study from the hrubý jeseník mountains (czech republic)

Abstract Debris flows and debris floods cause frequent geomorphic hazards, even in the mid‐mountains of Central Europe. In the Hrubý Jeseník Mountains (Eastern Sudetes, Czech Republic), strong anthropogenic interventions have created specific conditions for erosion, transport and accumulation of material released by debris flow/flood events. We present a detailed spatio‐temporal reconstruction of the hydro‐geomorphic process activity in two adjacent sub‐catchments using dendrogeomorphic methods applied to the steep, narrow channels. An analysis of 172 sampled trees [Picea Abies (L.) Karst.] revealed 14 torrential events since 1943 in the Klepáčský stream sub‐catchment and 11 events since 1897 in the Keprnický stream sub‐catchment. Identical events were identified in 1965, 1991, 1997, 2002 and 2010. The event return periods were comparable with return periods from the foothills of the European Alps. A higher frequency of events in the first sub‐catchment may be caused by the presence of a deep‐seated landslide, steeper slopes and a higher susceptibility to shallow slope deformations. Different spatial patterns of events were presented using the Kernel Density analysis in ArcMap 10.1. Clusters of affected trees in the valley floor during the last decades may be due to increased erosion below the check dams and increased accumulation above. The presence of check dams and slope stabilization works since the 1960s has mitigated the processes in several gullies, but due to the current non‐interventional management, the risk of their damage is increasing, particularly when increased activity is observed in the adjacent unprotected gullies.

Dendrogeomorphic chronologies of landslides: dating of true slide movements?

Dendrogeomorphic chronologies of landslide movements are frequently used to investigate past landslide activity. Slide areas are often affected by other slope movements (e.g., creep) simultaneously. Trees growing on landslides record all types of ground movements, which potentially creates significant noise in tree ring based chronologies of landslide movements. The effect of creep movements on dendrogeomorphic landslide chronologies was evaluated in a block-type landslide in the south-western foreland of the Orlicke hory Mts. In total, 272 trees (P. abies and F. sylvatica) were sampled (1088 increment cores) on the sub-horizontal surfaces of rotated slide blocks, which were presumably created only by slide movements, and on the steep internal scarps separating landslide blocks, which were presumably created and affected by a combination of slide and creep movements. Ground movements were dated based on growth disturbances identified in an analysis of eccentric tree growth. The trees growing on the internal landslide scarps separating the rotated blocks exhibited a significantly higher number and frequency of growth disturbances than those growing on the sub-horizontal block surfaces. All eight dated block surface movements were also identified on the internal scarps. Creep-based events represented as many as 70 % of the dated movement events on the internal scarps. Varying the It thresholds did not filter out more than 40 % of the noise without significantly reducing the number of true dated slide events. A significant difference was observed between the ability of P. abies and F. sylvatica to record ground movements by eccentric growth. Probably due its shallower roots (and weaker anchoring of the tree to landslide blocks), P. abies appears to be more sensitive to surficial ground movement, which potentially increases the proportion of dated creep events (noise). Thus, the careful selection of sampled tree species with different physiologies should be considered during dendrogeomorphic field sampling.

Open image in new windowAge and Reactivations of Catastrophic Complex Flow-Like Landslides in the Flysch Carpathians (Czech Republic/Slovakia)

Catastrophic complex flow-like landslides (CFLLs) are characterised by a deep-seated retrogressive landslide of structurally unfavourably oriented rocks and earthflows that occupy the lower slope positions and originate due to the liquefaction of material accumulated on the upper slopes. These landslides are locally important geomorphic agents of Late Quaternary mountain evolution in the Flysch Belt of the Outer Western Carpathians (Czech Republic/Slovakia). Most of the CFLLs dammed and steepened adjacent valleys. Radiocarbon dating suggests that a majority of them moved repeatedly throughout the Holocene, namely approx. every 1–2 ka. Dated events occurred during humid phases of the Younger Dryas/Holocene transition (11.5–9.4 cal ka BP), Atlantic chronozone (7.4–6.6 cal ka BP), the beginning of the Subboreal chronozone (ca. 4.6 cal ka BP) and, primarily, within the Subatlantic chronozone at ca. 2–0.8 cal ka BP (>50 % of all events). Our study suggests that slopes based on an unfavourable structural setting and affected by long-term deep-seated gravitational deformations may produce CFLLs, even if they are located in medium-high mountains. Although our chronological dataset is influenced by the erosion of older landforms, most of the dated reactivations correlate with regional increase in precipitation identified by previous palaeoenvironmetal studies.

Deciphering Dendroecological Fingerprints of Geomorphic Process Activity

The initial employment of tree rings in geomorphic studies was simply as a dating tool and rarely exploited other environmental information and records of damage induced by earth surface processes within the tree. However, these unique, annually resolved, tree-ring records preserve valuable archives of past process activity on timescales of decades to centuries. As many of these processes also represent significant natural hazards, understanding their distribution, timing and controls provides valuable information that can assist in the prediction, mitigation and defence against these hazards and their effects on society. This chapter provides an introduction to the topics, and illustrates it with three case-study examples, demonstrating the application of tree-ring records in studying earth-surface processes and illustrating the breadth and diverse applications of contemporary dendrogeomorphology. It also underlines the growing potential to expand dendrogeomorphic research, possibly leading to the establishment of a range of techniques and approaches that may become standard practice in the analysis and understanding of earth-surface processes and related natural hazards in the future.

Dendrogeomorphic Chronologies of Debris Flows for the Crimean Mountains (Ukraine)

The Crimean Mountains (Ukraine) are renowned for the highest occurrence in debris flows along the northern coast of the Black Sea, but information on their origin, frequency and triggers is widely lacking. This study therefore aims at reconstructing a regional time series of debris flows in eight catchments located on the slopes above Yalta. Dendrogeomorphic analyses were performed on 1,122 increment cores selected from 566 black pines (Pinus nigra ssp. pallasiana) with clear signs of damage induced by past debris-flow activity. We date a total of 215 debris flows back to AD 1701 and observe a mean decadal frequency of 6.9 events, with a peak in activity during the 1940s (20 events).