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Detail článku

Ročník:

56/2019

Název česky:

Datování recentní aktivity divočícího úseku řeky Úpy v Obřím dole v Krkonoších

Název anglicky:

Dating of recent activity of the braided Úpa River in the Obří důl valley

Autor (autoři):

Radek Tichavský; Karel Šilhán; Marcela Půlpánová

Klíčová slova česky:

povodně, divočící tok, dendrogeomorfologie, Obří důl

Klíčová slova anglicky:

fl oods, braided river, dendrogeomorphology, the Obří důl valley

Od strany:

145

Do strany:

162

Abstrakt česky:

Povodně na horských tocích jsou příčinou geomorfologické transformace koryt vodních toků a přilehlé
údolní nivy. Větší dynamiku transformace údolního dna lze očekávat ve zdrojových oblastech s nadprůměrným množstvím
mobilního materiálu. Takové toky pak vykazují charakter větvení nebo divočení s častými morfologickými
změnami údolního dna. Jedním z ojedinělých příkladů divočení v ČR je údolní dno pod soutokem Úpy a Rudného
potoka v Obřím dole v Krkonoších. Extrémní srážkové události za přispění mobility sedimentů z navážek hlušiny
důlních děl zapříčiňují zvýšenou transportní aktivitu. Cílem příspěvku je pomocí dendrogeomorfologických metod
(založených na identifi kaci událostí pomocí analýz letokruhů stromů) datovat recentní povodňové události, které
způsobily transport materiálu z rozplaveného dejekčního kuželu Rudného potoka do údolní nivy Úpy. Z 30 vzorkovaných
jedinců smrku ztepilého se podařilo identifi kovat celkem 10 povodňových událostí, přičemž většina z nich
byla zachycena v období od roku 1997. Největší vliv byl identifi kován v letech 1997 –1998, 2001, 2005 –2007 a 2013.
I přes omezené množství vzorkovaného materiálu byla potvrzena značná dynamika za posledních 20 let projevující
se čerstvými akumulacemi různé zrnitostní frakce podél hlavního koryta. Článek rovněž demonstruje možnosti
potenciálního budoucího výzkumu v dané lokalitě s ohledem na její jedinečnost v rámci ČR.

Abstrakt anglicky:

Floods in mountain catchments cause geomorphic transformation of river channels and fl oodplains.
Higher dynamics and transformation of the valley fl oor can be expected in areas with an abnormal amount of sediments.
Such conditions allow the development of braided and wandering rivers with frequent morphological
changes of the valley fl oor. The confl uence of the Úpa River and the Rudný potok stream in the Obří důl valley in
the Krkonoše Mts (the Giant Mts) is a rare example of such a pattern within the Czech Republic. Extreme rainfall
and an abnormal supply of sediments coming from heaps which remained after mining works caused a high transport
rate of the streams. The aim of this paper was to identify fl ash fl ood events based on dendrogeomorphic methods
(dating of tree-ring series to identify the activity of geomorphic processes). In total, 10 fl ood events were identifi
ed based on the analysis of increment cores from 30 trees (P. abies). In addition, eight events were found to have
occurred since 1997. The highest number of growth disturbances was identifi ed in 1997 –1998, 2001, 2005 –2007,
and 2013. Despite the limited number of sampled trees we were able to confi rm high transport dynamics during
the last 20 years manifested by fresh debris accumulations along the main channel. Moreover, the study demonstrates
the opportunities for future research in this area with respect to its uniqueness within the Czech Republic.

Souhrn anglicky:

Introduction The morphological features of mountain
rivers and surrounding fl oodplains are the result
of hydrogeomorphic processes such as fl oods, debris
fl ows and landslides. In addition, increasing anthropopressure
(infl uence of humans on natural processes)
leads to greater impact and changes to the erosion,
transport and depositional processes of the river continuum.
Mountain environments which produce an
abnormal amount of sediments transported by rivers
are typical of braided and wandering river systems
with bedload transport and a resulting multichannel
confi guration which evolves and changes during the
occurrence of fl ashy discharges. These river systems
frequently occur in glacial and periglacial environments,
but are rather scarce in the mild temperate climate
of mid-mountains due to abundant forest cover
which stabilizes the surface. One exception (within the
Czech Republic) can be found in the Obří důl valley
(the Krkonoše Mts) below the confl uence of the Úpa
River and the Rudný potok stream. Due to the presence
of an active alluvial fan supplied by material from
heaps, there is a frequent reactivation of sediments
during extreme rainfall events which create the wandering
and braided character of the Úpa River. If we
want to describe the patterns of such a dynamic environment,
we need to know the frequency and magnitude
of the hydrogeomorphic processes participating
in the morphological changes. Dendrogeomorphic
reconstructions represent an appropriate approach
to date past hydrogeomorphic events (such as fl oods
and debris fl ows) in ungauged, forested catchments.
Based on the presence and position of specifi c growth
disturbances within tree-ring series caused by fl oods,
we were able to accurately identify the event to a specifi
c year. The aim of this paper was to characterise
the geomorphic features of the above-mentioned site
and create a chronology of fl ood events responsible
for the higher transport rate causing disturbances to
riparian vegetation.
Material and methods The main morphological features
of the Obří důl valley were created during several
phases of glacial retreat and advance at the end of
the Pleistocene. Hydrogeomorphic and gravitational
processes (predominantly debris fl ows and snow avalanches)
have remodelled the slopes since the beginning
of the Holocene until now. The recent morphology
of the fl oodplain below the confl uence of the Úpa
River and the Rudný potok stream is a result of polygenetic
development which takes into account frequent
fl ood events, debris fl ows and mining works. The alluvial
fan at the confl uence was predominantly fed by
the material from debris fl ow paths during their reactivations
(the most known in 1897). Due to mining works during the period 1952 –1959, a lot of material
was stored as heaps just in the channel bottom of
the Rudný potok stream. In 1974 and 1977, extreme
rainfall triggered fl ood events resulting in the destabilization
of the heaps, high transport rate of sediments
and destruction of the alluvial fan. An enormous
amount of material was deposited even in the
fl oodplain of the Úpa River causing its braiding. Flash
fl oods of different magnitudes have occurred several
times since 1977 (e.g. in 1994, 1997, 2006, and 2013).
In the fi rst step, we used a digital terrain model
based on LiDAR data (resolution 2 × 2 m) to analyse
the basic morphometric features of the study
site. We especially focused on the basin area, slopes
and cross-sections of the fl oodplain. Publicly available
aerial images and orthophotos from 1936, 1955,
2000, and 2016 were analysed to identify temporal
changes in the development of the confl uence area.
In the next step, detailed geomorphic mapping was
performed, focusing on erosional and accumulation
forms within the fl oodplain. In total, 50 trees (Picea
abies (L.) Karst) were sampled for the purpose of dendrogeomorphic
dating. 30 samples were extracted from
the trees with visible scars on their stems as a consequence
of the transported material which injured the
tree. In addition, 20 trees growing in the neighbourhood,
but without geomorphic disturbances, were
used for the reference chronology to avoid misinterpretation
of the climate signal and identify false and
missing rings within the tree-ring series. All samples
for the dendrogeomorphic analysis were processed
in the laboratory. Tree rings were counted and measured,
and growth disturbances (scars, traumatic resin
ducts (TRD), compression wood, abrupt growth suppression
and release) related to the fl oods were identifi
ed. The identifi cation of fl ood event years was based
on the values of a calculated event-response index.
Results and discussion The headwater catchments
above the confl uence show different morphometric
features. The Úpa headwater catchment (183,1 ha)
is fi ve times bigger than the headwater part of the
Rudný potok catchment (36,6 ha). Several subfossil
debris fl ow paths (the last high-magnitude activity
more than 80 years ago) can be observed in both
catchments. From the aerial images and orthophotos
it is obvious that the transport rate of both catchments
has changed during the 20th century. Until the 1970s,
the Rudný potok stream fed only a limited amount of
material to the Úpa river fl oodplain from compared
to the headwaters of the Úpa River. However, it is evident
from the recent orthophotos that the transport
rate of the Rudný potok stream now prevails over the
Úpa River leading to the morphological transformation
of the valley fl oor below the confl uence. Dendrogeomorphic
analysis revealed 61 growth disturbances
with a predominance of scars and TRD. In total, we
were able to identify ten years of fl ood events. Due to
the sampling strategy, most of the events (eight) were
identifi ed during the period 1997 –2018. The oldest
event was dated to 1941, while the youngest event
was identifi ed in 2013. The highest It index values
were calculated for 2007 (40%), 1998 (33.3%), and
2006 (20%). The return period of fl ood events during
the last 30 years was 3.8 years. Despite the limited
number of sampled trees and past events (more
than 40 years old), this study is the fi rst tree-ring based
reconstruction of fl ood events in the Krkonoše Mts.
We identifi ed several consecutive event years (e.g.
1997 –1998 and 2005 –2007). Interpretation was diffi
cult in several cases, because such a pattern could be
the result of delayed responses of trees to fl ood disturbances
(e.g. abrupt growth release identifi ed in
fi ve cases in 1998 may be a delayed response to the
high-magnitude fl ood in July 1997). In contrast, it is
presumable that reactivations on the alluvial fan and
morphological changes of the fl oodplain below may
occur more frequently than geomorphically effective
fl oods in the whole region. Therefore, we assume that
local rainstorms may cause material transport in this
limited area without any traces in the lower reaches
of the Úpa River.
Conclusion and future challenges The resulting chronology
confi rmed some historically known fl ood events
(e.g. 1997, 2006, and 2013) but some of the fl ood
events were previously unknown or not described in
the literature (e.g. 2001, 2005, and 2010). Dendrogeomorphic
approaches are an alternative methods to
date past fl ood events in ungauged, forested mountains
and therefore their use can be further extended.
Given that the study site is considered as unique within
the Czech Republic, it deserves more attention for geomorphic
research. A more intense dendrogeomorphic
reconstruction, together with sedimentological
analyses, geophysical sounding and analysis of large
woody debris, could bring more comprehensive results
related to the development of the Úpa River braiding.

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