The sky kept crying over Smutná. What happened next?
Harsh mountain landscapes attract adventurers as well as scientists. The natural conditions there allow the formulation of unusual but also destructive phenomena and processes, often of large proportions. One such phenomenon is that of fast-moving masses of poorly sorted sediments saturated with water, known as debris flows. Studying these, along with the specific preconditions that give rise to them, is crucial for the prediction and prevention of devastating consequences. Therefore, geomorphological experts Tereza Dlabáčková and Zbyněk Engel from the Department of Physical Geography and Geoecology, Faculty of Science, Charles University have focused on mapping and evaluating the cause of a recent event.
Specifically, the experts went abroad into the inhospitable environment of the Western Tatras. In these mountains, they examined the recently formed debris flow in the area of Roháčská valley, in the Smutná valley. The extensive debris flow occurred after a rainfall event on 15 May 2014 and reshaped the local relief.
Author: Tereza Dlabáčková.
How can such a debris flow be characterised from a geomorphological point of view? Debris flow originates in a source zone, usually on the slopes of high mountains which are not protected by dense tree vegetation and have a slope of at least 20°. After release, the material moves through the transport zone, where erosion and partial deposition also occur, until it ends up in the accumulation area. The debris flow in the Smutná valley began to take place on the north-eastern slope of Plačlivé peak at altitudes of 1657–2013 m, before continuing north along the ridge to the valley bottom. The maximum size of the blocks transported by the debris flow reached 3.5 m.
What initiated such a massive movement and material detachment from the bedrock? For such phenomena, sediments saturated with water are essential. The cohesion of the material is compromised and subsequently the mass of the material sets into motion. The presence of a sufficient amount of unconsolidated regolith accumulated in bedrock gullies is also important. Rainfall lasting a full 29 hours was recorded in the Smutná valley, during which 120–135 mm of precipitation occurred. Such totals exceed the threshold values for initiating the formation of debris flows derived from global models. If we examine the precipitation totals for 24 hours, which reached 101–111 mm, these were relatively high compared to the reported values for previous events in the Tatras, Babí Hora, and the more distant Southern Carpathians. For comparison, the average precipitation totals in the Czech Republic in one year are less than 700 mm.
The researchers found that the debris flow that formed in Smutná valley in 2014 represents one of the largest accumulations in the northern part of the Western Tatra Mts. over the last 43 years. The erosion-accumulation zone extends all the way to the valley floor and stretches for more than half a kilometre.
The mountain ranges offer a unique scenery that is often breathtaking. However, visits need to be embarked upon with great humility and vigilance, especially in connection with significant meteorological phenomena. The recent event in the Western Tatras, described and evaluated by Czech scientists, is a notable example as extremely intense rainfall caused rapid, short-term gravitational movements, which were highly destructive and significantly reshaped the landscape.
Kateřina Fraindová
Published:
Apr 04, 2022 01:20 PM
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