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Popular Science: Unstable east of the Czech Republic

In extreme or prolonged rainfall events, the soil is saturated with water and the cohesion of the material decreasing, consequently these processes can cause very dangerous material movements on a slope, sometimes also called "debris flows". We hear about these phenomena mainly from foreign countries, but is occurrence of debris flows possible in our region? Experts in geomorphology led by Jana Smolíková and together with Vít Vilímek from the Department of Physical Geography and Geoecology, Faculty of Science, Charles University and other colleagues answered this question.

A debris flows also occurs in our territory, although only very rarely. Their occurrence is almost exclusively limited to the highest mountains, but their strength and suddenness can be very dangerous. Several debris flows, which followed precipitation events, were recorded, for example, in the Beskydy Mts., the Krkonoše Mts., the Hrubý Jeseník, the Jizera Mts. or in the Javorníky Mts.

Debris deposite of the investigated debris flow on Lemešná Mt.
Source: Authors of the original article.

Although extreme or prolonged rainfall is known as the main trigger of slope deformations, the exact meteorological preconditions have not been fully examined. Therefore, the researchers focused on a detailed analysis of previous conditions of one debris flow in the Javorníky Range on Lemešná Mt. on 2 June 2010. They used data since 1983 from three rain gauge situated 7, 12 and 20 km from the debris flow. However, as there is considerable spatial variability in total precipitation in mountain areas, a new rain gauge was installed in the immediate vicinity of the debris flow, measuring between 29 June 2012 and 16 July 2016. Comparing the data, it was possible to better understand local precipitation variability.

The researchers analyzed the precipitation amount and intensity in detail during the whole period from 1983 to 2018. For each day, they calculated cumulative totals for 2, 3, 5, 10, 20, 30 and 60 days and the so-called antecedent precipitation index (API) for 5, 10, 20, 30 and 60 days. API is influenced by the amount of precipitation and evapotranspiration and shows previous precipitation and soil moisture conditions, which are naturally influenced by the local climate and differ significantly on a spatio-temporal scale.

An interesting finding was that during the observed period, the rainfall totals, which exceeded the debris flow in 2010 triggering precipitation occurred several times, but no slope deformation was recorded during them. The instability of the material was not caused by only one parameter, but by their combination. In this case, it was mainly a combination of cumulative rainfall for 30 days together with 1-day and 3-day amounts, overall rainfall pattern and the development of the rainfall situation. Combination of these parameters were more important for triggering the Lemešná debris flow than the individual rainfall extremes.

In addition to a new perspective to the rainfall thresholds issue of debris flows, the researchers further emphasize the importance of rainfall gauge selection. Especially in the mountains, precipitation totals can be very different even over short distances due to orography, and the values ​​of local and measured precipitation can differ.

Although the occurrence of so-called "debris flows" is relatively unique in our country, current research focused on the rainfall preconditions triggering the debris flow in the Javorníky Range is considerably important for a more accurate prediction of these dangerous phenomena not only in our territory. Next time we will be ready!

Attention! Not only because of the state border, but also because of the area of dangerous debris flows occurrence, the Javorníky Range. Photo: K. Fraindová.

Smolíková, J., Hrbáček, F., Blahůt, J., Klimeš, J., Vilímek, V., Usuga, J.c.L. (2021): Analysis of the rainfall pattern triggering the Lemešná debris flow, Javorníky Range, the Czech Republic. Natural Hazards, https://doi.org/10.1007/s11069-021-04546-7

Kateřina Fraindová

Published: Apr 19, 2021 07:20 PM

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