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GEOLOGICAL AND GEOTECHNICAL FINDINGS OF THE CATASTROPHIC
DEBRIS FLOW NEAR TSKNETI, GEORGIA, JUNE 2015
NEUMANN, P.1, BAUER, M.1, HAIDN, M.2, KEILIG, K.1, 3, MENABDE, Z.4, DUMBADZE,
D.4
1 Baugeologisches Buero Bauer GmbH, Domagkstr. 1a, 80807 Munich (Germany),
mail@baugeologie.de
2 Trumer Schutzbauten GmbH, Handelsstr. 6, 5162 Obertrum (Autria)
3 Technical University of Munich, Arcisstr. 21, 80333 Munich (Germany)
4 Caucasus Road Project Ltd., 44 Leselidze Str., 0105 Tbilisi (Georgia)
In June 2015 a flash flood caused by a failure of a natural dam originated by a hazardous
debris flow in the Vere valley hit Tbilisi. 23 persons lost their lives and property damages were
huge. The catchment area is a region of high landslide susceptibility with a range of active and
expectable processes with differing intensities/volumes. The event of 2015 must be seen as mega-
event with recurrence periods of several 1000s of years or more. However, the landslide has
created even more unstable conditions and weakened an already semi-stable system. As
conclusion, the likelihood for medium to large subsequent events has risen significantly.
Along with the planning of the reconstruction of the Tskneti-Samadlo-road and the Tskneti-
Akhaldaba road some detailed geological investigations, e.g. large-scale engineering geological
mapping, laser scanning, monitoring of groundwater level, movement measurements etc. were
carried out or are still in progress.
These first results brought some evidence of geological, hydrogeological and geotechnical
setting in the Tskneti region and the types of processes (weathering, changes of water level) and
landslides (rock slides, rockfalls, creeping etc.) that provided debris for the catastrophic event in
2015.
This paper tries to give an idea of the general geomorphological and geological setting and
the processes and shows which measures have been taken already and what is planned in the future
to protect both roads and Tbilisi from further catastrophic hazards.
Keywords: debris flow, flash flood, geological mapping, geomorphology.
Introduction
On the night of 13-14 June 2015, a disastrous flash flood hit the Georgian capital Tbilisi
directly affecting more than 700 people, causing 23 fatalities and over USD 24 million in physical
damage [UNDP, 2015]. The flash flood originated in the Vere river west of Tbilisi [UNDP, 2015;
Gaprindashvili et al., 2016]. It was caused by exceptionally long and heavy rainfalls in the
previous ten days resulting in an already high discharge and a large landslide of approx. 1 million
m3 temporarily blocking the river and ultimately causing the flash flood after failing of the dam
[UNDP, 2015; Gaprindashvili et al., 2016]. Peak discharge during the event has been estimated
to be 468 m3/s almost doubling the discharge during the catastrophic flood in 1960 (259 m3/s
discharge) [UNDP, 2015]. Following a flood recorded on 4 June (155 m3/s discharge) this were
the highest consecutive floods ever recorded in the Vere river [UNDP, 2015]. Vere river flows
into Mtkvari river in Tbilisi.
The landslide leading to the blockage of the Vere river occurred between Tskneti and
Akhaldaba south of the Vere river and was a highly complex process of different types of
landslides, such as rock slides, debris slides, earth slides and debris flows [UNDP, 2015;
Gaprindashvili et al., 2016]. In the landslide area two important roads were completely destroyed
by rock slide (upper Samadlo road) and debris flow (lower Akhaldaba road), isolating Akhaldaba
from Tskneti, which is the villages main source for supplies and food [UNDP, 2015;
Gaprindashvili et al., 2016].
Caucasus Road Projects, Ltd. (CRP) is currently reconstructing the roads, while a group of
international experts including Trumer Schutzbauten from Austria and Baugeologisches Buero
Bauer (BBB) from Germany is consulting CRP in topics about landslide hazard assessment,
mitigation and development of an early warning system (EWS). This paper will give an overview
about the first geological and geotechnical findings regarding the landslide.
Study site
The landslide area is located about 10 km west of Tbilisi between the villages of Tskneti and
Akhaldaba (Fig. 1). The upper scarp of the landslide is located at 1410 m amsl, almost at the ridge
of the mountain range west of Tskneti. From the scarp to Akhaldaba road (910 m amsl) the average
slope angle is very steep at about 29°. From Akhaldaba road to Vere river (620 m amsl) the average
slope angle is much lower at about 6.5°.
Fig. 1: Location of the Landslide area between Tskneti and Akhaldaba. Orthophoto provided by
Municipial Development Fund.
Rock basement consists of two sedimentary formations of Tertiary age [Gudjabidze, 2003].
One formation is of Oligocene age and consists of clay- and thin layers of sandstone
[Gaprindashvili et al., 2016]. In the lower horizon of this formation claystones are more dominant,
while in the upper horizon sandstones are predominant [Gaprindashvili et al., 2016]. Thickness of
this formation can reach 2.000 m [Gaprindashvili et al., 2016]. The second formation is of Upper
Eocene age and is formed by thick-bedded sandstones with alternating layers of claystone as result
of flysch deposits [Gaprindashvili et al., 2016]. The rock formations are strongly faulted and
fractured [Gaprindashvili et al., 2016].
Methods
Geological-geomorphological Mapping
In order to develop an early warning system for the reconstructed roads BBB was assigned
by CRP to carry out detailed geological mapping with focus on spatial distribution and properties
of rock formations, geomorphological landslide features and possible landslide processes. The first
stages of mapping were performed in November 2017 and March 2018. About 80 ha were mapped
on a scale of 1 : 1.000. Mapping is fundamental and builds the essential basis for all further steps,
like safety of construction works or development of EWS.
UAV mapping
Additionally, UAV mapping and photogrammetric reconstruction was carried out in March
2018 to create an up to date digital elevation model (DEM) of the landslide area (Fig. 2 A). This
can provide a DEM at very high resolution and give us valuable information about the
geomorphology of the landslide area. Future surveys are planned to quantify geomorphologic
changes following the approach of multi-temporal DEMs of Wheaton et al. [2010]. About 27 ha
were mapped with the UAV from the scarp of the landslide until 600 m below Akhaldaba road.
Terrestrial laser scanning
Terrestrial laser scanning (TLS) was performed on two locations inside the landslide area in
March 2018 (Fig. 2 B). Above Samadlo road we scanned a suspected hazardous rock slide in order
to accurately determine the volume of the rock slide. With consecutive TLS surveys in the future
possible movement can be quantified, e.g. with the M3C2 algorithm developed by Lague et al.
[2014]. Furthermore, we surveyed the area above and below the Akhaldaba road to quantify
mobilizable volumes for debris flows and evaluate slope stability. Here we plan to carry out
additional surveys in the future as well to quantify geomorphic changes.
Fig. 2: UAV (A) and TLS (B) surveys were carried out in the landslide area in March 2018.
Results
While data of the UAV and TLS surveys is still in extensive post processing, first results of
geological and geomorphological mapping can already be presented (Fig. 4). The Oligocence
formation (Fig. 3 A & B) is very thinly bedded and is predominantly composed of clay- and
siltstones. Bedding is spaced between 0.5 cm and 15 cm (average ~0.02 m). Interlayers of
sandstones are common. The formation crops out mostly below Samadlo road and dips north with
and average angle of 30-35°, it is folded and faulted.
The bedding of the Eocene formation (Fig. 3 C & D) is spaced closely to widely. Sandstones
are predominant in the formation with interlayers of clay- and siltstones. Furthermore,
conglomerates (Fig. 3 D), partly very coarsely grained, were found in this formation.
Conglomerates exist both as concordant interlayers and unconformable fillings of erosive
channels. Bedding is spaced between 1 cm and 2.5 m (average ~0.8 m). The formation crops out
mostly above Samadlo road and dips north with and average angle of 30-35°, it is folded and
faulted. Up to now it is not completely clear if the contact of the two rock formations is concordant
or unconformable or even faulted.
As mentioned above, the rock formations dip north with an average angle of 30-35° and
therefore bedding is more or less parallel to the slope itself. This geological setting makes the
whole mountain range west of Tskneti highly susceptible to landslides. During mapping landslides
were detected, that have not been active during the 2015 event, but have a much older movement
history of a suspected 60 to 150 years (grey detachments in Fig. 4). Although there has been no
full reactivation of theses landslides there are indicators for recent movements, that could lead to
a reactivation in the future.
Fig. 3: Rock formations of the study site. A: Outcrop of thin-bedded rock formation at Samadlo
road. B: Close up of thin-bedded rock formation with predominant claystone. C: Sandstone of the
thick-bedded formation. D: Conglomerate of the thick-beddded formation. Photos: K. Keilig.
Fig. 4: Simplified geological-geomorphological sketch map of the landslide area (left), based on
the high-resolution mapping (A and B).
Fig. 5: Map showing different types of movement types (colours) and probabilities (symbols).
Based on the geological-geomorphological findings we generated a map that distinguishes
the different types of occurring landslides as well as their activity or the probability of failure (Fig.
5). This can help deciding which types of retaining structure, monitoring system or immediate
measure should be installed in the future.
As a result of mapping we developed a preliminary concept for the triggering mechanism of
the landslide (Fig. 6). We suspect groundwater and pore pressure to be the main trigger during the
2015 event. The groundwater level must have risen significantly during the ten days of heavy rain
prior to the event. The stratification and jointing of the rock formations lead to groundwater flow
parallel to the slope and possible clay layers could prevent groundwater from leaking on the
surface. Therefore, in the lower parts of the slope pore pressures must have risen immensely
leading to explosion-like escapes of water and destabilising slopes until failure in the area around
Akhaldaba road (“groundwater explosions”). The landslide then prograded to the upper parts of
the slope.
Findings about the degree of jointing and block sizes during mapping were used to quantify
rock fall energies and dimension a rock fall barriers for Samadlo road (Fig. 7). Construction of the
rock fall barrier started in April 2018 and it will be installed along the whole width of the landslide
area of 2015 above the Samadlo road.
Fig. 6: Preliminary model of triggering mechanism for the large landslide near Tskneti.
Fig. 7: Ongoing construction in April 2018 of posts for rock fall barrier by Trumer protecting the
Samadlo road. Photo: D. Dumbadze.
Conclusions
Climatic conditions before the disastrous flash flood in 2015 were exceptional and were
caused by the unfavourable combination of very high water discharge in the Vere river and a
landslide that temporarily blocked the river and eventually failed. The mountain range on which
the landslide occurred is very susceptible due to the unfavourable dipping of the bedding and the
alternating sequence of ductile claystones and brittle sandstones.
Mapping is essential for creating a geotechnical model of the slope, determining the factors
for slope failure, guaranteeing safety of construction works and developing an EWS. Since there
are indicators for further movements, mapping should be continued on the remaining parts of the
slope in our opinion.
Outlook
Mapping of the uppermost part of the landslide scarp and its souroundings have proven
essential, but extension of the mapping area will be needed to qualify the danger for Akhaldaba
village or get more information about other potentially unstable parts of the mountain range.
Further surveys with UAV and TLS are already planned for this fall and are going to be continued
in the next years. Installation of several crackmeters is currently in process in order to measure
deformations in suspected rock slide locations. Deformation of shallow and deeper landslides will
be obtained with wire extensometers to find out if movements still continue in new slides or old
slides are being reactivated. Boreholes for both deformation and groundwater measurements are
already planned and drilling and installation works commence. Additionally, it is planned to carry
out geophysical measurements in 2018. This will be fundamental for verifying the currently
preliminary model of the triggering mechanisms for the landslide as well as determining
deformations, depth of sliding plains and other important aspects of the slope.
Acknowledgment
The authors would like to thank Caucasus Road Project. Ltd., especially Mr. Paata
Trapaidze, for giving us the opportunity to work on this exciting and challenging project as well
as assisting us in terms of accommodation and transportation on site. We thank Prof. Kurosch
Thuro, Prof. Michael Krautblatter and Dr. Bettina Sellmeier from Technical University of Munich
for providing us with measurement equipment (UAV and TLS) as well as discussions about a
broad variety of topics regarding this project. Our thank also goes to Dr. Wolfgang Straka of
University of Natural Resources and Life Sciences, Vienna (BOKU) who gave us valuable
consultancy, especially about hydraulic problems and debris flows. We are grateful to the team of
HydroDiagnostics who assist us with planning and installation of crackmeters, boreholes and other
measuring units.
Gaprindashvili, G., Gaprindashvili, M. & Tsereteli, E. (2016): Natural Disaster in Tbilisi
City (Riv. Vere Badin) in the Year 2015. International Journal of Geosciences, 7, 1074-1087.
Gudjabidze, G. E. (2003): Geological Map of Georgia 1 : 500.000. Georgian State
Department of Geology and National Oil Company “Saqnavtobi”.
Lague, D., Brodu, N. & Leroux, J. (2013): Accurate 3D comparison of complex
topography with terrestrial laser scanner: application to Rangitikei canyon (N-Z). ISPRS Journal
of Photogrammetry and Remote Sensing, 82, 10-26.
UNDP (2015): Tbilisi Disaster Needs Assessment 2015 – Part 1 (Final draft).
Wheaton, J. M., Brasington, J., Darby, S. E. & Sear, D. A. (2010): Accounting for
uncertainty in DEMs from repeat topographic surveys: improved sediment budgets. Earth
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