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Wet weather spurs landslide movement

The reactivation of a prehistoric landslide has caused damage to buildings in Pissouri, Cyprus since 2012. In this post, Gareth Hearn and colleagues document a three-fold increase in the rate of land movement during and after an unusually wet winter.

The residential development of Limnes in Pissouri, Cyprus, (Fig 1) was constructed on a slope formed by a prehistoric landslide. Reactivated ground movements within this landslide, first noticed by residents in 2012, continue to cause significant damage to property. Ground movements accelerated from up to 25 mm per year between 2006 and 2010 to between 60 and 140 mm per year over large parts of the Limnes slope between 2014 and 2017.

Map of study area
Fig. 1 The Limnes study area, Pissouri, Cyprus

Since our initial study (Hearn et al. 2018), we have been using a combination of synthetic-aperture radar (SAR) and conventional ground survey to monitor slope movement. From the ground survey, we found that rates of movement increased by a factor of three both during and following the very wet winter of 2018 to 2019, with average rates of displacement of 7 to 9 mm per day between October 2018 and May 2019, compared to 2 to 3 mm per day for the same period the previous year (Hearn et al. 2020).

Abandoned building with massive crack
Fig. 2 Abandoned building located across a major tension crack

These rates of movement were too high to be reliably gauged from the analysis of SAR data,  emphasising the importance of independent ground truthing of remote sensing interpretation. The outcome has been a marked increase in architectural and structural damage to some properties (Fig. 2), as well as building rotation (Fig. 3).

Area of slope movement
Fig. 3 Horizontal rotation to building (X) between 2017 and 2019 due to movement along the main lateral shear of the landslide

We undertook damage surveys both before and after the 2018 to 2019 wet winter season and, as would be expected, most of the more significant damage was located around the periphery of the landslide and in the vicinity of the main tension cracks (as shown in Fig. 2), and especially among older buildings with pad foundations. Many of the more modern buildings have raft foundations, and these have tended to move monolithically, with minimal apparent structural effect.

Although we were unable to assess movements on the main landslide through our analysis of the SAR data captured between December 2017 and September 2019, we were able to use it to examine the condition of the 40 m-high back scarp and the ground behind it. The 80 mm per year ground movement contour is located at the crest of the back scarp in some areas (Fig. 4), with the 40 mm per year contour extending approximately 100 m to the west and northwest. These developments are, to an extent, supported by field observations, however there is a need for an accurate ground survey to confirm the pattern and progress of any continuing ground movements in this area.

Ground movements across main slip area
Fig. 4 Interpretation of December 2017 to September 2019 satellite InSAR results showing ground movement contours in mm per year for the back scarp and the area behind it

We can conclude that the situation at Limnes has worsened significantly as a result of the very wet winter season of 2018 to 2019, which was the second wettest on record, and probably resulted in the highest water condition on the slope since residential development began in 1980. Although some of the worst-affected buildings have been abandoned, there are some that remained occupied as of September 2019.

While the effects of movement within the landslide mass are plain to see, further investigation and monitoring of the back scarp and the residential areas behind it will be needed to confirm the extent and depth to which displacements might have become initiated in these areas as well.

Gareth Hearn is the director of Hearn Geoserve Ltd

References

Hearn, G.J., Larkin, H., Hadjicharalambous, K., Papageorgiou, A. & Elina Zoi, G. 2018. Proving a landslide: ground behaviour problems at Pissouri, Cyprus. Quarterly Journal of Engineering Geology and Hydrogeology, 51, 461–482, https://doi.org/10.1144/qjegh2017-134

Hearn, G.J., Larkin, H., Papageorgiou, A. & Elina Zoi, G. 2020. Acceleration of the Pissouri landslide, Cyprus. Quarterly Journal of Engineering Geology and Hydrogeology, https://doi.org/10.1144/qjegh2020-014

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