1 PGGH_SALZBURG_004

PanGeo D7.2.14: Geohazard Summary for Salzburg
1
1.1
PGGH_SALZBURG_004
GENERAL PROPERTIES OF THE MOTION AREA
The polygons are mainly concentrated between the Langwied and Gnigl central area of Salzburg and in the
municipality of Seekirchen am Wallersee.
The polygon covers an area of 0,397 km2.
The bedrock geology is characterised by Salzburger lake clay in Seekirchen and by moraine würmian
deposits in the city of Salzburg (figure 18).
Fig.20
Figure 18: geological map (1:200.000 Salzburg Region) for the city of Salzburg and surrounding showing the
mapped geohazard of shrink swell clays.
The land cover classes interested by the presence of lake clay are mainly discontinuous dense urban fabric
and industrial (figure 19).
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PanGeo D7.2.14: Geohazard Summary for Salzburg
Fig.20
Figure19: GMES urban atlas for the city of Salzburg and surrounding showing the mapped geohazard of
shrink swell clays.
1.2
SPECIFIC GEOHAZARD TYPE
3_2ShrinkSwellClays Instability
1.3
TYPE OF MOTION
Subsidence
1.4
THE DETERMINATION METHOD
1.Observed in PSI data
1.5
CONFIDENCE IN THE INTERPRETATION
High
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PanGeo D7.2.14: Geohazard Summary for Salzburg
1.6
GEOLOGICAL INTERPRETATION OF THE MOTION
The rate of subsidence indicated by the PSI data (only covering unfortunately the period of observation
1993-2000) it´s been also measured locally (the two yellow stars of figure 20). The measurement 12-40 mm
per year (April 2004) it´s in agreement with the maximum rate of subsidence observed with PSI
measurements (13 mm/y). The reason of this phenomenon is related to the lake clay sediment that
undergoes a process of shrink in summer and swelling in winter. The phenomenon is very well known since
20 years the so called "schiefen Häuser" in Seekirchen” (http://web.utanet.at/tothladi/daten/21.htm)
1.7
VALIDATION OF THE MOTION
The motion in Seekirchen (figure 20) is documented in two reports related to the improvement of the
properties of the town flood protection dam (Bautechnische Versuchs- und Forshungsanstalt Salzburg,
2006).
Figure 20 highlighted area in the municipality of Seekirchen with the mapped geohazard and the potential
geohazard overlaid.
The graph in figure 21 shows the geodetic levelling profile measurements (dotted black line in figure 20),
started on 07-10-2002 and ended on 25-04-2006, taken very near to the Seekirchen dam. Overall there is a
registered subsidence ranging between -150 and 250 mm (37,5 and 62 mm/y). This relatively high rate of
vertical displacement is due to the settlement of the propriety of the geotechnical characteristic of the
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PanGeo D7.2.14: Geohazard Summary for Salzburg
redesigned dam. In fact further inclinometer measurements taken between 2008 and 2009 (figure 22)
reported a reduction of the subsidence up to -18 in a year and a half (Bautechnische Versuchs- und
Forshungsanstalt Salzburg, 2009). We can in fact conclude that the monitoring campaigns showed a
reduction of the shrink and swell behaviour of the lake clay on the village of Seekirchen mainly due to the
improvement of the dam design.
Figure 21: Levelling profile measurements (highlighted in figure 20) in 6 geodetic stations parallel to the
dam position, with maximum cumulative vertical displacement of -250 mm in 4 year (July 2002 - April 2006)
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PanGeo D7.2.14: Geohazard Summary for Salzburg
Figure 22: Inclinometer NG_KB108 highlighted in figure 20, with a maximum vertical displacement of -18
mm in a year and a half (January 2008 - July 2009).
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