The purpose of this study is to introduce specific areas around the riverbeds of the urban hidden streams in the city of Athens presenting high risk in terms of ground subsidence. The study area comprises the centre of Athens and more specifically the Municipalities of Athens, Tavros, Kallithea, Daphne, Hymettus and Ilisia where the ancient rivers of Ilissos and Eridanus naturally flow through a heavily populated city. It wasn’t until the early 1960s that the two rivers were completely channeled and partly covered along their course during the urbanization process which allowed the construction of the modern boulevards, roads, recreation and open space areas as long as parking areas. The route of today's concealed rivers and streams poses a serious risk of potential ground subsidence concerning the erosion of streams’ bank permeable soil formations. Following this approach, areas of influence (buffer zones) were determined on each side of the subterranean urban rivers Ilissos and Eridanus. In particular, an inventory of literature and references such as, historical maps and drawings, technical studies as well as modern topographic and geological maps, land use / land cover maps was created and collected in a geospatial database. It is worth mentioning that a main part of this study was the utilization and analysis of satellite radar data of the time period 2012-2016, in order to detect vertical displacements (mm/year) and correlate these with the above-mentioned spatial data of Athens city

1. DETECTING AND MAPPING SPATIAL RISK DISTRIBUTION OFDETECTING AND MAPPING SPATIAL RISK DISTRIBUTION OF
GROUND DEFORMATION INDUCED BY URBAN HIDDENGROUND DEFORMATION INDUCED BY URBAN HIDDEN
STREAMS:A CASE STUDY FROM ATHENS CITY CENTRESTREAMS:A CASE STUDY FROM ATHENS CITY CENTRE
PP.. KrassakisKrassakis11
, S, S.. KazanaKazana22
, F, F.. ChenChen33
, N, N.. KoukouzasKoukouzas11
, I, I.. ParcharidisParcharidis22
, E, E.. LekkasLekkas44
11
CENTRE FOR RESEARCH & TECHNOLOGY HELLAS (CERTH/CPERI)CENTRE FOR RESEARCH & TECHNOLOGY HELLAS (CERTH/CPERI)
22
HAROKOPIO UNIVERSITY/ DEP. OF GEOGRAPHYHAROKOPIO UNIVERSITY/ DEP. OF GEOGRAPHY
33
CHINESE ACADEMY OF SCIENCESCHINESE ACADEMY OF SCIENCES
44
NKUA/ DEP. OF GEOLOGY AND GEOENVIRONMENTNKUA/ DEP. OF GEOLOGY AND GEOENVIRONMENT

2. Land surface subsidence events in AthensLand surface subsidence events in Athens
Land subsidence is a sudden sinking of the ground surface which could lead to hazards
such as disruption of buildings, roads, railroads, gas pipes and open space areas and other
infrastructures.
Recent cases of land subsidence observed on the junction of Mesogeion Avenue and
Zagoras street, in parking area of Tavros electric urban railway station and in Kasomouli
tram railway station.
Land subsidence event at parking area of Tavros electric urban
railway station

3. Land surface subsidence events in AthensLand surface subsidence events in Athens

4. Aim of the study and Data collectionAim of the study and Data collection
The aim of this study is to introduce specific areas around the riverbeds of the urban
hidden streams in the city of Athens presenting high risk in terms of ground
subsidence.
Data Collection
 bibliographic data
 historical maps and drawings
 technical studies
 topographic, geological, hydrological and land use maps
 A data set of SLC SAR stripmap images ofTerraSAR-X satellite

5. Study areaStudy area
The study area comprises
the centre of Athens and
more specifically the
Municipalities of Athens,
Tavros, Kallithea,
Daphne, Hymettus and
Ilisia where the ancient
rivers of Ilissos and
Eridanus naturally flow
through a heavily populated
city

6. Study areaStudy area
Geological Formations
 Quaternary alluvial deposits
(al)
 Pleistocene diluvial and scree
deposits (Pt)
 Pliocene marly limestones
and sandstones (Pl)
 Maestrichtian – Eocene
Athenian schists with
sandstones and polygenic
conglomerates and crystalline
limestones of Subpelagonian
Zone (K9.sch, K9.st.c, K9.k2)

7. Introduction to SAR InterferometryIntroduction to SAR Interferometry
Differential SAR interferometry, allows measurements of land
deformation very precisely with millimetre resolution. It has various
applications in the fields of volcanology, cartography, crustal dynamics
and land subsidence. By using large stacks of SAR images acquired
over the same area, long deformation time series can be analysed
using multitemporal differential SAR interferometry techniques.These
coherent methods exploit permanently coherent Persistent
Scatterers (PSs). PSs are typically artificial objects that reflect radar
energy well such as metal structures and buildings. PSI provides a
parametric estimation of the 3D location and velocity of each PS
along the line of sight (LOS) connecting it to the satellite. In urban
areas where there is a prevalence of PSs, PSI allows analysis of even
individual structures on the ground, in particular when high-resolution
SAR images were applied.

8. Data processing (1)Data processing (1)
Twenty scenes acquired by TerraSAR-X stripmap scenes, with HH
polarization, spanning the period 5/3/2012 - 30/08/2016, along an
ascending orbit were used to carry out an interferometric analysis .
A multitemporal SAR interferometry technique was used based on the
above dataset of scenes specifically a two-step Tomography-based
Persistent Scatterers Interferometry (Tomo-PSInSAR) approach
(Fornaro et al., 2010; Zhu et al. 2016, Chen et al. 2017).
Tomo-PSInSAR outperforms PSInSAR in the detection of PS points
by jointly analyzing SAR amplitude and phase information, which is
also essential for structural instability monitoring, taking advantage of
the increased observation samples in vertical by jointly extraction of
single PSs and double PSs (on overlaid pixels).
Assuming there is only vertical deformation, the LOS observed
motion (VLos) can be projected into vertical direction (VVer)
using the formulation: VVer= VLos/cos(sigma), where sigma
is the incidence angle of SAR data

9. Data processing (2)Data processing (2)
A geospatial database was created with the above-mentioned spatial
data of Athens city centre.
Areas of influence (buffer zones) were determined on each side of the
subterranean urban rivers Ilissos and Eridanos.
Satellite radar data of the time period 2012-2016 were analysed in
order to detect vertical displacements (mm/year)
Statistically significant vertical displacements are estimated and
correlated with the available spatial data.

10. ResultsResults
Identification of 3 areas with vertical displacements characterized by high
ground deformation rates with negative values ranging from -15,75 to -2,50
mm per year:
 Konstantinoupoleos av. in Tavros Municipality
Tavros electric urban railway station in Kallithea Municipality
NW of Agricultural University of Athens inVotanikos
In some areas high accumulation of ground deformation points follow the
hidden streams’ route in a linear development.
The ground deformation is mainly related to:
1.riverbed erosion of hidden and artificial filling streams
2.shrinkage and shifting of clay soil present in alluvial and diluvial deposits
3.belowground infrastructure impacts caused by riverbed erosion

11. Results-Deformation map (mm/year)Results-Deformation map (mm/year)
Tram’s
rail

12. ResultsResults

13. www.igme.gr
Harokopio
University of
Athens
Measurements of deformation or specific targets during the
period 2012-2016 showing almost stable

14. www.igme.gr
Harokopio
University of
Athens
Measurements of deformation or specific targets during the
period 2012-2016 showing relative high rate of subsidence

15. ConclusionsConclusions
 Following the multitemporal interferometry technique it
was possible to detected, measure and monitoring of
ground subsidence along the covered streams in the
center of Athens.
 Thousands of Coherence Scatteres offering the
measuraments through the backscattering of the radar
signal corresponds to buildings, infrastructures, railways
etc allowing the location and recognition of the
scatterers’ nature.
 A geographic information system has been developed
including al the above knowledge and contribute to
prevention and mitigation.