Date post: | 28-Apr-2018 |
Category: |
Documents |
Upload: | truongdiep |
View: | 217 times |
Download: | 4 times |
23 juni 2015
Land Subsidence
Simulation and Implications in Deltas.
Mahmoud Bakr
National Water Research Center
23 juni 2015
Contents
- Definition, causes, symptoms, and Data acquisition.
- Method of analysis and simulation
- Case study of Jakarta
- Land subsidence in the Nile Delta
23 juni 2015
Definition
Land subsidence is the lowering of the land
surface due to changes that take place
underground.
San Joaquin Valley subsidence
Mining ground water for agriculture has enabled the San
Joaquin Valley of California to become one of the world’s
most productive agricultural regions, while simultaneously
contributing to one of the single largest alterations of the
land surface attributed to humankind
9 m of land subsidence between 1925-1977 (52 years)
~ 17.3 cm/y
23 juni 2015
Land subsidence classification of Causes
23 juni 2015
Factors causing subsidence
Distribution of soft soil (e.g. clay) and organic soil (e.g. peat).
Excessive (unregulated, uncontrolled) groundwater abstraction for
domestic & industrial demand + reduced recharge (hard surface)
Drainage of soil oxidation of peat, soil compaction.
Erosion by rainwater runoff (sinkholes)
Construction of dams and changes of river channels reduced amount of
sediment supply no natural compensation of subsidence: fact of life in
city
23 juni 2015
Land subsidence effects on coastal areas
Pipeline Piles
Tall
buildings
Land
subsidence
High tide
Pipeline Piles
Tall
buildings
Land
subsidence
High tide
Aquifer salinization
High tide
Aquifer salinizationAquifer salinizationAquifer salinization
High tideHigh tide
Sinking velocity
> 5mm/year
Sinking velocity
> 5mm/year
Sinking velocity
> 5mm/year
Rising sea levels> 3mm/yearRising sea levels> 3mm/yearRising sea levels> 3mm/year
• Relative “sea level” higher due
to land subsidence
• Damage to flood protection
structures
• Different flood inundation maps
(land subsidence is a spatially
distributed phenomena).
• More significant sea water
intrusion.
23 juni 2015
Land subsidence effects on coastal areas; diagnosing
1. Predict change in topography in the future: impact on hydraulic
and hydrological models
2. Estimate effects of land subsidence of surface and deeper layers
on flood protection structures: impact on design of structures and
foundations
3. Possible damaging effects on sewerage/drainage systems and
other infrastructure.
23 juni 2015
Symptoms: Land subsidence observed at groundwater pumping wells
23 juni 2015
Data acquisition methods of land subsidence
1. Geometric leveling network (optical leveling ~0.3mm);
2. Global Positioning System Surveying;
3. Extensometers,
4. Remote sensing:
1. Interferometric Synthetic Aperture Radar (InSAR);
2. Differential InSAR (C-Band). It is capable of mapping centimeters to meters of contiguous deformation across large areas, with centimeter accuracy. DifSAR is capable of capturing wide are surface deformation, but unlikely to be able to resolve any highly localized surface deformation features; and
3. Permanent Scatterer (PS) InSAR measurements (L-Band). It is used to map order of millimeters subsidence trends in urban and semi-urban environments.
Geodetic station used by GPS; ~ 2-3 cmExtensometer; ~ 0.3cm
23 juni 2015
Contents
- Definition, causes, symptoms, and Data acquisition.
- Method of analysis and simulation
- Case study of Jakarta
- Land subsidence in the Nile Delta
23 juni 2015
Methodology: Integration of monitoring and modeling
Geological model
of the subsurface
Geomechanical
modelling
Forecasted
ground-motion
Ground-motion
monitoring data
Va
l ida
tion
&Im
pro
ve
me
nt
Comparison &
inverse modelling
Risk analysis &
Decision making
Specific requirements to satellite data
Geological model
of the subsurface
Geomechanical
modelling
Forecasted
ground-motion
Ground-motion
monitoring data
Va
l ida
tion
&Im
pro
ve
me
nt
Comparison &
inverse modelling
Risk analysis &
Decision making
Specific requirements to satellite data
STRATIGRAPHY HYDROGEOLOGY GEODESY GEOTECHNIQUESTRATIGRAPHY HYDROGEOLOGY GEODESY GEOTECHNIQUE
23 juni 2015
Methodology to predict subsidence
1. Modeling of causes
• Groundwater flow model
• Calibration of ground water flow model with head observations
• Prediction of drawdown development according to different groundwater management scenarios.
• Determine process contributing to compaction (i.e., primary, secondary)
• Oxidation potential of organic layers
• Estimation of location and thickness of unsaturated layers
2. Modeling of effects
• Either coupled or decoupled groundwater flow and geo-mechanics modeling
> Estimate compaction parameters
> Calibrate parameters with land subsidence observations
• Add oxidation (and erosion).
23 juni 2015
Terzaghi's Principle
Terzaghi's Principle states that when
a rock is subjected to a stress, it is
opposed by the fluid pressure of
pores in the rock.
σ = σ' + u
Also known as Terzaghi's theory of
one-dimensional consolidation; it
states that all quantifiable changes
in stress to a soil (compression,
deformation, shear resistance) are a
direct result of a change in effective
stress. The effective stress σ' is
related to total stress σ and the pore
pressure u by the relationship;
Consolidation of multiple layers
23 juni 2015
2
2
22 21
8 11 exp 2 1
42 1
v
i
c tU t i
di
Cv : consolidation coefficient [L2/T];
d : drainage length [L]; and
t : consolidation time [T].
1
1 ,
n
j nj i
jv v j
bb
c c
Calculating equivalent consolidation
coefficient
23 juni 2015
The NEN-Bjerrum method
• The NEN-Bjerrum model supports today’s international defacto standards for
settlement predictions, as contained for example in the Dutch standards.
• The model uses common linear strain soil parameters (Cc ,Cr, Ca).
• It assumes that the creep rate will reduce with increasing over-consolidation and
that over-consolidation can grow by unloading and by ageing.
• It decomposes total strain into two components; namely, a direct elastic contribution
(εd ) and a transient viscous contribution (εvp) where all inelastic compression is
assumed to result from visco- plastic creep.
• The Over-consolidation Ratio (OCR) is defined
as the ratio of pre-consolidation pressure and in-
situ.
23 juni 2015
Compressibility and specific storage; remarks
• Typically, skeletal compressibilities (and therefore storativities) of
interbeds and confining units are several orders of magnitude
larger than compressibilities of coarser-grained aquifers, which are
typically much larger than water compressibility, therefore, virtually
all of the water derived from interbed and confining-unit storage is
due to the compressibility of the granular skeleton.
• Skeletal specific storage is inversely related to effective stress.
• For deep sediments, σ will be large, and reductions in u resulting
from groundwater pumping are not likely to make large percentage
changes in σ. On the other hand, for shallow sediments where σ is
relatively small, changes in u could result in relatively large
percentage changes in σ
sk wS g n
23 juni 2015
Jakarta Case
23 juni 2015
Coastal flooding and land subsidence
Total2007-20251989-2007Pluit
8-12 cm4-6cm4-6 cmSea level rise
100-200 cm100-200 cmMinimum
Subsidence
200-400 cm
Total2007-20251989-2007Pluit
8-12 cm4-6cm4-6 cmSea level rise
100-200 cm100-200 cmMinimum
Subsidence
200-400 cm
Coastal Flooding!
Land subsidence and climate change…
23 juni 2015
Indications
Extensometer at old Geology Office Jalan Tongkol
41 cm
4,5 cm
3 cm
199
7
23 juni 2015
Data acquisition
• Leveling measurements in 1974, 1982, 1991, 1997
• GPS Surveys 1997, 1999, 2000, 2002, 2005, 2006, 2007, 2008,
2009, 2010
• Level on the tide gauge
• Extensometer, piezometric surface
• Remote sensing (InSAR, etc).
23 juni 2015
Data acquisition
Subsidence revealed from
Belakang Muka
Belakang Muka
Rambu-1
Rambu-2
Rambu-1
bb1
bm1
bb2bm2
TitikA
TitikB
HB – HA = (bb1 – bm1) + (bb2 – bm2)
Instrumensipat datar
Leveling survey
GPS survey
Extensometer
23 juni 2015
Measured land subsidence maps; 1974-1982
• Between 1974-1982, almost no land subsidence recorded around area of Jakarta. • This is could be because of less urban development at that time, and minimum usage of groundwater.
Subsidence map of Jakarta 1974-1982:
23 juni 2015
Measured land subsidence maps; 1982-1991
• Between 1982-1991, subsidence begin to be recorded around Jakarta;• at the same time urban development has been growing up, where indeed much used of groundwater is observed.
Subsidence map of Jakarta 1982-1991:
Total subsidence -5 up to -90 cm ; rate -0.5 up to -9 cm/year
23 juni 2015
Measured land subsidence maps; 1991-2000
• Between 1991-2000, wider areas with land subsidence and larger land subsidence rates were observed. • During this time, acceleration of urban development, and the tremendous used of groundwater were observed.
Subsidence map of Jakarta 1991-2000:Total subsidence -10 up to -150 cm ; rate -1 up to -16 cm/year
23 juni 2015
Measured land subsidence maps; 2000 - 2010
Between 2000-2010, generally constant rate of subsidence is recorded as compared to the period 1991-2000.
Subsidence map of Jakarta 2000-2010:Total subsidence -10 up to -160 cm ; rate -1 up to -17 cm/year
23 juni 2015
Measured land subsidence maps; 1974 - 2010
Subsidence map of Jakarta 1974-2010:
1974-1982
1982-1991
1991-2000
2000-2010
Total subsidence -25 up to -400 cm ; rate -0.5 up to -17 cm/year -4,1 meter
-1,4 meter-2,1 meter
-0,7 meter
-0.25 meter
-4,1 meter
-2,1 meter-1,4 meter
-0,7 meter
-0.25 meter
23 juni 2015
Land subsidence simulation in Jakarta
23 juni 2015
Research Questions
What is the expected additional land subsidence in Northern Jakarta in case:
• The groundwater abstraction is continued as present and piezometric heads in the deeper aquifers will continue to lower in the coming 20 years.
• The groundwater abstraction is partially reduced and piezometric heads in the deeper aquifer will stabilize.
• The groundwater abstraction is reduced to 10% of present abstraction and piezometric heads will recover.
• The groundwater abstraction is reduced to 10% and clean water is infiltrated in the former pumping wells resulting in a rapid recovery of the piezometric heads.
What is the estimated influence of large complexes of high rise buildings in Northern Jakarta on land subsidence.
23 juni 2015
Study area, and land subsidence and piezometric head data
23 juni 2015
Conceptual lithological sequence of aquifer/aquitard
formations
MS
ettle
7.3
: jwrm
sF
it2.sli
<N
ot R
eg
iste
red
>
<N
ot R
eg
iste
red
> <
No
t Re
gis
tere
d>
Ph
on
e<
No
t Re
gis
tere
d>
Fa
x<
No
t Re
gis
tere
d>
da
te
<N
ot R
egiste
red
><
No
t Re
gis
tere
d>
17/0
3/2
011
Annex
Input View
Layers
8. Aquifer 1
7. Aquitard 1
6. Aquifer 2
5. Aquitard 2
4. Aquifer 3
3. Aquitard 3
2. Aquifer 4
1. Aquitard 4
0.000 10.000
1
2
3
4
5
6
7
8
1 2
1
-4
-59
-69
-119
-129
-169
-179
-249
1
-4
-59
-69
-119
-129
-169
-179
-249
Level (m)
Piezometric heads of the considered scenarios.
23 juni 2015
1
1960 1970 1980 1990 2000 2010 2020 2030-50
-40
-30
-20
-10
0
Year
Pie
zom
etr
ic H
ead (
m)
"Aquifer 1"
"Aquifer 2"
"Aquifer 3"
"Aquifer 4"
1960 1970 1980 1990 2000 2010 2020 2030-80
-60
-40
-20
0
Year
Pie
zom
etr
ic H
ead (
m)
"Aquifer 1"
"Aquifer 2"
"Aquifer 3"
"Aquifer 4"
1960 1970 1980 1990 2000 2010 2020 2030-50
-40
-30
-20
-10
0
Year
Pie
zom
etr
ic H
ead (
m)
"Aquifer 1"
"Aquifer 2"
"Aquifer 3"
"Aquifer 4"
1960 1970 1980 1990 2000 2010 2020 2030-50
-40
-30
-20
-10
0
Year
Pie
zom
etr
ic H
ead (
m)
"Aquifer 1"
"Aquifer 2"
"Aquifer 3"
"Aquifer 4"
Case 1 Case 2
Case 3 Case 4
Average yearly drawdown in the area is about 1.2 m.
Land subsidence data at DNMG location.
23 juni 2015
Year Time step Subsidene
(m)
1974 3285 0.28
1991 9490 0.48
1992.4 10000 0.55
1995 10950 0.68
2000 12775 0.83
2005 14600 1.24
2010 16425 1.64
23 juni 2015
Geo-mechanical parameters for the Bejrrum method
Values in yellow cells are adopted as initial values
Soil Unit Weight Condolidation Coeff. Overconsolidation Reloading/Swelling Compression index Sec. compression
kN/m3
(Cv) m2/s ratio OCR (-) ratio RR (-) CR (-) coeff. Ca (-)
Aquifer 1 26 N/A 1.1 0.02 0.02 0.002
Aquifer 2 26 N/A 1.1 0.02 0.02 0.002
Aquifer 3 26 N/A 1.1 0.02 0.02 0.002
Aquifer 4 26 N/A 1.1 0.02 0.02 0.002
Aquitard 1 27 3.80E-07 1.3 0.02 0.16 0.002
Aquitard 2 27 3.80E-07 1.3 0.02 0.16 0.002
Aquitard 3 27 3.80E-07 1.3 0.02 0.16 0.002
Aquitard 4 27 3.80E-07 1.3 0.02 0.16 0.002
Layer RR CR C a OCR C v
[-] [-] [-] [-] m 2 /s
Aquitard 1 0.001 0.600 0.013 1.52 9.38E-07
Aquitard 2 0.002 0.044 0.030 1.27 4.31E-09
Aquitard 3 0.004 0.285 0.009 5.05 1.39E-08
Aquitard 3 0.048 0.225 0.005 4.06 2.52E-09
Calibrated parameter values
Calculated versus measured subsidence and
forecasted subsidence
23 juni 2015
1
1960 1980 2000 2020 2040 2060 2080 2100
0
1
2
3
4
Year
Subsid
ence (
m)
Simulated
Measured
1960 1980 2000 2020 2040 2060 2080 2100
0
1
2
3
4
Year
Subsid
ence (
m)
Simulated
Measured
1960 1980 2000 2020 2040 2060 2080 2100
0
0.5
1
1.5
2
2.5
Year
Subsid
ence (
m)
Simulated
Measured
1960 1980 2000 2020 2040 2060 2080 2100
0
0.5
1
1.5
2
2.5
Year
Subsid
ence (
m)
Simulated
Measured
Case 2 Case 1
Case 3 Case 4
ID Case Description
Case 1: Drawdown zero after 2010
Case 2: Drawdown increases 5m every 5 years from 2010 till 2030
Case 3: Piezometric heads are recovered to the values of 1995 by 2015
Case 4: Piezometric heads recovered to the maximum of all aquifers in
1995 by 2015
Year Case 1 Case 2 Case 3 Case 4
2020 1.97 2.48 1.74 1.73
2025 2.08 2.75 1.80 1.77
2030 2.18 2.92 1.85 1.81
2100 3.01 3.91 2.43 2.30
MS
ettle
7.3
: jwrm
sF
it2.sli
<N
ot R
eg
iste
red
>
<N
ot R
eg
iste
red
> <
No
t Re
gis
tere
d>
Ph
on
e<
No
t Re
gis
tere
d>
Fa
x<
No
t Re
gis
tere
d>
da
te
<N
ot R
egiste
red
><
No
t Re
gis
tere
d>
17/0
3/2
01
1
An
ne
x
Input View
Layers
8. Aquifer 1
7. Aquitard 1
6. Aquifer 2
5. Aquitard 2
4. Aquifer 3
3. Aquitard 3
2. Aquifer 4
1. Aquitard 4
0.000 10.000
1
2
3
4
5
6
7
8
1 2
1
-4
-59
-69
-119
-129
-169
-179
-249
1
-4
-59
-69
-119
-129
-169
-179
-249
Level (m)
Compaction of aquitard layers (Case 1)
23 juni 2015
1960 1980 2000 2020 2040 2060 2080 2100
0
0.5
1
1.5
2
Year
Com
paction (
m)
"Aquitard 1"
"Aquitard 2"
"Aquitard 3"
"Aquitard 4"
23 juni 2015
Degree of Consolidation
• Delay effect due to slow dissipation in the most bottom aquitard could be significant.
• This is due the assumption that the lower boundary is undrained. The undrained lower
boundary can be simply because the layer is overlaid on basement rock.
• Note that most aquitards have not reached full dissipation at end of simulation (2100).
This means a continuous subsidence till the layers reach hydrostatic condition.
1960 1980 2000 2020 2040 2060 2080 21000
0.2
0.4
0.6
0.8
1
Year
U (
%)
"Aquitard 1"
"Aquitard 2"
"Aquitard 3"
"Aquitard 4"
Subsidence for several drainage and creep
conditions of "Case 1"
23 juni 2015
1960 1980 2000 2020 2040 2060 2080 2100
10-4
10-3
10-2
10-1
100
101
0.0067
3.0129
1.3748
8.7908
0.0002
0.0240
0.0055
0.1733
Year
Subsid
ence (
m)
CrNoDr (Case 1)
CrDr
NoCrNoDr
NoCrDr
General conclusions (Jakarta Case)
23 juni 2015
• The NEN-Bjerrum method has been used to evaluate land subsidence in
deltaic environment due to change in pore water pressures as a result of
applying different groundwater management schemes.
• The method is combined with the Terzaghi’s consolidation theory to
account for consolidation of multiple homogeneous layers of soil between
drained layers
• The method decomposes total strain into a direct elastic contribution (d)
and a transient visco-plastic contribution (vp).
• The results showed significance of creep compaction on calculated land
subsidence where for the case studied here with the calibrated
parameters set, creep presented about 99% of total land subsidence for
the year 2100.
• The results also showed that coupled processes of consolidation and
creep produced a favorable situation where consolidation reduced total
land subsidence by 66% as compared to the case of drained layers (and
consequently 100% consolidation).
23 juni 2015
Land subsidence in the Nile Delta
(proposal stage)
Research Objectives
1. Establishing a national monitoring network to provide continuous
measurements of land subsidence in the Nile Delta
2. Identifying risk areas prone to land subsidence by:
1. Mapping the thickness of soft soils within the Nile Delta to help in
identification of areas with high risks to land subsidence,
2. To characterize clay-cap in the Nile Delta in terms of geo-mechanical
parameters,
3. Developing a preliminary coupled groundwater flow model and land
subsidence of the Nile Delta; this will be used to investigate the relationship
between groundwater abstraction and land subsidence.
4. Investigation processes (e.g. primary or secondary compaction) controlling
potential risk area of land subsidence,
5. Initial evaluation of consequences of land subsidence on flood mapping in
the coastal area of the Nile Delta; this includes how climate change and
groundwater management could affect flooding risks of the delta.
6. Evaluating the change in freshwater-seawater interface due to land
subsidence.
23 juni 2015
Research Approach and Methodology
23 juni 2015
1. Data inventory
2. Monitoring of land subsidence
1. Geodetic techniques
2. Remote sensing (Radar Interferometry Technique)
3. Tide gauges
3. Characterization of geo-mechanical properties of the clay layer,
4. Mapping risk areas of land subsidence.
5. Modeling land subsidence and consequences to flood inundation.
1. Theoretical development of a method to handle creep in clay
2. Hydrogeological conditions of the Nile Delta
3. Consequences of land subsidence and sea level rise on flood
inundation in the Nile Delta
GPS stations by National Institute of Astronomy
and Geophysics (NIAG)
23 juni 2015
Groundwater aquifers in the Nile Delta
23 juni 2015
23 juni 2015
Land Subsidence in the Nile Delta
Land Subsidence in the Nile Delta: Inferences from Radar Interferometry, The Holocene v.
19(6), doi:10.1177/0959683609336558
• Average elevation of
approximately 1 m above
sea level within 30 km of
the coast and
• A predicted rise in sea
level of 1.8—5.9 mm/yr
• Evaluate rates of
subsidence of sections of
the northeastern Nile
Delta (a total length of 110
km, up to 50 km from the
coastline) using Ps-InSAR
techniques applied to 14
ERS-1 and ERS-2 scenes.
B The highest subsidence rates (~8 mm/yr; twice average Holocene rates) do not
correlate with the distribution of the thickest Holocene sediments, but rather with the
distribution of the youngest depositional centers (major deposition occurred between
~3500 yr BP and present) at the terminus of the Damietta branch.
Expected Major Outputs
23 juni 2015
1. A national spatial geo-database system of land subsidence
measurements and parameters (NGDLS)
2. Several risk maps of land subsidence as inferred from different data
3. A complete geo-mechanical characterization of the clay-cap in the
delta (results are to be integrated into NGDLS),
4. Time series of land subsidence at different locations a long the
coastline of the Nile Delta (results are to be integrated into NGDLS),
5. Several modeling/data integrated tools to forecast spatial variability of
land subsidence in the Nile Delta, to forecast flood inundation due to
sea level rise and land subsidence, and to forecast impacts of
groundwater management on land subsidence and seawater
intrusion.
Thank you for your attention
23 juni 2015
well scheme in homogeneous aquifer and pore pressure
variation
23 juni 2015
Artesian well in confined aquifer and pore pressure
variation
23 juni 2015
Consolidation ratio as function of depth and time factor
uniform initial excess pore pressure
23 juni 2015
The Bjerrum method; explained
Idealized primary and secondary settlement during time (drained
conditions)
Idealized primary settlement during loading
(drained conditions)
23 juni 2015
Common parameter values (The Netherlands)
RR CR e0 Cr Cc sigma Ske Skv Percentage
Clay (Holocene) 0.08 0.24 0.1 0.0992 0.264 20 0.00196 0.00521 0.37576
Peat (Holocene) 0.6 1.81 0.3 1.686 2.353 20 0.02814 0.03928 0.71653
Clay (Pleistocene) 0.06 0.17 0.08 0.0702 0.1836 20 0.00141 0.00369 0.38235
Peat (Pleistocene) 0.3 0.9 0.25 0.57 1.125 20 0.00990 0.01953 0.50667
sk wS g n
23 juni 2015
Data requirements; The NEN-Bjerrum method
• Three dimensional model of the subsurface (lithology),
• Groundwater flow and related variability of water table and piezometric heads,
• Soil properties:
• The well-established constitutive models are based on common soil parameters for virgin compression, unloading/reloading and secondary creep. For example, for the NEN-Bjerrum method: the following parameters are required:
> RR: Reloading/swelling ratio
> CR: Compression ration
> Ca: Coefficient of secondary compression
> POP: Pre-overburden pressure (defines the over-consolidation by the difference between the initial vertical pre-compression stress and the initial field stress)
• Consolidation is either modeled by means of a consolidation coefficient or by means of permeability per layer.
• Total stresses is calculated using soil unit weights.
• Water unit weight to calculate effective stresses.
• Land subsidence data to calibrate/ validate relevant model parameters.
23 juni 2015
The Jakarta Case; Conclusions
• Groundwater abstraction is the main reason for land subsidence in Jakarta.
• Groundwater flow and land subsidence are coupled phenomena since dynamics of groundwater flow systems lead to a time-variant geo-mechanical system where change in effective stresses in time, and consequently evolution of land subsidence in time, follows the change in water levels/piezometric heads in porous media.
• In deltaic environment with extensive appearance of soft soils, secondary (creep) and delayed primary settlement play significant roles in the estimation of land subsidence and relation to different groundwater management schemes.
• The forward geo-mechanical model using the Bjerrum method is developed based on conceptual information about the subsurface as well as piezometric head data at the “DNMG” area located north-west of Jakarta.
• A time series of land subsidence at the studied location is used to condition the parameters of the forward model.
23 juni 2015
The Jakarta Case; Conclusions (cont.)
• The calibrated model estimates the compression ratio as 0.33, the re-compression ratio as 0.008, and the secondary compression ratio (creep) as 0.012.
• The developed model simulates the behavior of the measured land subsidence time series accurately with simulated subsidence value of 1.6m at 2010 (corresponds to 1.64m measured value).
• The model estimates 2.6m and 3.88m of land subsidence at 2030 and 2100, respectively, considering no further groundwater drawdown after 2010. This reflects the delay effect due to slow dissipation in aquitard systems.
• A moderate drawdown of 1m/y every 5 years till 2030 would cause land subsidence of 3.25m and 5.7m at 2030 and 2100, respectively.
• Several groundwater scenarios of stopping abstraction and artificial recharge of aquifers are examined. For example, recovering piezometric heads to measured values in 1995 (this corresponds to 15 m recovery), will result land subsidence (still) of 2.4m and 3.1m at 2030 and 2100, respectively.