Sinking coastal cities

Abstract

In many coastal and delta cities land subsidence now exceeds absolute sea level rise up to a factor of ten. A major cause for severe land subsidence is excessive groundwater extraction related to rapid urbanization and population growth. Without action, parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other coastal cities will sink below sea level. Land subsidence increases flood vulnerability (frequency, inundation depth and duration of floods), with floods causing major economic damage and loss of lives. In addition, differential land movement causes significant economic losses in the form of structural damage and high maintenance costs for (infra)structure. The total damage worldwide is estimated at billions of dollars annually.
As subsidence is often spatially variable and can be caused by multiple processes, an assessment of subsidence in delta cities needs to answer questions such as: what are the main causes? What is the current subsidence rate and what are future scenarios (and interaction with other major environmental issues)? Where are the vulnerable areas? What are the impacts and risks? How can adverse impacts be mitigated or compensated for? Who is involved and responsible to act? In this study a quick-assessment of subsidence is performed on the following mega-cities: Jakarta, Ho Chi Minh City, Dhaka, New Orleans and Bangkok. Results of these case studies will be presented and compared, and a (generic) approach how to deal with subsidence in current and future subsidence-prone areas is provided.
Sinking coastal cities. Available from: https://www.researchgate.net/publication/283771445_Sinking_coastal_cities [accessed Jun 2, 2016].

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Proc. IAHS, 372, 189–198, 2015
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doi:10.5194/piahs-372-189-2015
© Author(s) 2015. CC Attribution 3.0 License.
Open Access
Prevention and mitigation of natural and anthropogenic hazards due to land subsidence
Sinking coastal cities
G. Erkens1,2, T. Bucx1, R. Dam3, G. de Lange1, and J. Lambert1
1Deltares Research Institute, Utrecht, the Netherlands
2Utrecht University, Utrecht, the Netherlands
3WaterLand Experts, Amsterdam, the Netherlands
Correspondence to: G. Erkens (gilles.erkens@deltares.nl)
Published: 12 November 2015
Abstract. In many coastal and delta cities land subsidence now exceeds absolute sea level rise up to a factor of
ten. A major cause for severe land subsidence is excessive groundwater extraction related to rapid urbanization
and population growth. Without action, parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other coastal
cities will sink below sea level. Land subsidence increases flood vulnerability (frequency, inundation depth and
duration of floods), with floods causing major economic damage and loss of lives. In addition, differential land
movement causes significant economic losses in the form of structural damage and high maintenance costs for
(infra)structure. The total damage worldwide is estimated at billions of dollars annually.
As subsidence is often spatially variable and can be caused by multiple processes, an assessment of subsidence
in delta cities needs to answer questions such as: what are the main causes? What is the current subsidence rate
and what are future scenarios (and interaction with other major environmental issues)? Where are the vulnerable
areas? What are the impacts and risks? How can adverse impacts be mitigated or compensated for? Who is
involved and responsible to act?
In this study a quick-assessment of subsidence is performed on the following mega-cities: Jakarta, Ho Chi
Minh City, Dhaka, New Orleans and Bangkok. Results of these case studies will be presented and compared,
and a (generic) approach how to deal with subsidence in current and future subsidence-prone areas is provided.
1 Introduction
Currently, global mean absolute sea- level rise is around
3 mm yr−1, and projections until 2100 based on Intergov-
ernmental Panel on Climate Change (IPCC) scenarios ex-
pect a global mean absolute sea-level rise in the range of 3–
10 mm yr−1(Church and White, 2011; Slangen, 2012). How-
ever, currently observed subsidence rates in coastal megaci-
ties are in the range of 6–100 mmyr−1, and projections until
2025 expect similar subsidence rates (Fig. 1).
In coastal cities around the world, land subsidence in-
creases flood vulnerability (flood frequency, inundation
depth, and duration of floods), and hence contributes to major
economic damage and loss of lives. Land subsidence is ad-
ditionally responsible for significant economic losses in the
form of structural damage and high maintenance costs; it af-
fects roads and transportation networks, hydraulic infrastruc-
ture, river embankments, sluice gates, flood barriers, pump-
ing stations, sewage systems, buildings, and foundations. The
total damage associated with subsidence worldwide is esti-
mated at billions of dollars annually.
There are no indications that neither subsidence nor the
resulting damage will reduce in the near future. In fact, both
are likely to increase. Ongoing urbanization and population
growth in delta areas, in particular in coastal mega-cities,
continues to fuel economic development in subsidence-prone
areas. Consequently, economic development drives both the
growing demand for groundwater, thereby increasing subsi-
dence rates, and the growth of the total value of assets at risk.
These impacts are aggravated on the long term in coastal ar-
eas, by expected future climate change impacts, such as sea-
level rise, increased storm surges, and changes in precipita-
tion.
In this paper, we focus on land subsidence in the urban en-
vironment, rather than land subsidence in rural agricultural
areas, where the drivers may be similar, but the impact very
Published by Copernicus Publications on behalf of the International Association of Hydrological Sciences.

190 G. Erkens et al.: Sinking coastal cities
Figure 1. Drivers, processes and impacts of land subsidence in coastal cities. Land subsidence can exceed global absolute sea-level rise
(SLR) with a factor 10.
Figure 2. Subsidence history (cumulative) in a series of coastal cities around the world. Absolute sea level rise is depicted as reference.
Subsidence can differ considerably within a city area, depending on groundwater levels and subsurface characteristics. Values provided here
can be seen as average for the local subsidence hotspots. Some cities are currently seeing an acceleration of subsidence as a result of economic
growth. Tokyo stands out as an example where subsidence has stopped after successful mitigation measures were implemented. The caption
of Table 1 provides references.
Table 1. Subsidence in coastal cities. Estimated additional mean cumulative subsidence until 2025 (mm) are linear interpolations of the
current rates, notwithstanding any policy changes. Sources: Bangkok: MoNRE-DGR (2012), Aobpaet et al. (2013); Ho Chi Min City: van
Trung and Minh Dinh (2009); Jakarta: Bakr (2011); Manila: Eco et al. (2011); West Netherlands: van de Ven (1993); Tokyo: Kaneko and
Toyota (2011).
City Mean cumulative Mean current Maximum Estimated additional mean
subsidence in period subsidencerate subsidence rate cumulative subsidence
1900–2013 (mm) (mm yr−1) (mm yr−1) until 2025 (mm)
Jakarta 2000 75–100 179 1800
Ho Chi Minh City 300 up to 80 80 200
Bangkok 1250 20–30 120 190
New Orleans 1130 6 26 >200
Tokyo 4250 ≈0 239 0
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G. Erkens et al.: Sinking coastal cities 191
different. Figure 2 and Table 1 show that land subsidence
rates widely vary from city to city. In many cases, the under-
lying processes and the relative contribution of the different
drivers is not well understood. Similar to the level of techni-
cal understanding, policy formulation and governmental en-
gagement in cities is equally diverse. Whereas some cities are
in an early state of research and policy development on land
subsidence, others have already implemented measures mit-
igating subsidence and the resulting damage. The observed
different stages in development mean that cities can learn
from each other, thereby avoiding re-inventing the wheel.
Cities that actively pursue a policy on subsidence have valu-
able experiences to share with cities that have just started to
address their subsidence.
This is exactly the thought behind the assessment that was
carried out for this research. We compared five cities regard-
ing their state of subsidence research and policy develop-
ment: Jakarta, Ho Chi Minh City, Dhaka, New Orleans and
Bangkok. The assessment aimed at getting insight into the
processes causing subsidence in the urban environment, ob-
taining a (generic) research agenda for this topic, and listing
best practice cases. Results of these case studies will be pre-
sented and a (generic) approach how to cope with subsidence
in current and future subsidence-prone areas is provided.
2 Results of the review
For the quick assessment we used published reports on both
the technical and the policy aspects of subsidence in the focus
cities. In addition, we interviewed local scientist and policy-
makers to obtain their perspective. It became quickly clear
that all cities tried to answer similar questions. We com-
piled the interview results into seven interrelated questions
(Fig. 3). They include questions such as: what are the main
causes for subsidence? How much is the current subsidence
rate and what are future scenarios? Where are the vulnerable
areas? What are the impacts and risks? How can adverse im-
pacts be mitigated or compensated for? Who is involved and
responsible to act? How to monitor the effect of the imple-
mented measures? The interrelation between the questions is
indicated with the arrows in Fig. 3. The indicated interrela-
tion does not necessarily mean that each question needs to be
answered in a specific order, but it merely indicates that each
answer may be valuable input for a next question.
In this paper we follow this framework (in seven steps)
and illustrate how these questions are addressed in example
cities, thereby discussing both technical and policy aspects
of subsidence. In this way, this framework could serve as a
blue print for cities to shape their policy and research agenda
regarding subsidence.
Figure 3. Seven questions that need to be addressed to pursue a suc-
cessful policy to coop with subsidence. This is loosely based on the
policy cycle, a popular framework to analyse policy development.
3 Measuring and monitoring
The first step towards a successful strategy for subsidence is
to establish if a certain area is actually subsiding. This may
not me evident from the field, particular if subsidence is non-
differential and no structural damage (cracks, tilting) is ob-
served in buildings or infrastructure. Typically, the loss of
elevation, which may have been observed, is interpreted as
the result of climate-driven sea level rise instead of the result
of subsidence.
To determine land subsidence rates, accurate measuring
techniques are required. Continuous subsidence monitoring
provides the necessary insight into changes – ranging from
minor to very significant changes – in the topography of the
urban area. These observations are also essential to validate
subsidence prediction models in a later stage.
The following geodetric observation methods are being
used:
–optical levelling;
–Global Positioning System (GPS) surveys;
–Laser Imaging Detection and Ranging (LIDAR);
–Interferometric synthetic aperture radar (InSAR) satel-
lite imagery.
Following early work with systematic optical levelling
nowadays GPS surveys and remote sensing techniques (LI-
DAR and InSAR) are deployed with impressive results. In
contrast to surveys, LIDAR and InSAR images give a spa-
tially resolved subsidence signal. InSAR images date back to
the 1990s and can now be used to establish subsidence since
that time. Application of this technique in soft soil areas is
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192 G. Erkens et al.: Sinking coastal cities
for the moment limited to the build-up environment, as a re-
sult of the need for stable reflectors. Ideally, multiple obser-
vation techniques are combined, for instance absolute mea-
surements from GPS and Optical Levelling Spatially can be
combined with remotely sensed, relative displacement mea-
surements from InSAR. In this way, spatially resolved subsi-
dence maps with respect to a global reference frame can be
produced. InSAR measurement can therefore not replace pe-
riodic and systematic ground surveys, as they remain essen-
tial for ground truthing subsidence rates derived by remote
sensing and as an independent source for validating subsi-
dence prediction models.
Systematic observation of elevation forms the base for
subsidence monitoring systems. Monitoring results can be
used to develop a so-called dynamic digital elevation model
(dDEM). This is not a static, one-time only (preferably high
resolution) recording of the local topography, but an eleva-
tion model that can be corrected and updated from time to
time, and that can be used in hydraulic models for flood pre-
diction and urban water management.
All techniques mentioned above measure land surface el-
evation change, but give no information on the source of the
subsidence. Subsidence benchmarks or extensometers can
provide in-situ information of ground movement, as they
record the volume reduction across a certain stretch in the
subsurface, or even of individual geological layers. Ideally
the benchmarks or extensometers need to be connected to
surface movement observations, for instance by using a com-
bined extensometer and continuous GPS station (e.g. Wang
et al., 2014). Monitoring total subsidence at these “super-
sites”, where a terrestrial network of site specific measure-
ment stations is combined with remote sensing, forms the
backbone of a spatially resolved subsidence measurement
system (Allison et al., 2014). To support subsidence mod-
elling, hydraulic heads of different aquifer systems and the
phreatic groundwater level need to be monitored at these su-
per sites as well. Measurements of geotechnical parameters
at the same site provide additional necessary input for model
studies.
4 Unravelling the subsidence signal
Subsidence can have natural as well as anthropogenic causes.
The natural causes include tectonics, loading by ice sheets,
by sediments, of by the ocean/sea (isostatic adjustment),
and natural sediment compaction (autocompaction). Anthro-
pogenic causes include compression of shallow soft layers
by loading (with buildings for instance), or as a result of
drainage and subsequent oxidation and consolidation of or-
ganic soils and peat. Alluvial or coastal sediments consisting
of alternating layers of sand, clay, and peat are specifically
compressible and vulnerable for oxidation. This is related
to the physical characteristics of these sediments and makes
low-lying coastal and delta areas specifically prone to subsi-
Figure 4. A distinct relation between falling hydraulic heads and
subsidence in Ho Chi Minh City (Vietnam). This is indicative of
an important contribution of groundwater over-exploration to sub-
sidence, although it is not necessarily the only component contribut-
ing to the total subsidence signal.
dence. In deeper layers subsidence is caused by extraction of
resources such as oil, gas, coal, salt, and groundwater.
In most of the large delta cities where land subsidence is
severe (Jakarta, Ho Chi Minh City, Bangkok, Dhaka, Shang-
hai, and Tokyo), the main cause is extraction of groundwater.
Rapidly expanding urban areas require enormous amounts of
water for domestic and industrial water supply. This need of-
ten leads to over-exploitation of groundwater resources, es-
pecially when surface waters are seriously polluted (Jakarta,
Dhaka). Dhaka (Bangladesh) is an example of a city that
started to discover that it subsided after the flood frequency
increased. In this rapidly expanding city data on subsidence
and its impacts are currently largely lacking. Large-scale ex-
tractions cause groundwater levels to fall by 2–3myr−1. At
present, 87 % of the supplied water is from groundwater ex-
traction, and it has been acknowledged that a shift to using
surface water instead is necessary. However, treating the pol-
luted surface water is much more technically complex and
expensive than extracting groundwater.
Although groundwater extraction is often not the sole
source of subsidence, studies in many cities have revealed a
distinct relation between falling groundwater levels and sub-
sidence, indicative of an important contribution of aquifer
compaction (Fig. 4). The resulting spatial pattern of subsi-
dence and its progress over time are strongly related to the
local composition of the subsurface and the number and po-
sitions of groundwater abstraction wells.
New Orleans (USA) is a prominent example of a city
where an array of processes contributes to the total subsi-
dence of the city. The Mississippi Delta subsides as a result
of natural processes, such as autocompaction, faulting, sedi-
ment loading and isostacy (e.g. Törnqvist et al., 2008; Yu et
al., 2012). Within the urban area of New Orleans, there ad-
ditionally is anthropogenic induced subsidence as a result of
drainage of shallow soft soils (Stuurman and Erkens, 2015)
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G. Erkens et al.: Sinking coastal cities 193
Figure 5. Subsidence components in the urban area of New Orleans. Values are derived from studies of Tulane University, New Orleans,
and are indicative. The total subsidence rate is derived from InSAR measurements (Dixon et al., 2006). It shows that in the urban area natural
subsidence forms the smaller portion of the total subsidence and that human induced subsidence dominates.
and extraction of deeper groundwater in confined aquifers,
for industrial use mainly (Dokka, 2011). After drainage of
the organic rich soils, they start to oxidize and lose volume,
and this process will continue to cause subsidence as long as
organic material is available in the drained subsoil.
The average measured subsidence rate in the city of New
Orleans (including the urban area of Jefferson and St Bernard
Parishes) is 6 mm yr−1(Dixon et al., 2006). Many studies
try to quantify one or more of the different components con-
tributing to the total measured subsidence. Figure 5 shows
how this may look for New Orleans, when components are
quantified step by step (source: Tulane University, New Or-
leans). In-situ observation data may provide an independent
valuable source of information to unravel the total subsidence
signal, as argued in Sect. 3. Another approach to unravel the
subsidence signal is inverse modelling, whereby with the use
of a careful inversion scheme, the available knowledge on
the geology and hydrological dynamics of a system can be
quantitatively constrained with subsidence observations (e.g.
Fokker et al., 2007).
From Fig. 5 also follows that in the urban area of New Or-
leans, human induced subsidence has a much larger contri-
bution to the total subsidence signal than natural subsidence.
This is often the case, as natural subsidence rates are mainly
limited to tens of millimeters per year, to millimeters per year
in exceptional cases. Human induced subsidence rates can
easily reach centimeters per year, to even tens of centime-
ters per year (e.g. Jakarta). For policy development this is an
important notion: it is worthwhile to implement measures to
reduce human-induced subsidence.
5 Modelling subsidence to make predictions
In step three, once the causes for land subsidence have been
established (see Sect. 4), predictions can be made to get in-
sight in future land subsidence. Land subsidence modelling
and fore-casting tools are being progressively developed that
enable quantitative assessment of medium- to long- term land
subsidence rates, and determination of multiple causes. Mod-
elling tools are ideally complemented with monitoring tech-
niques (i.e., GPS leveling, the use of InSAR -monitoring
techniques), see Sect. 3.
Because land subsidence is in many places closely linked
to excessive groundwater extraction, we focus in this paper
on modelling of aquifer compaction. One of the most widely
used computer program to simulate vertical compaction in
models of regional ground-water flow is MODFLOW SUB-
WT (Leake and Galloway, 2007). MODFLOW SUB-WT is
developed by the US Geological Survey and uses changes
in groundwater storage in subsurface layers (aquifers and
aquitards) and accounts for temporal and spatial variability
of geostatic and effective stresses to determine layer com-
paction.
In soft soils, such as unconsolidated Holocene layers of
peat and clay, the classical consolidation theory by Terza-
ghi is unable to explain observed consolidation behaviour.
These lithology form the aquitards and interbed units in con-
fined aquifer complex systems, albeit often more consoli-
dated, which start to compact after groundwater is extracted
from the confined aquifers. Creep deformation is one of the
typical processes that occur when the effective stress is in-
creased in clay or peat soils. The creep deformation (also
known as secondary strain) of soils is a secondary consoli-
dation process that leads to a reduction in void ratio at con-
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194 G. Erkens et al.: Sinking coastal cities
Figure 6. The performance of a series of models used to calculate settlement compared to Oedometer test results. Prediction made by
classic models such as Koppejan fit the measurements less well compared to models based on the isotachs method, such as NEN-Bjerrum or
abc-Isotachs.
stant effective stress, and consequently, to the development
of an apparent pre-consolidation pressure (Den Haan, 1994).
It is seen as visco-plastic behaviour and is considered a slow
process, compared to primary or elastic consolidation. The
inclusion of creep behaviour in numerical descriptions of the
consolidation process has a long history, which is excellently
described in Bakr (2015). An important aspect of the creep
based models is that, due to secondary compression, there
is a family of stress-strain curves rather than a single curve
describing the relationship between stress and strain. Each
of these curves, called “time lines” (i.e. isochrones), corre-
sponds to a different duration of the applied load in a standard
oedometer test. For soft soils, model predictions that make
use of the isochrones method tend to match the oedometer
test results best, specifically on longer time periods (Fig. 6).
Deltares Research Institute modified the US Geological
Survey SUB-WT module by including isotachs (line of equal
speed) based consolidation predictions. This model, MOD-
FLOW SUB-CR (SUBsidence Creep), is used to determine
medium- to long-term land subsidence trends under differ-
ent scenarios of groundwater usage. In this way, the conse-
quences of groundwater extraction for urban flood manage-
ment become clear.
Because the SUB-CR model works with isotachs to calcu-
late consolidation, it differs from the SUB-WT model in two
ways:
–It predicts on the longer term more consolidation, thus
subsidence, in clay and peat layers, as creep is a slow
and largely irreversible component of subsidence
–Creep may continue for some time even after the hy-
draulic heads increased, introducing a time lag in con-
solidation.
As a result of these differences, aquifers with many fine-
grained interbeds, creep forms a considerable part of the to-
tal amount of settlement over time and should not be ne-
glected. An example is the subsidence predictions conducted
for Jakarta, Indonesia, using isotachs-based consolidation
calculations. Bakr (2015) calculates the subsidence occur-
ring in four future groundwater management scenarios for
Jakarta. The four scenarios are:
1. drawdown for all aquifers are kept zero till 2100 by
maintaining piezometric levels at their values of 2010
(no change);
2. drawdown for all aquifers increase 5m every 5 years
from 2010 till 2030 (business as usual);
3. piezometric heads are recovered for all aquifers by 2015
to their values of 1995 (recovery),
4. piezometric heads are recovered for all aquifers by 2015
to the maximum level of all aquifers in 1995 (full recov-
ery).
In Table 2, we report the predicted cumulative subsidence
for Jakarta as calculated by Bakr (2015), calculated with the
inclusion of creep. The results indicate that (i) if the hy-
draulic head declines continue with the current rate (sce-
nario 2) parts of North Jakarta will sink an additional 3.9 m,
and (ii) even if hydraulic heads remain the same (scenario 1)
or are restored (scenarios 3 and 4) subsidence continues,
up to 2.3 or 2.4m in the recovery scenarios in 2100. This
residual subsidence is the result of both delayed pore wa-
ter pressure dissipation and visco-plastic creep compaction.
This means that even if effective stresses do not change, land
subsidence will continue till all layers reach hydrostatic equi-
librium and creep compaction of all layers vanishes by time
due to aging (Bakr, 2015).
This has important implications for policy development in
the city of Jakarta. The significant predicted subsidence in the
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G. Erkens et al.: Sinking coastal cities 195
Table 2. Cumulative subsidence (m), modelled including creep behaviour, for 4 groundwater management scenarios for Jakarta, Indonesia,
by Bakr (2015). Because of the slow creep rates, subsidence continues even after hydraulic heads are restored (scenarios 3 and 4).
Year Scenario 1 Scenario 2 Scenario 3 Scenario 4
(no change) (business as usual) (recovery) (rapid recovery)
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
business as usual scenario justifies a subsidence mitigation
policy. But the forecasted subsidence values in the recovery
scenarios indicate that for the remaining residual subsidence
an adaptation strategy must be developed too. Because of
these far reaching implications for policy development, it is
important that subsidence predictions are as accurate as pos-
sible. Although the inclusion of creep behaviour in prediction
models aimed to increase accuracy of predictions, they are
also sensitive for the geo(hydro)logical schematisation used
in the model. This is because the fine-grained interbeds and
aquitards are most sensitive to creep, and their exact distri-
bution both in vertical and lateral direction determines the
model outcome. The 3-D distribution of fine-grained deposits
in the subsurface, and their geo-mechanical properties, are
therefore key to reliable subsidence predictions for cities.
6 Impact and damage
With subsidence predictions for different management sce-
narios (step 3, see Sect. 5), damage estimates (step 4 in the
framework, Fig. 3) provide additional information for policy
decisions. The estimation of costs associated to subsidence
is very complex. Subsidence is a “hidden threat” because in
practice, costs appear on financial sheets as part of ad hoc
investments or planned maintenance schemes, but are not la-
belled as subsidence-induced damage. Dedicated damage es-
timates of subsidence can help to raise awareness among pol-
icymakers and initiate policy development.
Generally, two (very different) types of damage as a result
of subsidence can be recognised: (i) increased flood risk (due
to increased flood frequency, floodwater depth, and duration
of inundation) and more frequent rainfall-induced floods due
to ineffective drainage systems, and (ii) damage to buildings,
foundations, infrastructure (roads, bridges, dikes), and sub-
surface structures (drainage, sewerage, gas pipes, etc.). The
former is mainly the result of non-differential subsidence,
which is characteristic for large subsidence bowls that ex-
ist when groundwater or hydrocarbons at greater depth are
extracted. Examples of cities that have increased flood risk
as a result of subsidence include Jakarta, Ho-Chi-Minh and
Bangkok. The second type of damage, to structures, is the re-
sult of differential subsidence. This commonly happens when
fault systems are (re)activated, or when the subsidence is
the result of shallow processes (loading or drainage of soft
soils). Examples of cities in which structures are damaged
include New Orleans, Venice (Italy) and Amsterdam (the
Netherlands). Note that the construction site preparation and
construction costs in soft-soil areas should be considered as
subsidence-related costs, as these are mainly incurred to pre-
vent consolidation. On the longer term, however, cumulative
subsidence of soft soils may also increase flood risk as for
instance happened in the Netherlands (subsidence over the
last ∼1000 years) or in New Orleans (subsidence over the
last ∼150 years). The extent of the damage is different in
the two cases: increased flood risk usually applies to a larger
area than structural damage that applies to single structures
or parts of the network. The owner of the problem is also dif-
ferent: it is the local government who is investing in reducing
flood risk, whereas local communities, (utility) companies or
even home owners pay for the damage to (infra)structures.
Making an estimation of costs associated with subsidence
is notoriously complex. Some bulk estimates are available.
For instance, in China, the average total economic loss due to
subsidence is estimated at around USD 1.5 billion per year, of
which 80–90 % is from indirect losses. In Shanghai, over the
period 2001–2010, the total loss cumulates to approximately
USD 2 billion. In the Netherlands, new estimates based on
subsidence modelling, try to unravel the bulk costs. For in-
stance, it is calculated that damage to foundations (as a result
of subsidence) has been more than EUR 5 billion thus far,
and might reach EUR40 billion in 2050 (although this is a
theoretical maximum, Hoogvliet et al., 2012). The communi-
ties in soft soil areas in the Netherlands spend EUR0.25 bil-
lion per year more on maintenance than the communities
on supportive soils. This values consists of EUR0.17 bil-
lion per year maintenance for roads and water networks and
EUR 0.08 billion per year for sewage systems (Lambert et al.,
2014). The total damage associated with subsidence world-
wide is unknown, but estimated based on the aforementioned
values suggest billions of dollars annually. Because of ongo-
ing economic and urban development, the potential damage
costs of subsidence will increase considerably in the future,
especially in subsidence-prone areas such as flood plains.
Damage estimates form the core of cost-benefit analyses.
For subsidence, cost-benefit analyses will help to systemati-
cally calculate and compare benefits and costs of a decision
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196 G. Erkens et al.: Sinking coastal cities
or government policy on the short and long term. Being a
gradual process, usually mitigation measures for subsidence
are costly on the short term, but cost-effective on the longer
term. Cost-benefit analyses could provide this insight in a
quantitative way.
7 Measures and monitoring
Once the damage caused by subsidence is quantified
(Sect. 6), the responsible actors (step 5 of the framework,
Fig. 3) can work out a policy on subsidence (step 6), that
should be evaluated after implementation (step 7). In this sec-
tion we focus on action necessary for steps 6 and 7.
There are generally two policy strategies for subsid-
ing cities: mitigation and adaptation – analogue to the
climate change policy discussions. A successful strategy,
however, probably includes both. Mitigation only works
for human-induced subsidence (Sect. 4). Typical mitigation
measures include restrictions of groundwater extraction, ar-
tificial recharging aquifers, or raising (phreatic) water levels
in areas with organic rich soils, thereby reducing oxidation
of organic matter. Building with lighter materials decreases
the load on soft soils, thereby decreasing consolidation and
subsidence (Lambert et al., 2014).
For the human induced subsidence that cannot be miti-
gated, either because of technical difficulties (for instance the
use of lighter building materials in high rise buildings), or
because of financial reasons (i.e. the mitigation costs are too
high), an adaptation strategy should be considered. This is
also true for residual subsidence after a successful mitigation
of subsidence (see Sect. 5) or for natural subsidence, where
mitigation is not possible.
Adaptation must focus on reducing the impact of sub-
sidence, for instance by decreasing the vulnerability of a
certain asset to the negative impacts of subsidence. For in-
creased flood risk as a result of subsidence, adaptation mea-
sures include the strengthening or heightening of embank-
ments, building on mounds or piles, or conduct spatial plan-
ning in such a way that new constructions are only built on el-
evated areas. For damage to structures, adaptation strategies
may include the use of flexible pipes and cables (specifically
for connection points), the use of better foundations for struc-
tures, or again careful spatial planning, whereby building is
limited to areas with supportive soils (for instance channel
belt deposits within a delta).
Adaptation strategies are commonly applied in subsiding
coastal cities, for instance most of them have network of em-
bankments that reduces the flood risk. Cities that pursue an
active policy on subsidence mitigation are less common, but
successful examples do exist. In Tokyo, after taking regula-
tions measures restricting the groundwater use were imposed
in the early 1960s, the groundwater levels began to rise as a
result (Fig. 7). Subsidence came to hold 10 years later as a
result of the delayed response in the compacting layers (see
Figure 7. Land subsidence and groundwater levels in the Tokyo
area (Japan), modified after Kaneko and Toyota (2011). The effect
of the reduction of groundwater extraction on groundwater levels is
clearly visible. Note that land subsidence completely stops 10 years
after the groundwater level recovery started.
also Sect. 5). The restrictions on groundwater use meant that
a replacement water source had to be found. Dams were con-
structed in several river basins that were designated for wa-
ter resources development. During the 1970s and 80s numer-
ous dams were built to provide storage to avoid future water
scarcity and to supply the growing cities with sufficient wa-
ter. Beginning in the 1960s an additional investment in waste
water treatment was initiated.
Shanghai in China is another example of a city with
a successful subsidence mitigation strategy. Following the
increased understanding of the close relationship between
groundwater extraction and land subsidence in Shanghai
(e.g. Shi et al., 2008), groundwater levels were restored with
active recharge techniques. Although this approach reduced
the further lowering of groundwater tables and limited sub-
sidence, it did not completely eliminate the effects of subsi-
dence on infrastructure, roads, and buildings. The Shanghai
case shows that, with active and substantial recharge, sus-
tainable groundwater use is achievable, without severe sub-
sidence, provided that average yearly pumping rates are in
balance with the average yearly recharge.
In Bangkok, Thailand, regulation of and restrictions on
groundwater extraction have successfully reduced severe
land subsidence. A specific law (the Groundwater Act) was
enacted in 1977. The most affected areas were designated
as Critical Zones, and the government was given more con-
trol over private and public groundwater use in these areas.
Groundwater use charges were first implemented in 1985 and
have gradually increased. Currently about 10% of the to-
tal water use is supplied by groundwater extraction, and this
mainly used by the industry in Bangkok. In most urban areas,
subsidence is now reduced to 1cmyr−1, with local increased
subsidence rates of 2 cm yr−1in the aforementioned indus-
trial sites.
Proc. IAHS, 372, 189–198, 2015 proc-iahs.net/372/189/2015/

G. Erkens et al.: Sinking coastal cities 197
Jakarta (Indonesia) and Ho-Chi-Minh City (Vietnam) are
considering similar subsidence mitigation strategies. In the
Greater Jakarta area, metropolitan authorities and technical
agencies are advocating the reduction of groundwater extrac-
tion in vulnerable areas. The goal is to completely phase out
the use of groundwater by taxing groundwater consumption.
This would require developing an alternative water supply
for large industrial users or relocation of large groundwa-
ter users, outside the so-called “critical zones”. The num-
ber of “unregistered” users is still a problem. To some ex-
tent, spatial planning measures have been applied to avoid
subsidence-prone areas, but the fast growth of informal set-
tlements has made many of these plans obsolete. Recently,
the Jakarta province government started to clear out the wa-
ter management structures to reduce flood risk. In 2015, the
Governor of Jakarta announced the reduction of the usage of
deep groundwater in all government and public buildings, as
a first step in the transition to piped water supply. The ex-
pected delayed response of subsidence to groundwater head
recovery (Bakr, 2015; Sect. 5) asks for accurate subsidence
prognosis. They form a vital component for any integrated
flood management and coastal defence strategy (Dam, 2012).
Although land subsidence in Ho Chi Minh City has been
observed since 1997, there is still – similar to Jakarta – con-
siderable disagreement about the underlying processes and
impacts. This is partly due to poor land level and groundwa-
ter extraction monitoring data (Ho Chi Minh City Flood and
Inundation Management, 2013). Restrictions of groundwater
extraction have been initiated, but it is too early to observe
any effects.
In the Netherlands, with arguably the longest history of
human induced subsidence in the world (since 1000 AD), the
focus has been on adaptation strategies for more than nine
centuries. In the coastal peatlands, after ∼1200 AD, adapta-
tion measures included improving drainage (digging canals),
the closing of (tidal) creeks and rivers, raising dikes and cre-
ating polders, and the improvement of foundations of build-
ings and infrastructure. Only in the last 50 years, with ever
increasing damage to structures, mitigation measures were
implemented. Nowadays, groundwater is sustained as shal-
low as possible in the peatlands. This means careful land use
planning (less productive grassland and nature development
in the most sensitive areas and considering alternative crops
elsewhere) and the inlet of fresh water in polders in dry peri-
ods. Complete mitigation of subsidence is probably not pos-
sible, because that would end agriculture in a major part of
western and northern Netherlands. The associated high eco-
nomic losses are socially and culturally not acceptable. In
the northern part of the Netherlands, gas extraction results in
significant subsidence. Here policy similarly developed to-
wards mitigation measures, albeit on a shorter time scale.
Gas extraction started in the 1960’s, but until about 2010 the
governmental response to subsidence was limited to adapta-
tion of the surface water management system. After 2010,
subsidence was accompanied by more frequent and power-
ful induced seismicity (earthquakes). The resulting damage
of houses and other constructions forced the government in
2014 to start additional mitigation measures in the form of a
significant reduction of gas exploitation in the most critical
fields. Again, full mitigation was very difficult as stopping of
the gas exploitation would endanger the national energy sup-
ply and would reduce the gas revenues by several billion of
euro’s per year. In addition, even if the gas exploitation was
completely phased out, the subsidence and earthquakes are
likely to continue. Concluding, for the Netherlands full mit-
igation of subsidence is far more expensive than implement-
ing adaptation measures and an adaptation strategy combined
with limited mitigation is a much more feasible option.
For all measures taken to reduce subsidence or its impacts,
it is important that the effectiveness of these efforts is mon-
itored. This implies that a subsidence monitoring network
(see Sect. 3) need to be installed before the measures are
implemented. The monitoring data form an important contri-
bution to any subsidence monitoring network that has been
established in step 1 (Sect. 3). Preferably, the monitoring
data and analytical results (of the various modelling tools)
are stored in a central database.
8 Concluding remarks
–In urban areas, human induced land subsidence domi-
nates the total subsidence signal.
–Land surface elevation measurements need to be com-
bined with in-situ measurements in order to be able to
unravel the total subsidence signal.
–There are two types of damage as a result of subsidence:
increased flood risk (with non-differential subsidence)
and damage to structures (with differential subsidence).
–Analogue to climate change policies, a successful policy
on subsidence consists of adaptation measures (reduc-
ing the damage and vulnerability) and mitigation mea-
sures (actively reducing subsidence).
–Delayed response of aquitards and interbed compaction
may introduce unwanted additional subsidence after im-
plementing mitigation measures, which is presently un-
accounted for.
Acknowledgements. This study is based on research conducted
by scholars working in the cities mentioned in this paper. They
shared data and insights from which this study greatly benefitted.
They are all gratefully acknowledged.
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198 G. Erkens et al.: Sinking coastal cities
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