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REVIEW ARTICLE
Heat waves and floods in urban areas: a policy-oriented review
of ecosystem services
Yaella Depietri •Fabrice G. Renaud •
Giorgos Kallis
Received: 3 June 2011 / Accepted: 28 September 2011 / Published online: 22 October 2011
Integrated Research System for Sustainability Science, United Nations University, and Springer 2011
Abstract Urbanisation is increasing and today more than
a half of the world’s population lives in urban areas. Cities,
especially those where urbanisation is un-planned or poorly
planned, are increasingly vulnerable to hydro-meteorolog-
ical hazards such as heat waves and floods. Urban areas
tend to degrade the environment, fragmenting and isolating
ecosystems, compromising their capacity to provide ser-
vices. The regulating role of ecosystems in buffering
hydro-meteorological hazards and reducing urban vulner-
ability has not received adequate policy attention until
now. Whereas there is a wide body of studies in the
specialised biological and ecological literature about par-
ticular urban ecosystem features and the impacts of hazards
upon people and infrastructures, there is no policy-driven
overview looking holistically at the ways in which eco-
system features can be managed by cities to reduce their
vulnerability to hazards. Using heat waves and floods as
examples, this review article identifies the aggravating
factors related to urbanisation, the various regulating
ecosystem services that buffer cities from hydro-meteoro-
logical impacts as well as the impacts of the hazards on the
ecosystem. The review also assesses how different cities
have attempted to manage related ecosystem services and
draws policy-relevant conclusions.
Keywords Heat waves Floods Ecosystem services
Urban areas Inland water systems Environmental
vulnerability
Introduction
Nowadays, more than 50% of people live in urban areas
with some 3.5 billion people having settled in cities
throughout the world (UNFPA 2009). This urbanisation
trend continues today and is likely to continue in the
coming decades (UNFPA 2007). As a consequence, large
changes in exposure to natural hazards and vulnerability of
social-ecological systems are taking place in urban areas.
At the local and regional levels, consumption of natural
assets and disruption of ecosystems
1
by cities have con-
tributed to the modification of the surrounding environ-
ment. Urban development fragments, isolates and degrades
natural habitats and disrupts hydrological systems (Alberti
2005). The occupation of floodplains, land conversion,
deforestation and loss of ecosystems are anthropogenic
factors that contribute to the loss of buffering capacity of
ecosystems to hazards. For instance, the impairment of soil
functions in urban areas causes the loss of water perme-
ability (i.e. soil sealing), which increases the impacts of
Handled by Victor Savage, National University of Singapore,
Singapore.
Y. Depietri (&)G. Kallis
Institut de Cie
`ncia i Tecnologia Ambientals (ICTA),
Universitat Auto
`noma de Barcelona (UAB), ETSE,
QC/3103, 08193 Bellatera, Barcelona, Spain
e-mail: depietri@ehs.unu.edu
Y. Depietri F. G. Renaud
United Nations University, Institute for Environment
and Human Security (UNU-EHS), UN Campus,
Hermann-Ehlers-Str. 10, 53113 Bonn, Germany
G. Kallis
Institucio
´Catalana de Recerca i Estudis Avanc¸ats (ICREA),
Universitat Auto
`noma de Barcelona (UAB), ETSE,
QC/3103, 08193 Bellatera, Barcelona, Spain
1
Ecosystems are the dynamic complexes of plant, animal, and
microorganism communities and the nonliving environment interact-
ing as a functional unit, including humans (MA 2005, p. 27).
123
Sustain Sci (2012) 7:95–107
DOI 10.1007/s11625-011-0142-4
potential floods and the likelihood of urban floods. Besides
this, ecosystems can also be affected by hazards that can be
an important determinant of vulnerability when commu-
nities have high dependencies on specific ecosystem ser-
vices in and around cities. Extreme, large-scale weather
events are likely to trigger ecosystem level disturbances,
which may affect the organisation (species composition
and diversity) and the functional attributes of ecosystems
(Parmesan et al. 2000). Overall, ecological effects of
extreme events have been identified as one of the main
gaps of knowledge in community ecology (Agrawal et al.
2007).
While there is growing literature on climate change and
vulnerability assessment (Adger 1999; Handmer et al.
1999; O’Brien et al. 2004;Fu
¨ssel and Klein 2006), com-
paratively little of it concerns cities (Kallis 2008) and even
fewer address the importance of urban ecosystem services
(Niemela
¨et al. 2010). There is extended literature on
ecosystem services,
2
classification and valuation (Costanza
et al. 1997; de-Groot et al. 2002;MA2005; Fisher et al.
2009), but likewise little of it focuses on the contribution of
buffering hydro-meteorological hazards,
3
and much less in
cities. This gap is covered by the present review. Our
starting point is that there is a large volume of relevant
material in specialised literatures, not least ecology, about
particular components of the urban ecosystem, but no
overarching interdisciplinary and policy-oriented synthesis
targeting disaster risk reduction. To ground our analysis we
focus on two important hazards, especially for European
cities: heat waves and floods.
Section 2presents the conceptual framework we used
for the review. Section 3discusses concepts related to
ecosystem services and urban systems. Sections 4and 5
review heat waves and floods, respectively. Both sections
address the following components of the framework pre-
sented in Fig. 1: a description of the hydro-meteorological
hazard, the aggravating factors related to urbanisation, the
regulating services (e.g. climate regulation, air quality
regulation and water regulation) and the impacts of the
hazard on the ecosystem. Section 5reviews related policy
initiatives for the protection of urban ecosystem services.
The review ends with a concluding discussion and policy
recommendations (Sect. 6).
Conceptual framework
Figure 1provides a conceptual framework for under-
standing the relationships between urban systems, ecosys-
tems and hydro-meteorological hazards. While we
recognise more integrated definitions of urban ecosystems
that include both human and non-human elements in their
inter-relation (e.g. Pickett et al. 2001), for the sake of this
analysis we maintain a distinction between the human
(‘‘urban systems’’) and ecosystem components. As Fig. 1
indicates, urban areas are vulnerable to hydro-meteoro-
logical hazards but ecosystems offer services that can
buffer these potential impacts. However, urbanisation as a
process erodes these ecosystem services, as do hazard
impacts, especially in a context of climate change and
intensifying extremes, as indicated in Fig. 1. Urbanisation
fragments ecosystems, which diminishes their regulating
capacity and increases the vulnerability of urban areas
themselves. As shown in Fig. 1, we are also interested in
the vulnerability of ecosystems to hazards, defined in
relation to the human component, i.e. their capacity to
withstand stresses and maintain important regulating and
other services. The vulnerability of the urban system
derives therefore from the effect of the impacts of the
hydro-meteorological hazard on the ecosystem and on the
urban system combined with the potential role of regulat-
ing services. It is possible to act in different ways to reduce
this vulnerability. Conventionally policy has focussed on
the urban system component in terms of adaptation policies
seeking to reduce the vulnerability of infrastructures or
particular segments of the population to hydro-meteoro-
logical hazards. This article shifts attention instead to
policies that can act on different leverages, such as urban
ecosystem restoration and preservation by protecting
ecosystems themselves from hazards directly (arrow 1 in
HYDRO-
METEOROLOGICAL
HAZARDS
ECOSYSTEMS
URBAN
SYSTEM
Impact
Vulnerability
Regulating ser vices
Impact (increase in
vulnerability)
Services
Impact
POLICY
LEVERAGE
1
2
3
Fig. 1 Conceptual framework highlighting the relationships between
hydro-meteorological hazards, ecosystems and urban systems
2
Ecosystem services are the benefits people obtain from the
ecosystem (MA 2005, p. 27).
3
Hydro-meteorological hazards are processes or phenomena of
atmospheric, hydrological or oceanographic nature that may cause
loss of life, injury or other health impacts, property damage, loss of
livelihoods and services, social and economic disruption, or environ-
mental damage (UNISDR 2009, p. 18).
96 Sustain Sci (2012) 7:95–107
123
Fig. 1), ecosystems restoration and preservation (arrow 2)
or indirectly by reducing urbanisation pressures (arrow 3).
Urban areas and ecosystem services
An urban area is defined as ‘‘a set of infrastructures, other
structures, and buildings that create an environment to
serve a population living within a relative small and con-
fined geographic area’’ (Albala-Bertrand 2003, p. 75).
Cities can be additionally defined as settlements that are
permanent. The definition of urban areas adopted in
national census varies from country to country. Three main
classifications of localities as urban can be identified
according to: (1) the size of the population (e.g. civic
district which is in general greater than 2,000, 2,500 or
5,000 inhabitants); (2) the proportion of population of a
civic district engaged in agriculture and the predominance
of non-agricultural workers; and (3) administrative, legal
criteria (e.g. type of local government) (2007 United
Nations Demographic Yearbook). Delimitating urban
areas, from a multidisciplinary perspective, is not a
straightforward task (Pelling 2003). The city can be con-
sidered as a single ecosystem or composed of individual
ecosystems: all natural green and blue areas in the city,
including street trees and ponds; seven ‘‘natural ecosys-
tems’’ are identified: street trees, lawns/parks, urban for-
ests, cultivated land, wetlands lakes/sea and streams
(Bolund and Hunhammar 1999). These ecosystems provide
a variety of services including climate regulation, air
purification, water regulation and carbon dioxide (CO
2
)
sequestration. For instance, urban green areas can buffer
extreme events such as heat waves and floods by reducing
temperatures, increasing ventilation, storing water and
reducing run-off (EEA 2010b). The Millennium Ecosystem
Assessment (MA) has identified the following classes of
ecosystem services: ‘‘provisioning services such as food
and water; regulating services such as regulation of floods,
drought, land degradation, and disease; supporting services
such as soil formation and nutrient cycling; and cultural
services such as recreational, spiritual, religious and other
nonmaterial benefits’’ (MA 2005, p. 27). The supply of
these services are the result of the functioning of ecosys-
tems, representing the products of processes that occur
within every ecosystem and, because the processes depend
on organisms and the organisms are linked by their inter-
actions, the services themselves are also linked (Fitter et al.
2010). For the sake of this review we focus mainly on
regulating services and the multiple benefits they provide
for the buffering of heat waves and floods in urban areas.
Of all ecosystem services, regulating services are amongst
the least investigated and assessed, and regulating
service indicators are weaker (i.e. low ability to convey
information and data availability) overall than provisioning
service indicators (Layke 2009). The analysis is based on
the classification of ecosystem services proposed by the
MA and the relevant services for each hazard have been
identified. For heat waves we considered: air quality
regulation and climate regulations, while for floods we
analysed water regulation.
Heat waves
Heat waves as a hazard
Heat waves are extreme events associated with particularly
hot sustained temperatures able to produce notable impacts
on human mortality and morbidity, regional economies and
ecosystems (Koppe et al. 2004; Meehl and Tebaldi 2004).
In Europe, heat waves have been the most prominent
hazard with regards to human fatalities in the last 10 years
(EEA 2010a). One well-documented example is the
European 2003 heat wave when more than 70,000 excess
deaths were reported during the summer (EEA 2010a) and
15,000 excess deaths in France alone (Fouillet et al. 2006).
A large precipitation deficit during spring 2003 contributed
to a rapid loss of soil moisture (Ciais et al. 2005; Zaitchik
et al. 2006; Fischer et al. 2007). As a result, the summer
2003 was by far the hottest since 1500 AD in Europe
(Luterbacher et al. 2004), and it seems that heat waves will
become more intense, longer lasting and/or more frequent
in future warmer climates (Meehl and Tebaldi 2004; Luber
and McGeehin 2008).
Urbanisation as an aggravating factor
In urban areas, the impacts of heat waves are aggravated
and the vulnerability of ecosystems and urban communities
are increased (see Fig. 1). Urban development modifies
land surface, leading to the creation of distinct urban
climates (Grimmond et al. 2004). Urbanisation has
quickly transformed ecosystems to infrastructures and
buildings that increase thermal-storage capacity (Luber and
McGeehin 2008). Built up and impervious surfaces are
stronger absorbers and the radiation is then slowly
re-emitted as long-wave radiation that is responsible of
warming up the boundary layer of the atmosphere within
the urban canopy layer (Oke 1988), producing the so called
‘‘Urban Heat Island’’ (UHI) effect. The UHI effect con-
cerns the magnitude of the difference in temperature
between cities and their surrounding rural regions and the
temperature difference increases with the number of
inhabitants and the building density: in Europe, the maxi-
mum UHI goes from 2 to 12C (Koppe et al. 2004). Due to
this effect, the highest morbidity and mortality associated
Sustain Sci (2012) 7:95–107 97
123
with extreme heat appear to occur in cities (Clarke 1972).
Harlan et al. (2006) examined the relation among the
microclimate of urban neighbourhoods, population char-
acteristics, thermal environments that regulate microcli-
mates and the resources people have to cope with climate
conditions in Phoenix, AZ. Neighbourhoods with few open
and green spaces, which have been proven to have cooling
functions, contribute to increasing the impacts of heat in
cities. Heat waves’ mortality rates, neighbourhoods’ envi-
ronmental quality and population characteristics are thus
spatially correlated (Harlan et al. 2006).
Climate regulation
As mentioned, temperatures in cities are higher than in the
surroundings, which cause higher impacts of extreme heat
events. Ecosystems in urban areas contribute to reducing
the UHI effect (Bolund and Hunhammar 1999). However,
urban forests, a common term to characterise all of the
vegetation of an urban region (McPherson et al. 1994), play
a particularly important role in regulating climate, energy
and water between the land surface and the atmosphere
(Zaitchik et al. 2006). According to various authors,
greening can cool the environment at least at the local scale
(Oke 1989; Akbari et al. 2001; Bowler et al. 2010), pro-
viding a climate-regulating service that can buffer the
impacts of heat waves. This is because plants and trees
regulate their foliage temperature by evapo-transpiration,
leading to a reduction of the air temperature. In addition,
green vegetation absorbs up to 90% of the photosyntheti-
cally active radiation while reflecting up to 50% of the near
infrared radiation (Braun and Herold 2003), thus absorbing
less heat than built infrastructures. The size of the green
area contributes to the magnitude of the cooling effect,
although it is not clear if there is a minimum size threshold
or if there is a simple linear relationship between these two
factors: on average an urban park would be around 1C
cooler than a non-green site (Bowler et al. 2010). Gomez
et al. (1998) observed that, in green areas, there was a drop
of 2.5C with respect to the city of Valencia (Spain)
maximum temperature. Wong and Yu (2005) observed a
maximum difference of 4.01C between well planted area
and the central business district area of Singapore, while
according to Hamada and Ohta (2010) the temperature
difference between urban and green areas in Nagoya
(Japan) was large in summer and small in winter. The
maximum air temperature difference was 1.9C in July
2007, and the minimum was -0.3C in March 2004.
Renaud and Rebetez (2009) compared open-site and
below-canopy climatic conditions from 14 different sites in
Switzerland during the 11-day August 2003 heat wave.
Maximum temperatures were cooler under the canopy and,
the warmer the temperature, the stronger the impact of the
forest. For maximum temperature, the difference was
higher in deciduous and mixed forests compared to conif-
erous forests. For minimum temperature, in contrast, the
discrepancy was higher in coniferous forests (Renaud and
Rebetez 2009). Similarly it is worth noting that, during heat
wave days, the increase in sensible heat flux is initially
much larger over forests than over grasslands (Teuling
et al. 2010). In the long term, however, grasslands become
the main heat source due to the fact that elevated evapo-
rative cooling accelerates soil moisture depletion (Teuling
et al. 2010).
According to Alexandri and Jones (2008), although
parks manage to lower temperatures within their vicinity,
they are incapable of significantly cool the surrounding
areas where people live. Therefore, the authors suggest
that it would be more effective to place vegetation within
the built space of the urban fabric; thus raised urban tem-
peratures can decrease within the human habitats them-
selves and not only in the detached spaces of parks. For
single trees, evapotranspiration and tree shading are impor-
tant control measures in heat-island mitigation in Tel-
Aviv (Shashua-Bar and Hoffman 2003). According to these
researchers, the cooling effect depends mainly on the
amount and extent of the partial shaded area. For instance
in Athens, during a short exceptionally hot weather period
in 2007, the highest cooling effect of 2.2C was found to be
reached in a street with high tree shaded area and minimal
traffic load (Tsiros 2010). These results imply the passive
cooling potential of shade trees. Akbari et al. (2001) esti-
mated that 20% of the cooling demand of the USA can be
avoided through the implementation of heat island miti-
gation measures for instance by planting trees. In general,
frequentation and use of green spaces could generate
benefits and well being on people, especially during heat
waves periods, and this could be explained by the capacity
of green spaces to provide better thermal comfort
(Lafortezza et al. 2009).
Air quality regulation
Air quality regulation refers to the role ecosystems play in
regulating the gaseous portion of nutrient cycles that affect
atmospheric composition. Air quality plays an important
role during heat waves and can be a source of human ill-
nesses during these extreme events. During a heat wave in
urban areas, hot days are often followed by hot nights
because of the heat island effect. These conditions can
produce a high degree of heat and air pollution stress,
especially for people with cardiovascular and respiratory
disorders (Piver et al. 1999). For instance, in The Nether-
lands, 1,000–1,400 deaths were estimated because of the
hot temperatures that occurred during the 2003 summer
period, and of these, the number of deaths attributable to
98 Sustain Sci (2012) 7:95–107
123
the ozone (O
3
) and particular matter (PM
10
) concentrations
in the period June–August were estimated at around
400–600 deaths (Fischer et al. 2004). In France, the relative
contribution of O
3
and temperature in the high mortality
during the 2003 heat wave was heterogeneous among cities
(Filleul et al. 2006). For the nine cities considered in their
study, the excess risk of death for an increase of 10 lg/m
3
in O
3
level is significant (Filleul et al. 2006). In particular,
between 3 and 17 August 2003, the excess risk of deaths
linked to O
3
and temperatures together ranged from 10.6%
in Le Havre to 174.7% in Paris, while the contribution of
O
3
alone varied, ranging from 2.5% in Bordeaux to 85.3%
in Toulouse (Filleul et al. 2006). In Croatia, a significant
part of excess mortality, during the same period, was
attributed to PM
10
and O
3
in the air (Alebic
´-Juretic
´et al.
2007).
Due to their large leaf areas and their physical properties
trees can act as biological filters. These can remove large
numbers of airborne particles and hence improve the
quality of air in polluted environments (Beckett et al. 1998;
Nowak et al. 2000; Brack 2002; Jim and Chen 2008;
Escobedo and Nowak 2009). In particular, trees can be
effective in reducing the impacts of damaging forms of
particulate pollution such as PM
10
or gasses such as
sulphur dioxide (SO
2
), nitrogen oxides (NO
x
), carbon
monoxide (CO) and CO
2
, and are effective in reducing O
3
concentrations in cities (Nowak et al. 2000). The effec-
tiveness of this ecosystem service varies according to plant
species, canopy area, type and characteristics of air pollu-
tants, and local meteorological environment. Larger trees
extract and store more CO
2
from the atmosphere and their
greater leaf area traps air pollutants, casts shade and
intercepts rainfall run-off (Brack 2002). Uptake happens
mainly through dry deposition, a mechanism by which
gaseous and particulate pollutants are transported to and
absorbed into plants mainly through their surfaces. In urban
areas, districts with more extensive urban trees capture
more pollutants from the air, and this capacity is increased
as trees gradually reach final dimensions (Jim and Chen
2008). In general, the effectiveness of uptake by trees of
particles [5lm is increased if their leaf and bark surfaces
are rough or sticky (Beckett et al. 1998). For smaller par-
ticles the most effective uptake happens in the needles of
conifers. Due to the larger total surface area of needles,
coniferous trees have a larger filtering capacity than trees
with deciduous leaves, with pines (Pinus spp.) capturing
significantly more material than cypresses (Cupresses spp.)
(Beckett et al. 2000). In addition, this capacity is also
greater because the needles are not shed during the winter,
when the air quality is usually worst (Bolund and
Hunhammar 1999). According to Jim and Chen (2008),
most removal occurs in the winter months mainly due to
the higher pollutants concentrations. However, coniferous
trees are also more sensitive to air pollution and deciduous
trees are better at absorbing gasses (Bolund and Hunham-
mar 1999). Veteran trees (i.e. trees that have lived a long
time and are significant elements of the landscape) often
contribute substantially more benefits to society relative to
other (smaller) trees in the landscape (Nowak 2004). For
instance, veteran trees will store, due to their increased
size, larger amounts of carbon in their tissues. Interception
of particles by vegetation seems also to be much greater for
street trees, due to their location in proximity to high road
traffic (Beckett et al. 1998). Trees situated close to a busy
road capture significantly more material, especially larger
particles, than those situated in a rural area (Beckett et al.
1998). In Chile, it has been demonstrated that Santiago’s
urban forests are effective at removing PM
10
(Escobedo
and Nowak 2009). In 1991, trees in the city of Chicago
(11 percent tree cover) removed an estimated 15 metric
tons of CO, 93 tons of SO
2
, 98 tons of NO
2
, 210 tons of 0
3
and 234 tons of PM
10
(McPherson et al. 1994). Similarly,
the peri-urban vegetation of the Madrid region constitutes a
sink of O
3
, with evergreen broadleaf and deciduous tree
species removing more atmospheric O
3
than conifer for-
ests. Nowak et al. (2000) modelled the effects of increased
urban tree cover on O
3
concentrations (13–15 July 1995)
from Washington, DC (USA), revealing that urban trees
generally reduce O
3
concentrations in cities.
Jim and Chen (2008) assessed the capability and mone-
tary value of the removal by urban trees of air pollution in
Guangzhou city in South China. The researcher found that
an annual removal of SO
2
,NO
2
and total suspended par-
ticulates of about 312.03 mg, and the benefits were valued
at RMB 90.19 thousand. Overall, there are few known
studies that analyse differences in urban forest structure and
air pollution removal in sub-regions of a city, and there are
even fewer studies that link a city’s urban forest structure
and socioeconomic activity with site-specific pollution
dynamics through time (Escobedo and Nowak 2009).
Impacts of heat waves on ecosystems in and around
urban areas
As showed in Fig. 1, hydro-meteorological hazards can
also affect ecosystems, such as forests and water systems,
and their services in urban and peri-urban areas. In the
summer 2003, the drought experienced by the vegetation
was worsened by the length of the period with scarce
precipitations and humidity, by the heat during the summer
and the longer duration of the sunshine period (Rebetez
et al. 2006). In the course of a drought, the gradually
decreasing rate of passage of either water vapour or CO
2
through the stomata, or small pores of the plant, assimila-
tion and growth are observed (Leuzinger et al. 2005). Fires
can also affect vegetation during heat waves or periods of
Sustain Sci (2012) 7:95–107 99
123
drought. During the 2003 heat waves, small-scale forest
fires were observed all over central Europe and the western
Mediterranean (Fink et al. 2004).
Trees can be directly affected by air pollutants
depending on the types and concentrations. The most
common effects of plant exposure to O
3
are modifications
of stomata behaviour, which leads to a reduced photosyn-
thesis and an increased respiration. As gas, NO
x
and SO
2
damage cuticles and stomata and, most importantly, they
penetrate through stomata and alter tissues. SO
2
can cause
both acute (e.g. cell plasmolysis) and chronic injury (e.g.
reduced gas exchange, chlorophyll degradation, chloroplast
swelling, and alteration of cellular permeability). As
mentioned above, after a certain threshold is reached trees
can be affected by nutrient stress, reduced photosynthetic
or reproductive rate, predisposed to entomological or
microbial stress, or direct disease induction (Smith 1974).
In the urban context, water bodies and wetlands are often
beneficial for transportation, recreation, dilution and puri-
fication. On the other hand, urbanisation is thought to cause:
‘‘reduced baseflows, increased frequency and magnitude of
peak discharges, increased sediment loads, impaired water
quality, reduction in channel and floodplain complexity’’
(LeBlanc et al. 1997). Water quality may further deteriorate
to critical values during periods of prolonged low-flow
conditions in combination with high water temperatures. In
the Meuse river basin, which flows through the cities of
Namur and Lie
`ge in Belgium, during the 1976 and 2003 heat
waves, deterioration of water quality by high water tem-
peratures, eutrophication, increased concentrations of major
elements and some metals or metalloids (selenium, nickel
and barium) were measured (van-Vliet and Zwolsman
2008). However, concentrations of nitrate and some heavy
metals with high affinity for adsorption onto suspended
solids (i.e. lead, chrome, mercury and cadmium) decreased,
which also positively affected chemical water quality during
drought (van-Vliet and Zwolsman 2008). Overall eutro-
phication, following hot spells, causes major impacts on the
water system.
During a heat wave event, temperature of lakes can rise
until reaching record temperatures (increase varies between
1 and 3C on average) (Jankowski et al. 2006). Jankowski
et al. (2006) investigated the consequences of the 2003
European heat wave for lake temperature profiles, thermal
stability (i.e. suppressing downward turbulent mixing) and
hypolimnetic oxygen depletion. Warming of water bodies
can restrict lake overturning and lead to anaerobic condi-
tions, thus potentially seriously impacting aquatic ecosys-
tems, leading, in the summer, to the development of
harmful cyanobacterial blooms. Oxygen depletion may
lead to negative ecological consequences such as phos-
phorous dissolution from the sediments leading to internal
loading and algal blooms (Jankowski et al. 2006). Surface
blooms of toxic cyanobacteria in eutrophic lakes may lead
to mass mortalities of fish and birds, and affects cattle, pets,
and humans (Jo
¨hnk et al. 2008). This may have impacts
on recreational activities, fishing and surface water sports.
In sport fisheries, increased water temperature has been
associated with decreased activity and movement to deeper
cooler waters, which reduces fish catches.
Concerning water quantity, most of the studies agree
that wetlands reduce the flow of water in downstream
rivers during dry periods. In fact evapotranspiration from
wetlands is shown to be higher than from other portions of
the catchment during these periods (Bullock and Acreman
2003). For groundwater resources, prolonged heat stress
may lead to lowered water table levels. In urban areas, the
impairment of soils aggravates the magnitude of this
impact because the reduced infiltration reduces the water
table. Lower water table levels were measured in urban
areas (Scalenghe and Marsan 2009).
Floods
Floods as a hazard
The European Union Floods Directive defines a flood as a
temporary covering by water of land not normally covered
by water. According to Few et al. (2004), flood disasters
and their mortality impacts are heavily concentrated in
Asia, where there are high population concentrations in
floodplains, such as the Ganges, Brahmaputra, Mekong and
Yangtze basins, and in cyclone-prone coastal regions such
as around the Bay of Bengal and the South China Sea.
Floods affecting urban areas can be either generated locally
or in other locations in the watershed and basin. Urban
areas often generate impacts on watershed-wide ecosys-
tems such as through land use changes and infrastructure
development, which affects watercourses. When dealing
with floods, it is therefore important to consider the role of
ecosystems not only within the urban areas themselves, but
also in the entire landscape of the watershed and the
influences urban areas have on them (PEDRR 2011). These
two scales of analyses are considered in this section with a
focus on urban areas.
Floods are the result of meteorological and hydrological
factors, but anthropogenic modifications can also play a
role in defining the magnitude of the event. Therefore,
floods in urban areas are the result of natural and man-
made factors. Although these influences are very diverse,
they generally tend to aggravate flood hazards by accen-
tuating flood peaks. As a result of different combinations of
factors, urban floods can basically be divided into four
categories: local floods, riverine floods, coastal floods and
flash floods. Floods in urban areas can be attributed to one
100 Sustain Sci (2012) 7:95–107
123
or a combination of the above types. The main cause of
urban flooding is a severe thunderstorm, which is generally
preceded by a long but moderate rainfall that saturates the
soil. Therefore floods in urban areas are, in general, flash
floods that expand both on impaired surfaces and in sur-
rounding parks and streets (Andjelkovic 2001). Other
causes of urban floods are: ‘‘inadequate land use and
channelization of waterways; failure of the city protection
dikes; inflow from the river during high stages into urban
drainage systems; surcharge due to blockage of drains and
street inlets; soil erosion generating material that clogs
drainage system and inlets; inadequate street cleaning
practice that clogs street inlets’’ (Andjelkovic 2001).
Major destructive flooding events occurred in Europe in
the last few years: floods in the Elbe basin in 2001 that
produced losses of over 20 billion Euros; floods in Italy,
France and the Swiss Alps in 2000 costing around 12 bil-
lion Euros and a series of floods in the UK during summer
2007 accumulating losses of more than 4 billion Euros
(EEA 2010a). Losses as a consequence of floods have
increased in the past decades in Europe. While there is no
evident trend over time in respect to the number of fatal-
ities and observations do not show a clear increase in flood
frequency (Mudelsee et al. 2003), increases in population
and wealth in the affected areas are the main factors con-
tributing to the increase in losses (EEA 2010a).
Impacts of urbanisation on ecosystems
When degraded by a variety of human activities and
changing climatic conditions, hydrologic regimes typically
have increased frequency and severity of flooding, lowered
water tables and reduced groundwater recharge compared
to previous, more ‘‘natural’’ conditions (Cai et al. 2011).
At the river basin level, urbanisation directly affects
catchments hydrology by changes in surface runoff,
groundwater runoff, groundwater levels and water quality.
The introduction of impervious surfaces inhibits infiltration
and reduces surface retention (Packman 1980). Thus the
proportion of storm rainfall that goes to surface runoff is
increased, and the proportion that goes to evapotranspira-
tion, groundwater recharge and base flow is reduced. This
increased surface runoff is combined with an increase in
the speed of response and increased peak discharge, which
can lead to floods (Packman 1980; Nirupama and Simo-
novic 2006). For instance, in the Upper Thames River
watershed in Canada, urban areas have increased to 22% of
the total watershed area in the year 2000 compared to only
10% in 1974, enhancing the risk from river flooding
(Nirupama and Simonovic 2006). On the other hand, in the
Dead Run watershed (14.3 km
2
) in the Baltimore, MD,
region, the current tree cover is 13.2% with an impervious
cover of 29%. Increasing tree cover in the watershed to
71% is estimated to reduce total runoff in the watershed
by about 5% for the simulation period of the year 2000
(Nowak 2006). Li and Wang (2009) analysed the urban
expansion of St. Charles County, a suburb of St. Louis,
MO, located in the Dardenne Creek watershed. A rapid
increase of urban areas in the watershed took place from
3.4% in 1982 to 27.3% in 2003, and model simulations
suggested an increase in more that 70% in average direct
runoff in the watershed from 1982 to 2003, correlated with
urban expansion.
At the urban scale, more radical changes in surface
characteristics and soil sealing (i.e. the covering of soil for
housing, roads and parking lots, etc.) increase the imper-
meability of soils, drainage and water run-off and lead to
rapid precipitation run-off, flooding, erosion and impervi-
ous surfaces cause: ‘‘local decreases in infiltration, perco-
lation and soil moisture storage, reductions in natural
interception and depression storage and increases in run-
off’’ (Brun and Band 2000). Severe storms may also yield
discharges exceeding the capacity of the sewer system,
causing choking of the flow and increased attenuation in
localised ponding. The soft ground of vegetated areas
allows water to seep through, and the vegetation takes up
water and releases it into the air through evapo-transpira-
tion (Bolund and Hunhammar 1999). Urban sprawl with
moderate to high soil sealing over a large area reduces the
infiltration potential of the soil and increases the flood risk
of urban areas (EEA 2010a). Soil sealing also impacts the
porosity of soils by reducing it or by modifying its pattern,
which reduces water infiltration (Scalenghe and Marsan
2009). In the surroundings of urban areas the amount and
the speed of flooding water that arrives on unsealed sur-
faces are increased and increase the risk of ponding and
erosion (Scalenghe and Marsan 2009). In these areas, when
human population increases with urban sprawl, wetland’s
functions can easily be affected with pollution or recreational
activities (Mitsch and Gosselink 2000). In these conditions,
wetlands can no longer effectively reduce floods, sequester
pollutants or host different biota (Mitsch and Gosselink
2000). Thus wetland value appears to be maximum when
close to the river system and distributed spatially across an
environment that is not dominated either by cities or agri-
culture, but one that balances nature and human aspects
(Mitsch and Gosselink 2000).
Water regulation
The role of forests and soils
On the local scale, forests and forest soils are capable
of reducing runoff generally as the result of enhanced
infiltration and storage capacities. At the river basin scale,
this holds true for small-scale rainfall events in small
Sustain Sci (2012) 7:95–107 101
123
catchments, which are not directly responsible for severe
flooding in downstream areas (FAO and CIFOR 2005). The
geomorphology of the area and the preceding rainfall seem
to be the two most important factors in determining the
magnitude of the flooding event: the amount of storm flow
is most directly linked to the area in the watershed and
volume of precipitation or snowmelt deposited on the site,
stored or transported to the stream (Eisenbies et al. 2007).
Overall, forests seem not to be able to stop large-scale
floods, which are caused by severe meteorological events
(Eisenbies et al. 2007).
According to the physical properties of soils, some, not
heavily affected by human activities, have a large capacity
to store water, facilitate transfer to groundwater, and pre-
vent and reduce flooding (MA 2005). The initial conditions
of soil saturation in a basin determine the manifestation and
the intensity of a flood event. In conditions of low soil
saturation, water percolates in the soil and flood risk is
diminished (Lahmer et al. 2000). While in conditions of
high soil saturation, additional precipitation is rapidly
transferred to the river through surface runoff or by inter-
flow (Lahmer et al. 2000). When the soil becomes saturated
and loses its ability to store further water, there does not
seem to be any evidence that forests and their soil have a
noticeable effect in regulating floods for the extreme
rainfall conditions that lead to major flooding events
(Balmford et al. 2008). However, evidence indicates that
the key factor linking land use and flood regulation is soil
condition rather than the trees, and that much of the soil
degradation associated with deforestation results from poor
land use practices (e.g. soil compaction during road
building, overgrazing, litter removal, destruction of organic
matter, clean weeding) (Balmford et al. 2008).
The role of wetlands
Wetland, floodplain, lake and reservoir ecosystems play an
important role in the regulation of floods in inland systems
and provide protection from the adverse consequences of
natural hazards to humans, even for urban areas. In par-
ticular, wetlands are significant in altering water cycle and
perform hydrological functions (Bullock and Acreman
2003). Floodplain wetlands reduce or delay floods; on the
other hand most of the studies show that wetlands located
upstream in a watershed tend to be quickly saturated and
increase the risk of flash floods (Bullock and Acreman
2003). If wetlands are too small, functions, such as storage
of floodwater (for mitigation of floods), no longer exist: it
has been assessed that 3–7% of the area of a watershed
in temperate zones should be maintained as wetlands to
provide both adequate flood control and water quality
improvement functions (Mitsch and Gosselink 2000).
Effective control is more often the result of the combined
effect of a series of wetlands within a catchment area
instead of single units. Some authors argue that, to main-
tain the pulse control function of wetlands, a greater
number of wetlands in the upper reaches of a watershed is
preferable to fewer larger wetlands in the lower reaches
[Loucks (1989) cited in Mitsch and Gosselink (2000)].
A modelling effort on flood control suggested the opposite:
the usefulness of wetlands in decreasing flooding increases
with the distance of the wetland downstream [Ogawa and
Male (1986) cited in Mitsch and Gosselink (2000)]. Further
research is therefore needed in order to determine the real
effects of wetlands and their position in the landscape in
terms of buffering urban areas from floods.
Impacts of floods on ecosystem services
While floods provide a series of benefits (e.g. nutrients
deposition), these hazards can also affect ecosystems
especially when the environment has been degraded. For
instance, flooding and submergence are responsible for
major abiotic stresses and, together with water shortage,
salinity and extreme temperatures are the major factors that
determine species distribution (Visser et al. 2003). Overall,
only pioneering species are able to locate in zones close to
the river (Blom and Voesenek 1996), and floods have a
greater impact on plant species during the growing season
(Kozlowski 1997). Plant responses to flooding during the
growing season include: ‘‘injury, inhibition of seed ger-
mination, vegetative growth, and reproductive growth,
changes in plant anatomy, and promotion of early senes-
cence and mortality’’ (Kozlowski 1997). Adaptive strate-
gies and flood tolerance of plants depend on plant species
and genotype, age of plants, frequency, duration, timing
and conditions of flooding (i.e. soil flooding, water logging,
total submergence of vegetation) (Kozlowski 1997;
Vervuren et al. 2003). Underwater light intensity and depth
of the water column may also affect survival during periods
of submergence (Vervuren et al. 2003).
Flooding also changes the physical status of soils, which
may severely affect peri-urban agricultural practices. For
instance, water logging causes the breakdown of large
aggregates into smaller particles, deflocculation of clay
and destruction of cementing agents. As the water level
declines, these small parts of soil are redistributed in a new,
more dense structure, creating: ‘‘smaller soil pore diame-
ters, higher mechanical resistance to root penetration, low
O
2
concentrations and the inhibition of resource use’’
(Blom and Voesenek 1996) and the accumulation of CO
2
in soils (Kozlowski 1997). Gas diffusion is severely
inhibited during flooding (Blom and Voesenek 1996):
oxygen remains in the soils and is consumed by aerobic
processes (roots and soil organisms), nutrient availability
for plants strongly decreases and anaerobic processes take
102 Sustain Sci (2012) 7:95–107
123
place producing toxic substances for plants [lowering of
soil redox potential (Eh)]. Chemical changes also include
the increased solubility of mineral substances, reduction of
Fe, Mn and S, and anaerobic decomposition of organic
matter. The reduction condition (i.e. low soil Eh) is also a
major factor in wetland ecosystems that influences plant
survival, growth and productivity (Pezeshki 2001). Floods
can also cause secondary impacts, for instance affecting
industries and in particular petrochemical ones, causing
the release of toxic chemicals, which further impact the
environment.
Policy initiatives for the protection of urban ecosystem
services for disaster risk reduction
At the international level, policy initiatives that target any
of the three points of policy leverage identified in Fig. 1, are
rare. The United Nations International Strategy for Disaster
Reduction’s (UNISDR) Hyogo Framework for Action
2005–2015 lists ‘‘sustainable ecosystems and environmen-
tal management’’ as one of the main pillars for reducing
underlying risk factors (UNISDR 2005). Calling for an
improved management of ecosystems and their services for
disaster risk reduction, this initiative directly targets arrow 2
of the framework in Fig. 1. Paragraph 19 of the Hyogo
Framework can be situated in correspondence to arrow 3 of
the framework as it establishes the Priority for Action 4:
‘‘Reduce the underlying risk factors’’, which include:
‘‘Incorporate disaster risk assessments into urban develop-
ment planning and management of disaster-prone human
settlements’’ …‘‘rural development’’ …‘‘major infra-
structure’’ …‘‘including considerations based on social,
economic and environmental impact assessments’’. In
addition the UNISDR campaign on Making Cities Resilient
proposes a 10-point checklist to serve as a guide for com-
mitment by Mayors (UNISDR 2010). Point 8 makes explicit
the need to consider the environmental dimension: ‘‘Protect
ecosystems and natural buffers to mitigate floods, storm
surges and other hazards to which your city may be vul-
nerable. Adapt to climate change by building on good risk
reduction practices’’ (UNISDR 2010,p.9).
At the local and regional levels, policy initiatives are
very few for the incorporation of ecosystem preservation
for disaster risk reduction in urban areas. At the European
level, the Communication from the Commission to the
European Parliament ‘‘Options for an EU vision and target
for biodiversity beyond 2010’’ identifies four policy
options to halt the loss of biodiversity and ecosystem ser-
vices by 2020 but with no reference to disaster risk
reduction (EC 2010). For European countries, notably the
UK, The Netherlands and Germany, affected by severe
flooding in recent years, have made policy shifts to
‘‘making space for water’’, represented by arrow 2 of the
framework (Fig. 1). New risk management policies and
practices favour a more holistic approach to flood risk
management based on River Basin Management Plans,
Integrated Coastal Zone Management to enhance natural
processes.
Regarding heat waves, the London local government
authorities have developed the ‘‘Right Trees for a Changing
Climate’’ database and website to provide advice on
planting the right trees in the right place, based on the fact
that planting more trees, alongside increasing other green
cover, is one of the ways in which London can adapt to
climate change (http://www.right-trees.org.uk/default.aspx),
using vegetation to keep the city cool. The city has set
ambitious targets to: increase tree cover by 5% by 2025;
increase greenery in the centre of London by 5% by 2030
and a further 5% by 2050; create 100,000 m
2
of new green
roofs by 2012; and enhance 280 hectares of green space by
2012. Stuttgart has planned to exploit the role of natural
wind patterns and dense vegetation in reducing problems of
overheating and air pollution. A Climate Atlas was devel-
oped for the Stuttgart region, presenting the distribution of
temperature and cold air flows according to the city’s
topography and land use. Based on this information, a
number of planning and zoning regulations are recom-
mended that aim to preserve open space and increase
the presence of vegetation in densely built-up areas
(Kazmierczak and Carter 2010). In Stuttgart the preserva-
tion of the natural environment in urban areas is principally
guided by the Federal Nature Conservation Act
(BNatSchG), which prohibits the modification or impair-
ment of protected green spaces, or changing land use in
these protected areas (i.e. ‘‘zones in settlement areas, parks,
cemeteries, significant gardens, single trees, lines of trees,
avenues or groves in settled or unbuilt areas; and
some plantings and protective wood outside forests’’)
(Kazmierczak and Carter 2010). London and Stuttgart have
thus put in place policies that focus on arrow 2 and 3 of the
framework presented in Fig. 1, meaning that they call for
ecosystem conservation and an improvement of urban
planning.
Other examples of the implementation of projects and
policies to protect cities from hydro-meteorological hazards
through the restoration and management of ecosystem ser-
vices come from the US and Lao PDR. The ‘‘Grow Boston
Greener (GBG)’’ is a collaborative effort of the city of
Boston (USA) and its partners to increase the urban tree
canopy cover in the city by planting 100,000 trees by 2020.
The planting of these trees will increase Boston’s tree
canopy cover from 29 to 35% by 2030 as the planted trees
mature. The goals are to: ‘‘increase the tree canopy cover in
low canopy areas; mitigate the urban heat island effect
and reduce energy consumption through the appropriate
Sustain Sci (2012) 7:95–107 103
123
placement of trees on residential and commercial proper-
ties; improve air quality; and improve storm water man-
agement through strategic neighbourhood plantings’’
(http://people.tribe.net/phoenix_fire_nectar/blog/bfacb2b9-
300a-4a28-9ac9-48681de04b07). This case is interesting as
it considers the multiple benefits and services that can be
derived by the same ecosystem, in this case the urban forest.
‘‘Integrating Wetland Ecosystem Values into Urban
Planning: The Case of That Luang Marsh’’ is an economic
assessment of the goods and services provided by the
marshes in an attempt to examine the economic value of
urban wetland biodiversity and its importance to people
living around the wetland as well as the larger urban area of
Vientiane (Gerrard 2004). Wetlands and marsh areas in and
around the city are important physical features and provide
hydrological functions such as flood control. There are
currently 175 flood-prone areas within the city limits, 70 of
which are located in the city’s core area. Flooding occurs at
least six times a year but in many cases flood-prone areas
will flood every time it rains. In the urban area of Vientiane
flooding is not deep, but frequent flooding causes damage
to buildings and roads, and interrupts transportation. The
value of the regulating ecosystem service has been mea-
sured as the annual value of flood damages avoided in these
areas and it will amount at close to US$ 3 million by the
year 2020 (Gerrard 2004).
Conclusions
Basing our analysis on the conceptual framework of Fig. 1,
we reviewed how locally and regionally generated and
well-managed ecosystem services can contribute to lower
the vulnerability of urban communities to hydro-meteoro-
logical hazards such as heat waves and floods. We also
reviewed studies that quantify, when possible, the role of
these ecosystem services.
Urban areas will continue to attract people who want to
settle in them because of the many economic advantages
they provide. It is therefore critical for urban areas to pro-
vide safe livelihoods to their populations, and although
urbanisation will always imply a change in ecosystems and
the services they provide, urban planning should system-
atically consider the role of ecosystems in buffering and
mitigating the effects of environmental hazards such as heat
waves and floods. In this respect, urban areas would benefit
from the restoration and adequate management of ecosys-
tems. Urban planners and managers should take into con-
sideration the role of ecosystems in reducing risks and
vulnerabilities (arrow 2 of Fig. 1). When establishing urban
development plans the presentation of landscape plans and
green open space structure plans should be taken into
account (arrow 2 and 3 of Fig. 1). Although urban areas will
generally create similar types of effects when it comes to
heat waves and flooding, the specific local climatic condi-
tions (e.g. urban heat island effect) will dictate the magni-
tude and frequency of the hazards and the urban ecosystems
the potential to buffer these. There are therefore no generic
sets of solutions to address the problems globally, but
adapted solutions need to be sought regionally or locally.
On the other hand, while designing strategies that make use
of ecosystem services to buffer cities, it is important to take
into consideration the effects of hazards on the ecosystem
itself (arrow 1 of Fig. 1). For instance, the right type of trees
(i.e. in general native species) or the right succession that is
more resistant to the impacts of hydro-meteorological
hazards should be identified to be planted. Overall we have
sufficient ecological knowledge on how cities can be buf-
fered by ecosystem services with respect to the reviewed
hydro-meteorological hazards. But we know little on how to
measure ecosystem services for hazard regulation, which is
an obstacle towards the design and implementation of
appropriate policies and plans. We propose here a series of
general recommendations to fill this gap:
•ecosystem conservation and restoration play an
important role in lowering the vulnerability of urban
areas to hydro-meteorological hazards. A meta-analysis
of 89 restoration assessments in a wide range of eco-
system types across the globe indicates that ecological
restoration increased provision of biodiversity and
ecosystem services by 44 and 25%, respectively
(Benayas et al. 2009);
•the vulnerability of urban communities and ecosystems
to hydro-meteorological hazards is influenced and
aggravated by the impacts of these hazards on ecosys-
tems and urbanisation. It is therefore necessary to
integrate disaster risk reduction, appropriate urban
planning and ecological restoration. For instance, in
areas where land-use pressure is considerable, ecosys-
tem services can be secured by leaving green and blue
areas in close proximity to one another, so that these
areas form larger nature and landscape entities (Nie-
mela
¨et al. 2010);
•one ecosystem my provide services for the regulation of
more than one type of hydro-meteorological hazard, as
is the case for green areas in cities. It is therefore
recommended to adopt a multiple-hazard approach;
•generally cities are located in a watershed. The
management of ecosystems for the protection of cities
from hydro-meteorological hazards at this scale, which
transcends administrative boundaries, can provide
important elements of comparison of the vulnerability
of different cities.
Urban ecosystems provide essential services to cities
and city dwellers that are exposed to heat waves and floods.
104 Sustain Sci (2012) 7:95–107
123
Given the rapid urbanisation throughout the world and the
likely impacts of climate change in terms of these two
hazards, it is urgent to manage these ecosystems in a better
way than has been done in the past through integration of
ecosystem management in urban planning and disaster risk
reduction.
Acknowledgments The work presented in this paper was carried
out as part of the EU funded Seventh Framework project MOVE
(Methods for the Improvement of Vulnerability Assessment in Eur-
ope, project no. 211590).
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