Ur b a n f l o o d i n g
By Carlos E.M. TUCCI*
Main causes and impacts
Flood disasters in urban areas may arise from floodplain occupation or be generated by changes in land
use, such as urbanization and deforestation. The main
impacts of flooding on the population occur when
there is insufficient knowledge regarding the frequency of water levels associated with floods and
planning for space occupancy according to the risks
of flood events, especially on new migrants to the city.
A common scenario of uncontrolled urbanization is that floodplain occupation takes place during
a succession of years with low flood levels that are
confined within normal banks of rivers and streams.
When higher flood events take place, damage
increases and the authorities have to invest in flood
relief, which is followed by public demand for protection from floods. The metropolitan area of Curitiba
(state of Paraná, Brazil) has 2.5 million inhabitants.
Most of this urban area is in the Upper Iguaçu River
Basin, which has an area of 1 000 km2. The greatest
urban concentration is in the Belem Basin and the
neighbouring basins of Atuba and Palmital. The river
has a large natural floodplain due to the small river
conveyance and bottom slope. As a result, during
floods, the hydrograph is attenuated by the storage
capacity of the floodplains. The regional administration ruled against occupation of the floodplain but, as
of 1980, heavy pressure was applied to occupy the
floodplain once more. The public invaded green
Table 1
Flood losses at União da Vitória and Porto
União (JICA, 1995)
Year
Losses
US$ millions
1982
1983
1992
1993
10.365
78.121
54.582
25.933
Q (m3/s)
areas, making unapproved developments and habitations.
In Figure 1 the floods levels of Iguaçu River at
União da Vitória are presented. For a long time, floods
remained below the five-year return period. The
floods after 1982 caused significant damage in the
community (Table 1).
Floods are accentuated by urbanization because
of increased impermeable areas, flow obstructions,
such as bridges and landfills, and man-made
drainage such as conduits and channels. These are
not natural obstacles but are created to accommodate
population growth and related development imperatives. Usually, the land surface in small urban basins
is made of roofs, streets and others impervious surfaces. Runoff flows through these surfaces to the
storm sewers at high velocity and increase the peak
flow, overland flow volume and
6 000
decreased groundwater recharge
and evapotranspiration. Under
5 000
these conditions, total runoff and
peak discharge increase, together
4 000
with the flood frequency (Figure
2). There is as much as a six-fold
3 000
increase in the flood magnitudes
2 000
due to urbanization in the present
case. In addition to these impacts,
Historic data
1 000
the washed surfaces during rainy
Return period : 5 years
Return period : 50 years
days increase the pollution load in
Return period : 100 years
0
the urban environment and down1980
1990
2000
1950
1960
1970
1910
1920
1930
1940
1890 1900
stream rivers.
Figure 1 — Maximum flood discharges in the Iguaçu River at União da Vitória (a basin of approximately 25
Usually, urban flooding
000 km2, Tucci and Villanueva, 1997)
results from the misconception of
urban drainage design, which is based on the princi* Institute of Hydraulic Research, Federal University of Rio
ple of draining water from urban surfaces as quickly
Grande do Sul
as possible through pipe and channel networks that
1
3 000
25
•
20
Flood
Floods
events
events
15
10
1 000
5
0
1920
events
2 000
Events
Inhabitants (1 000)
iInhabitants
IInn itants
0
1940
1960
1980
2000
2020
2040
Year
years
Figure 2 — Increase in flood events in Belo Horizonte, Brazil (Ramos, 1998)
2
increase the peak flow further downstream. There is
no control of increased peak discharges at the minor
drainage level and most of the impacts appear downstream in the major drainage. To cope with this problem, city and state administrations undertake works
such as channels in the major drainage and pipes in
the secondary drainage network. This type of solution
helps only in transferring the flood problem from one
section of the basin to another, at high cost. In addition, the water quality impact is larger, since the overflow has a higher amount of solids, including metals
and other toxic components.
Since the 1970s in developed countries the
source control of urban drainage has been developed
by detention and retention ponds, permeable surfaces, infiltration trenches and others source control
measures. In developing countries, this type of control rarely exists and the impacts are transferred
downstream in the major drainage system. The cost
of controlling this impact is transferred from the
individual to the public, since the country has to
invest more in hydraulic structures to reduce the
downstream flood impacts.
Experience in many countries has now led to
certain accepted principles in urban drainage management. These are:
• Drainage evaluation should be carried out for
the basin as a whole and not be confined to specific flow sections;
• Flood-control and drainage-management measures should not transfer the flood impact to
downstream reaches, and should give priority to
source control measures;
• Control management should start with implementation of the Urban Drainage Master Plan in
the municipality;
• Urban drainage planning should take into
account future city development scenarios;
The impacts caused by urban surface washoff
and others related to urban drainage water quality should be reduced;
• Appropriate emphasis should be given to nonstructural measures such as floodplain zoning,
insurance and real-time flood forecasting.
• Public participation in urban drainage management should be increased;
• Urban drainage development should be based on
cost recovery.
These principles have been applied to a certain
degree in developed countries. However, urban
drainage practices in most developing countries do
not fulfill these principles. The main causes are the
following:
• Urban development in developing countries
occurs too fast and is unpredictable. Usually, the
tendency of this development is from downstream to upstream which increases damage
impacts (Dunne, 1986);
• Peri-urban and risk areas (floodplains and hillside slope areas) are occupied by low-income
populations with no infrastructure. Spontaneous
housing development in flood-risk areas can be
seen in Bangkok, Bombay, Guayaquil, Lagos,
Monrovia, Port Moresby and Recife. Some of the
developments prone to landslides are Caracas,
Guatemala City, La Paz, Rio de Janeiro and Salvador (WHO, 1988);
• Municipality and population usually do not have
sufficient funds to meet the basic supply of
water, sanitation and drainage needs;
• Lack of appropriate solid waste collection and
disposal reduces water quality and the capacity
of the urban drainage network (clogging);
• Lack of institutional organization in urban
drainage at a municipal level such as regulation,
capacity building and administration;
• Lack of law enforcement or unrealistic regulation (see Box 1).
In most Asian cities there is a lack of comprehensive project organization and clear allocation of
responsibilities; adequate urban land-use planning
and enforcement; and capability to cover all phases
and aspects of technical and non-structural planning
(Ruiter, 1990).
Integrated urban drainage management
Integrated urban drainage management planning is
based on the goals and objectives related to the wellbeing of the population and environmental conservation. An Urban Drainage and Flood Control Master
Plan (UDMP) is developed based on urban space,
BOX 1
URBAN OCCUPATION PRESSURE ON REGULATED AREAS
Regulations in the city of Curitiba (Brazil) have restricted land occupation for the preservation of the basins used
for urban water supply and in flood-prone areas. Urban development had, to a certain extent, encroached on
these areas and increased their real-estate value. The property owners adopted the following strategy: (a) clandestine development; and (b) helping their land to be “invaded” by poor populations in order to break down the
regulations and then selling the land to the municipality as a social solution (this usually occurs during election
years, when the political pressure is greater).
This situation comes about mainly because of the low compensation for private landowners in the regulations as they have to preserve the space unused and at the same time pay land taxes without obtaining any economic benefits. Lower taxes and appropriate land use which does not degrade the water quality would have
provided more incentive for land-use conservation.
hydrological conditions, hydraulic network and environmental conditions in order to reduce flood risks.
The main goals usually are:
• Regulation of the use of floodplain areas
through legislation and other non-structural
measures;
• Prevention and relief measures for low frequency floods;
• Improvement in the urban drainage water quality.
Urban development condition factors are not
discussed here, since they belong to the Urban Master
Plan (UMP) but there should be a strong interaction
between this plan, UDMP, the flood control plan and
others city plans such as water supply and sanitation
and solid waste management. Land use is strongly
related to urban drainage and the UMP also has to
take into account the restrictions of the UDMP in
which the latter is a component of the former.
The Integrated Urban Drainage Management
Plan includes:
• Non-structural measures included in the county
legislation or in the city building code;
• Structural measures for each sub-basin in the
city in which the works are planned, including
environmental and economic evaluations;
• Capacity-building programmes, which provide
long-term support to the Plan.
Non-structural measures
Non-structural measures are developed to regulate
the land use of the floodplains and to control the
impact of urbanization on drainage. Floodplain regulation usually restricts the use of the flood-prone
areas for new developments and plans new areas for
occupation in the city using tax incentives (see Box
2).
Regulations related to urban drainage can also
serve the objective of reducing the downstream
impact on peak discharge and water quality degradation, taking into account the socio-economic conditions. The best regulation is that which increases public participation (see Box 3). One of the basic aspects
of this type of regulation is that the new development
keeps the peak discharge equal to or below the predevelopment scenario by limiting the impervious
surfaces.
Structural measures
An urban drainage flood control plan is developed
sub-basin-wise, each sub-basin being evaluated for
the risk and scenario selected. Based on these individual conditions, the works required to control these
impacts are planned. Usually, the measures are a combination of upstream dams, detention ponds, dikes,
river channel changes, conduits and channels, based
on available hydrometeorological conditions, space
BOX 2
TAX INCENTIVE
In Estrela, (Rezende and Tucci, 1979), a study was
prepared for the city, together with the Urban
Master Plan and included in the municipal regulations. After the legislation was implemented, the
areas at risk were preserved and the remaining population was gradually removed to safe areas using
tax incentives. The tax incentives were the exchange
of building construction area permits downtown with
flood risk areas. Flood damage losses and population affected have decreased over the years since
1979.
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BOX 3
PUBLIC PARTICIPATION
União Vitória and Porto União (Tucci and Villanueva, 1997) are on the border of the State of Parana and Santa
Catarina and have a population of about 150 000 inhabitants. This urban area is subjected to frequent floods
but, in 1980, a large hydropower reservoir was constructed downstream. In 1983, there was a major flood,
which had an important economical impact (60 days of flooding). The population began to blame the Electric
Company (COPEL), which claimed that it was a natural flood and that the dam did not create any additional
impact. But, in 1992, another major flood took place, smaller than that of 1983 but also with a high damage
impact and created a major conflict between the city and COPEL. A non-governmental organization was created
by the population and a study was undertaken for it to carry out a diagnosis of the flood conditions, negotiations with COPEL for operational rules and flood-zone planning for the city. The study brought some results and
the negotiations improved the city’s capability of dealing with floods.
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a river delta draining basin of about 80 000 km2. It is
protected by a system of dykes, storm water and
pumping stations designed and constructed before
1970. The city developed from downstream to
upstream. The actual capacity of the drainage is not
enough to discharge the upstream increase of flood
peak and volume in some parts of the city.
Capacity building
Porto Alegre county covers an area of about
400 km2 and there are 26 basins. The Urban Drainage
Capacity building is required at all levels. Starting
with the urban drainage manual for use by planners
Master Plan was formulated in phases. The first phase
and engineers to advise on city restrictions and prowas the proposal for non-structural measures such as
cedures accepted by the city in urban drainage
legislation for new developments. The second part
design. There is a need for capacity building at the
consisted of a review of the design capacity of the
community level for enabling their participation.
storm water drainage of the basin which is pumped
out from inside the dyke system, and the Plan of six
Urban drainage master plan of Porto
important sub-basins of the city. The non-structural
Alegre, Brazil
measures consisted of (a) new legislation on source
control for developments which has been implePorto Alegre is the capital of the State of Rio Grande
mented since March 2000; (b) capacity building in the
do Sul in Brazil. The metropolitan area has some
form of urban drainage education at engineer level;
3 million inhabitants and the city county has about
and (c) preparation of a design manual. The county
2 million inhabitants. The city is located in the side of
already had a department for
45
urban drainage development and
Actualscenario
scenario
Actual
40
Plan scenario with increased
maintenance. The structural
Plan
scenario
with
Increase
conduits
capacity
conduit capacity
measures were in the form of the
35
Planscenario
scenario
detentions
Plan
withwith
detentions
Urban Drainage Plan for six basins
Q
30
Q
(m3/s)
For example, the Areia basin
m /s
25
has an area of about 12 km2 and
20
high population density. There is a
pumping station to drain the low15
est basin. The drainage from the
10
upstream basin flows inside a
5
pressure pipe (which is located
0
below the airport lanes and can0
10
20
30
40
50
60
70
80
90
not be increased without major
Time (minutes)
cost) up to the Jacui Delta. The
Figure 4 — Hydrographs for the scenarios (future scenarios with 10 years rainstorm) basin A (IPH, 2001)
basin was divided into 11 subavailability, existing drainage and topography. However, they have higher costs and are economically
viable only when the damage prevented is greater
than development costs or when there are special
social considerations. Non-structural measures have
lower costs, but are politically difficult to implement.
3
basins and the study scenarios were created on the
basis of actual occupation and the projected occupation as per the Urban Development Plan.
Increasing the pipe capacity along the major
drainage system would increase peak flow up to
140 m3/s and would cost US$ 14 million (not taking
downstream impacts into account ). Using detention
ponds in the major drainage system at some public
open spaces would cost US$ 8 million and peak flow
would be 42 m3/s. Figure 4 shows the hydrographs for
these scenarios in one of the basins.
In the case of simulations for the prediction of
runoff frequencies, it is of great importance that rainfall data have sufficient length, preferably at least
three times the length of the return periods of interest. The optimal raingauge network for such operations should have a spatial resolution of about 0.1 to
1.0 km2 and temporal resolution of the order of 1-5
minutes.
Implications for Child Health: Potential for Action. Geneva.
Conclusion
Urban flooding is a major threats to cities. Most of the
existing public policies in developing countries are
not technically, socially or economically sustainable.
Integrated urban drainage and floodplain master
plans are the main instruments for developing a sustainable policy to manage flood impacts in urban
areas.
Urban flood management in developing countries also requires evaluation of socio-economic
issues related to land use and urban development.
Most of the control can be developed through legislation and its enforcement, public participation and
capacity building.
References
IPH, 2001: “Plano da bacia do Areia”. In: Plano Diretor de
Drenagem Urbana de Porto Alegre. 1° Fase. Instituto de
Pesquisas Hidráulicas/UFRGS DEP/Prefeitura Municipal de
Porto Alegre.
JICA, 1995: The master study on utilisation of water resources in
Parana State in the Federative Republic of Brazil. Sectoral
Report Vol. H—Flood Control.
RAMOS, M.M.G. 1998: Drenagem Urbana: Aspectos urbanísticos,
legais e metodológicos em Belo Horizonte. Master’s thesis,
Engineer Faculty Federal University of de Minas Gerais.
REZENDE, B., C.E.M TUCCI, 1979: “Análise das Inundações em
Estrela”. Technical report, Estrela County. 30 pp.
RUITER, W. 1990: “Watershed: flood protection and drainage in
Asian Cities”. Land & Water In’l 68:17-19.
TUCCI, C. E. M., R. L. PORTO, 2000: “Storm hydrology and urban
drainage”. In: Humid Tropics Urban Drainage, UNESCO.
TUCCI, C.E.M, A. Villanueva, 1998): “Controle de Inundações da
cidade de União da Vitória”. Technical Report. CORPRERI.
135 pp.
WORLD HEALTH ORGANIZATION, 1988: Urbanization and its
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