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drainage systems. The extent and frequency of urban

Journal of Hydrology 29

Agern Alle 11, DK-2970 Hrsholm, Denmark. Tel.: C45-4516-0022-1694/$ - see front matter q 2004 Elsevier B.V. All rights reseAbstract

Urban flooding is an inevitable problem for many cities around the world. In the present paper, modelling approaches and

principles for analyses of urban flooding are outlined. The paper shows how urban flooding can be simulated by one-

dimensional hydrodynamic modelling incorporating the interaction between (i) the buried pipe system, (ii) the streets (with

open channel flow) and (iii) the areas flooded with stagnant water. The modelling approach is generic in the sense that it handles

both urban flooding with and without flood water entry into houses. In order to visualize flood extent and impact, the modelling

results are presented in the form of flood inundation maps produced in GIS. In this paper, only flooding from local rainfall is

considered together with the impact in terms of flood extent, flood depth and flood duration. Finally, the paper discusses the data

requirement for verification of urban flood models together with an outline of a simple cost function for estimation of the cost of

the flood damages.

q 2004 Elsevier B.V. All rights reserved.

Keywords: Cost function; Digital elevation model; Flood map; GIS; Sewers; One-dimensional urban drainage; Stormwater; Urban flooding

1. Introduction

The problems arising from urban flooding range

from minor ones, such as water entering the base-

ments of a few houses, to major incidents, where large

parts of cities are inundated for several days. Most

modern cities in the industrialized part world usually

experience small scale local problems mainly due to

insufficient capacity in their sewer systems during

heavy rainstorms. Cities in other regions, including

those in South/South-East Asia, often have more

severe problems because of much heavier local

rainfall and lower drainage standards. This situation

continues to get worse because many cities in the

developing countries are growing rapidly, but without

the funds to extend and rehabilitate their existing* Corresponding author. Address: DHI Water and Environment,Ole Marka,*, Sutat WeesaSurajate Boonya Aroonn

aWater Engineering and Management Program, Asian Institute obSchool of Engineering, Computer Science and Mathematicmodelling of urban flooding

,1, Chusit Apirumanekula,, Slobodan Djordjevicb,2

nology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand

iversity of Exeter, North Park Road, Exeter EX4 4QF, UK

9 (2004) 284299

www.elsevier.com/locate/jhydrolas flood data are available and the impact of

alleviation schemes can be evaluated straight away.

rved.

doi:10.1016/j.jhydrol.2004.08.014

(S. Weesakul), s.djordjevic@exeter.ac.uk (S. Djordjevic).1 Tel.: C66-2-524-5554; fax: C66-2-524-6425.2 Tel.: C44-1392-263664; fax: C44-1392-217965.flooding in large cities in developing countries make

them good case studies for urban drainage modelling,

9373; fax: C45-4516-9292.

E-mail addresses: ole.mark@dhi.dk (O. Mark), sutat@ait.ac.th

am

which forced 200,000 people from their homes and

killed 50 nationwide (Bangkok Post, 3rd February

2002). A final example is Houston, Texas (USA)

spreads eggs to new victims (Kolsky, 1998). Today

Hydrwhere the storm Allison in June 2001 caused urban

flood damages in the order of $2 billion on the Texas

Medical Center in the Harris Gully watershed (Holder

et al., 2002).

2. The impacts on society from urban flooding

Water flowing on the urban surface is a problem

when it causes damage. The perception of damage

varies from person to person. Konig et al. (2002)

divided damages from urban flooding into categories:

Direct damagetypically material damage causedby water or flowing water.

Indirect damagee.g. traffic disruptions, adminis-trative and labour costs, production losses, spread-

ing of diseases, etc.

Social consequencesnegative long term effectsof a more psychological character, like decrease of

property values in frequently flooded areas and

delayed economical development.

It is often difficult or impossible to provide an

accurate estimate of the cost of the flood damages.stagnant water. In 1983, Bangkok (Thailand) was

flooded for nearly 6 months and it caused the loss of

life and infrastructural damages of approximately

$146 million (AIT, 1985). In February 2002, five

people were killed in Jakarta (Indonesia) as heavy rain

extended floods to the city center, deepening a crisis,mo

parNep water in the streets. In September/October 1996,

st of the daily activities in Dhaka were nearly

alyzed and heavy traffic jams occurred due tohas

deeflood. In Dhaka City (Bangladesh), even a small

ount of rain may cause serious problems. Dhaka

often experienced flooding with ankle to knee-the

theA few examples of historic urban flood problems

are: Mumbai (India), 2000 where nearly 17,000

telephone lines in Mumbai City ceased to function

after flooding occurred and electric supply was cut off

for safety purposes. The water depth reached 1.5 m at

worst inundated locations and 15 lives were lost in

O. Mark et al. / Journal ofvertheless, a very basic cost estimation procedureantihelmintica kills parasites, but the parasites may

gradually develop resistance to the drug and impose

new and more severe problems. The best way to

manage parasite problems is to break the life cycle of

the parasites, that is, to remove their natural

environment by reducing the frequency and duration

of flooding. For example, Moraes (1996) found that

reduced flooding reduced the prevalence of round-

worm and hookworm by a factor of two and

hookworm alone by a factor of three.

3. What can be done to understand and reduce

urban flooding?

With todays advances in computer technology,

many cities in the developed part of the world manage

local and minor flooding problems using computer-

based solutions. This involves building computer

models of the drainage/sewer system, for instance by

using software like MOUSE (Lindberg et al., 1989);

InfoWorks (Bouteligier et al., 2001) and the SWMM

models (EPA SWMM, MIKE SWMM, and XP

SWMM), (Huber and Dickinson, 1988). These types

of models are used to understand the frequently

complex interactions between rainfall and flooding.

Once the existing conditions have been analyzed and

understood, alleviation schemes can be evaluated and

the optimal scheme implemented. Nevertheless, atis presented in this paper, to provide a first estimate of

flood costs.

Urban flooding may create considerable infrastruc-

ture problems and huge economic losses in terms of

production, as well as significant damage to property

and goods. The water depth in some inundated city

areas is commonly in the order of 5070 cm. In

addition, diseases spread and impose problems to the

population, for instance, diarrhoea or Leptospirosis,

which can be spread by bacteria in the urine of rats.

In September 2000 flooding in the north east of

Thailand, 6921 cases of Leptospirosis were reported,

244 of these resulting in loss of human life (Bangkok

Post, 20th September 2000). Last, but not least,

parasites seem to thrive when urban flooding occurs

regularly. Moist soil provides a good environment for

worm eggs to flourish, and water flooding open drains

ology 299 (2004) 284299 285present there are few studies on urban flooding that

pipe a

Hydrology 299 (2004) 284299deal with both the conditions in the surcharged pipe

network and the extensive flooding on the catchment

surface. Even fewer projects have dealt with model-

ling urban flooding in developing countries. Some of

the few case studies dealing with of modelling of

Fig. 1. Layout ofO. Mark et al. / Journal of286urban flooding which both includes the pipe system

and extended surface flooding are: Bangkok

(Thailand) (Boonya-Aroonnet et al., 2002); Dhaka

City (Bangladesh) (Mark et al., 2001); Fukuoka and

Tokyo (Japan) (Ishikawa et al., 2002); Harris Gully

(USA) (Holder et al., 2002); Indore (India) (Kolsky

et al., 1999) and Playa de Gandia (Spain) (Tomicic

et al., 1999). These studies treated urban flooding as a

one-dimensional (1D) problem. Schmitt et al. (2002)

considered a 2D model as a benchmark for 1D model.

A model, which dynamically couples a 1D pipe flow

model with a 2D hydrodynamic surface flood is

currently under development (Alam, 2003).

4. A methodology for simulation of urban flooding

Urban flooding may be due to various causes. The

runoff generally starts as overland flow on the street

before entering the underground pipe system through

catch pits. Fig. 1 shows a street system connected to a

pipe system through manholes/catch pits.If the intake capacity of the drainage system is

limited, only a fraction of the water can flow into the

pipes and a large runoff volume will be transported on

the surface during and after a heavy rainfall. This may

happen even if the underground pipe system has

nd street system.sufficient capacity, see Fig. 2. The water in the pipe

system may return to the street system if the capacity of

the pipe system is insufficient. In this case the water

will flow from the pipe system to the street system,

causing surface flooding, see Fig. 3. The duration of

flooding on the street depends on the intake capacity of

the catch pits, the drainage capacity of the pipe system,

infiltration and evaporation in the catchment area.

In the present modelling approach, the urban

drainage system consists of two networks, one

Fig. 2. Flow from the street system into a partly full pipe.

representing the free surface flow in the streets and

one for the pipe network. The drainage system is

modelled as two dynamically interconnected net-

works. The hydrodynamic model is based on an

implicit solution of the St Venant equations. The two

networks route the rainfall runoff simultaneously in

the pipes and on the streets. Manholes (network

nodes) function as points of flow exchange between

the pipe and the street systems. Water from the street

system can enter the pipe system by flowing through

Fig. 4 shows the modelling approach for urban

flooding. Two models are needed, i.e. a hydrological

model, which simulates surface runoff from rainfall

and a hydraulic model describing flows in pipes,

streets and storage of water on the surface.

In urban flooding simulation, the hydrological

process is separated conceptually from the hydraulics

of the drainage system. The computation of the

surface runoff from rainfall can be carried out by a

standard surface runoff model, e.g. a time/area,

kinematic wave or linear reservoir model. A surface

runoff hydrograph is computed for each sub-catch-

ment. Runoff hydrographs from each sub-catchment

are then used as input for the hydrodynamic model,

simulating flows in the pipe and street systems. The

runoff from the catchments is entered in the model

either on the streets or directly in the sewers

depending on the local layout of the drainage systems.

Hence, the initial flooding will be generated due to

insufficient capacity of either the pipes themselves or

of the inlets to the piped system. As the pipe and inlet

capacities can differ significantly, it is important to get

Fig. 3. Flow to the streets from a pipe system with insufficient capacity.

O. Mark et al. / Journal of Hydrology 299 (2004) 284299 287catch pits or manholes and vice versa.Fig. 4. Interactions between various stages in the modellthis part of the schematization right.ing approach for a flooded urban drainage system.

5. The digital elevation model

The digital elevation model (DEM) represents land

elevation data, which are essential for estimation of

flood volumes on the surface areas. In addition, the

result presentation in the form of a flood inundation

map is based on water levels from the model

simulation in conjunction with the DEM. Thus,

quality of the model results depends on the quality

of DEM. To generate the DEM, spot elevations (X, Y

coordinates and the ground level Z) covering all of the

catchments are needed. The data for the DEM can be

obtained by field survey data, by digitising a contour

map or by some other techniques (Hale, 2003). The

interval of spot elevation for analysis of urban

elevation of low and high spots.

details so that the DEM can yield sufficiently accurate

representation of elevations along the streets and flood

prone areas. The size of a 1!15!5 m resolution isrecommended for urban flood analysis since it can

cover the width of the road, the width of sidewalks, and

houses or buildings. However, using a finer resolution

like 1 m does not necessarily provide results which are

significantly more accurate in terms of flood levels, but

does provide a much better visual presentation of the

flood extent. A coarser 5!5 m DEM can thus be usedfor quick assessment of the model results, while

detailed analyses should be based on the 1!1 m DEM.It makes sense to create both a fine DEM and a coarse

DEM and to use each for various purposes.

O. Mark et al. / Journal of Hydrology 299 (2004) 284299288The DEM may be developed based on a distance

weighted interpolation routine. This means that a

Z-coordinate is interpolated from the adjacent X, Y, Z

points for each and every grid cell. The resolution of

the DEM should be fine enough to cover importantcatchment,

elevation of bottom and curb level of road system,

elevation and general topography of eachflooding should be in the range of 1040 cm, to

achieve a resolution sufficiently accurate to cover all

important details in the city area, e.g. the distance

between the road and the curb level. For a field survey,

it is especially important to obtain the following data

in the flood prone areas (Fig. 5):Fig. 5. Spot elevation map used for generatiIt is essential to have an accurate description of the

streets in the DEM. If the DEM is based on spot

elevations on the ground level, e.g. on the sidewalk,

the streets must be burned into the DEM with an

elevation which corresponds to the height of the curb.

Major roads in the study area where floods occur must

be included in the DEM, as the streets act as drains for

the surface flooding. If the water from street rises

above the curb level of the street, the water will flow

to adjacent areas and cause flooding. Fig. 6A and B

show sample DEMs from real models, i.e. a DEM

from Dhaka City with and without the street system

and a DEM from Ballerup, Denmarkwhere the

houses also have been added to the DEM. The houses

are on purpose not included in the DEM for Dhaka

Cityas it is believed that the water flows into many

of the houses during floodingwhereas it is essentialon of the DEMcase Bangkok City.

O. Mark et al. / Journal of Hydrto include the house in the DEM for Ballerup as only

smaller amounts of water flow into the houses during

the flood events.

A typical procedure is to interpolate and generate

the DEM using ground points only, i.e. without taking

into account the heights of man-made objects. DEM

must then subsequently be corrected, by raising of

pixels to include buildings. This operation is called

height correction of DEM, and is illustrated in

Fig. 6B. The procedure presented in this paper is

generic in the sense that when the final DEM has been

made, the areaelevation curves are automatically

computed based on the DEM (with or without houses)

and the correct flood zones will automatically be

Fig. 6. (A) The DEM for Dhaka City, without and with the road system. (

houses.ology 299 (2004) 284299 289attached to the model. This method thus provides

modelling of flooding with or without flood water

inside the houses depending on the reality in the field.

6. Catchment delineation for urban flooding

The catchment definition depends on topographical

and drainage network data. In urban areas the

catchments must be further divided into sub-catch-

ments connected to the appropriate location in the

local drainage channel. This means that it will be

necessary to overlay different types of mapped

information from the study area. Difficulties arise in

B) A 3D view of the DEM for Ballerup, Denmark, with streets and

the flat areas, where the boundaries are unclear, and

often determined as much by small local drainage

paths as by topography (Kolsky, 1998). The main

concept in sub-dividing of catchments is to under-

stand the topography and drainage network to define

the areas contributing flows to different portions of the

network. Once the draft catchment delineation is

sketched on a map, it is important to survey the study

area and inspect the past flooded areas to check that

the runoff is flowing in the expected direction and that

there is an agreement between the model and real life.

Apart from a manual procedure, automatic catchment

delineation can generally be laid out in three different

ways (Djordjevic et al., 1999):

in

del

Cit

delineate based on topography and land use maps. The

distance based delineation was applied for Bangkok,

as the model for the downtown area of Bangkok has

more than 2000 catchments, but very little information

is presently available in a digital form concerning local

terrain and land use. Hence, the distance based

method was convenient for Bangkok to define the sub-

catchments for each manhole, but at the same time it

defined the catchment without considering the topo-

graphy and land use of the area. This may lead to

inaccurate physically unreasonable catchment delinea-

tion. For instance, automatic delineation does not

consider features, like natural flow paths, houses and

hill traces as boundaries for sub-catchment delineation.

Hence, the result from the distance based delineation

the analyzed catchment, but also for a certain zone

surrounding its assumed boundary. This should ensure

O. Mark et al. / Journal of Hydrology 299 (2004) 284299290y, as the catchment is rather small and easy tobasTwo types of catchment delineation were evaluated

this study. The first procedure was the manual

ineation and the second procedure was distance

ed. The manual delineation was applied for Dhaka1. Distance-based, i.e. based on the distance to the

drainage network.

2. DEM-based, i.e. based on an algorithm that

traces most probable flow paths depending on the

information in the DEM concerned terrain aspects

and slopes.

3. DEM plus cover image (land use). This is the same

as the DEM-based procedure, but with the

addition of impacts from objects in the digital

image, like buildings, cascades, etc.Fig. 7. Dhaka City catchment: manual delineation (left)that the real catchment boundary is precisely

determined.

7. Modelling of routing and flooding

The drainage network is geographically and

topologically fully determined so that a networkmust be checked and if necessary, verified through a

manual procedure. Fig. 7 shows the sample of manual

and automatic catchment delineation.

When DEM-based delineation is applied, it is

essential to have a reliable DEM created not only forand automatic (distance-based) delineation (right).

8. Flow exchange between the street

and the pipe system

Water from the pipe system may flow to the streets

through a manhole when flooding takes place. On the

other hand, when water in the pipe system is drained,

surface flooding water in the street system can flow

through the manholes to the pipe system. In modelling

of urban flooding, the manhole may be described as a

broad crested weir, where the crest length of the weir is

represented by a perimeter of the manhole and the weir

crest is set to the bottom level of the street as described

in Fig. 9A and B. Discharge through manholes can also

be described by a common Weir equation, which

handles both free and submerged flows. The use of a

weir for the description of the connection between pipe

and street systems ensures that a restriction exists both

for water from streets entering the pipe system and for

water flowing from pipes to the streets.

When the sewer system becomes fully surcharged,

it may be more correct to shift the Weir equation to an

orifice equation, where the driving head is the

difference in head between the pressure in the sewer

Hydrmodel actually resembles a schematized view of a real

system. The flow in a flooded pipe system is complex,

and the computations in the present urban flood model

are based on an implicit finite difference scheme,

adopted from the St Venant equations.

Traditionally, when surcharge water from a pipe

system flows into the street system, most of the

models stores flood water from the underground

system in a virtual reservoir and the stored volume

returns to the pipe once the system resumes free

surface flow. To make a significant advance on this

approach, and to model the dynamics of flooding in

urban areas reliably, high resolution data on terrain

model are needed, as stated by Maksimovic (2000).

His paper concluded that a new methodology for

simulating the storage of surface flooding on the street

system is needed, not using a virtual reservoir

approach at each computational nodal point on the

surface. By the application of GIS features like a

DEM and a simulation module, modelling of real

storage and routing of surface flooding can be

achieved.

If water from a pipe network flows through a

manhole and reaches the street ground level, then

surface flooding takes place. The flooded surface area

is gradually increased following the DEM and hence a

model can accurately describe the rising water level

along the street and its borders, and therefore simulate

the surface inundation. An areaelevation relation is

required for the surface topography, in order to define

the storage capacity for surface floodingas input of

the model, e.g. as a storage function for a basin. This

relation is developed from the DEM with the

application of GIS. Areaelevation relations must be

defined for each sub-catchment connected to the street

network. The sub-catchments are assumed to be the

adjacent areas around manholes, where surface

flooding may spread. Fig. 8 shows a sample of an

areaelevation relation, which is used as input of the

urban flooding model, in order to simulate storage of

water next to the street system.

When surface flooding occurs, the flow along the

street can be in either direction along the street; it is

not necessary that the flow direction in the street

coincides with the street slope, or that it is the same as

in any buried pipe flow. After all, it is not necessary

that the layout of the streets matches that of the buried

O. Mark et al. / Journal ofpipe network.Fig. 8. An example of an areaelevation relation.

ology 299 (2004) 284299 291and the water level on the surface. However,

Hydrology 299 (2004) 284299292the

ori

9.

pro

req

su

Fig

(B)

theO. Mark et al. / Journal oforifice equation breaks down in cases where the

fice is not full flowing, e.g. as in Fig. 9B.

Model requirements

Based on the discussion above of the physical

cesses involved in urban flooding, the technical

uirements in an urban flood model can be

mmarized as:

Dynamic flow description: when urban flooding

occurs, surface water can flow in both street and

. 9. (A) The principle behind the application of a Weir formula for the de

Water flowing into a catch pit in Dhaka City. This is an illustration from rea

description of the flow exchange between pipe and street system.pipe systems with flow exchange between these

two systems through manholes. This means that

simulation of backwater effects is needed in

modelling of urban flooding. By using a dynamic

wave model, the model includes backwater effects

and surcharge from manhole including rapid

change of water level.

Parallel flow routing: while surface flooding takes

place, water from the pipe system flows through

manholes or catch pits to street system. Flow along

the street (e.g. right above the pipes) can be in either

direction along the streets, i.e. it can flow following

scription of the flow exchange between pipe and street system.

l life of the principle behind the application of a Weir formula for

the

tak

(th

sim

Hydrroximately 0.5% of the accumulated rainfall during

3 days of flood. Hence, if only a little evaporation

es place compared to the accumulated rainfall

e volume of flood water), then it does not affect theand

app10. Other physical processes in urban catchments

exposed to flooding

Important physical processes like evaporation and

infiltration must be considered if they affect the urban

flood conditions. In some cities, evaporation and infil-

tration may be predominant, e.g. depending on the

season, the land use, location of the city, soil type, etc.

Evaporation happens on all surfaces exposed to preci-

pitation, such as parks, buildings, houses and paved

area. However, the evaporation rate depends on tem-

perature, wind and atmospheric pressure. To consider

whether evaporation should be included in the model

simulation, comparison of accumulated evaporation to

accumulated rainfall during the rain and flooding

periods is needed. Apirumanekul (2001) found that

evaporation was insignificant for the maximum flood

depth in Dhaka City during the October flood in 1996

he found that the evaporation per unit city area wasa slope of the street or against it. It is not necessary

that the flow direction in the street has to be the same

as the flow direction in the pipe system.

GIS interface: GIS is important in simulation ofurban flooding. It is used as a tool to provide input

data and display simulation results. Surface storage

for simulating surface flooding can be calculated

by the application of GIS together with the DEM of

the study area, i.e. find areaelevation relation

from DEM. In addition, results of the simulation

can be easily understood in form of flood inunda-

tion maps. Model output in term of water level

along the streets are transferred to GIS and with the

interpolation routine, water surface is able to be

developed. Flood inundation maps can be gener-

ated by overlaying of water surface and DEM,

introducing flood depth map which is an easy

method to visualise flood situations.

The importance of these elements has also been

realized and pointed out by Maksimovic and Proda-

novic (2001).

O. Mark et al. / Journal ofulated maximum flooding. soil typemore infiltration can occur in sandy soilcompared to clay or silt,

soil moisture contentif the soil moisture contentis high, infiltration is less than in dry soil.

For Dhaka City, most of the areas are commercial

areas and government office areas, which consist of

houses, buildings and paved areas and the soil is

clayey. Therefore, only a minor flood water infiltra-

tion takes place in the central part of Dhaka, e.g. the

infiltration rate of clay is in the order of 15 mm/day

(Daniel, 1980). The groundwater level in Dhaka City

is around 25 m below the ground surface (Khan and

Siddique, 1999). This is due to accelerated utilization

of ground water as a resource for supplying drinking

water to the city. At the same time the recharge to the

ground water has been reduced due to increased

impervious city areas, thus infiltration from ground-

water to the pipe system has decreased.

In Playa de Gandia, Spain, the city is located along

the Mediterranean coastal areas and driven by theboland use typemost urban areas are commercial

areas, industrial areas and government office areas,

which comprise buildings, houses and paved

areasthis means that less infiltration may occur

in these areas,othconsidered. This type of infiltration rate depends on

er factors like:wa

beThe infiltration term is generally used for two

different processes in modelling of urban sewers, i.e.

it may be used for seepage from groundwater to the

pipe system or to describe seepage of surface flooding

water to pipe system and groundwater. The two cases

are discussed briefly.

Knowledge of the groundwater level is necessary

for determining the effect of infiltration into sewer

pipes. If the water table level is higher than the level

of the drainage pipes, infiltration from groundwater to

the pipe system may take place depending on the

condition of the pipes.

In highly urbanized areas, during small rain events

that do not generate flooding, often only a small

amount of water from rainfall and runoff infiltrates

the soil due to imperviousness of the surface.

However, infiltration from surface flooding to ground

ter is one of the physical processes, which should

ology 299 (2004) 284299 293oming tourist industry. Because of the intensive

during flooding the capacity of the system may be

Hydrtotally different because the water flows both in the

pipes and on the streets. During a flood, the flow

condition for a pipe might be pressurized flow, while

the flow condition for the street is open channel flow.

The pipe discharge is limited by pipe size and the

difference in head between the upstream and down-

stream end of the pipe, but street flow is only limited by

the width of the street and the slope of the water

surface. Fig. 10 shows water level and discharge on the

street and in the pipe from the simulation of a rain event

with a return period of 1 year for Bangkok, Thailand.impermeabilisation of urban catchments, there is less

infiltration of surface water to the pipe system. The

drainage conditions in the area are not very favourable.

The terrain is quite low, sandy and the groundwater

level is high. This means that infiltration effect (from

groundwater to pipe system) must be considered. Since

the city of Gandia is located over sandy soil, surface

flooding water can predominantly infiltrate to ground-

water and hence the effect of infiltration to flooding

must be considered (Tomicic et al., 1999).

11. Calibration and verification

Calibration involves minimization of deviation

between observed data and simulated results by

adjusting parameters within the model. The urban

drainage model calibration can be carried out by

calibration of the surface runoff model and sub-

sequently the pipe flow model is calibrated towards

measured flow and water level at the specific

locations. The surface runoff model can be calibrated

by adjusting hydrological parameters, for example

time of concentration, until the computed hydrograph

agrees closely to observed runoff data. Next, the

runoff hydrograph is used as input data for the pipe

flow model to simulate discharge and water level in

the pipe system by changing pipe flow parameter,

such as Manning number. This step is iterated until the

calculated discharge and water level outputs are

agreeably close to the observed data. The main

objectives in calibrating urban flooding models are

to match the flood extent and the flood depth.

The capacity of a drainage system normally

depends on pipe sizes and pumping capacity. However,

O. Mark et al. / Journal of294Water starts to flow in the pipe only in a positivedirection (downward). When the downstream node

floods, backwater effects retard the pipe flow because

of a reduced head difference. From this moment, flow

on streets will occur in a negative direction (upward).

However, the water from upstream still keeps flowing

downward. This upstream water will push both street

and pipe flows back to a positive direction. In Fig. 10, at

the end of the simulation, the discharge in the street is

an order of magnitude higher than the pipe flow. This

means that contribution to the drainage capacity from

the street network is extremely important to define the

flow capacity during urban flooding and that measuring

the flow in the pipes provide only a part of the

information required for calibration of an urban flood

model. Hence, in order to calibrate an urban flood

model it is not enough to have only a well calibrated

pipe network, but the flow paths, the flow extent and the

flow capacity of streets must also be estimated

accurately.

Model verification is a process of testing the quality

of a calibrated model against observed data, using the

model parameters derived during the calibration.

Another set of data, for instance the water level in a

manhole, flow velocity and discharge in specific

locations in the pipe system should be recorded for

model verification purposes. Model verification may

be more cumbersome for urban flooding due to sparse

flood data. However, verification is still required in

order to have confidence in the modelling results.

Model validation requires comprehensive and

detailed field data sets. Ideally, these would include

rainfall as well as level and discharge measurements

at several locations both in the underground pipe

system and on the surface. Often the desire to model

urban flooding occurs after a flood incident, i.e. flow

gauges and water level meters are rarely in place to

measure flooding when it occurs. An exception to this

is the flood in Bangkok during October 2000, where

measurement during a flood incident paralysing

Bangkok was carried out (Chingnawan, 2003). If

high-tech equipment is not available, the areas

affected by flooding and the highest flood levels can

be cheaply recorded by tools such as resident gauges

and chalk gauges (Kolsky, 1998). Whenever the

collection of high quality data was not carried out

during a flood event the model may be verified by

comparing predicted and observed flood extent and

ology 299 (2004) 284299maximum water levelsthese data are typically

HydrO. Mark et al. / Journal ofavailable. An example of a model verified against

observed flood extent is the flooding in Dhaka City

during October 1996 (Fig. 11). In addition the model

was compared to marks of highest flood levels where

the accuracy of the model was within 510 cma

very good result.

12. Result presentation and model application

The model results are geo-referenced and related

through a coordinate system linked to the DEM grid.

The results are presented in the GIS interface as flood

inundation maps, based on the water levels computed

by the urban drainage model. Flood inundation maps

provide a most effective medium for visualizing

flooding.

The water levels on the surface mainly cause

flooding on the streets and the adjoining areas.

Fig. 10. Illustration of the complexity of the flow conditions during the flo

during the event: at tz13:10 the downstream node is flooded; at tz13:20 tstreet.ology 299 (2004) 284299 295Output of the simulation in the form of simulated

water levels along the street system is transferred to GIS.

Using interpolation routines, continuous three-dimen-

sional water surfaces can be constructed based on the

simulated street water level from the model and the

DEM. The DEM elevations are subsequently subtracted

from the water level surface delineating inundated areas

by flood extent and flood depth. Results (water levels)

from the simulation are available along the streets as

shown in Fig. 11, which shows a flood inundation map

for urban flooding in Dhaka City in 1996.

Based on the verified flood model for Dhaka a

number of alternative scenarios were evaluated to

reduce the flooding of the city. Surprisingly it was

found that provision of additional pumping capacity

had only a local effect. This was due to the fact that

the pipes in the most upstream part of the catchment

acted as bottlenecks and that the ground slopes in that

area were unfavourable for drainage the floods waters

od event as both the sewer flow and the street flow change direction

he head in the sewer pipe exceeds the water level in the overlapping

HydrO. Mark et al. / Journal of296in the streets. In other words, the small pipes were the

points controlling the drainage of the larger upstream

catchments. As a logical consequence it was found

that real time control of the pumping station had very

little impact on reduction of flooding.

For the Bangkok flood modelling study, the flood

inundation maps was overlaid on property maps of the

city (Fig. 12).

A cost function for flood damage per establishment

(in Thai baht) for the Sukhumvit 0.4 km2 area in

Bangkok has been developed by Tang et al. (1990) as:

Flood damagebahtZ a Cb depth of flood in centimeters

Cc duration of flood in days

Fig. 11. Flood inundation (maximum flood depth) map for Dhaka City in

were recorded by the local authorities.ology 299 (2004) 284299where a, b, c are parameters given in Table 1,

estimated based on previous flood. Hence, even

though this is a simple empirical model, it provides

very useful information for the city planners concern-

ing cost estimation of the flooding and the return of

the investment from implementation of alleviation

schemes.

13. Drawbacks and limitations

Clearly the greatest inaccuracy of the described

approach lies in the treatment of street channels as

prismatic and of flow in those channels as one-

dimensional. Irregular street geometry and/or catch

pits situated in gutters on two sides of the road may

September 1996the red bold lines show the flooded areas as they

y ma

Hydrology 299 (2004) 284299 297lead to water in two parallel gutters flowing in

opposite directions. When the surface channel is

prismatic and the boundary conditions for it are close

to the real situation, the limit in validity of the 1D flow

assumption is not sharproughly speaking, it is

reasonably realistic as long as water remains within

Fig. 12. Flood map for Bangkok on the top of a propert

O. Mark et al. / Journal ofthe street profile. When the curbs are overtopped, not

only is the flow no longer 1D, but also the water

probably reaches pervious areas where roughness is

significantly higher, and where infiltration may be

possible.

Separation between the hydrological and hydraulic

phases of the runoff, outlined earlier, is absolutely

acceptable for simulation of events without flooding.

However, surface runoff parameters calibrated via

measurements during moderate rainfall, might not be

valid when the underground system cannot capture all

the runoff, since the excessive amounts of water on the

surface would induce both decreased hydrological

losses and quicker response (shorter concentration

times). In other words, storm sewer overflows interact

with surface runoff. One possible approach to handle

this was described by Djordjevic et al. (1999).

The Weir formula describing the link between the

pipe network and a surface channel network is only a

rough approximation of reality, because one such link

represents the holes on the manhole cover and/orseveral catch pits. So, whichever expression is used to

relate discharge as a function of water levels at two

nodes (one in the manhole and another on the street),

it is never unique. Even in the situation when such a

detailed system description is achieved to have one

link representing only one catch pit (corresponding to

psimulated for a rainfall with a 1 year return period.one manhole), depending on the type of the inlet

structure, it may have several openings that may work

in different regimes in time. Furthermore, during the

outflow the pressure force of the water rising in the

manhole may easily be able to lift and partly remove

the manhole cover (Guo, 1989). Consequently, the

surface flow might be strong enough to remove the

cover completely. In such situations, it is clearly very

complicated to include all those phenomena in the

simulation.

Local losses in both free-surface and surcharged

manholes have been subject to extensive experimental

Table 1

Estimated flood damage function parameters for different land use

types in Bangkok (Tang et al., 1990)

Land use type a b c

Residential K300.5 45.4 33.5

Commercial K2.2 88.1

Industrial K1739.9 522.8 180.5Agricultural K1047.2 553.5

at

loc

sur

of simplification. Urban flooding is certainly a very

maps. This integrated approach for modelling of

Hydrthe water level in the storage, and this in turn is almost

impossible to include in the model.

Where open channels (e.g. small streams) are used

as part of a drainage system the chances are high that

during floods culverts or bridge openings become

partly clogged by large items brought by flooding

water and hence in this way obstructing the predefined

modelling assumptions.

A comparison was made between three different

model layouts for the Bangkok sewer system for a

rainfall event with a 1 year return period, i.e.

35 mm/h. The comparison included:

1. Simulation by application of a standard approach

in commercial urban drainage software as outlined

by Maksimovic (2000). In other words, no

provision of streets and surface routing/storage

outside the streets.

2. Simulation by application of a standard approach

as described just above but with streets added.

3. Simulation of the urban flooding as suggested in

this paper.

The maximum simulated water levels are reduced

by up to 60 cm when streets are included in the

modelproving the statement by Maksimovic (2000)

that a standard flood simulation without any concern

for the surface flood routing and flood storage heavily

overestimates the flood depth. The difference in

maximum flood depth between the model with andstre

surwidraulic grade lines will reach the ground level first

nodes that are in local depression. At those

ations the water initially accumulates (filling the

face storage) before a possible flow further down-

am. In such cases, the actual lengths of joining

face channels are in fact variablethey depend onhyerent to sub-critical flow applied in the model.

It is likely that, during the flooding event, theass

inhand theoretical research (e.g. Pedersen and Mark,

1990), meaning that they can be included in the model

in a fairly accurate manner. However, even when the

flows in adjoining street channels are more or less

one-dimensional, local flow geometry at street junc-

tions is always complex, so the energy equations for

surface nodes are less sound. In particular, this is the

case with supercritical flow, having in mind the

umptions on the boundary conditions structure

O. Mark et al. / Journal of298thout the provision of flood storage next to theurban flooding provides a methodology for systema-

tic and consistent analyses of the causes for the

urban flooding together with evaluation of flood

alleviation schemes. It is believed that the method

presented in this paper, a combination of GIS and

1D hydrodynamic modelling, constitutes a cost

efficient system for planning and management of

drainage systems suffering from urban flooding.

15. Future perspectives

Today, a few studies have been carried out to

simulate urban flooding by application of mathemat-

ical models. In urban areas the flow paths on the

surface are often complex to define because ofcomplex phenomenon, but incapability to include all

details in modelling should not discourage attempts to

use a 1D approach, at least for internal floods caused by

heavy rainfall. Basically, the limitations mentioned in

this paper make it very difficult to accurately simulate

local conditions on a small scale, whereas simulation

of larger scale urban flooding based on the principles

outlined in this paper gives very promising results.

14. Conclusion

This paper has outlined the potential and

limitations of a special modelling technique, where

a hydrodynamic urban flood model built in two

layers describes the conditions both in the sur-

charged pipe system and flooding on the catchment

surface. It can be concluded that the modelling of

urban flooding is feasible on a large scale and the

model is a powerful tool combination with GIS.

Complex hydrological-hydraulic mechanisms are

encapsulated into the model and results can be

presented as easily understandable flood inundationstreets is in the order of 1020 cmbut the model

with the flood storage next to the streets gives a

significantly better picture of the real flood extent and

flood damaged areas, providing more accurate and

more valuable information.

Engineering predictions always imply some degree

ology 299 (2004) 284299crowded buildings, houses and roadsand during

heavy flooding a 1D modelling approach may be

insufficient. Future approaches to modelling urban

City. Master Thesis No. WM-00-13, Asian Institute of

Technology, Bangkok.

02-15, Asian Institute of Technology, Bangkok.

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urban drainage system with large tailwater effects under

extreme rainfall conditions, Ninth International Conference on

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Hale, J., 2003. Urban flood routing . the next step. WaPUGAutumn Conference, Blackpool, UK.Boonya-Aroonnet, S., Weesakul, S., Mark, O., 2002. Modelling of

urban flooding in Bangkok, Ninth International Conference on

Urban Drainage, Portland, Oregon, USA.

Bouteligier, R., Vaes, G., Berlamont, J., van Assel, J., Gordon, D.,

2001. Water quality model set-up and calibrationa case study,

The Autumn WaPUG Conference, Blackpool, UK.

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Sukhumvit area, Bangkok, Thailand. Master Thesis No. WM-flooding may use a hydrodynamic pipe flow model

below ground in conjunction with a full 2D hydro-

dynamic model in order to describe the surface flow.

The results of such a model would be usefully

compared with those of a 1D urban flood model to

provide insight for the selection of an appropriate

approach for modelling of urban flooding.

Acknowledgements

The last authors work on this paper was partly

supported by the U. K. Engineering and Physical

Sciences Research Council (EPSRC), Project No.

GR/R14712/01 (the Platform Grant).

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Potential and limitations of 1D modelling of urban floodingIntroductionThe impacts on society from urban floodingWhat can be done to understand and reduce urban flooding?A methodology for simulation of urban floodingThe digital elevation modelCatchment delineation for urban floodingModelling of routing and floodingFlow exchange between the street and the pipe systemModel requirementsOther physical processes in urban catchments exposed to floodingCalibration and verificationResult presentation and model applicationDrawbacks and limitationsConclusionFuture perspectivesAcknowledgementsReferences