Floods, from Defence to Management – Van Alphen, van Beek & Taal (eds)© 2006 Taylor & Francis Group, London, ISBN 0 415 39119 9

Rainfall loads in flood-control project for urban areas
Shoji Fukuoka
Research and Development Initiative, Chuo University, Tokyo, Japan

Yasushi Tanioka
Pacific Consultants Co., LTD. Tokyo, Japan

ABSTRACT: In urban areas, since intense land use is remarkable and improvement to small and medium sized
rivers and expansion of sewer drainage infrastructures are very difficult, storage facilities, which performmomentary storage of a flood water, such as a regulation pond and storage pipe, play an important role, for theurban flood control. The present projects for rivers and sewers are made based on each design rainfall scale anddesign hydrograph, and the present condition is that mutual adjustment and increase in efficiency are notattained. DAD analyses of time-spatial scales (approximately 10–180 minutes and up to about 100 km2) forsmall and medium-sized rivers and sewer watershed are performed. Moreover, the difference in the rainfall
characteristics by the rainfall sources (typhoon and thunderstorm) is clarified, and the way that should be as
rainfall load for the project, which can reflect these characteristics, is proposed.

1

flood-control plans that consider rivers and sewers as
an integrated whole.

OBJECTIVESThe urbanization of watersheds is resulting in larger,
faster discharges, leading to an increase in so-called“urban flooding”, particularly cases in which brief,
local downpours cause small and medium-sized rivers
to overflow concurrently with sewer flooding. Thisurgently necessitates both improvements to small andmedium-sized urban rivers and the expansion of sewerdrainage infrastructures. However, because of the difficulty of securing land for such purposes in urban
areas—due to exorbitant real estate prices and theintense use of land—an important role can be playedby regulation ponds, storage pipes, and other facilities
that temporarily store overflow from rivers and sewers.
Although such storage facilities interdependently affectthe discharge and storage capacity of rivers and sewers
and thereby improve overall flood-control capabilities,
current planning for small and medium-sized rivers
and sewers has focused on the design frequency ofoccurrence (i.e., an n-year-frequency storm) and design
rainfall of each type of infrastructure separately; thetwo have yet to be integrated and optimized for greater
effectiveness and efficiency.
Therefore, the authors have analyzed the temporalspatial properties of rainfall on the scale of the small
watersheds of small and medium-sized rivers and sewers in urban areas. This work presents a method for
calculating relevant rainfall for realistic, efficient

2

ISSUES IN CURRENT PLANNING
METHODOLOGY

Current flood-control and rainwater-drainage planning for small and medium-sized rivers and sewers is,in general, based on rational formulas. This section
discusses some of the issues relating to the analysis ofdesign rainfall load and discharge in flood-control
planning based on rational formulas.
2.1

Rainfall load

Planning for rivers and sewers is generally based onstorms of different annual probabilities: range of the
30- to 50-yr recurrence interval for small and mediumsized rivers, and range of the 5- to 10-yr recurrence
interval for sewers. Estimating capacity for floodwater
storage requires a rainfall distribution, and it is often
the narrowly concentrated rainfall distribution basedon rational formulas that is used. However, such a
rainfall distribution is at odds with the reality that therainfall distribution differs according to the flood
travel time, and that typhoons, thunderstorms, and otherrainfall sources produce rainfall of differing temporaland spatial distributions. In flood-control planning

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that involves storage facilities, the interdependent effectof those facilities on the river and sewers in question
should be properly assessed. It is questioned to form
the planning using the different rainfall of each and thetemporal-spatial distribution of assumed, as opposed
to historical, rainfall.

Clear differences in rainfall properties were usedaccording to rainfall source (e.g., typhoon and thunderstorm), and these properties were incorporated intoplanning for the relevant rainfall.
3.1

2.2

Flood discharge calculation

Current methods that use rational formulas in floodcontrol planning for small and medium-sized rivers
and sewers cannot depict the interdependent effect offlood control facilities on flood discharge and storage.
Furthermore, rational formulas assume a uniform rainfall load over a watershed and constant rainfall during
the flood travel time and so do not reflect actual conditions in small and medium-sized rivers, as having awatershed 100 to 200 km2 in size. The need therefore
exists for a flood discharge calculation method thattreats rivers and sewers as an integrated whole andthat accurately reflects the effects of storage facilities,
relative to differing temporal and spatial distributionsof rainfall.
3

RAINFALL PROPERTIES ON THE SCALEOF SMALL AND MEDIUM-SIZED URBAN
RIVER AND SEWER WATERSHEDS

Using comparatively dense telemetric data from rainfall observation stations in the Kanda River watershedin Tokyo, Japan, we conducted depth–area–duration(DAD) analysis on the temporal-spatial scale of small
to medium-sized river and sewer watersheds (i.e., durations of 10 to 180 minutes and areas up to 100 km2).

Subject of investigation

The subject of our investigation was the Kanda River(watershed size: 105 km2), a small to medium-sized
river system typical of the Tokyo area (Fig. 1). We
examined seven different sub-basins in this river system, including tributaries and sewer mains and ranging in surface area from 7 to 105 km2 (Table 1).
As rainfall data, we used rainfall statistics on 91
rainfall incidents occurring in the Tokyo area over a
17-year period (1979 through 1995). Ten-minute rainfall was measured at intervals of 3 to 5 km by the rain
Table 1. Data on the watershed investigated.
Watershed
Kanda River
Upstream of Minami
Kotaki Bridge
Upstream of
Wadami Bridge
Myoshoji River
Zenpukuji River
Upstream of Kanda
River and ZenpukujiRiver confluence
Momozonogawa
Sewer Main

Figure 1. The watershed investigated.

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Watershedarea (km2)

Approximate travel
time (minute)

105
47

180
120

30

100

22
19
11

90
80
60

7

40

gauges shown in Figure 1. Weather maps for the daysin question and other meteorological data were used
to categorize the rainfall sources as “thunderstorm”
or “typhoon or other source.”
We calculated the 60-minute maximum rainfall distribution and inundation damage distribution for thoserainfall incidents that resulted in flood inundation damage due either to typhoon or thunderstorm; an example
is shown in Figure 2. The data show that thunderstormrelated local downpours have a very high degree of
local concentration of rainwater, and that centeredaround this zone of concentration are sporadic occurrences of sewer flooding (i.e., sewer or storm-drain
overflow that results in the inundation of roads andother surfaces) located a distance from the river. Thissewer flooding happens because the brief, intense rainfall associated with thunderstorm-related local downpours quickly overwhelms the drainage capacity ofsewers, which indicates that river capacity is notreached in such situations because of the failure of massive volumes of floodwater to reach the rivers. In contrast, the short-term intensity of typhoon-related rainfall
is not as great as that of thunderstorms. Consequently,

sewers designed for brief, high-intensity rainfall
according to rational formulas gradually drain away
the rainwater, causing discharge to concentrate in theriver and thereby causing the river to overflow its
banks. This, too, indicates the need of countermeasures to consider the difference of rainfall distribution
and different regional distributions according to interdependent discharge in rivers and sewers, storage
conditions, and rainfall source.
3.2

Relationship of travel time of observed
rainfall (Tanioka,Y., Fukuoka, S. 2003)

Figure 3 shows the relationship of, for instance, 30minute and 180-minute maximum rainfall observed at
ground rainfall observation stations in the Tokyo area.
Here, the 30- and 180-minute points roughly correspond to the flood travel times in a 2-km2 sewer and a
100-km2 river. The data clearly show the difference inrainfall properties between thunderstorm rain andtyphoon rain: Typhoons result in a rainfall distributionthat is large relative to travel time in rivers, whereas
thunderstorms result in a rainfall distribution that islarge relative to travel time in sewers. This difference
between types of rainfall is also apparent in the differences in flood patterns shown in Figure 2. Figure 3
also shows the probable rainfall depth at the 30- and180-minute points, with a dashed line indicating thepoint of intersection for the same return period. Thisline represents rainfall depth with a narrowly concentrated-rainfall distribution, i.e., the rainfall depth foreach travel time. It can be seen that such rainfall occurs
only very rarely.

220

Flooding along a river

Rainfall Contours(60minite Maximum Rainfall)

Sewer flooding locateda distance from the river

140
120

1/10

100

1/5
1/3

80

1:1

60

30

40

50

1/50

20

1/100

Thunderstorm
Typhoon

40
1/5

River

1/100
1/50

160

1/10

Numeral number describes 60-min. maximum rainfall depth (mm)Concentrated-Rainfall Distribution

1:6

180

1/3

180-m imutes maximum rainfall
(mm)

200

0
0

10

20

60

70

80

90 10030 minutes maximum rainfall (mm)Figure 2. Example distribution of the rainfall investigated.

Figure 3. The relationship between 30- and 180-minutemaximum rainfall measured at observation stations.

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3.3

3.5

Relationship between travel time andrainfall intensity during travel time

Figure 4 shows the relationship between travel time
and watershed-averaged rainfall intensity during
travel time for the thunderstorm and typhoon data
shown in Figure 2. As can be seen, thunderstorms
produce a rainfall intensity that falls sharply in a shortperiod, whereas typhoon rainfall intensity is large but
declines in intensity at a slower rate over time.
3.4 Relationship between watershed size andrainfall intensity

Watershed-average Rainfall
Intensity Within travel Time (mm/hr)

As shown in Figure 5, which presents the tendency ofwatershed-averaged rainfall intensity to decline aswatershed size increases, the decline in rainfall intensitywith watershed size is extremely sharp in the case of
30-minute thunderstorm rain on the small temporalspatial scale of small and medium-sized rivers and sewers. In comparison, typhoon rain does not decrease inwatershed-averaged intensity as sharply as watershedsize increases.

The relationship between watershed size, travel time,and intensity for historical rainfall (Figure 6) is suchthat thunderstorm rainfall is highly concentrated ina brief time period and a small area, as indicated byits sharp temporal-spatial form. We categorized thehistorical rainfall data mentioned above as either“thunderstorm-related intense downpour” or “typhoonor other rain.” For the seven watershed sizes listed inTable 1 (ranging from 7 to 105 km2), we calculated
the greatest watershed-averaged rainfall volume foreight different travel times (from 10 to 180 minutes)
and performed hydrological statistical analysis according to the rainfall source. An example is given in
Figure 7: temporal-spatial distribution of probable rainfall intensity according to watershed size and travel
time for a 30-year storm. The DAD relationship ofprobable rainfall from thunderstorms and typhoons
shown in Figure 7 corresponds well with the temporal-

120
2Thunderstorm(7km2)

80

Typhoon 1982.9.12 (7km )2
1982.9.12 (105km )2Thunderstorm1987.7.25 (7km )2
1987.7.25 (105km )Typhoon (7km2)

40 Typhoon(105km2)

Thunderstorm
2(105km )

0
0

30

60

90

120

150

180

(a) Thunderstorm Rainfall (1987.7.25)

Figure 4. The relationship between travel time and rainfall
intensity during travel time.

140
Watershed-average Rainfall
Intensitymm/hr

Relationship between DAD for historicaland probable rainfall

Thunderstorm(30min)

120
100
80

Typhoon 1982.9.12 (30min )1982.9.12 (180min)
Thunderstorm1987.7.25 (30min)
1987.7.25 (180min)

Typhoon (30min)

60

Typhoon180min

40
20Thunderstorm180min

0
0

20

40

60
Area (km2)

80

100

120

(b) Typhoon Rainfall (1982.9.12)
Figure 5. The relationship between watershed size andrainfall intensity.

Figure 6. Example DAD relationship in historical rainfall.

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spatial distribution properties of historical rainfall
shown in Figure 6. This, too, indicates the clear difference between the temporal-spatial distribution properties of thunderstorm and typhoon rain. Figure 8 is arainfall-intensity curve generated from the relationship between approximate travel time and watershedsize shown in Table 1. This shows that thunderstorm
rainfall is more intense when approximate travel timeis shorter than 60–100 minutes and watershed size isless than 20–30 km2, but at longer travel times andlarger watershed sizes, typhoon rain is more intense.
Because probable rainfall intensity differs greatly
according to the watershed size and travel time in theriver or sewer and according to the rain source, it isnecessary to select the appropriate storm magnitude(i.e., frequency of occurrence) and to identify therainfall source in question.

4

A METHOD FOR ESTABLISHING
RAINFALL LOADS IN FLOOD-CONTROLPLANS FOR URBAN AREAS

Consolidating everything discussed in the preceding
sections about rainfall loads, temporal-spatial distribution properties, and the discharge and flooding ofrivers and sewers, we will now consider a method of
establishing rainfall loads in flood-control plans thattakes all of these factors into account.
Figure 9 is the flood-control planning process forurban areas that the authors propose. Instead of using
rational formulas to determine rainfall loads and discharge, we propose DAD analysis that, in analyzing
rain properties, also takes into account rain sources in
the investigated area and that uses historical rainfall
data for the rainfall relevant to the flood-control plan.
Furthermore, discharge calculation is conducted using
a flood flow and flood inundation model that treats
rivers and sewers as an integrated whole, that calculates the mutual impact and effect of storage facilities
and other infrastructure, and that also takes sewer
flooding into account. This section describes theauthors’ method.
4.1(a) Thunderstorn Rainfall(Recurrence Interval 30 years)(b) Typhoon Rainfall and other(Recurrence Interval 30 years)
Figure 7. DAD relationship in probable rainfall.

Analysis of rainfall properties

Conventional flood-control planning based on rational
formulas uses the rainfall-intensity curve of a single
representative location in a watershed to determine
the rainfall load and consequently assume a uniform
distribution of rain throughout the watershed during
the travel time in question. In actuality, however—particularly in the case of local downpours caused bythunderstorms – watershed-averaged rainfall intensity
tends to be lower in larger watersheds. Furthermore,thunderstorm and typhoon rain have considerably

Figure 8. The relationship between travel time, watershed
size, and probable rainfall intensity. (For comparison, probable rainfall intensity (Tokyo Met. Gov. 1985) at the Tokyo
Observation Substation of the Tokyo Weather Station isindicated by a dashed line.)505

the probable rainfall separately for each source so as
to produce a plan that reflects reality.
The aim is to determine, on the scale of the riverand sewer watersheds in question, how frequently andwith what sort of temporal-spatial distribution rain
will fall.

different temporal-spatial properties. Design rainfall
loads must properly reflect all of these factors.
4.1.1

DAD analysis to determine rainfall
properties
It is necessary to collect historical rainfall data on
small watersheds (small to medium-sized rivers andsewers) measured at short (5- to 10-minute) intervals,organize the data according to the temporal-spatial
rainfall properties of the rainfall source, and thenidentify and analyze the rainfall properties of the watershed in question.

4.2

Determining the relevant rainfall

4.2.1

Extracting historical rainfall distribution
according to rainfall source
To determine the rainfall that is likely to occur in the
future, data on historical rainfall are analyzed to identify the different rainfall sources. Figure 10 is an example of the data used in this identification process. In thiscase, the authors plotted the relationship of sewerwatershed-averaged rainfall intensity during the flood
travel time (e.g., 30 minutes) and of the river-watershed-averaged rainfall intensity during the flood
travel time (e.g., 180 minutes) in order to identify therespective ranges of the thunderstorm and typhoon rain
(encircles with dashed lines in the figure). Within theseranges of data, the relevant rainfall was identified by

4.1.2

Probable rainfall intensity according
to rain source, watershed size, andtravel time
Probable rainfall intensity is calculated by statisticallyarranging watershed-averaged rainfall intensity bywatershed size and travel time. When doing so, because
the time and geographic distribution properties of rainfall differ greatly according to rainfall source (e.g.,thunderstorm or typhoon), it is important to calculate

Basic Policy of Planning river basin and drainage area of sewer to be investigated design storm event of river and sewer

Analysis of rainfall properties

Determining flood and inundation properties

 rainfall properties by DAD analysis

 condition of flood and inundation

 probable rainfall according to rain soure, travel time in rivers and sewers

watershed sizeand travel time
Determing the relevant rainfall

Constracting flood and inundation model historical rainfall hyetograph according torain source flood and inundation model treating

 establishment of regional rainfall

distribution
Substitution of rational formula
Establishment of flood-control measures[flood water carrying facilities]
river channels and sewers[water-storage facilities]
regulation ponds,storage pipes

assessment
 flood-control function
 reduction in damages
 economyOK
Decision of flood-control planning
Figure 9. The flow chart of flood-control planning for urban areas.

506

NG

determining the areas (A through D) in the figure ofhistorical rainfall that correspond to the probable rainfall intensity of the design storm event (e.g., 30-year
storm for rivers and 10-year storm for sewers) determined separately for each rainfall source using hydrological statistical analysis, as mentioned earlier. Table 2
lists the types of rainfall and notes on each. Essentially,a flood-control plan should be designed so that sewer
systems will not be flooded by typhoon or thunderstorm rain produced by the sewer’s design storm event(10-year storm). In the case of rivers, floodwater equivalent to the river’s design storm event (30-year storm)
should not result in river overflow. At the same time,other measures are necessary to handle rainfall thatexceeds sewer capacity, such as monitoring sewer
overflow and taking steps to minimize the intensityand extent of damage.

4.2.2 Establishing regional distribution
Discharge varies greatly depending on the regional
distribution of rainfall (Tanioka,Y., Fukuoka,S., et al.
1998). On the scale of a small to medium-sized riveror sewer watershed, it is not known where in the watershed the rainfall will be concentrated. Figure 11 is a
proposed method of determining the rainfall distribution in a watershed.
In the case of a typhoon, uniform rainfall throughout the watershed can be assumed because almost no
variation in rainfall intensity occurs. In the case of athunderstorm-related local downpour, however, thelocation of greatest concentration can determine
whether the runoff concentrates in the sewers or in theriver system. This necessitates considering scenarios
in which rainfall is concentrated in locations considered

Figure 10. The relationship between watershed-averaged
30- and 180-minute maximum rainfall intensity.

Figure 11. Conceptual diagram of determining the area ofconcentration of design storm rainfall.

Table 2. Extraction of relevant types of rainfall with notes on each.
Sewer

Thunderstorms

Typhoon rains

Sewers(1/10 plan)

A : Thunderstorms 1/10


B : typhoon 1/10
Rivers(1/30 plan)

D : Thunderstorms 1/30
C : typhoon 1/30


• Is it possible for sewers todischarge or store flood water
when thunderstorm of 10-year
storm attacks in the sewerdrainage area?

• Do rivers floods locally• Does sewer flooding cause ahuge damage in inhabitant areas.

• Possible for sewers
to discharge out or store
flood water against highlevel of river water?

• Possible for rivers todischarge flood flow?
• Does sewer flooding cause
a huge damage due tohigh level of river water.

507

Remarks• Flood-control plan is made notso as to arise sewer flooding• Flood-control plan is made notso as to arise river flooding.
• Countermeasures to prevent hugedamage from sewer flooding.

the most dangerous (e.g., the center or downstream
reaches of a watershed or the upstream reaches ofwater-storage facilities).
4.2.3

Determining discharge and flood
properties and constructing a discharge
calculation model
It is crucial to use data on historical water levels anddischarge in rivers and sewers—including data on river
overflows and sewer flooding—to identify discharge
and flooding phenomena, determine the causes, andgenerally ascertain actual conditions in the watershedin question. Once this is done, instead of using rational
formulas, it is necessary to construct an integrated
model of discharge and flooding in the river and sewer
systems that incorporates the regional and temporal
distributions of the rain load. A precondition is theability to accurately incorporate the effects of storage
facilities. In addition, because of the large impactof sewer flooding on river discharge (Fukuoka, S.,
Tanioka, Y. 2001), the model should be able to determine
how extensive sewer flooding will be and what effectit will have on the river and sewer systems. Furthermore, the model should also reflect the temporalspatial distribution of historical rainfall, which is considered in analyzing rain load.

5
5.1

ISSUES FOR FUTURE INVESTIGATION
How to draft flood-control measures

Flood-control measures should use the appropriate
combination of floodwater-carrying facilities (e.g.,river channels and sewers) and water-storage facilities(e.g., regulation ponds and storage pipes). Furthermore,combinations of different types of water-storage facilities are also needed depending on the rain source—
for instance, to control sewer flooding in the case ofthunderstorm-related local downpours, and to prevent
river flooding in the case of typhoon-related rain.
Flood-control measures should also strive to interlink
various facilities throughout a watershed, such as tunnel-type water-storage facilities, underground rivers,and diversion channels, thereby ameliorating regionalvariation in rainfall and affording flexibility (Tanioka,
Y., Fukuoka, S. 2003).
5.2

Systematization of flood-control planning
methods for urban areas

The methods of flood-control planning discussed inthis paper are by no means applicable to every type ofriver or sewer system or every set of geographic or
meteorological factors. These techniques must be systematized to yield logical, appropriate flood-control
plans that also strive to use existing infrastructure

more effectively. The availability of only limited historical data on short-term rainfall properties, sewer
flooding, and other information about small andmedium-sized rivers, sewer systems, and other small
watersheds may present difficulties in the investigation of rainfall, discharge, and flooding properties inthose watersheds. Nevertheless, the calculation of simple, general design indices (e.g., DAD analysis on
rainfall) for local areas should further the formulationof more logical plans in the future.
5.3

Accumulation and utilization ofobservations and data

In many cases, very little data are available on thetemporal-spatial scale of rain, discharge, and flood
properties of a small river or sewer watershed.
Consequently, an important issue in designing more
appropriate flood-control plans is conducting detailed,appropriate measurements and monitoring to gather
and archive such data.
5.4

Flood-control planning that also utilizes
rainwater storage and percolation facilities

In addition to rainwater-storage facilities for rivers
and sewers, watershed-wide rainwater-storage andinfiltration facilities and other technologies will also
play an important part in the urban flood-control planning of the future. Therefore, these techniques should
also be systematized into a coordinated whole for use
in realizing comprehensive flood-control plans forurban areas.
Acknowledgements: For providing the historicalprecipitation data used in this paper, the authors wish
to thank the River Planning Division of the Bureau ofConstruction, Tokyo Metropolitan Government.

REFERENCES
Tanioka, Y., Fukuoka, S. 2003. Flood Control Projects
through Coordination between Rivers and Swear in Urban
Areas, Journal of Hydraulic, Coastal and Environmental
Engineering, Japan Society of Civil Engineers 733(II63): 21–35 (in Japanese).
River Planning Division of the Bureau of Construction,
Tokyo Metropolitan Government, 1985, 85 small andmedium-sized rivers of Tokyo: 22–24.
Tanioka, Y., Fukuoka, S., Taniguchi, M., Koyama, Y. 1998.
Characteristics of Floods in Urban Rivers, Journal of
Hydraulic, Coastal and Environmental Engineering, Japan
Society of Civil Engineers 586(II-42): 1–12 (in Japanese).
Fukuoka, S., Tanioka, Y. 2001. On the Flood Control
Measures in Urban Areas due to Integration of Rivers andSewers, Advances in River Engineering, Japan Society of
Civil Engineers Vol.7: 149–154 (in Japanese).

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