Inundation analysis of urban areas by using GIS– Application to the Tokai Heavy Rain Flood
H. Kiyomiya
Yokohama City, Yokohama, Japan

A. ManoTohoku University, Sendai, JapanT. Katada
Gunma University, Kiryu, Japan

ABSTRACT: A numerical method to construct automatically the mesh cohered with roadnetworks has been developed, which enables the chase of inundation caused by levee breach aswell as local rainwater through taking into account of sewage systems. This method was applied
to 2000 Tokai Heavy Rain Flood and validated through the comparison with the data obtainedby the questionnaire survey. The pumping effect of the sewage system in this inundation is also
discussed.

1 INTRODUCTION
Continuous concentration of population and property in urban areas ever increases disaster
vulnerability. The 2000 Tokai Heavy Rain Flood caused damage to property estimated at about
850 billion Yen at Nagoya City with two million population and the surrounding communities
in Aich Prefecture. About the half of the damage was due to automobile wreck. The water
spilled from levee failure as well as local rainwater expanded the inundation, and sank cars andlower floors of houses in the area.
To get good understanding on complex mechanisms of urban inundation disasters we havedeveloped a numerical model that takes account of levee failure and drainage by sewagesystems. The mesh cohered with road networks, MCRN is our important inclusion, because thesubsurface sewage networks are mainly constructed under the surface road networks. Different
behavior between roads and resident areas in rainfall infiltration and water spread also requires
this mesh.
2 MESH COHERED WITH ROAD NETWORKS
Although there are many advantages in MCRN, the construction of the mesh mainly
done by handwork was exhausting and time consuming work especially if we consider
wide area. Many countries are recently developing Geographical Information System,
GIS that contains the database on the positions of roads and resident areas, and on theinformation connecting these. This has the potential to develop MCRN through acomputer program.
The Geographical Survey Institute in Japan has developed Digital Map 2500 which
has the same spatial resolution as the topographic map of a scale 1:2500 and contains
vector data on the centerlines of roads and rivers, and on the polygons enclosing
resident areas for all cities and towns. The MCRN used in our inundation analysis iscomposed by a set of polygons of residential areas, roads and crossings. By using themap, we have developed a method to construct automatically MCRN in the following
steps;

(1) Input arcs indicating residential area boundaries and connect them to form closed
polygons. Then simplify each of the polygons by reducing nodes of the sides into such aquadrilateral, pentagon, hexagon, etc. This procedure eases the correlating process
between roads and sides of residential areas because the map expresses curved sides by
a number of short straight lines.
(2) Input centerlines of road segments. Then find the adjacent residential areas and thecorresponding sides for each segment. This is realized by comparing the distances
between the middle points of the road centerline and the sides, and by selecting theminimum distance. Through this correspondence, all the connectivity among theadjacent roads and residential areas is completed.
(3) Compute the polygons of crossings from the sides of residential areas and complete
the connectivity among the joining roads.
Thus the objective area is covered by polygons of residential areas, roads, andcrossings the connectivity of which is known. This method is applied to the 2000 Tokai
Heavy Rain flood.

3 APPLICATION FOR THE OF THE TOKAI HEAVY RAIN FLOOD
3.1 Flood outline
The stationary front stimulated by the typhoon 14 brought heavy rainfall reaching
567mm in total at Nagoya City on September 11 to 12, 2000. Figure 1 shows the hourly
rainfall and cumulative rainfall observed at Nagoya. The rain caused record-breaking
floods in the Shonai river passing through the city and breached or spilled over a dozenof levees of the major and minor rivers. Tsujimoto (2001) reports the damage of morethan 63,000 houses inundated and ten people killed in Aich Prefecture.
The discharge split from the Shonai river through the spill-weir exceeded the flow
capacity of the branch of the Shin River. Spilled water breached the left levee of theShin river over 100 meter length and inundated the adjacent urban area where low land
stretches along the river. Although there exists a drain system for rainfall in this area,the operation was stopped because of high stage of the river. Thus the inundation wasgrown by the river water inflow at the breaching as well as the rainfall collection in thearea.

Figure 1. Hourly rainfall intensity and cumulative rainfall at West Public Work Office ofNagoya City. The elapsed time in the abscissa originates at 12:00, September 11.
3.2 Computational method
The object of the analysis is the inundated area along the Shin river. The residential
area polygons in Figure 2(a) and the road network in Figure 2(b) are used to produce theMCRN. Superimposition of 10-meter mesh Digital Elevation Map gives the polygons
elevation attribute. The register chart and book on the sewage system relates the roadand sewage networks major channels of which was taken into account as shown in
Figure 3. The two networks are connected by manholes mainly installed at crossings.
The water surface elevation on the ground is computed at the center of polygons byusing the continuity equation and discharges at the sides. The discharge on the groundis computed at the midpoints of the polygon sides by the Manning formula. The
Manning roughness factor n employed is 0.043 for roads and 0.067 for residentialareas. The discharge between the residential area and road is modified by the reduction
coefficient   1   with the occupation ratio of houses  after Inoue (1999).
The discharge at the breach point is computed by using the Honma spilling
formula(1940) and the observed data on water stage at the Shin River.

(b) Road network
Figure 2. The residential area polygons along the Shin river and road network

the Shonai R.

the Shin R.

PumpManhole

Sewage channel

Figure 3. The sewage system with pumping stations, manholes, and sewage channels.

The water surface elevation in the manholes is computed by using the continuity
equation and the discharges from / to the ground surface and sewage channels. Thedischarge from / to the ground surface is evaluated by the Homma formula. It contains
flowing out from the manholes to the ground surface. The discharge in the sewage
channel is computed by using the Manning formula. If the depth is less than thediameter of the channel, then the hydraulic characteristic relationship between thedimensionless depth and discharge is used. The roughness coefficient was 0.012.
3.3 Data collection
Questionnaire survey in this area were conducted in October, 2000 by Katadaetal.(2001). 9743 questionnaires were distributed and the 32% collected. The question
articles are on the times when the inundation started under the floor, spilled over thefloor, reached the highest, and disappeared. The answer shows more than 59 % of thehouseholds in this area suffered inundation over the floor, which were caused by thelevee breach. Significant inundation under the floor also occurred even before thebreach due to the insufficient drainage. The each time in the answer was averaged ineach residential area and set in GIS format for the comparison with our computational
results.

4 RESULTS AND DISCUSSION
4.1 Validation of the computation
The objective area bounded by the Shin River and Shonai River has depressions along
the Shin River which collect the rainwater fallen in the area as shown in Fig.(a) The
residential areas for the inundation depth comparison are sampled as shown in Fig.(b).
The inundation depth for each residential area was obtained by taking average over all
the answers in the area.

unit(m)Figure 4. Topography of the area shown by 10m-mesh DEM. The central and broadrivers are the Shin and Shonai rivers respectively. (b) shows the sampling positions forcomparison of inundation depth.
Figure 5 show the temporal changes of the inundation depth at Nos. 2 and 8, obtained
from the questionnaire survey and computation. The computation considers pumping of
the discharge, 30m3/s in the actual operation. Due to the heavy rainfall after 5 hours,
the inundation depth increases despite the pumping. It reached about the steady state
after 10 hours, however the levee breach at 15.5 h increased the depth rapidly. Thecomputation at both positions reproduces the observed depth. The maximum depth atthe lower land at No.2 is deeper than that of No.8.

2

1.6
No.8 Position

1.5

1Computed
Observed

0.5

Inundation depth (m)Inundation depth (m)No.2 Position
1.2

0.8

0.4

With pumpObserved

0005

10Time (h)152005101520

Time (h)Figure 5. Temporal changes of the inundation depth at No.2 on the left and No. 8 on theright The origin of the time is same as Fig.1.
4.2 Effect of pumping
The effect of the pumping is examined here. Figure 6 compares the inundation depth
distributions at 3 h of the computations without and with the pumping of 30m3/s. The
effect is obvious for the earlier stage of inundation due to the local rainwater. 1 to 1.5m
reduction of the depth was obtained at the depressed region by the pumping.

unit(m)Figure 6. Computed inundation depth distributions at 3 h without pumping on the left
and with the pumping of 30m3/s on the right.

32

No.3 Position

No.8 Position
1.6
Inundation depth (m)Inundation depth (m)

2.5
2
1.5
1with pumpwithout pump

0.501.20.80.4

With pump
Without pump

0051015Time (h)202505101520Time (h)Figure 7. The effect of pumping.
However, rapid increase of the inundation depth after the levee breach diminishes theeffect of the pumping. For example, at No.3 Position in Fig. 7, one meter gain in theinundated depth by the pumping decreases to few tens centimeter at the final depth.
This is due to the large volume of breached water.
5 CONCLUSION
We have proposed the method to construct automatically the mesh cohered with roadnetworks by using the GIS data. The mesh enables the connection with the sewage
network. This method was applied to the 2000 TOKAI HEAVY RAIN FLOOD andsolved the inundation caused by the levee breach as well as the local rainwatercollection. The computed inundation depth was validated by the observed data obtainedby the questionnaire survey. The pumping by the sewage system is shown to be effectivein the initial stage of the rainwater collecting inundation, however less effective in thelatter stage after the levee breach and in the reduction of the maximum inundation
depth.

REFERENCES
Honma, H., 1940. Discharge coefficients on low spill weir, Proc. JSCE, Vol. 26, No6 &9.
Inoue, K., Kawaike, K, & Hayashi, H., 1999. Inundation flow modeling, Annual Journalof Hydraulic Engineering, JSCE, Vol. 43: 533-538.
Katada, T., & Kuwasawa N., 2001. The actual situation of flooding due to heavy rainfall
disaster around Nagoya City by the questionnaire survey, River Technology, Vol. 7:41-46.
Tsujimoto, T. edi. 2001. Survey and research on the disaster due to 2000 September
Tokai Heavy rainfall, Report on the National Science Foundation #12800012: 1-6.