PREDICTION OF URBAN INUNDATION IN YOKOHAMA AND KAWASAKI CITY BY USING “S-UIPS”
Introduction

Masato SEKINE, Moe YOSHINO, Naoki TAKEMURA, and Wataru BABA (Waseda University, Tokyo, Japan)
Results

The damage caused by torrential rain has been occurring frequently in Japan with theextremes of weather on a global scale in 2019. In order to prepare for the unprecedented
heavy rainfall countermeasures in the future, accurate flood risk assessment and hazard
map creation are desired. Previous studies by Shibuo et al. (2018) aimed at improving theaccuracy of river water level prediction for the Tsurumi River have been conducted. Inaddition, Murase et al. (2018) conducted a study on inland water inundation analysis in
Kasugai City, Aichi Prefecture, using the results of sewer internal water level measurement.
In this study, using the sophisticated inundation prediction method "S-uiPS"(Sekine, 2011)
developed by the first author, we evaluated the inundation risk in the eastern part of
Yokohama and the eastern part of Kawasaki, and clarified the inundation process.

Analysis overview
Overview of the target area Target area is made up 5 drainage
treatment zones in Yokohama City and2 drainage zone in Kawasaki City.
 The total area is 239.3 km².
 The Tsurumi River that is the typicalurban rivers in Japan exists in thecenter of this area, and the total lengthof river is 42.5 km.
 The Katabira River, the Ooka Riverand some more small urban rivers arein this area.

Prediction calculation method

In this study, we calculate the inundation
prediction using the sophisticated inundation
prediction method "S-uiPS" developed by thefirst author. In highly urbanized areas,
rainwater hardly penetrates into the ground dueto the asphalt paved roads and the densely
packed blocks. Therefore, the flow of water inthe target area can be solved only based on theprinciple of hydraulics. In this research, theroad database, the sewer database, and otherurban infrastructure information were reflectedin the calculation, and the reproduction
calculation was performed. S-uiPS does notinclude any parameters to be tuned.

Overview of the target area

In this study, computation was conducted by using XRAIN data which was measured by
the X-band radar system of the Ministry of Land, Infrastructure, Transport and Tourism ofJapanese Government (see in Figure 1 (b) and (c)).

Figure 2 is the results of computation by using XRAIN data to reproduce what
inundation occurred on October 6, 2014.
In the period from 6:15 to 8:00
Rain clouds have moved northwest. During 30 minutes from 7:30 to 8:00,
rainfall intensity became over 50 mm/h mainly in the western part of this area.
 Inundation started in the entire area at 8:00, but the inundation depth wasabout 0.1 m or less, and no serious inundation occurred.
 A red drainage pipe in the contour map of the drainage fullness at 8:00 denotes
the water in the pipe has already been filled with rainwater. The drainage pipe
attains such full-packed state due to the local rainfall exceeding the designed
rainfall intensity. At this stage, the rainwater removal system worked normally.

In the period from 8:00 to 10:00

Heavy rainfall of 75 mm per hour and then the one more than 100 mm per hour
occurred in this area.
 The number of drainage pipe in full-packed state increased. Inundation
occurred on the road in lowland area during this time. At 10:00, inundation
depth in the center of Yokohama City became about 0.1-0.5m. In some area
there, there are some location whose inundation depth is 0.8m or more than it.
 Flooding started to occur at the same time in several urban rivers including theTsurumi River. It is because rainwater had been transported in drainage pipes
to the rivers and the flow discharge in the river increased rapidly.

In the period from 10:00 to 11:00

Inundation became more serious. During the first 30 minutes from 10:00, rainfall
exceeding 50 mm/h occurred over the entire area. In the southern part of this area,
the rainfall exceeding 75 mm/h occurred for the 20 minutes from 10:00.
Figure 2 Contour map of Inundation depth on road and relative The remarkable inundation area is the same as above, and the depth there water depth of the river (in left) and state in drainage pipe (inbecame much larger. When the rain weakened at 11:00, the inundation area right) predicted for the rainfall event on October 6, 2014.
decreased because the ability to remove the rainwater on road became larger
than the rainfall intensity, the area of Inundation was reduced. the local rainfall
exceeding the designed rainfall intensity. At this stage, the rainwater removal system worked normally.
 As to the river flooding, overflow was not predicted from the Katabira River and the Ooka River, which is coincide with the actualevent.

Summary

Even if the rainfall exceeds the designed rainfall intensity, the rainwater removal system works relatively well for about 30 minutes
or so. But if such level of rainfall keeps for more than one hour, the drainage pipe attains the full-packed state of water. And then,inundation depth grows larger in lowland area because drainage pipe cannot remove the water on the ground surface. It was foundfrom this computation that the inundation area is limited and no large inundation depth was predicted in the areas far from the river.
And it was verified that the state of drainage pipe is improved and inundation itself starts to disappear gradually after the rain stops.
Figure 1. Contour maps in the target area: (a)elevation (b)(c)rainfall intensity on October 6, 2014 at 8:45 and 10:10

Conclusion

In this study, inundation prediction was conducted in Yokohama City and Kawasaki City in Kanagawa Prefecture on October 6, 2014by using the XRAIN data. As this study, the area where inundation depth is relatively higher were found in these cities. It was also
confirmed that the inundation depth becomes 0.8m or more than it in the lowlands. In addition, the characteristics of rainwater removal
system including drainage pipe and urban river was made clear. when the rainfall weakened, the sewage fill rate decreased at highaltitudes, but there were still many pipes with a fill factor of 1 in lowlands, and the drainage function did not recover immediately.

References
Murase, M. and Takeda, M. (2018). Journal of JSCE, Ser. B1 (Hydraulic Engineering).
Sekine, M. (2011). Journal of JSCE, Ser. B1 (Hydraulic Engineering).
Sekine, M., and Nakamori, N. (2017). Journal of JSCE, Ser. B1 (Hydraulic Engineering).
Shibuo, Y. and Lee, S. (2018). Journal of JSCE, Ser. B1 (Hydraulic Engineering).

Sekine Laboratory, Department of Civil and Environmental Engineering, WASEDA UNIVERSITY