IJERD – International Journal of Environmental and Rural Development (2020) 11-1

Research articleerd

Estimation of Flooded Areas and Water Levels after theHeavy Rain in July 2018 in Okayama
TAKAHIKO KUBODERA*
Faculty of Science and Engineering, Toyo University, Saitama, Japan
Email: kubodera@toyo.jp

SHUMPEI HIRAMICHI
Faculty of Engineering, Osaka Sangyo University, Osaka, Japan
Received 19 February 2020

Accepted 23 May 2020(*Corresponding Author)

Abstract Paddy fields in the Osaka Prefecture were heavily flooded by heavy rain in July
2018 due to levee breaches. To estimate the flooded area and water level, it was necessary tocombine aerial- and ground-based surveying. Accordingly, aerial photographs of the flooded
area were taken on the day after the heavy rain. In this study, we developed a digital surface
model (DSM) of the flooded area using the structure-from-motion (SfM) software, which canproduce three-dimensional (3D) models based on multiple aerial photographs and multiple
ground control points (GCPs). Using the global navigation satellite system (GNSS), wesurveyed the target area to observe the 3D coordinate values of the GCPs. The DSM of theflooded area could be made by the SfM with the aerial photographs and GCPs. The DSM wasinput into the geographic information system to analyze the flooded area and water level. Thewater levels were calculated from the differences between the DSM and the digital elevation
model (DEM). We used airborne laser surveying to build the DEM because it provides higher
accuracy than the SfM software. To check the accuracy of the DSM, we compared theelevations of the DSM on the ground points of non-flooded areas with those of the DEM, andthe differences were found to be less than 0.2 m. Then, the water levels on the actual waterlevel observation points were compared to verify their accuracy, and the differences were
found to be less than 0.4 m. The flooded area was found by overlaying the water level on theexisting map.
Keywords structure-from-motion, digital surface model, digital elevation model,
geographic information system, paddy field

INTRODUCTION
Heavy rain in July 2018 seriously affected the Okayama Prefecture in the west of Japan (particularlythe paddy fields) due to levee breaches. As a result, nearly half of harvest of the paddy fields was lost.
A combination of aerial- and ground-based surveys was necessary for a prompt understandingof the complete picture of the damaged area (Kubodera et al., 2017). Therefore, we conducted anaerial photogrammetry and a global navigation satellite system (GNSS) survey. Aerial
photogrammetry is a surveying technology that utilizes three-dimensional (3D) ground-based models,
which are constructed from aerial photographs, camera coordinate values, and coordinate values ofground control points (GCPs). Based on the aerial photogrammetry, we also built orthophoto mosaics
and a digital surface model (DSM), which captures the land surface elevations, including the heightsof buildings and vegetation. An orthophoto mosaic is a single aerial photograph on which several
orthographically projected aerial photographs are assembled. To obtain the coordinate values of theGCPs in the devastated areas, we conducted a GNSS survey (Yamazaki et al., 2019), which is amethod to determine the positions using signals from positioning satellites, such as the global
positioning system (GPS), global navigation satellite system (GLONASS), and Galileo. The
accuracy of these satellites ranges from several meters to several millimeters. In this research, we
conducted a network-based real-time kinematic (RTK) GNSS survey to determine the coordinate
values of the GCPs with an accuracy of several millimeters. We used the geographic informationⒸ ISERD176

IJERD – International Journal of Environmental and Rural Development (2020) 11-1system (GIS) to determine the difference between the DSM and digital elevation model (DEM),
which captures the ground elevations (excluding the heights of buildings and vegetation), to analyze
the flooded area and water level in the paddy fields quantitatively.
We considered that aerial photogrammetry by airplane would be preferable than by satellite ordrone for the analysis of the flooded area and water level. The aerial photographs used were captured
from an airplane immediately after the heavy rain. The Geospatial Information Authority of Japan(GSI) has built a detailed database of Japan’s land surface from photographs taken from airplanes andhas released the DEM (5 m mesh) from airborne laser surveying throughout Japan. Furthermore, theGSI has released aerial photographs of Okayama taken immediately after the heavy rain in July 2018.
METHODOLOGYFigure 1 shows the target area of the survey and analysis. The Okayama Prefecture is located to thewest of Osaka and Kyoto. Of the large area damaged by the heavy rain, we selected Mabi City tofocus on the damage to paddy fields. Figure 2 shows the levee breach at the Oda river in Mabi City
that caused the flooding of the paddy fields. The first author took the picture on November 10, 2018.

Fig. 1 Target area in Okayama

Fig. 2 Levee breach at Oda River in Mabi City
The first author took the picture in Novemeber 10, 2018.

Aerial photo

GCP
Paddy fields

Oda river

Point of observation of thewater level

Fig. 3 3D model by aerial photogrammetry

Fig. 4 GNSS surveying

We used the photogrammetry structure-from-motion (SfM) software PhotoScan Professional,
which allows the aerial triangulation based on multiple aerial photographs and multiple GCPs. It is
used to build DSMs through image matching and create orthophoto mosaics automatically. Figure 3Ⓒ ISERD177

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shows the 3D model built in this study. The Oda river and paddy fields are in the center of Fig. 3;
the point of water level observation, which was performed every hour, is in the right of Fig. 3. The
twelve blue rectangles in the sky show the 3D positions from which the aerial photographs weretaken. The 15 circles on the ground surface indicate the 3D positions of the GCPs where weconducted the GNSS survey. AS coordinate system, we used the Japanese geodetic datum (JGD)
2011 plane-rectangular coordinate system Area 5 (Matsumura et al., 2004; Tsuji and Matsuzaka,2004; Imakiire and Hakoiwa, 2004; Geospatial Information Authority of Japan, 2011). We
successfully set a group of 3D points on the ground surface through aerial photogrammetry. The
aerial photographs were taken at 12:00 on July 9, 2018, immediately after the heavy rain with a wellcalibrated digital aerial camera. Using the RTK GNSS surveying and inertial measurement unit(IMU) technology, we set the coordinate axes and altitudes of the digital aerial camera with anaccuracy of several tens of millimeters.
As shown in Fig. 4, we conducted a network-based RTK GNSS surveying at GCPs in Okayama
on November 10, 2018. We used the Trimble R10 for the GNSS surveying, and three types ofsatellites: GPS, GLONASS, and the quasi-zenith satellite system (QZSS). In addition, we used two
frequency bands: L1 (1575.42 MHz) and L2 (1227.60 MHz). We used the geoid model GSIGEO2011(Miyahara et al., 2014; Kubodera et al., 2016). For the GNSS survey at the GCPs, we selected thefifteen points enclosing the flooded area and the point of observation of the water level, sites
accessible by car, and pavements to obtain clear photographs.
We conducted aerial triangulation based on the 3D coordinate values of the camera and theGCPs. Table 1 shows the standard deviation (SD) values of the residual errors of the most probablevalues of fifteen GCPs. The SD values for the X and Y coordinates and elevation H were lower than
0.10 m, indicating high accuracy.
We created a DSM with 0.4 m mesh to determine the differences between the DSM and DEM.
Furthermore, we created orthophoto mosaics to obtain more information on the flooded area.
RESULTS AND DISCUSSION
Specifying the Position of Levee Breaches with Orthophoto Mosaics
Figure 5 shows the orthophoto mosaic developed on the GIS. For the aerial photogrammetry, weused the coordinate axis JGD2011 Plane-rectangular Coordinate System Area 5. The GIS software
used was ArcGIS 10.7. By comparison with an existing map, we confirmed that the mosaic wasdeveloped on the designated positions correctly. The black areas in Figure 5 were either not in theaerial photograph field of view or covered by cloud or water, because SfM could not create the clouds
and water. As shown in Figure 6, we successfully specified the exact positions of the levee breaches,
and the orthophoto mosaic indicates that the paddy fields were flooded.
Table 1 SD values of the residual errors of the most probable valuesof fifteen GCPs

SD

X [m]

Y [m]

H [m]

0.097

0.061

0.078

DSM accuracy verification
We compared the elevation values of the DSM to those of the actual water surface at the point ofwater level observation to verify the accuracy of the constructed DSM. Figure 7 shows a photographof the instrument used to observe the water level in Yakatabashi, which observed the actual waterlevel every hour during the heavy rain in July 2018. The first author took the picture on October 29,
2018. Figure 8 shows the DSM of the area around Yakatabashi. The areas of not-heat-map were thosethat PhotoScan Professional could not create the DSM.
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Out of aerial photographfield of viewCloudWater

Fig. 5 Orthophoto mosaic developed from the GIS

Flooded area
Water flow
Not analyzed
Water flow

Levee breach
Water flow

Fig. 6 Levee breaches and flooded paddy fields based on the GIS
Table 2 shows the elevation values in the DSM around Yakatabashi at 12:00 on July 9, 2018.
The aerial photographs were taken at 12:00 on July 9, 2018. So, this DSM was at 12:00 on July 9,
2018. The elevation values of DSM were measured at 10 points in the water surface of theverification area of Fig. 8. The average and SD of the ten elevation values were 8.208 and 0.096 m,
respectively. Table 3 shows the elevation values of the actual water surface at Yakatabashi on July9, 2018. As shown in the table, the elevation values of the actual water surface at 12:00 was 8.167
m. Therefore, the difference between the elevation values of the DSM and those of the actual water
surface was +0.041 ± 0.096 m.

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Vegetation
Verification area
Not analyzed“Yakatabashi”

Fig. 7 The water level observation inYakatabashi
The first author took the picture onOctober 29, 2018.

Fig. 8 DSM around Yakatabashi
Table 2 Elevation values in the DSM around Yakatabashi at 12:00 on July 9, 2018
No.

12

3

4

56

7

8

9

10Ave.

SDElevation [m] 8.122 7.923 8.514 8.220 8.125 7.731 8.024 8.318 8.799 8.302 8.208 0.096

Table 3 Elevation values of the actual water surface at Yakatabashi on July 9, 2018
Time

09:00

10:00

11:00

12:00

13:00

14:00

15:00

Elevation of water surface [m]

8.507

8.357

8.247

8.167

8.107

8.047

7.987

Not analyzed

Flooded area

Levee breach
BuildingBridge

Fig. 9 Differences between DSM and DEMⒸ ISERD180

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Differences between DSM and DEM
To build the DEM before the heavy rain, we used the results (5 m mesh, elevation value in units of0.01 m) obtained from the GSI. We deducted the elevation values of the DEM before the heavy rainfrom those of the DSM after the heavy rain to obtain the differences. Figure 9 shows the results ofthe analysis in the same area as that shown in Fig. 6. The black areas indicate areas for which thePhotoScan Professional software could not create the DSM. We whited out the objects with a height
difference greater than 3 m because these were assumed to be buildings or vegetation. The analysis
results indicate that the vertical water level was 0−2 m and vertical levee breaches levels werebetween -5 and -6 m. Based on the analysis of the differences between the DSM and DEM, we
successfully clarified the vertical water level both dimensionally and quantitatively.
CONCLUSION
The contributions of this study are as follows:(1) The exact levee breach in flooded paddy fields was specified based on the orthophoto mosaic.
(2) We compared the elevation values of the DSM and those of the actual water surface at the pointof water level observation to verify the accuracy of the constructed DSM. We determined that thedifference between the elevation values of the DSM and those of the actual water surface was +0.041±0.096 m.
(3) The analysis of the differences between the DSM and DEM indicated that the vertical water level
was 0−2 m and the vertical levee breaches levels were between -5 and -6 m.
ACKNOWLEDGEMENTS
We express our gratitude to the Information Provision Support Team of the Major Disaster
Management Headquarters of the GSI for providing the aerial photographs used in this research inorder to support the recovery project implemented by the Okayama Prefecture.
REFERENCES
Geospatial Information Authority of Japan. 2011. Retrived from http://www.gsi.go.jp/ENGLISH/page_e
30030.html
Imakiire, T. and Hakoiwa, E. 2004. JGD2000 (vertical), The new height system of Japan. Bulletin of the GSI,51, 31-51.
Kubodera, T., Okazawa, H., Hosokawa, Y., Kawana, F., Matsuo, E. and Mihara, M. 2016. Effects of surveying
methods between GNSS and direct leveling on elevation values over long distance in mountainous Area.
International Journal of Environmental and Rural Development, 7 (1), 62-69.
Kubodera, T., Suzuki, T., Masaharu, H. and Matsuo, E. 2017. Analysis on surface deformation and cracks inpaddy fields by 2016 Kumamoto earthquake using GNSS and photogrammetry. International Journal ofEnvironmental and Rural Development, 8 (1), 85-92.
Matsumura, S., Murakami, M. and Imakiire, T. 2004. Concept of the new Japanese Geodetic System. Bulletinof the GSI, 51, 1-9.
Miyahara, B., Kodama, T. and Kuroishi, Y. 2014. Development of new hybrid geoid model for Japan,GSIGEO2011, Bulletin of the GSI, 62, 11-20.
Tsuji, H. and Matsuzaka, S. 2004. Realization of horizontal geodetic coordinates 2000. Bulletin of the GSI, 51,11-30.
Yamazaki, Y., Okazawa, H., Sekiyama, A. and Fujikawa, T. 2019. Accuracy verification of UAV-SfM surveyof terrace paddy fields in a hilly and mountainous area. International Journal of Environmental and RuralDevelopment, 10 (1), 153-159.

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