IAHR-APD 2020, Sapporo, Japan

Dynamic state of river-mouth bar in Yuragawa River andapplication of drone photography image to obtain its topographical data
Hiroshi MIWA (Tottori University, Japan), Mio OMURA (WESCO Co., Ltd., Japan), Takashi WADA (Tottori University, Japan),
Yoshinori KATO (National Institute of Technology, Maizuru, Japan) and Keiichi KANDA (National Institute of Technology, Akashi, Japan)
3. Topographical data by using drone photography images

1. Introduction
Topographic changes of the river-mouth bar in the Yuragawa River of Japan (Fig.1) arecontinuously activated by sediment transport due to river flows and sea waves. The bar
has formed on the right bank only since 2004 (Fig.2) and its size has gradually decreased
as a long-term tendency. In resent years, the expansion of the bar has been identified at
the upstream of the bar. In order to clarify the characteristics of the topographical changesof the bar and their causes, we have measured the bar topography by using an RTKGNSS receiver. However, this method may have disadvantage for swift actions upon adisaster investigation. In this study, temporal variations in geometrical properties (e.g.
area) of the river-mouth bar were analyzed based on the hydrological and topographicaldata in the Yuragawa River estuary. As for the measurement of the bar topography, we
proposed the method of calculating coordinate values of stations on the shoreline by acoordinate transformation using aerial photography images. The estimated results of theshorelines of the river-mouth bar and coastal zones were compared with the measurement
results by the RTK-GNSS survey. The estimation results of their areas based on theestimated shorelines were also compared with those by using the measurement results ofthe shorelines, and then effectiveness of the proposed method was discussed.

3.2 Estimation of shoreline and area of Yuragawa river-mouth and Kanzaki coastal zones

Figure 4.
Phantom 4 (DJI).

Figure 5. Target zones for topographical data.
(a) Yuragawa river-mouth zone,(b) Kanzaki coastal zone.

(a) Yuragawa river-mouth zone(b) Kanzaki coastal zone
Figure 8. Examples of estimation and measurement results of shoreline and their comparison.

The estimation results can roughly follow the shorelines measured by the RTK-GNSS
surveys in the both zones. The relatively low accuracy in Kanzaki coastal zone (b) might becaused by not only the distortion of the image but also the unbalanced arrangement of GCPs.

River-mouth bar

Figure 6. Examples of combined images of Yuragawa and Kanzaki zones.

3.1 Coordinate transformation method and estimation of shorelines

Figure 1. Location and basin of the Yuragawa River.

Figure 2. Changes in river-mouth
topography.

2. Field measurements on river mouth topography
The area of the river-mouth bar kept on fluctuating until July 2018, whereas thefluctuation is not so large after that. As concerns of the short-term tendency, the area
increased in the winter seasons of 2016, 2017 and 2018, and it decreased in their summerand rainy season. Increase of the area may be activated by an increase of longshore
sediment transport rate due to high wave heights in the winter seasons and decrease ofthat was caused by the flooding in rainy and summer season. A possible reason why thearea hardly increases in the winter seasons of 2019 and 2020 is that the drift sand from theoffshore may be accumulated on
the seabed eroded by the flood
flow on July and September
2018. The expansion of the rivermouth bar has been identified at
the upstream of the present bar
in resent years. The red symbols
in Figure 3(a) show the area ofthe river-mouth bar including theupstream bar, which might havebeen formed since the winterseason of 2017-2018. This may
be because the drift sand fromthe offshore has more easily
entered to the estuary.

The coordinate values of the stations on the shoreline in the combined image aretransformed into the plane rectangular coordinate system as following procedure:(1) Some GCPs are chosen in the combined images.
(2) The affine coefficients are calculated so that the coordinate values of GCPs in thecombined image match their coordinate values in the plane rectangular coordinate system.
(3) The coordinate value (U, V) in the combined image is transformed into the coordinatevalue (X, Y) in the plane rectangular coordinate system by using the following relation: X  aU  bV  c(1)Y  dU  eV  fa  xu  b   A1   xv  , c x  d   yu  e   A1   yv  , f y    u2A    uv
 u

 uv

u

v

v

2v

 (2)


1 


where a - f = affine coefficients, u, v = lateral and longitudinal coordinate values of GCP in
the combined image, respectively, x, y = those in the plane rectangular coordinate system.

(a)

(b)

Figure 3. Temporal variations in (a) river-mouth bar area,(b) significant wave height and (c) water discharge.

The ratio of the estimated area AC to the measured one by the RTK-GNSS survey AG is
0.96 - 1.07 for Yuragawa zone. As for the Kanzaki zone, the value of AC/AG is 0.95 - 1.05.
These results indicate that since the estimation of areas by means of the combined images
and the affine coordinate transformation give a few percent error, the small-scale variations
in areas may not always be grasped. However, a relatively large-scale variations in areas
due to a flood and high wave events can be grasped.

4. Conclusions

(a)

(c)

(a) Yuragawa river-mouth zone(b) Kanzaki coastal zone
Figure 9. Estimation results of areas in their temporal variations.

(a) Yuragawa river-mouth zone(b) Kanzaki coastal zone
Figure 8. Effects of GCP on accuracy of estimation results of shoreline(a) When the GCP were Nos.1, 4 and 6, the shoreline around No.2 shifted in the East
direction from the shoreline of the RTK-GNSS survey. This is improved by addition of No.2.
(b) The shoreline colored in green was estimated by using GCPs of Nos.6, 12, 15 and 16, it
was away from the shoreline measured by the RTK-GNSS survey. The blue line wasestimated by addition of GCP No.7, which is located away from the linear position of theother GCPs. The addition of Nos.1 and 10 caused the further improvement of the estimation
as shown by the red line.

1. The area of the river-mouth bar of the Yuragawa River kept on fluctuating (increase inwinter season and decrease in rainy and summer season) until 2018, whereas thefluctuation is not so large after that. Most of the river-mouth bar has been flushed out bysome floods in recent years, whereas the expansion of the bar has been identified. Thismay be because the drift sand from the offshore has more easily entered to the estuary.
2. We proposed the method of calculating coordinate values of stations on lands by acoordinate transformation using aerial photography images and grand control points(GCPs). The shorelines of the Yuragawa river-mouth zone and the Kanzaki coastal zone
could be estimate by the method. However. the reproducibility of the estimation results
depends on not only the distortion of the image and the personal attributes but also theunbalanced arrangement of GCP. Therefore, the GCP needs to be arranged dispersively.
In particular, since the coastal zone widely extends in one direction, the GCP in theonshore or offshore direction are also required.
3. The estimation results of areas of the Yuragawa and Kanzaki zones method keep anerror within a few percent. Although the small-scale variations of the areas may not begrasped, a relatively large-scale variations due to a large flood could be grasped.