# Unmanned Aerial Vehicle Depth Inversion to Monitor River-Mouth Bar Dynamics
## 6 Conclusions  
We developed a simple method to estimate two-dimensional coastal bathymetry from UAV-derived video imagery. The estimation procedure can be divided into two phases. In the first phase, the original video was converted into sequential orthogonal images using GCPs. The effect of wind-induced UAV motions was removed by automatically detecting the GCPs. In the second phase, a wave-celerity vector field was obtained by the cross-correlation analysis of pixel intensity signals, whereas the corresponding wave period was determined via cross-spectral analysis. Finally, the estimated wave parameters were inverted to water depth based on the linear dispersion relation.  
The method was tested for the four cases with varying wave and bathymetry conditions. The coastal videos were taken from a UAV hovering 100-150 m above mean sea level, which typically covered a horizontal area of 600 \(\times\) 1000 m. The method robustly worked  
Figure 14: Relationship between RMSE and the mean cross-correlation coefficient for different transects.  
under different bathymetry conditions and yielded two-dimensional bathymetry with a RMSE less than 0.5 m. The applicable range was limited mainly by wave breaking on the shore and up to a water depth of \(\sim\)8 m offshore, depending on the dominant wave period. We compared the estimation results with those obtained by cBathy and confirmed a better performance for the tested cases under dominant swells. Furthermore, with a compact array and a small number of reference points, the present method is expected to reveal finer topography at a low computational cost.  
We can apply the method to monitor the evolution of highly two-dimensional bathymetry, such as a deltaic bar in front of a river mouth. Although it does not work in the vicinity of a river mouth, the method reveals the outer bar topography with effortless UAV-based observation. The accuracy level is insufficient for it to be a promising alternative to conventional bathymetric surveys. However, the low operation cost is advantageous in monitoring drastic topographic changes induced by an extreme event. A drawback of the method may be that its accuracy and applicable range vary with the incident wave condition. The method was made simple and efficient on the assumption that intensity signals are generated dominantly by a single wave component. The UAV's observational flexibility can mitigate this shortcoming, as we can adjust the camera settings to strengthen the dominant swell signals. Further tests will be needed to clarify how the method's performance changes with the observation settings and environmental conditions.  
Conceptualization, T.S. and K.H.; methodology, K.H., Y.M. and T.S.; formal analysis, K.H.; Field data acquisition, K.H., Y.M., T.S. and T.O.; writing--original draft preparation, K.H.; writing--review and editing, K.H. and T.S.; project administration, T.O.; funding acquisition, T.O. and T.S. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Grant for R&D on River and Sabo Engineering Technologies from the Ministry of Land, Infrastructure, Transport and Tourism, Japan.  
Wave observation data at Omaezaki Station are available from the website of the Nationwide Ocean Wave information network for Ports and Harbors, Japan ([[https://nowphas.mit.go.jp/eng/](https://nowphas.mit.go.jp/eng/)]([https://nowphas.mit.go.jp/eng/](https://nowphas.mit.go.jp/eng/))). Tide observation data at Omaezaki Station are available from the website of Japan Meteorological Agency ([[http://www.data.jma.go.jp/gmd/kaiyou/db/tide/genbo/index.php](http://www.data.jma.go.jp/gmd/kaiyou/db/tide/genbo/index.php)]([http://www.data.jma.go.jp/gmd/kaiyou/db/tide/genbo/index.php](http://www.data.jma.go.jp/gmd/kaiyou/db/tide/genbo/index.php))).  
The authors appreciate the support of [PERSON] of the University of Tokyo for the field data acquisition.  
The authors declare no conflict of interest.