Session1: River Engineering and Management
Title: DYNAMIC STATE OF RIVER-MOUTH BAR IN YURAGAWA RIVER AND APPLICATION OF DRONE PHOTOGRAPHY IMAGE TO OBTAIN ITS TOPOGRAPHIAL DATA
Hiroshi Miwa, Mio Omura, Takashi Wada, Yoshinori Kato, Keiichi Kanda
Topographic changes of the river-mouth bar in the Yuragawa River of Japan are continuously activated by sediment transport due to river flows and sea waves. A large part of the bar was eroded by the flood flow due to Typhoon No.23 in October 2004. The bar has formed on the right bank only since then. The size of the bar has gradually decreased as a long-term tendency, whereas the expansion of the bar due to transportation of drift sand into the estuary has been identified in recent years. These situations may cause problems such as washout of bank protection works, intrusion of drift sand into irrigation canals. In order to clarify the characteristics of the topographical changes of the bar and their causes, we need to analyze the temporal variations in geometrical properties of the bar on the basis of the topographical and hydrological data in the estuary. The topographical data have often been obtained by using a RTK-GNSS receiver, which can show high precision and accuracy. However, this method may have disadvantage for swift actions upon a disaster investigation. In recent years, an aerial photography image can be easily taken by a drone. We propose a method of introducing the images and a coordinate transformation into obtaining shoreline data, and then compare the obtained results with the RTKGNSS data. The DRMS values concerning with the coordinate values of GCPs were approximately a few meters in maximum. As for the area of the river-mouth bar, the estimation results based on the proposed method kept an error within a few percent, the large-scale variations in the area due to large flood could also be grasped.