A wide tidal flat is located in Nakatsu, Kyushu, and faces the Suo-nada Sea. The Maite River flows into this tidal flat and the Oshinden coast extends west of the river mouth. In the past, on the Oshinden coast, sand was transported by eastward longshore sand transport generated by waves incident from the NW direction, and then sand deposited at the river mouth was flushed away during floods. With this, the sand dune next to the river mouth was severely eroded. The sandy beach next to the river mouth has been a spawning ground for horseshoe crab, and its hinterland was used as a habitat for endangered species, requiring the protection of the area. Since 2016, therefore, a training jetty was extended at the mouth to prevent sand transport into the river mouth and to protect the sandy beach next to the river mouth. As a result of this improvement, the sandy beach has recovered immediately west of the jetty. In this study, field observation was carried out at this river mouth to monitor the effect of the measure in April 2024. Longitudinal profiles across the foreshore were measured using an RTK-GNSS and seabed material was sampled. A UAV survey of the tidal flat near the river mouth was also carried out. Finally, we proposed a method to naturally form a spawning ground of horseshoe crab, a sandy beach, and coastal vegetation through the deposition of sand originating from tidal flat.