Effective excavation for restoring the former watercourses of a river where the watercourses are established (#123)
In recent years, the area of gravel dry riverbeds has been rapidly decreasing because of the invasion by riparian forests on the Satsunai River, a tributary of the Tokachi River in Hokkaido, Japan. The braided streams have concentrated into one main watercourse and many of the branched streams have already disappeared though braided streams with double-row bars had been the typical bed configuration of the river before. As part of a project for the restoration of gravel dry beds on the Satsunai River, artificial floods from Satsunai Dam has been implemented since 2012. In the project, the entrance of former branched watercourses has been additionally excavated in the attempt to restore and maintain the passages from the main watercourse with the purpose of enhancing the watercourse mobility and disturbing the bars already fixed due to the disappearance of branched streams. In this study, onsite surveys and theoretical analysis were conducted to clarify the characteristics of bifurcation flows to the branched watercourses related to double-row bars, and to make selecting the optimum excavation locations for effectively restoring former watercourses already closed by sediment deposition at their entrance. It was found to clearly identify riffles and pools related to the double-row bar formation in the longitudinal profile of the main watercourse in the Satsunai River. Excavation at locations slightly downstream of a pool, where a riffle starts to emerge, were found to be effective to restore the branched flows seen in the braided streams as before.