In recent years, citizen groups have attracted attention as public service providers as they collaborate with river administrators for river management. However, as time has passed since their establishment, some citizen groups are facing various difficulties in continuing their activities. For this study, we administered a questionnaire survey about the active mass, which evaluates their activities quantitatively, and factors for activity continuation, which were extracted from earlier studies of citizen groups related to rivers. Stepwise multiple regression analysis and correlation analysis were applied to the responses of 64 citizen groups to estimate the factors affecting the active mass variation.Partial correlation network analysis was conducted to estimate the relations among the factors, based on the assumption that a certain relation exists between activity continuation and the variation of the active mass of citizen groups. The factors were discussed using results of these analyses.

Land use regulation is a fundamental measure of flood mitigation and climate change adaptation for reducing the amount of exposure in floodplains. In Japan, land use regulation for flood mitigation has been considered in the classification area under the City Planning Act and the Location Normalization Plan under the Act on Special Measures Concerning Urban Reconstruction. However, mitigation effects and operational challenges have not been fully elucidated. In this study, we analyzed the characteristics of flood damage in the urbanization control area in Naganuma District of Nagano City, which was severely damaged by the Chikuma River levee break triggered by Typhoon Hagibis in 2019. The analysis, which adopted building point data, determined that land use regulation by area classification played a role in reducing damage in the area. However, at the community level, it was confirmed that more buildings were located in areas with high flood hazards owing to the development of new houses and businesses associated with the development of infrastructure.