Critical relative channel width to ensure reservoir sustainability in flood retention dams: Insights from a nationwide inventory in Japan.

Flood retention dam (FRD) reservoirs are usually dry, minimizing sediment deposition through fluvial flushing and thereby keeping maintenance costs low. However, when the Relative Channel Width (RCW)-the ratio of the flushing channel width to the reservoir bottom width-is extremely small, the flushing process becomes ineffective, leading to substantial sediment accumulation and costly maintenance. Nevertheless, the relationship between RCW and sedimentation in FRD reservoirs has largely remained qualitative. This study quantifies the relationship between RCW and reservoir sedimentation status to identify the critical RCW level at which mechanical sediment removal becomes necessary for long-term reservoir sustainability. Using a nationwide dataset of 24 FRDs in Japan, we compiled information on reservoir sedimentation status and statistically examined its relationship with RCW. As a result, reservoirs with RCW <0.20 experienced pronounced fine-sediment accumulation and substantial loss of storage capacity (mean > 7%), with the capacity loss continuing to increase after major flood events. Based on this relationship, we propose RCW = 0.20 as a critical threshold for long-term reservoir sustainability. Furthermore, we demonstrated that flushing channel width can be estimated from the flushing discharge, which is calculated from the channel slope and bottom-outlet dimensions. Using this relationship, RCW can be estimated immediately once the dam construction site and the bottom-outlet dimensions are determined. This finding can thereby support preliminary decision-making by project authorities at the early stages of FRD planning.