The Impact of Long-Term Flood Forecasting on Waterfront Utilization and Stakeholder Decision-Making - A Case Study of the Oto River in Okazaki City, Japan

In recent years, with improvements in weather forecasting technology, the development of long-term flood forecasting has advanced globally. This technology is expected to mitigate damage from large-scale floods, which occur infrequently but cause immense damage. However, it is also anticipated to be highly effective against frequent high-water events that occur routinely. This is particularly relevant for urban rivers, where frequent high-water limits the utilization of waterfront areas. Therefore, this study aims to expand the scope of long-term flood forecasting to address these high-frequency events, using the Oto River in Okazaki City, Aichi Prefecture, Japan, as a case study. We analyzed the impact of long-term flood forecast information on the decision-making of waterfront stakeholders through group interviews and workshops with 28 participants, including riverside business owners, municipal river managers, and academic experts.

The findings revealed that the level of demand for long-term flood forecasts varies significantly depending on the type of riverside use. For use that contains many physical installations or hardware, such as urban furniture and temporary structures, the evacuation process requires significant physical effort and time. Therefore, a high accuracy forecast with a lead time of 24 hours or more is essential, as it ensures a safe evacuation timeframe while avoiding unnecessary evacuations due to false alarms. Conversely, for “soft operations” like event hosting or rental businesses, a shorter lead time of 12 to 18 hours was shown to be an ample amount of time to determine event feasibility the day before and notify customers, allowing continued operations while controlling business risk.

A notable finding was that, regardless of whether the usage style was physical installations or soft operation based, when prediction accuracy exceeded 40-60%, users became more willing to accept risk, and the number of waterfront usage ideas increased dramatically. Furthermore, private businesses demonstrated a flexible stance, accepting false alarms in forecasts as an “insurance” cost for business continuity. With this approach, the construction of physical installations, which previously have been impossible due to high risks and strict standards, can broaden the types of businesses that can operate on the riverside, realizing a future urban landscape where permanent installations are standard. Based on these findings, it can be concluded that implementing long-term flood forecasting has the potential to significantly enhance the value of river spaces in daily life, extending beyond providing disaster prevention information for evacuation actions. By presenting appropriate lead times and accuracy levels, it suggests the potential to foster a new urban culture that coexists with waterfronts while accounting for flood risks, ultimately creating more diverse and resilient riverside urban spaces.