Forestry Agency Deploys USV Sonar to Survey Wanli Creek Barrier Lake, Revising Volume to 4.52 Million m³

To monitor the Wanli Creek barrier lake in Hualien, the Forestry and Nature Conservation Agency's Hualien Branch conducted its first underwater bathymetric survey using an unmanned surface vessel equipped with sonar. After equipment was delivered to the lake area by airborne helicopter, operators remotely controlled the vessel to explore underwater topography. The survey confirmed that due to right-bank slope failures and upstream sediment accumulation, water capacity decreased from an initial estimate of approximately 5.2 million cubic meters to 4.52 million cubic meters. The branch director explained that depth was interpreted through color gradient, noting the dam remains intact with stable overflow. Addressing local community appeals regarding riverbed red alerts during continuous overflow, the branch stated that the bathymetric survey will be used to calculate downstream runoff impacts during the non-flood season and carefully assess alert adjustments, while emphasizing alerts will resume during heavy rain or typhoons.

Key takeaways

• The Hualien Branch transported a USV via helicopter to Wanli Creek barrier lake, conducting its first sonar survey and revising capacity to 4.52 million cubic meters.
• Volume reduction stems from continuous right-bank slope collapse and upstream siltation, with the natural dam remaining intact and overflowing steadily.
• The agency will utilize the bathymetric findings to estimate downstream impact and evaluate the feasibility of lifting riverbed red alerts during the non-flood season.