Reservoir-Release and Dam-Breach Flood Hazard to Highway Infrastructure in Central Vietnam: Integrated HEC-HMS and HEC-RAS Modeling and LULC Exposure Assessment

Reservoir releases and hypothetical dam-breach scenarios can intensify flood hazard to downstream transportation infrastructure in data-scarce mountainous basins. This study assesses the Khe Ngang Reservoir-Tu Ca Bridge corridor in Central Vietnam using an integrated HEC-HMS and HEC-RAS framework, contemporary land-use/land-cover exposure analysis, and a separate Sentinel-1 surface-change assessment. HEC-HMS parameters calibrated and validated in the nearby Binh Dien donor basin (NSE = 0.94 and 0.90) were transferred to the ungauged Khe Ngang watershed; therefore, the resulting November 1999 inflow is treated as regionalized forcing rather than a locally validated hydrograph. Three deterministic hydraulic scenarios were compared: no breach (S1), a broad hypothetical breach (S2), and a narrower hypothetical breach (S3). Inundation increased from 9.753 km2 in S1 to 10.599 km2 in S2, while maximum depth at Tu Ca Bridge increased from 4.332 to 5.800 m. A targeted mesh check for S3 compared the original 100 m grid with a 30 m grid along a fixed 37.97 m profile near Tu Ca Bridge. Profile maximum depth changed from 5.723 to 5.998 m (+4.81%), whereas maximum velocity changed from 1.502 to 1.031 m/s (−31.36%), indicating greater mesh sensitivity of local velocity. Contemporary Sentinel-2 exposure and a separate 2025 Sentinel-1 surface-change analysis provide present-day spatial context, not validation of the 1999 simulation. The results are consequently interpreted as conditional hazard indicators rather than infrastructure-design values.