Scenario-Based 2D Hydrodynamic Modelling of Dam-Breach Flood Hazard and Risk under an Extreme Storm

Dam breach flooding is low probability yet high consequence hazard particularly in arid and semi-arid regions where urban and rural population is deployed along the ephemeral streams. The study analyses the flood hazard and flood risk assessment of the Wadi Baysh Dam in southwestern part of Saudi Arabia, based on a dam breach analysis as a result of an observed extreme rainfall event happened in July 2011. The target is to evaluate the flood impacts on the downstream under various breach scenarios and to support risk informed mitigation and emergency planning.

Three dam breach scenarios were studied using HEC-RAS 2D hydrodynamic model simulations: (i) full breach (S1), (ii) half breach (S2), and (iii) one-third breach (S3). The 2D simulations produced spatially distributed inundation depth, maximum flow velocity and inundation duration, which were later used to derive composite dam-breach flood hazard maps. By integrating composite flood hazard and landuse map as a vulnerability measure (including urban areas, agricultural land, roads, rangeland, bare ground and tree cover classes), risk maps of dam-breach floods were created and finally exposure analysis was conducted.

Results show strong non-linear relationship between breach severity and downstream impacts. Under full-breach conditions, the total inundated area reaches approximately 371 km², with large portions classified as moderate to high flood hazard. Agricultural land and rangeland exhibit the greatest exposure, while urban areas, although spatially limited, experience locally elevated hazard levels due to high flow depths and velocities. Partial-breach scenarios substantially reduce inundation extents (~278 km² for half breach and ~32 km² for one-third breach); however, hazardous conditions persist along the main wadi channel and low-lying floodplain zones, indicating that partial structural failure still poses significant downstream risk.

Composite flood risk assessment shows that low-to-moderate risk dominates across all scenarios, yet localized high-risk zones emerge near critical infrastructure and densely cultivated areas, particularly under full- and half-breach conditions. The results further demonstrate that reductions in breach size do not translate linearly into risk reduction, underscoring the importance of explicitly considering multiple breach scenarios in dam-safety assessments.

The study highlights the value of scenario-based 2D dam-breach modelling for flood-risk assessment in arid environments, where observed extreme storms can act as credible compound triggers. The proposed framework supports advances in flood-risk modelling by integrating hazard characterization, land-use exposure, and risk classification, providing actionable insights for emergency action plans, evacuation zoning, and long-term risk mitigation strategies.