Structure-based flood damage mapping for Ngan Truoi downstream housing risk assessment, Vietnam

Flood damage to residential buildings is strongly influenced by both hydraulic conditions and structural characteristics, yet most flood risk assessments in Vietnam rely primarily on inundation depth and overlook hydrodynamic effects. This study develops a structure-based flood damage assessment framework for the downstream area of the Ngan Truoi reservoir by integrating coupled 1D–2D hydraulic modelling with analytical- mechanical derived fragility curves that account for both flood depth and flow velocity. Three dominant housing types, including reinforced concrete, masonry, and temporary structures, are analysed under design flood and dam-break scenarios, including slow-rise and rapid-rise flooding mechanisms. The results reveal that communes along the main river corridors and confluence zones, particularly Lam Thanh, Thien Nhan, Vu Quang, Thuong Duc, and Duc Minh, experience the highest damage levels due to deep, high-velocity flows. Temporary structures are identified as the most vulnerable building type, showing widespread collapse even under design flood conditions. Masonry (brick) buildings exhibit moderate resilience but suffer severe damage in high-momentum flows, while reinforced concrete buildings perform best overall, remaining largely intact under slow-rise depth but experiencing collapse in localized corridors during rapid-rise dam-break scenarios. The resulting building-scale damage maps provide critical insights for spatial planning, flood risk mitigation, and housing resilience strategies in reservoir downstream regions. • Integrated Assessment Framework: The study develops a structure-based flood damage assessment framework for the Ngan Truoi reservoir downstream area, moving beyond traditional depth-only models to include hydrodynamic effects by integrating coupled 1D–2D hydraulic modeling with analytically derived fragility curves. • Bivariate Analysis: Unlike previous studies in Vietnam that rely on depth alone, this research utilizes the product of flood depth and flow velocity (vxd) as the primary intensity measure to account for both hydrostatic and hydrodynamic pressures on buildings. • Scenario Comparison: The analysis evaluates three dominant housing types (Reinforced concrete, masonry, and temporary) under "Design flood" (spillway operation) and "Dam break" scenarios, distinguishing between slow-rise and rapid-rise flooding mechanisms to capture different structural loading conditions.