Geohazards and coastal dynamic: a geo-engineering assessment of the southern Iraqi shore (Ras al-Bisha zone)

The coastal zone of Ras al-Bisha, located between the mouth of the Shatt al-Arab River and the eastern breakwater of the Grand Faw Port, exhibits complex interactions of tidal forces, sediment transport, and anthropogenic modifications. This study develops an engineering geological framework to assess sediment stability and geohazard potential under semi-diurnal tidal conditions. Field measurements, including in situ vane shear tests at 41 stations, were used to determine undrained shear strength and derive critical shear stress for surface sediments. Hydrological data provided ebb and flood current velocities and water levels. A dual factor of safety (FS) approach was introduced to evaluate sediment stability separately for ebb and flood tides, producing spatially explicit maps of stable (FS > 1.5), critical (1.0 ≤ FS ≤ 1.5), and unstable (FS < 1.0) zones. The results reveal an inland-to-seaward gradient in sediment strength and resistance, with very soft to soft sediments dominating the nearshore environment. Flood tides generate higher applied shear stresses than ebb tides, leading to expanded unstable zones along the shoreline front. Erosion rate analyses confirm greater sediment displacement during flood conditions, while ebb tides partially mitigate instability. The dual-FS hazard maps offer a refined way for prioritizing monitoring and mitigation efforts, directly informing coastal management and infrastructure planning in estuarine settings affected by bidirectional tidal dynamics.