Site assessment and evaluation of the structural damages after the flood disaster in the Western Black Sea Basin on August 11, 2021

On August 11, 2021, one of the most destructive flood disasters occurred in the Western Black Sea Basin of Turkey. The flood resulted in the death of 76 individuals, 30,000 people being affected by the disaster. A maximum precipitation depth of 400 mm/day was recorded at one station, indicating a return period exceeding 500 years for the rainfall event. During a two-day site visit immediately following the flooding event, damages to infrastructures, water structures, bridges, retaining walls, roads, and private houses were observed in the Bozkurt and Ayancık regions. Based on the observations, the flood wave propagated through the initial meandering river bed and floodplain, exceeding the channelized river bed capacity. Due to the massive sediment transport and drifting of trees, several bridges have been blocked and overflown where the basements of the structures in these regions were flooded. The enormous flood flow triggered extensive scouring on bridge piers, building foundations, and retaining walls, eventually causing the walls and bridges to collapse. The collapse of structures blocked the waterway and amplified the backwater effect when combined with the sediment transport. The total collapse of the retaining walls in some sections of the stream caused accelerated scouring in the foundations of the nearby buildings. Damages were also observed on the side roads along the river beds. This paper evaluated the driving mechanism of damages caused by flood flow from hydrological, structural, and geotechnical perspectives. Based on these observations and assessments, recommendations on engineering design guidelines for structures close to the floodplain, such as bridges, retaining walls, and side roads, were elaborated. Emphasis was placed on the flood-resistant design of these structures to develop a comprehensive approach for flood risk management.