Tracing the culprits over the destruction of three coastal communities during a rouge rainstorm on August 2021 in north central Türkiye

Abstract

Extreme floods resulting from one of the heaviest ever documented rain storms over several urbanized coastal sub-districts in north-central Türkiye in August 2021, caused severe destruction, considerable property damage and tens of casualties. Peak discharges following the event were mostly greater than the projected 50-year and 100-year ones. This study aimed to examine the causes and consequences of this catastrophic event that occurred in the sub-districts of Bozkurt and Catalzeytin within Kastamonu, and Ayancik within Sinop provinces. Real-time 24-h precipitation follow-up by two neighboring Doppler Radar stations disclosed the spatial distribution of the phenomenon as it unfolded. Relative damages that occurred within each district following the event were evaluated, taking into account the meteorological, topographical and hydrological variables. Steadily increasing precipitation from 0 mm/h at midnight on August 11th to 35–40 mm/h around 4 p.m. in all sub-districts, kept on falling without slowing down until the following midnight, even further increasing to 45–47 mm/h in Bozkurt. Heavily forested feeder basins, 91.4% in Ayancik, 92.2% in Bozkurt and 88.2% in Catalzeytin were overwhelmed by the amount of precipitation, and quickly saturated surfaces allowed the excessive runoff to reach the sub-districts down the main channel. Tracing the entire phenomenon as it progressed over the sub-districts showed how site selection and unfavorable topography worsened the outcome. Topographic indices calculation showed that the main channels and their immediate surroundings in all sub-districts were the confirmed no-go zones for urban developments. The events highlighted several lessons for city and flood managers regarding flood planning, mitigation, response and recovery.