Critical storm thresholds for morphological changes in the western Black Sea coastal zone

摘要

Storms are one of the phenomena determining the short-term evolution of coasts by influencing the erosion and accretion patterns of the beaches. Some severe storms could cause loss of beach sediments, partial or complete destruction of coastal structures and even threaten human life and occupation. Bearing this in mind, it is essential to comprehend, which storms could evoke transient, although in some instances, significant morphological changes, and which could be considered as hazardous and result in catastrophic damage. This issue implies establishment of a set of critical thresholds for storm impact on coastal morphology. The paper presents a regional scale analysis of the near-shore morphological response of five beaches to storms of varying intensity. The analysis is accomplished by coupling of offshore wave and cross-shore profile data obtained for the Bulgarian Black sea coast. The historical storm pattern is reconstructed through hindcast using a coupled wave model system, while resulting morphological changes, such as shoreline alteration and cross-shore profile evolution, are estimated by analysis of long-term series of coastal measurements performed in 1970-1977 and 2008-2010. The storm impact thresholds are set in terms of integral wave energy taking into account both storm pattern and duration. It was found that the morphological response to storm impact is site specific and single-value thresholds are difficult to be established even at regional scale. Nevertheless, a range of critical storm thresholds is proposed. Thus, storms with integral wave energy varying within threshold values 0.4-0.7 × 10~6 J m~(-2) are regarded as capable of significant morphological changes, while less energetic events are supposed to influence predominately the seasonal beach dynamics. Furthermore, storms with integral wave energy higher than 0.7 × 10~6 J m~(-2) represent potentially destructive events that can change irreversibly the beach pattern or damage impact.