Sedimentary record of historical extreme storm surges on the Gulf of Gdańsk coast, Baltic Sea

Abstract

The Baltic Sea is commonly viewed as a region with a low frequency of coastal hazards such as tsunamis or extreme storm surges. However, historical sources indicate that in the past, several catastrophic storm surges resulted in coastal floods and related casualties. Their sedimentological records and reconstructions of paleostorminess were so far studied mainly in the westernmost (Denmark) and easternmost (Estonia) parts of the Baltic Sea. The present study focus on southern Baltic coast (Gulf of Gdańsk), where storm surge sedimentary deposits left within coastal peatlands were investigated, and related to historical record in order to compare recent risk estimations and predictions of storm surge levels to geological data. To achieve this aim, the collected sediment cores were subjected to sedimentological, grain size, and diatom analyses, supported with ¹³⁷Cs, ²¹⁰Pb, and ¹⁴C datings. Two types of sandy event deposits were identified, and interpreted to be storm deposits formed under an inundation regime and overwash regime, respectively. The deposits formed under the inundation regime were characterised by an erosional lower boundary, massive structure and presence of rip-up clasts of underlying sediments, whereas the sediments formed under the overwash regime were low-angle planar cross-stratified medium sands with intercalations of massive fine sand. The recognized sandy storm deposits were dated mainly to the Little Ice Age period (XVIth–XIXth centuries). They were most likely left during the historical storms of 1825, 1872, and 1914 CE. It extends the sedimentary record of known historical events as mainly storms from 1497 CE and 1872 CE were reported previously from Baltic Sea. Notably, none of the historical storm events during the past 100-year period have formed a distinguishable sedimentary record. These findings clearly show that risk models and storm return period predictions, which are based on instrumental measurements from the last century, should also include geological evidence. The strongest storm surges from the past century may not be adequate reference of a worst case scenario.