Rapidly Transforming Holocene Strandplain, Affected by Port Jetty and Hydroclimatic Shifts – Natural Laboratory of Past and Future Shoreline Behavior

Tõnisson, H.; Luik, K.; Suursaar, Ü.; Buynevich, I.; Kont, A.; Männikus, R.; Rivis, R.; Suuroja, S., and Vilumaa, K., 2024. Rapidly transforming Holocene strandplain, affected by port jetty and hydroclimatic shifts – Natural laboratory of past and future shoreline behavior. In: Phillips, M.R.; Al-Naemi, S., and Duarte, C.M. (eds.), Coastlines under Global Change: Proceedings from the International Coastal Symposium (ICS) 2024 (Doha, Qatar). Journal of Coastal Research, Special Issue No. 113, pp. 715-719. Charlotte (North Carolina), ISSN 0749-0208. Coastal strandplains that have been dominated by progradation for most of the Holocene, may be entering into a new development phase of erosion due to sea-level rise, changes in climate-related hydrodynamic conditions, or anthropogenically induced sediment deficit. During the past several millennia, on the postglacially uplifting (>3 mm/yr), functionally tideless Baltic Sea coast, scores of shore-parallel sandy ridges have formed at the Lehtma study site on the NE-facing coast of the Tahkuna Peninsula, Hiiumaa Island, Estonia. Both uplift-driven emergence and abundant sediment supply from marine and glacial depocenters, have contributed to the progradation of the strandplain. However, the Lehtma port jetty constructed on the NE tip of the Tahkuna Peninsula in the 1950s and reinforced in past decades, severely hampered the southerly longshore sediment flux. In addition, recent increase in air temperature (by 2.0–2.5°C on Estonian coasts over the past ∼70 years) have significantly reduced the duration of the sea-ice season (when the sea and sediments are frozen) thereby increasing exposure to northerly winter storms. Cartographic analysis and repeated coastal surveys revealed that the shoreline 2–3 km south of the Lehtma port has receded ∼70 m since 1955, while underwater sandbars have migrated up to 170 m landward. Total loss of sand is estimated at ∼165,000 m3 (2,500 m3/yr), including the nearshore. Shoreline orientation has also changed, which in turn, has favored a rapid (∼25 m/yr) elongation of a sandy spit, which is currently ∼1600 m long. Along the eroding strandplain margin, heavy-mineral concentration (HMCs) provide evidence of elevated energy regime. At the Lehtma study site, high magnetic susceptibility anomalies and thick HMC layers may indicate long-term shoreline erosion (instead of individual storms). By comparing these trends with the recent findings from the Järve site, Saaremaa Island, our research documents an intricate interplay of hydroclimatic shifts, anthropogenically induced erosion, and progradation-driven changes in coastal morphodynamics.