Sandy beach evolution in the low-energy microtidal Baltic Sea: Attribution of changes to hydrometeorological forcing

Abstract

We analyse the response of small embayed sandy beaches in marginal seas to various hydrometeorological drivers to isolate a specific mechanism that may stabilise beaches under bi-directional wind patterns. The focus is on the evolution of two sandy beaches (Pirita and Russalka) in Tallinn Bay, northeastern Baltic Sea, that are about 3 km apart and oriented at an angle of 35° with respect to each other. A large proportion of annual wave energy flux is packed into a very few days in the year. The wind direction, and thereby the direction of the wave fields, of the strongest storms in the year determines which coastal areas are most affected. We quantify the response of the beaches to hydrometeorological conditions over 12 years, 2008–2020 based on annual high-resolution measurements using airborne and terrestrial laser scanning technology. The fastest sediment removal from the subaerial beach occurs during time periods with elevated water level and strong wave energy flux. Even though annual variations in the sand volume of the subaerial beach are significant, both beaches are stable during the decade studied but respond differently. We outline specific mechanisms that maintain the beaches, best illustrated at Pirita beach where there is strong longshore sediment transport component driven by waves that often arrive the beach at a large angle. Sand that is removed from the higher subaerial segments of the northern part of Pirita beach and moved to the south by north-west storms during periods of elevated water levels, is deposited in the shallow nearshore (above about −0.5 m of the mean sea level). This sediment is then able to be transported back to the north during periods of lower water levels where even relatively small waves generated by moderate south-westerly winds can entrain and move the sediment. This mechanism, different from the classic cut and fill cycle, may implicitly stabilise beaches where there is a bi-directional wind regime and a small tidal range. Another, less explicit cross-shore mechanism, is found for the differently oriented Russalka beach where waves usually approach at a small angle with respect to the shore normal. This beach is maintained by a synchronisation of intense waves and water level that limits the depth of deposited sand with respect to mean sea level. The two mechanisms are thus fairly similar, however Pirita beach has extensive alongshore excursions of sand.