New insights into coastal processes in the southern Baltic Sea: relevance to modelling and future scenarios

Abstract

Un der stand ing the re la tion ships be tween shore line changes and morpho-geo log i cal fea tures of the sea bed and hy dro dynam ics is im por tant for pre dict ing coastal dy nam ics. The south ern Bal tic bar rier coast is dis tin guished by rhyth mic shore line fea tures of dif fer ent scales, form ing an ero sion-ac cu mu la tion sys tem. In this study we ex plore the re la tion ships be tween shoreface-con nected, obliquely ori ented sand ridges, the dis tri bu tion of wave en ergy, and the im pact of waves on shore line changes. A 28.5 km stretch of the Pol ish coast, up to 2 km off shore, was in ves ti gated us ing a multibeam echosounder, a sub-bot tom profiler, side-scan so nar, and sed i ment cor ing. Anal y ses of dig i tal ter rain mod els, ae rial pho to graphs and maps from the late 19th cen tury show that the bound aries be tween ac cre tion and ero sion zones on the coast have shifted eastwards at a rate of ~10-11 m/year. Hy dro dy namic mod el ing shows that de pres sions be tween the ridges are "en ergy windows" through which higher en ergy waves reach the shore. The asym me try of the ridges and their ori en ta tion rel a tive to the pre vail ing wave di rec tion sug gest that they have been mov ing eastwards. Mea sure ments of the 137 Cs con tent in the sed iment cores show that the thick ness of the dy namic layer ex ceeds 1.5-2.0 m. When large-scale sand ridges mi grate, the "energy win dows" also mi grate with them, as does the en tire ero sion-ac cre tion sys tem. In con clu sion, a new type of al ter nat ing shore dy nam ics has been iden ti fied in the dissipative, multibar south ern Bal tic coastal zone. It is highly likely that the av er age shore line re treat mea sured on the scales of a few cen tu ries and tens of kilo metres may be smaller than we sus pect based on ob ser va tions made in re cent de cades.