Beach-Foredune Sediment Budget Response to Sea Level Fluctuation
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water Article Beach-Foredune Sediment Budget Response to Sea Level Fluctuation. Curonian Spit, Lithuania Darius Jarmalaviˇcius*, Donatas Pupienis, Gintautas Žilinskas, Rasa Janušaite˙ and Viktoras Karaliunas¯ Laboratory of Geoenvironmental Research, Nature Research Centre, Akademijos 2, LT-08412 Vilnius, Lithuania; [email protected] (D.P.); [email protected] (G.Ž.); [email protected] (R.J.); [email protected] (V.K.) * Correspondence: [email protected] Received: 14 January 2020; Accepted: 17 February 2020; Published: 20 February 2020 Abstract: Beach-foredune sediment exchange maintains a coastal system’s stability. Sea level fluctuation is one of the most important factors that modifies the beach and foredune sediment budget. This study aims to assess beach and foredune sand budget changes depending on sea level fluctuations. On the basis of annual measurements of cross-shore profiles on the Curonian Spit in Lithuania, the sediment volumes on the beach and foredune and their changes between 2002 and 2019 were calculated. The sea level fluctuations were examined in parallel. The obtained data revealed that in the case of a sand surplus, a relatively low sea level rise does not have a significant impact on the development of a foredune (and a minimal impact on a beach) on a decadal time-scale. Short-term sea level fluctuations are reflected in year-to-year variability in a beach sediment budget. However, no significant relationship between year-to-year variability in sea level fluctuation and the foredune sediment budget has yet been identified, nor is there a reliable year-to-year variability relationship between the foredune and beach sediment budget. The foredune sediment budget remained positive both through an increase and a reduction in the sediment volume on the beach. Keywords: sea level; coastal erosion; coastal morphometry; Baltic Sea 1. Introduction The global mean sea level has been rising during the past century [1]. Based on satellite data, the rate of sea level rising has peaked at 3.36 mm/yr since 1993 [2]. The sea level trend in the Baltic Sea is similar to the global mean sea level trend [3]. Global climate change and sea level rise are often related to increasing coastal erosion and associated socio-economic transformations in coastal regions [4,5]. One of the most important problems in predicting future coastal evolution is determining the impact of a potential rise in sea level on coastal systems. However, there are other significant factors that also influence coastal dynamics including the geologic framework [6,7] and sand availability [8–13]. It is worth noting that coastal changes on short-term and decadal time scales are relevant to coastal managers and planners [14]. Within a decadal time period, there are other factors besides sea level fluctuation and geologic framework: extreme events [15–19], vegetation cover [20–23], human activity [24–26], and river discharge [27]. Since there is a complex interrelationship between sea level change and other factors on a short-term time scale, it is difficult to separate the direct impact of sea level on coastal systems from other factors. Depending on the interaction between various factors, accumulation processes may predominate on the coast despite the rising sea level [9,13,17,28,29]. Due to different driving forces, there can be high uncertainties related to the prediction of sea level rise impact on coastal systems [30]. The sand spits with predominant alongshore sediment transport are good examples for illustrating erosion and Water 2020, 12, 583; doi:10.3390/w12020583 www.mdpi.com/journal/water Water 2020, 12, 583 2 of 12 Water 2020, 12, x FOR PEER REVIEW 2 of 12 accretion processes along the shore [31–33]. Similar shoreline dynamic processes can be observed at illustrating erosion and accretion processes along the shore [31–33]. Similar shoreline dynamic the Curonian Spit where at the base of the spit erosion dominates, whereas on the distal end of the processes can be observed at the Curonian Spit where at the base of the spit erosion dominates, spitwhereas accretion on dominatesthe distal end [34 of,35 the]. Duespit accretion to the reduction dominates of sediment[34,35]. Due supply to the inreduction the southern of sediment part of the spitsupply and sea in levelthe southern rise, there part is intensificationof the spit and ofsea erosion level rise [36, ].there Alongshore is intensification sediment of transport erosion [36]. directed northwardAlongshore creates sediment favorable transport conditions directed for northward accumulation creates near favorable the distal conditions part of thefor spit.accumulation Thus, a huge sandnear mass the isdistal involved part of in the the spit. formation Thus, a of huge coastal sand landforms mass is involved [37]. The in development the formation ofof foredunes coastal is closelylandforms linked [37]. to beach The development evolution and of foredunes the formation is closely of a linked beach-foredune to beach evolution system and [38– the40]. formation Despite close interactionof a beach between-foredune beach system and [38 foredune,–40]. Despite these close two interaction components between of the beach system and do foredune not necessarily, these two evolve in onecomponents direction. of Forthe system example, do thenot necessarily foredune sand evolve budget in one may direction. be positive For example, while thethe beachforedune sand sand budget is inbudget equilibrium may be orpositive slightly while negative the beach [29 ,sand39,41 budget–43]. is in equilibrium or slightly negative [29,39,41– 43].Since 2002, annual changes in the beach and foredune sand volume have been monitored along the Since 2002, annual changes in the beach and foredune sand volume have been monitored along Lithuanian part of the Curonian Spit Baltic Sea coast. Between 2002 and 2019, the coastal dynamics in the Lithuanian part of the Curonian Spit Baltic Sea coast. Between 2002 and 2019, the coastal dynamics coastal systems with different geomorphologies were determined based on the obtained data. The aim in coastal systems with different geomorphologies were determined based on the obtained data. The of this paper is to evaluate the interrelationship between the beach and foredune sand budget, and aim of this paper is to evaluate the interrelationship between the beach and foredune sand budget, theand influence the influence of annual of annual mean mean sea level sea level fluctuation fluctuation on the on Balticthe Baltic Sea Sea coast coast in thein the Lithuanian Lithuanian part part of the Curonianof the Curonian Spit on aSpit decadal on a decadal time scale. time scale. 2. Study2. Study Area Area OfO thef the Lithuanian Lithuanian part part of of thethe CuronianCuronian Spit Spit (51 (51 km km long) long),, the the foredune foredune ridge ridge occupies occupies an area an of area of aboutabout 262.3 262.3 ha ha (Figure (Figure1 ).1). A A continuouscontinuous foredune foredune ridge ridge along along the theentire entire Curonian Curonian Spit was Spit formed was formed in in thethe second half half of of the the 19th 19th century. century. The The formation formation of the of foredune the foredune was started was started along the along shore the in shore in orderorder to to protect protect the the coastal coastal settlements settlements from fromsand being sand blow beingn and blown the andcoast thefrom coast wave from action. wave Semi action.- Semi-permeablepermeable sand sand fences fences were wereconstructed constructed along the along shore the between shore between 1810–1892 1810–1892 [41]. [41]. Figure 1. Location map. The black lines and numbers represent cross-shore profiles (Google Earth, earth.google.com/web/.). Water 2020, 12, x FOR PEER REVIEW 3 of 12 Figure 1. Location map. The black lines and numbers represent cross-shore profiles (Google Earth, Water 2020, 12, 583 3 of 12 earth.google.com/web/.). When the ridge of accumulated sand reached 3 m in height it was planted with marram grass. Later on, the ridge developed without human intervention,intervention, except for being reinforced with tree branches in some places. Therefore Therefore,, human activity activity has has no crucial effect effect on foredune de development.velopment. The height of thethe foreduneforedune rangesranges fromfrom 66 mm atat JuodkrantJuodkrantėe˙ toto 1616 mm atat AlksnynAlksnynėe˙ (Figure(Figure2 ).2). StrikingStriking didifferencesfferences have alsoalso occurredoccurred inin thethe foreduneforedune volume.volume. The largest foredune is at SmiltynSmiltynėe˙ (up to 2200 m3/m)/m) and the smallest is atat JuodkrantJuodkrantėe˙ (110(110 mm33//m).m). The foredunes are densely covered with marram grass. The The widths widths of of the the beaches beaches vary vary from from up up to to 65 65 m m at at Smiltynė Smiltyne˙ to to 30 30 m at Juodkrantė. Juodkrante.˙ The volumevolume of of beach beach sediment sediment ranges ranges between between 124 m 1243/m atm the3/m Smiltynat the e˙Smiltynė and 42 m 3and/m at42 the m Juodkrant3/m at thee.˙ TheJuodkrantė. Curonian The Spit Curonian is formed Spit exclusively is formed ofexclusively Quaternary of deposits.Quaternary The deposits. upper part The ofupper the Quaternary part of the depositsQuaternary in thedeposits Curonian in the Spit Curonian is composed Spit is of composed sediments of that sediments have been that formed have been in the formed basins ofin the variousbasins of stages the various of the Balticstages Sea’sof the development—starting Baltic Sea’s development from—starting the Baltic from Ice the Lake Baltic and Ic endinge Lakewith and recentending marine with recent sediments marine [34 sediments]. The beach [34]. of theThe Curonian beach of Spitthe Curonian is composed Spit of is finecomposed and medium of fine sand. and Meanmedium sand sand. grain Mean size sand ranged grain from size 0.2 ranged mm at from Smiltyn 0.2 e˙mm to 0.5 at Smiltynė mm at Juodkrant to 0.5 mme.˙ at Juodkrantė.