Intensity of geodynamic processes in the Lithuanian part of the Curonian Spit

The paper considers conditions and intensity of aeolian and dune slope transformation processes occurring in the wind-blown sand strips of the dunes of the Curornian Spit. An assessment of the intensity of aeolian processes was made based on the analysis of climatic factors and in-situ observations. Transformations in aeolian relief forms were investigated based on the comparison of 15 geodetic measurements and measurements of aerial photographs. Changes in micro-terraces of dune slopes were investigated through comparison of the results of repeated levelling and measurements of aerial photographs. The periods of weak, medium and strong winds were distinguished, and sand moisture fluctuations affecting the beginning of aeolian processes were investigated. The wind-blown sand movements were found to start when sand moisture decreased by 2 % in the surface sand layer and by up 20 to 5 % at a depth of 10 cm. In 2004 – 2016, the wind-blown sand movements affected the size of reference deflation relief forms: scarp length by 8 %, scarp width by 35 %, pothole length by 80 %, pothole width by 80 %, roll length by 17 %, roll width by 18 %, hollow length by 17 %, and hollow width by 39 %. The elementary relief forms in the leeward eastern slopes of the dunes experienced the most intensive transformations. During a period of five months, the height of micro-terraces of the eastern slope of the 25 Parnidis Dune changed from 0.05 to 0.64 cm. The change was related with fluctuations in precipitation intensity: in July – August 2016 the amount of precipitation increased 1.6-fold compared with the multiannual average, thus causing the change in the position of terrace ledges by 21 %. References 24, figures 7, tables 3.

Southward transportation of sand formed a shallow littoral zone on the south-eastern coast of the Baltic Sea. In the zone, fine-and medium-grained sand was drifted against glacigenic hills and onto emerged sand, grain and pebble shallows of marine origin (Bitinas and Damušytė, 2004;Bitinas et al., 2005) (Fig.  40 1). Fig. 1. Cross-section in the northern part of the Curonian Spit (Vingiakopė environment) (by Bitinas et al., 2002;Badyukova et al., 2007).

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Favourable conditions for the formation of the spit and its dunes occurred before the start of the third stage of transgression of the Littorina Sea, when water level on the south-eastern coast was 6-7 m lower compared to the today's water level. With water level rising, the Semba Peninsula suffered intensive erosion and sediments were transported northwards. The germ of the spit formed near Šarkuva settlement (phase I); it reached Rasytė Island and grew further (phase II). 4-4.5 thousand years ago, the spit extended 50 up to Juodkrantė settlement (phase III), and 2 thousand years ago, it reached the continental coast (stage IV) (Gudelis, 1979(Gudelis, , 1998Kabailene 1967;Kunskas, 1970;Kliewe and Janke, 1982;Mojski, 1988;Müller, 2004;Starkel, 1977) (Fig. 2).
Wind-blown sand movement is the main geomorphological geodynamic process that currently occurs in the Curonian Spit. Since the 16th century, 14 villages have been entirely buried under the sand, 55 and dwellers of other villages had to move from one place to another to escape wind-blown sand (Fig. 3).
The first efforts to slow down wind-blown sand movement go back to the first half of the 19th century when dunes started to be overgrown with vegetation, which stabilized intensive movement of sand throughout the 19th century. The first scientific research into aeolodynamic processes in the Curonian Spit appeared in the second half of the 19th century (Berendt, 1869). Since the middle of the 20th century, 60 the Curonian Spit has become an object of comprehensive investigations by Lithuanian geographers who put their main focus on the morphogenesis of dunes and the mineral and granulometric composition of sand (Gaigalas, Pazdur, 2008;Gudelis, 1998;Mardosienė, 1988;Michaliukaitė, 1967;Minkevičius, 1982;Minkevičius et al., 1996). Though by the end of the 20th century only four segments of wind-blown dunes were left in the Curonian Spit, a dramatic increase in the number of visitors to the dunes not overgrown 65 with vegetation brought about significant relief transformations. At the initiative of the Curonian Spit National Park, since 2003 the monitoring of aeolian processes has been pursued in the Lithuanian part of the Curonian Spit, which enables identifying regularities of changes in aeolian deflation and accumulation processes (Morkūnaitė, Česnulevičius, 2005;Česnulevičius et al., 2016).  1911;Gudelis, 1998;Kabailiene, 1967;Kunskas, 1970). 75 Currently, specific tectonic subsidence phenomena related to deformations of marl accumulated in the freshwater basin are taking place south of Nida settlement (Fig. 4). Due to this reason, marl squeeze formed on the coast of the Curonian Lagoon. Before the Second World War, Vladas Viliamas described the phenomena and pointed out a rather wide distribution of the phenomena south of Juodkrantė 80 settlement (Viliamas, 1932). In 1985, marl squeeze was still observed south of Juodkrantė (Kabailienė, 1997). To date, marl can only be detected south of Nida. The pressure of a sand layer on the plastic marl layers causes deformations on the surface of dunes and gives rise to the formation of stepwise microterraces. Such geological-geomorphological formations can also be found in other areas of the Baltic Sea (Lampe et al., 2011;Sergeev et al. 2016).
alterations of climate components in the wind-blown areas of the Curonian Spit.

Methods
The intensity of aeolodynamic processes was assessed using repeated precise levellings in the reference strips of wind-blown dunes and analysing the digital topographical large-scale maps ( An analysis of aeolian microform variations (depressions, hummocks, gullies, ridges) was carried out in a wind-blown sand strip of Naglis and Vinkis dunes. Quantitative changes of microforms that occurred in the period of 1999-2016 were compared. The morphometric microform indices were obtained 110 by performing route measurements from natural benchmarks and using a GPS receiver and a ±1 mm accuracy distance measurer Leica Disto D510.
An assessment of dune surface changes was made based on meteorological factors: the beginning and end of strong wind periods and the moisture of the surface sand layer. An analysis of sand samples taken from the surface sand layer and at a depth of 0.15 m in windward and leeward dune slopes enabled 115 determining the beginning of local and massive wind-blown sand movements. Sand moisture and groundwater level in the dunes were estimated using georadar scanning data (Dobrotin et al., 2013).
To specify in greater detail the periodicity and intensity of wind-blown sand movements, an analysis of climatic parameters (wind speed, wind direction, air temperature, and precipitation) from meteorological stations of Klaipėda, Nida and Šilutėų for the period 1991-2015 was performed: In 120 addition, the data from a temporary meteorological station established on the seacoast of Nida for the summer of 2016 were used. The assessment of climatic parameters was necessary for the purpose of forecasting possible meteorological situations. Unfavourable weather conditions (wind, rain, snow) made a direct impact on the flight of an unmanned aerial vehicle and the quality of aerial photographs.
To measure the local wind regime in the coastal area of the Baltic Sea, a temporary mobile 125 meteorological station was established. It operated only in the summer of 2016. The data about wind speed, wind direction, air temperature, and precipitation taken by self-recorders of the station were compared with the data from the Nida meteorological station, and correlation indices were determined.
Occasional wind speed measurements were also taken by an unmanned aerial vehicle. A comparison of short-term and occasional measurements of wind speed and direction with the data of meteorological 130 stations allows stating that the wind regime recorded at the Klaipėda meteorological station was closest to the wind regime recorded in the dune ridge of the Curonian Spit. The correlation coefficient was equal to r = 0.988 for wind speed and r = 0.998 for wind direction.

3.Climatic factors 135
An analysis of thirty-year data of climatic factors from meteorological stations of Nida, Klaipėda, and Šilutė enabled distinguishing the most intensive periods of aeolian processes. In particular, three main factors that made a direct impact on aeolian processes, i.e. wind regime, precipitation, and air temperature, were analysed. 140 Compared with the remaining territory of Lithuania, the Curonian Spit distinguishes by a specific climate with mild and often snowless winters, frequent strong winds and storms, a longer period of abovezero temperatures. We described the climate of the northern part of the Curonian Spit based on the data from the Klaipėda meteorological station and the climate of the central part of the spit based on the data from Nida and Šilutė meteorological stations. 145 Air temperature was an important factor affecting aeolian processes. In the cold season of the year, aeolian processes were very slow and occurred only with storm winds. Due to low temperature, the surface of moist sand got frozen and the crust formed, which protected sand from being blown by wind.
The cold season of the year lasted from 18 (1991 -1992) to 114 (1995 -1996) days. Cold seasons were often accompanied by thaw periods when the frozen sand crust lost its stiffness and aeolian processes 150 revived. The thawing of the sand crust required longer (4 -16 days) periods of temperatures above zero (1 -4 Cº). Such situations were recorded in 1986-1987, 1989-1990, 1992-1993, 1999-2000, and 2008-2009. In 1991-1992 Wind was the main climatic factor affecting aeolian processes. Wind speed, direction and duration of blowing varied between different seasons of the year (Fig. 5). An analysis of multiannual weather parameters from the Nida meteorological station showed that the Curonian Lagoon made a great impact on wind dynamics. E-SE-S winds of the average speed of 5 -6 m/s prevailed in all years. 170 According to the data from the Nida station, the maximum speed of winds lasting three hours and longer  The storms influence on extreme values of sea and lagoon level when the water level rise higher than 100 cm. In end of 20 th century storms have been more frequent than before (Dailidienė et al., 2006, Jarmalavičius et al., 2014. Raised water level in Curonian Lagoon directly influence the foot of dunes. Dunes foot are washed and at the middle of the slope parts moves down. 195 The recorded differences in wind regime were due to the location of meteorological stations: Klaipėda and Šilutė meteorological stations were exposed to the winds of all directions, whereas the Nida meteorological station was located at the foot of the dune ridge of the Curonian Spit, and the dune ridge blocked EW -W -NW winds. Curonian Spit was particularly windy. According to the data from the Nida meteorological station (wind speed and direction were measured every 6 hours), winds were recorded to blow for a total 4221 hours, and winds stronger than 6 m/s were recorded to blow 3966 hours during the warm season (Table 1).

Commented [A4]:
Added information and references about sea level impact to dunes slope changes during of storm.

Commented [A5]:
Revised and corrected wind speed measurement frequency. The wind speed measured every 6 hours (212 line).

Commented [A6]: Corrected summary of windy hours (211 line).
Permanent strong winds caused fast sand drying and intensified deflation processes. Our measurements showed that one hour after rain the surface sand layer was about 50 per cent drier compared 215 with sand at a depth of 10 cm (Table 2).  Because The relief microforms reflecting the intensity of short-term deflation processes have formed in nearly all investigated deflation and accumulation hollows in the northern Juodkrantė -Pervalka segment.
Scraps (up to 2 m height and up to 40° inclination), deflation mini-gullies, potholes, accumulative steps, small ridges and rolls formed in the hollows. Deflation hollows were found to be mostly distributed between the Lydumo ragas peninsular and Vinkis Dune. The deflation hollows were 35 -40 m a.s.l., 245 which shows that the best conditions for deflation were on the top of the dune ridge intensively affected by W winds.
Deflation hollows formed in each seaward or lagoonward relief depression in the Lydumo ragas -Vinkis Dune segment. We compared the measurements of 1999 -2016 and found that the length and width of deflation hollows were the most variable parameters. Besides, the length and width of passages 250 connecting the hollows were also very dynamic. They could change by some to several dozens of meters per year (Table 3). In the Juodkrantė -Pervalka segment, the blowing of sand away from hollows was due not only to frequent strong winds, but also to the position of hollows in dune slopes. The most intensive deflation processes took place in the hollows located in the leeward eastern slopes of the dune ridge. The hollows in the crest of the dune ridge were blown out at a lower degree, and the hollows in the 255 windward western slope of the dune ridge suffered the lowest degree of deflation. The degree of deflation also depended on whether hollows were open or closed. The hollows with W-E deflation passages opening thereto suffered the fastest transformations. The wind quickly transformed such hollows into deflation gullies. Rather sharp changes in micro-terraces of eastern slopes were related with the amount of precipitation exceeding the average. In April-September of 2016, precipitation amount exceeded the multiannual average by 110%. Still greater differences were observed in July-August of 2016 when the amount of precipitation exceeded the multiannual average by 160% (Fig. 7). 2. In the northern segments of the Curonian Spit (Nagliai and Vinkis dunes), aeolian microforms (scraps, gullies, potholes, steps, small ridges and rolls) appeared in the deflation and accumulation hollows of 35 -40 m in absolute height. Such microforms appeared in each deflation hollow on a windward or leeward slope of the dunes. The most intensive deflation processes took place in the hollows of the northern slopes of the dune ridge exposed to winds blowing from the Curonian Lagoon. The 315 hollows on the crest of the dune ridge experienced a lower deflation degree, and those in the western windward slope of the dune ridge suffered the lowest degree of deflation. The deflation process largely depended on whether the hollow was closed or open. The hollows with W-E deflation passages opening thereto underwent the most intensive transformations. W winds quickly transformed such hollows into deflation gullies. 320 3. The measurements of repeated aerial photographs showed that the change in morphometric parameters of micro-terraces was related to fluctuations in precipitation intensity. A greater infiltration of precipitation sharply increased the weight of the sand layer and its pressure on the marl layer. In the summer (July -August) of 2016, the amount of precipitation increased 1.6-fold compared with the multiannual average, which gave rise to changes in microterraces and in the scrap of the coastal marl. 325 The sinking of terrace ledges reached 21%, and marl scrap height increased by 25% in some places.