Factors Controlling the Development of Foredunes along the Łeba Barrier on the South Baltic Coast of Poland

Journal of Coastal Research SI 64 308-313 ICS2011 (Proceedings) Poland ISSN 0749-0208 Factors Controlling the Development of Foredunes along the Łeba Barrier on the South Baltic Coast of Poland J. Rotnicka Institute of Geology Adam Mickiewicz University Maków Polnych 16, 61-606 Poznań, Poland joanrot@amu.edu.pl ABSTRACT ROTNICKA, J., 2011. Factors controlling the development of foredunes along the Łeba Barrier on the south Baltic coast of Poland. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal Symposium),. Szczecin, Poland, ISSN 0749-0208 The paper aims to show the roles of various factors controlling the evolution of foredunes along the Łeba Barrier and explain why parts of the coast prograde while others suffer erosion. SW and W winds dominated the effective wind (> 5 m/s) regime of the area, as determined for the years 2007-2009 on the basis of data from a weather station, with a total frequency higher than 60%. As the barrier coastline changed from SW-NE to nearly W-E, these winds generated alongshore and oblique landward sediment transport. The aeolian mass flux on a beach was evaluated by field experiments. These experiments included measurements of the sand transport rate (by means of a 0.5-m-high vertical sand trap) as well as the wind speed and direction at 1 m height. On the basis of the established relationship and the wind data from the meteorological station, the potential annual sand transport was calculated. It appeared that the most effective winds accounted for more than 90% of the total transport. Taking into account the coastline orientation, the landward sand transport theoretically exceeded the seaward and alongshore transport only on the beach oriented 50º-230º, suggesting that this part of the coast should favour foredune formation. However, along the barrier zones, foredune development and erosion alternate regardless of the coastline orientation. A possible explanations including fluctuation of the littoral sediment budget along the shoreline and/or rhythmic changes in bathymetry need future investigations. ADDITIONAL INDEX WORDS: Aeolian transport rate, aeolian sand budget, beach accretion and erosion INTRODUCTION Generally, foredunes develop under prevailing onshore winds, when sand transported landwards is trapped by vegetation. Another important factor controlling their evolution is the sand transport rate on the beach, which exhibits a great spatial and temporal variability as a function of the wind speed and angle of approach as well as the beach width, geometry (e.g., Arens et al., 1995; Bauer and Davidson-Arnott, 2003), surface roughness and moisture (e.g., Nickling and Davidson-Arnott, 1990). Apart from the aeolian processes acting in a beach-dune system, the sand budget in the littoral zone is of high importance for coastal dune evolution (Psuty, 1988). Therefore, every coast exhibits a very complex and unique system, and individual approaches are needed for their study. The main objective of this study was to find the major factors controlling the evolution of the foredunes along the Łeba Barrier on the south Baltic coast of Poland (Figure 1). At some sections of the barrier, there are up to four foredune ridges marking shoreline progradation, whereas other parts of the coast suffer intensive destruction during storms and neither the beach nor the foredunes is soon rebuilt during interstorm periods. Previous research by Hildebrandt-Radke (1999) and Borówka II and Rotnicki (2001) have shown that, in the case of this barrier system, seaward aeolian transport exceeds landward transport and, on an annual scale, only short episodes of onshore winds are responsible for landward sediment transport. Although the absolute values of this landward sediment transport are small, the bulk of sand carried by the wind is sufficient to build the new foredunes. However, these studies do not explain why some parts of the coast are dominated by sand accumulation and others by wave erosion. Therefore, a question about the factors favouring the formation of foredunes along the Łeba Barrier remains unanswered. STUDY AREA The Łeba Barrier, some 40 km long and 0.6-2 km wide, is part of a well-developed barrier-lagoon coast (Rotnicki, 1995). The beach and foredunes, 4-15 m high, are WSW-ENE oriented with the axis orientation changing from 50-230 in the western part of the barrier to 90-270 in the eastern part (Figure 1). The beach is tideless, low gradient and tens of metres wide (locally up to 150 m), switching between dissipative and reflective during the year. Up to three sand bars exists in the shoreface. Both the foredunes and the beach are composed of fine or medium quartz sand, locally pebbles occur. Because the area studied is located within Słowiński National Park, the beach and foredunes are not human altered, excluding artificially planted vegetation and fascine fences along some parts of the coast. 308

Factors Controlling the Development of Foredunes Figure 2. Arrangement of anemometers and sand traps during experiments. The distance between anemometers An1 and An3 varied between 30 m and 90 m depending on the beach width. axis. Because any deviation of wind direction from the beach axis reduces the maximum fetch significantly, alongshore sectors were narrowed to 30, and other sectors were broadened to 50. Figure 1. A middle Polish Baltic coast, B the Łeba Barrier with zones of foredune growth and erosion. METHODS Research on factors controlling the evolution of foredunes along the Łeba Barrier started in November, 2006. To characterise the general and effective wind regimes of the studied area and to calculate the potential sand transport on an annual scale, a meteorological station was placed in the beach hinterland on the meadow in Smołdziński Las, ca. 2 km south from sites of beach experiments (Figure 1b). The anemometer was installed at a height of 15 m above the ground, and wind parameters were sampled over 10-s intervals and recorded as the averages of 10-min intervals. The wind data set used for determining the aeolian sand budget covered 3-year period (2007-2009). To simultaneously define the wind regime of a beach and estimate the sand transport rate, field experiments were performed on the beach. On the basis of their results, the wind conditions favourable for foredune development were determined, and the relationship between the wind velocity and the sand transport rate was established. The field experiments were performed at four places along the western part of the barrier (Figure 1b). They involved measurements of: 1) the wind speed and direction at 1-m height (with a 1-s sampling interval and 30-s logging interval), 2) the sand transport rate by means of 0.5-m-high vertical sand traps (Rotnicka, 2011). The anemometers and sand traps were deployed in a shore-normal line (Figure 2). The distance between the anemometers at each site remained the same during successive measurements. The experiments covered a wide range of wind events characterised by different angles of approach and wind speeds average wind velocities between 4.5 and 13.5 m/s (with gusts up to 19.0 m/s). Air temperature and relative humidity varied in the range of 1.6ºC to 19.8ºC and 61.7% to 91.9%, respectively, but none of the runs presented was performed during rainfall. All 653 measurements of sand transport rate were taken during 134 runs; among them 462 represent autumn-winter conditions, 191 summer conditions. Both the wind data and the measurements of mass flux were grouped and analysed in eight wind sectors related to the beach WIND REGIME OF THE ŁEBA BARRIER The wind structure of the Łeba Barrier was analysed in 10 angular intervals, each centred on a multiple of 10. The data confirm that the area is dominated by westerly and south-westerly winds, though in analysed period the year 2009 was exceptional, as the frequency of NE, E and SE winds exceeded 45%. The mean annual wind speeds recorded by the meteorological station in the three successive years amounted to 4.89 m/s, 5.04 m/s and 4.52 m/s. The strongest winds (up to 22 m/s) occurred only in the autumn-winter periods. To evaluate the structure of the effective winds causing sand transport, it was necessary to determine the value of the wind speed at the weather station, which would correspond to the critical threshold shear velocity (u c ) for the beach sand. As the average diameter of the sand was 0.26 mm, u c calculated from Bagnold s (1941) equation amounted to 0.235 m/s and the corresponding velocity at 15 m, computed from the equation for the wind profile (Von Karman, 1934), was 4.7 m/s. Assuming for simplicity that sand-transporting winds are stronger than 5 m/s, it appeared that in the years 2007-2009 less than 40% of winds were effective. Among them, the prevailing directions were W and SW with a combined frequency of more than 60%, and the most frequent direction was 260 (Figure 3). Taking into account the orientation of the beach axes, winds in these directions should generate alongshore and oblique onshore sediment transport. AEOLIAN SAND TRANSPORT RATE AT ŁEBA BEACH Field experiments carried on the beaches of Łeba Barrier allowed verification of the role and significance of alongshore and oblique onshore winds in the formation of foredunes. The sand transport rate in a given wind regime exhibited a great spatial variability on the beach, particularly due to topographic forcing of airflow and beach width. The results of these experiments are presented in detail by Rotnicka (2010). In general, during perpendicular and oblique onshore winds approaching the shore at angles higher than 50, beaches 50 m and 80 m wide provide fetches of less than 65 m and 100 m, respectively, and measurements showed that the near-bed wind flux frequently did not reach the state of maximum saturation. 309

Rotnicka Figure 3. Regime of effective winds in the area of the Łeba Barrier and distribution of all winds (solid line). Low-speed winds (< 8-10 m/s depending on the beach profile) generated highest rates of sand transport in the middle part of the beach, and, due to deceleration toward the foredune, they caused sand deposition near the dune foot. Therefore, the upper beach was the place of sand accumulation and embryo dune development, particularly in places where vegetation occurred. It is important to note that only strong winds (> 9-10 m/s) fed the existing foredune ridge with sand and redistributed sand within the ridge. However, even though they were able to transport sand over the foredunes, they were still unsaturated. On the one hand, as the wind speed increases, the distance needed to attain the maximum transport rate also increases (Dong et al., 2004), and, on the other hand, these winds often generated storms leading to significant beach narrowing or destruction. The most significant differences in amount of sand transported on the backshore and upper foreshore were measured during alongshore winds and oblique winds approaching the shore at a low angle (< 30 ). The most intense transport was observed in the lower and middle part of the beach; toward the foredune ridges it decreased considerably. Despite this decrease, during alongshore winds of 10 m/s, the transport rate near the dune foot was an order of magnitude higher than the rates measured during any other onshore winds. Even sparse vegetation present on the upper beach trapped blowing sand, and an intensive growth of embryo dunes was observed; otherwise, the upper beach acted as a sediment transfer zone. Thus, for the beaches of the Łeba Barrier, which are mainly 50-80 m wide, the general pattern of foredune evolution seems to be very complex and must take into account the great significance of alongshore winds in foredune formation. Keeping in mind that alongshore wind sectors involve wind directions with an onshore component (up to 15 to the beach axis), these winds and oblique onshore winds of moderate strength are responsible for sand deposition on the upper beach. During conditions of strong oblique onshore winds, when the beach and fetch are reduced by storm waves, the sand accumulated earlier at the dunefoot is the only available source of sediment and is redistributed toward and within the foredunes. As alongshore winds cause sand transport that is not fetchlimited, the measured rates reflected conditions of maximum transport rate Q m (Rotnicka, 2011). The relationship between Q m (kg/s/m) and the wind speed at a height of 1 m (U 1 ), established by regression analysis, was expressed as: Q m = 4 10-9 U 1 6.7705, with R 2 = 0.8347 (1) It is important to note that the above relationship does not take into account the impact of the beach surface type on the sand transport rate. However, it shows a strong positive correlation, meaning that it expresses the real rate of aeolian sand transport on this particular beach, and a calculation of annual sand transport based on this formula will provide a better estimate than any other model based on the results of tunnel investigations and simplified transport conditions (e.g., Bagnold, 1941; Kawamura, 1951). Figure 4. Aeolian sand budget calculated for the analysed period. 310

Factors Controlling the Development of Foredunes Under- and overestimates in the sand budget Figure 5. Calculated sand budget with reference to the beach axis. Wind directions: onw, one oblique onshore W and E; onn perpendicular onshore; offw, offe oblique offshore W and E; offs perpendicular offshore; aw, ae alongshore W and E. AEOLIAN SAND BUDGET Predicted annual sand transport rate Using the established relationship, the potential sand transport rate was calculated (Figure 4). The results of this calculation suggest that: 1) W and SW winds were theoretically responsible for 90-95% of the total sand transport and 2) the angle of approach of the most effective winds ranged from 240 to 280, and these winds made up more than 75% of the total transport. It is also noteworthy that the potential mass transported in 2009 was less than half that in the last years. If the beach axis is considered, the distribution of the mass transported among all sectors was strongly differentiated. On a beach orientated 50-230, oblique landward sand transport dominated, accounting for more than 65% of the total transport, while alongshore sand transport amounted to ca. 20% (Figure 5a). The coast orientated 70-250 presents the opposite case, as the alongshore westward transport dominated (54-63%), while the oblique landward transport was much lower (ca. 24%) and partly balanced by seaward transport (Figure 5b). As the shoreline changes direction to latitudinal, the oblique offshore transport acquires significance; in an analysed period, the value of the oblique offshore transport was comparable to that of alongshore transport (Figure 5c). The calculated sand budget may be overestimated, as it was based on data collected by a weather station 15 m above the ground, and the relationship used in the calculation was established for velocities 1 m above the ground. However, simply comparing the wind velocities measured 1 m above the beach with those measured at the same time by the weather station (Figure 6) shows that on the beach, most of the onshore winds are stronger (by up to 4 m/s) and the offshore winds are weaker (by even 5 m/s). This finding suggests that the actual mass of sand transported landwards by onshore winds is higher than that arising from the calculated sediment budget, whereas the seaward transport is nearly negligible. In fact, however, there are at least two factors limiting the real landward transport. The formula (1) represents the conditions of the maximum transport rate, but in the case of onshore winds, they are attained only on wide beaches, which are not very common on the Łeba Barrier. Furthermore, the spatial diversity of beach surface moisture and roughness also influences the sand transport across a beach. An increase in moisture and the presence of a dense cover of pebbles both control the threshold value of shear velocity required for initiation of sand grain movement (McKenna Neuman and Nickling, 1989) and, to some extent, limit the potential source of sand susceptible to aeolian transport (Nickling and Davidson-Arnott, 1990). It is particularly important on the studied beaches, as surfaces with different characteristics (i.e., dry, moist or covered with pebbles) extend in shore-parallel zones. However, all of these factors exhibit considerable spatial and temporal variability due to weather conditions and short-term sea level fluctuations, so it is difficult to evaluate them without continuous beach monitoring, and, thus, they are excluded from transport equations (e.g., Bagnold, 1941; Kawamura, 1951; Fryberger and Dean, 1979). Another factor that may suppress aeolian transport on the beach and makes the calculated sand budget overestimated is rainfall, though its role in aeolian transport is not sufficiently clear. Most wind erosion models assume no aeolian transport during rain, but research by van Dijk et al. (1996) and Jackson and Nordstrom (1998) contradict this. Also some measurements taken on Łeba beach suggest that during heavy rain the aeolian sand transport rate may even exceed that one measured over a moist or dry surface and this is probably due the splash-erosion, which initiate Figure 6. Comparison of wind speed measured on the beach, ca. 20 m from the dune foot, at 1-m height (v B(1m) ) with speed logged at the same time by weather station (beach hinterland) at 15-m height (v MS(15m) ). 311

Rotnicka the saltation (Jungerius and Dekker, 1990; Riksen and Goossens, 2007). However, the available data are not sufficient to quantify the sand transport intensity during rainfall and to determine limits of aeolian transport in such conditions. OBSERVED FOREDUNE GROWTH AND DECAY Keeping in mind the above limitation and assuming that onshore winds are of high importance in nourishing the foredunes with sand and their shaping, the calculated sand budget suggests that a coastline orientation of 50-230 should favour foredune formation. If alongshore winds with onshore components are additionally considered as feeding the foredunes with sand, the shoreline of this orientation is all the more favourable for dune growth. However, this situation is not the case for the Łeba Barrier, where zones of foredune growth and erosion alternate regardless of the coastline orientation (Figure 1b). It is important to note that these zones are spaced regularly, every 1.2-1.6 km, as measured in spring 2010. The study shows that factors other than wind influence foredune development along the Łeba Barrier and that the potential for foredunes to grow must be examined in connection with all processes acting in the foredune-beach-shoreface system. A rhythmic behaviour of the erosion and accumulation zones may be related to rhythmic bathymetrical changes in the nearshore and/or to the littoral sediment budget, which determines whether the shoreline is transgressive, stable or progrades (Psuty, 1988). However, it should be stressed that no detailed data on shoreface bathymetry for the area studied are available and so far no attempt has been made to assess the littoral sediment budget in the area. DISCUSSION AND CONCLUSIONS The relationship between the maximum sand transport rate and the wind velocity, established on the basis of field experiments carried out on the beach, represents conditions of saturated flux reached during alongshore winds. The formula shows that mass transport is proportional to the wind velocity at 1 m above ground level raised to the 6.77 power. This power exceeds that in Bagnold s (1941) mass transport equation (cube) and in Borówka s (1990) relationship (4.68) established on the basis of experiments carried out on in the area of active barchan dunes on the Łeba Barrier. The higher the power is, the faster the sand transport rate increases with wind velocity. One possible explanation why the obtained power is so high is that the sand transport rate was measured by means of a higher sand trap than those used in the other experiments. As the wind increases and the layer in which sand is transported becomes thicker, lower traps catch less sand. The formula obtained shows a strong correlation, meaning that it expresses the real maximum rate of aeolian sand transport on the studied beach, and, therefore, it was used for estimation of the annual sand budget. However, it must be stressed that the sand transport rate across the beach exhibits strong spatial variability, and further measurements are needed to elaborate the relationship between mass flux, wind speed and beach width in the case of unsaturated flux. The aeolian sand budget calculated on the basis of the established relationship and the wind data from the weather station, as considered in connection with the coastline orientation, reveals that landward sand transport theoretically exceeded seaward and alongshore sand transport only on the beach oriented 50º-230º. As the coastline switches to the W-E direction, the magnitude of seaward transport increases, becoming comparable with the magnitude of alongshore transport. Previous research by Hildebrandt-Radke (1999) and Borówka II and Rotnicki (2001) reveals that in particular years, seaward aeolian transport exceeds landward transport and only short periods of strong winds nourish the foredunes. This finding implies that the sediment budget on the beach is negative, and, if so, the foredunes should not form as considering their evolution on a longer time scale. This study proves the importance of short-duration episodes of high-speed onshore oblique winds (approaching the shore at an angle of less than 40-50 ) in feeding dunes with sand, but it does not confirm the dominance of seaward transport. Its magnitude in the estimated sand budget is even overestimated, as the calculation is based on the formula established for saturated flux, which is not the common case for across-beach transport, and on winds measured by the weather station, which are stronger than winds on the beach because it is sheltered by the hinterland topography and the forest. Even though the estimated sand budget suggests that the coastline oriented 50º-230º is privileged in terms of foredune formation, zones of foredune growth and decay alternate along the Łaba Barrier regardless of the coastline orientation. A possible explanations for this alternation including fluctuation of the littoral sediment budget along the shoreline and/or rhythmic changes in bathymetry are at present not more than a hypothesis. To prove it, future research on that issue is needed. ACKNOWLEDGEMENT This research was supported by a grant from the Polish Ministry of Science and Higher Education (2 PO4D 008 28). The author is grateful to Władysław Pawłów, Patryk Markiewicz, Karol Rotnicki and Jolanta Czerniawska for their help with the field experiments and Jerzy Terefenko for financial support and the renovation and assembly of the 15-m mast for the weather station. The help of the staff of the Słowiński National Park is greatly acknowledged. 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