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Dune coast changes caused by weak storm events in Miedzywodzie, Poland
Dune coast changes caused by weak storm events in Miedzywodzie, Poland Natalia Bugajny† and Kazimierz Furmańczyk Institute of Marine and Coastal Sciences University of Szczecin Szczecin, Poland †
[email protected]
www.cerf-jcr.org
ABSTRACT Bugajny, N. and Furmańczyk, K., 2014. Dune coast changes caused by weak storm events in Miedzywodzie, Poland In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 211-216, ISSN 0749-0208. www.JCRonline.org
This paper describes the impact of weak storm events on morphological changes of the sandy, non-tidal Southern Baltic coast. It was initially assumed that weak storms do not cause any significant changes on the coast, i.e. dune erosion. For this purpose, 14 RTK-GPS surveys were carried out during June to December 2012, consisting of a cross-shore profile every 100 m along a 2 km stretch of coast. Offshore wave data (WAM model) and water level data (tide gauge) were also collected. The hydrodynamic conditions were grouped into 3 groups by Ward’s hierarchical cluster analysis to investigate relationships with morphological changes on the coast. Correlation between volume changes and shoreline displacement for profiles and each group of hydrodynamic conditions were obtained. The correlation coefficient value for profiles ranged from R=0.41-0.91; and for groups R=0.79-0.82. It was found that for the first group (Hs 1 m). No significant storm events, i.e. events that cause dune erosion (Furmańczyk et al., 2012), were recorded within the period of the study. However, weak storm events occurred that did not cause dune erosion. Due to difficult meteorological conditions during the last two surveys measurements were taken only on the every second profile, i.e. every 200 m. Each profile was interpolated every 1 m, which allowed comparison of particular measurement series at the same elementary points. Widely applied statistics were deployed for each created elementary point. The statistics were: mean value, standard deviation, difference between particular measurements, difference between maximal and minimal height. Additionally, an average of the shoreline position along the investigated area and its deviations as well as minimum and maximum location values were defined. The shoreline in the studies was a datum-based shoreline, which is defined as the intersection of a specific elevation datum equal mean sea level (zero Kronstadt). Moreover, the sand volume in each profile was calculated between mean sea level (zero Kornstadt) point and dune base point, in order to calculate the correlation coefficient, similar to Farris and List (2007).
RESULTS AND DISCUSSION Hydrodynamic characteristics The period from June to December 2012 was characterized by predominance of waves of significant wave height less than 0.5 m (63%). Waves of 0.5-1.0 m height accounted for 23% of the wave height, whereas waves of over 1.0 m occurred only in 10% of the study period. Easterly, south-easterly, south-westerly directions
were dominant wave directions over that time. These directions accounted for 20%, 30% and 20%, respectively. Water level changes recorded during the studies oscillated between -0.43 and +0.56 m relative to mean sea level (zero Kronstadt).
Hydrodynamic classification Classification of the hydrodynamic conditions was carried out by means of Ward’s hierarchical cluster analysis (Figure 3). Five parameters that specify each time interval were considered. They included: maximum significant wave height for the time interval, coefficient of trend of water level variations for the time interval, time spans from the last storm event within the time interval to the end of the period (days), duration time of the last storm event for the time interval (hours) and number of storms for the time interval (when Hs > 1 m). Three classes of hydrodynamic conditions were identified. The first class (3 occurrences) is characterised by lack of storm events, so that the significant wave height did not exceed 1 m. The second class (4 occurrences) is characterised by the occurrence of 2-3 weak storm events of significant wave height from 1.4 m to 1.9 m. The third class (6 occurrences) includes 1 to 5 storms of significant wave height from 1.5 to 2.4 m.
Morphological changes Beach volume changes between the first and last survey are shown on figure 4a. Maximal changes that occurred there amounted to 1.6 m3/m. Fig 4a shows the situation, where beach accumulation appeared among the middle beach, while erosion focused on the lower beach and shallow water. Volumetric changes ranged from +0.8 m3/m to approximately -1.2 m3/m. The shoreline position changed from the very rough in the beginning of the measuring season to very smooth at the end of it, while the western part was receding, the eastern part accreted. Mean width of the beach over the entire study period oscillated between 29 and 53 m, with the lowest mean values at the edges of the study site, during the investigated period.
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10 m. The first group is consists of very dynamic profiles. These profiles are characterized by above mean value of shoreline displacement oscillation and above mean standard deviation values of volumetric changes. The second group contains profiles that are characterized by below mean values of the aforementioned parameters, which means that they are of low dynamics. The third group is composed of profiles that are diverse in terms of dynamics, so that they received values over or below the mean values of the parameters. The spatial analysis of the results proves that the most dynamic profiles are located in the western and central-eastern parts of the investigated area, while the stable ones are in the central part. The location of dynamic profiles in the western part can be related to dominance of storm waves that arrive from the western sector during storm events (see wave characteristics), whereas the eastern part of the investigated area is protected by a group of groynes (Figure 1). Furthermore, correlation coefficients between shoreline displacement and beach volume changes were calculated for all Figure 3. Cluster dendrogram presenting the separation of the three surveys with division into particular profiles. Their spatial groups of hydrodynamic conditions using hierarchical Ward distribution is presented on figure 5. The value of correlation cluster analysis. Hydrodynamic intervals are listed on vertical axis. coefficient along the investigated area oscillates between R=0.410.91 and its mean value is R=0.79. Most of the profiles reveal good correlation between shoreline displacement and volume Analysis of the standard deviation values of volumetric changes changes. Only 2 profiles stand out and they have R values less for each elementary point shows that the greatest changes occur in than 0.6 shoreline displacement (Figure 4b). Three types of profile with Correlations between shoreline displacement and volume distinctive dynamics were distinguished on the basis of maximal changes of the beach for all measurements and with the division standard deviation values of volumetric changes and maximal into 3 profile groups of hydrodynamic conditions are changes of shoreline displacement for the entire investigated demonstrated on figure 5. The correlation coefficient for all period of time. Limit values between particular groups were: mean measurements amounts to R=0.79 (Figure 5a), which means a value of maximal standard deviation value, 0.36m3/m and mean good correlation. value of maximal change of shoreline displacement in the profile, The group which contains no storm events, i.e. Hs < 1 m
Figure 4. Morphological changes between first and last RTK-GPS survey with shoreline position at these particular surveys (left), standard deviations of volume changes with maximum and minimum positions of shoreline (right). Types of dynamic profiles are marked by triangles and crosses.
Journal of Coastal Research, Special Issue No. 70, 2014
Dune coast changes caused by weak storm events in Miedzywodzie, Poland
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Figure 5. Beach volume changes vs. shoreline displacement. a) all observations, b) the first group of hydrodynamic conditions, c) the second group of hydrodynamic conditions, d) the third group of hydrodynamic conditions. (Figure 5b), experiences accumulation, both in terms of shoreline displacement (30/7) and beach volume changes (46/7). The values in brackets refer to the number of profiles with registered accumulation/erosion in time interval qualified to particular groups. The correlation coefficient for this group is R=0.82. Maximal differences in shoreline accumulation values amounts to 6 m, while beach volume changes are approx. 4 m3/m. The second group (Figure 5c), where time intervals contain 2-3 weak storm events of significant wave height smaller than 1.9 m, reveals similar accumulation and erosion changes, both for shoreline displacement (30/41) and beach volume changes (41/41), so the processes are in equilibrium. The correlation coefficient for this group is R=0.79. Maximal differences of shoreline accumulation values amount to 6-7 m, while beach volume changes are up to 8 m3/m. The third group (Figure 8d), where time intervals can contain up to 5 weak storm events and significant wave height oscillates between 1.5 and 2.4 m, also shows accumulation and erosion changes. The changes of beach volume are similar (57/59), whereas the changes of shoreline are dominated by erosion (31/69). The correlation coefficient for this group is R=0.80. Maximal differences of shoreline accumulation values amounts to 10 m, while beach volume changes are up to 8 m3/m. Therefore, waves with heights
