Variability of Hydrodynamic and Lithodynamic Coastal Processes in the East Part of the Gulf of Gdańsk - PDF

Archives of Hydro-Engineering and Environmental Mechanics Vol. 57 (2010), No. 2, pp IBW PAN, ISSN Variability of Hydrodynamic and Lithodynamic Coastal Processes in the East Part of the

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Archives of Hydro-Engineering and Environmental Mechanics Vol. 57 (2010), No. 2, pp IBW PAN, ISSN Variability of Hydrodynamic and Lithodynamic Coastal Processes in the East Part of the Gulf of Gdańsk Rafał Ostrowski, Zbigniew Pruszak, Marek Skaja, Marek Szmytkiewicz Institute of Hydro-Engineering, Polish Academy of Sciences, Kościerska 7, Gdańsk, Poland, s: (corresponding author), (Received April 08, 2010; revised May 05, 2010) Abstract The paper presents new findings concerning motion of water and sediment in the coastal zone of the east part of the Gulf of Gdańsk, from the Vistula River mouth at Świbno (Poland) to Cape Taran (Russia, Kaliningrad Oblast). The presented study deals with spatial variability of parameters of hydrodynamic and lithodynamic processes which have been subject to theoretical modelling. For the considered coastal segment, deep-water wave conditions reconstructed for a 44-year period have been analysed and transformed to the nearshore zone. Next, velocities of wave-driven longshore currents for the mean statistical year have been calculated, along with the longshore sediment transport rates. Regarding the net longshore sand motion, its detected direction is from Cape Taran towards the Vistula Spit. Very high annual rates of longshore sediment transport have been obtained for the coastal segment at Sambian Peninsula. These rates decrease considerably along the Vistula Spit, reaching zero at a distance of about one third of the Polish part of the Spit, measured from its root. At this location, the net longshore sediment transport (resulting from net longshore wave-current impact) reverses from westward to eastward. Key words: sediment transport, waves, currents, deep-water wave climate 1. Introduction The coastal zone of the east part of the Gulf of Gdańsk, from the Vistula mouth at Świbno (Poland) to Cape Taran (Russia, Kaliningrad Oblast) is less developed than the west part of the Gulf, spreading westwards of the Vistula mouth to Cape Rozewie (PL). There is only one Russian harbour located close to the open coast, namely the Baltiysk harbour, while there are no Polish harbours at all. As regards inland harbours (on the shores of the Vistula Lagoon, including the internal coastline of the Vistula Spit), there is one big harbour in Kaliningrad (RU, KO) and a smaller one and economically less important in Elbląg (PL), as well as a few very small fishery harbours 140 R. Ostrowski, Z. Pruszak, M. Skaja, M. Szmytkiewicz and marinas both on the Polish side (Piaski, Krynica Morska, Kąty Rybackie, Tolkmicko, Frombork) and on the Russian side of the lagoon (Krasnoflotskoye, Ushakovo, Pribrezhnyy). Aside from harbour industry, the touristic infrastructure is also less developed than in the west part of the Gulf of Gdańsk. For instance, the Hel Peninsula (a coastal form similar to the Vistula Spit) is visited by tens of time more tourists than the Polish part of the Vistula Spit. The Russian part of the Vistula Spit is almost untouched by tourism. The region has huge economic and touristic potential and its role will presumably grow in the nearest future. One should expect rapid development of the coastal touristic infrastructure at a few villages and towns which are becoming ever more fashionable, both in Poland (e.g. Jantar, Stegna, Krynica Morska) and in Russia (e.g. Yantarnyy). The number of harbour investments will also increase. Currently, these investments on the open sea side are related mainly to maintenance of the Baltiysk (former Pilawa) Strait (RU, KO), an inlet in the Vistula Lagoon. In the Vistula Lagoon, engineering activities concentrate on modernisation of small harbours and upkeep of waterways (dredging works in navigable canal and at harbour areas). In Poland, the growing popularity of yachting along with plans for the full activation of the harbour in Elbląg, as well as the other harbours and marinas, have given rise to an idea of an artificial cross-cut through the Polish part of the Vistula Spit. In this way, an alternative passage for Polish boats from the Vistula Lagoon to the Gulf of Gdańsk would be available, independent of the Russian waterway through the Baltiysk Strait. Besides, in further perspective, new small harbours and marinas can be constructed on the open sea side, both in Poland and Kaliningrad Oblast. All the above activities, either planned or currently being carried out, have a considerable impact on the sea shore. To minimise possible drawbacks of future investments and to optimise anticipated coastal engineering ventures, more detailed knowledge on coastal hydro- and lithodynamic processes occurring in this part of the Gulf of Gdańsk is required. In particular, identification of directions and rates of the longshore sediment fluxes is crucial in the design of coastal structures and in predicting potential damage to the sea shore by these structures. According to previous studies concerning the Vistula River mouth, the net longshore sediment transport in the south part of the Gulf of Gdańsk is directed from west to east. The sandy material is transported from the Vistula River mouth towards the Vistula Spit. There is a common opinion among scientists that the north-east edge of the Gulf of Gdańsk, namely Cape Taran (RU, KO), constituting the north-west corner of the Sambian Peninsula, represents a point at which longshore sediment fluxes diverge distinctly. This denotes mean annual resultant sediment transport along the north and west coasts of the Sambian Peninsula, directed eastwards and southwards, respectively. According to all concepts and ideas, the latter southward direction of longshore sediment motion continues until the Baltiysk Strait and farther along Vistula Spit. A question exists whether such a longshore sediment flux reaches a region nearby the Polish-Russian border and meets the eastward directed opposite flux. Opin- Variability of Hydrodynamic and Lithodynamic Coastal Processes ions of various Polish and Russian coastal researchers on this issue differ considerably, but all researchers agree that somewhere between the Baltiysk Strait and the Vistula River mouth, the longshore sediment fluxes converge. The location of this convergence point (being an objective of the scientific argument) is very important for understanding of coastal morphodynamics in the region and all resulting consequences in the domain of coastal engineering. In order to clarify the above uncertainties, the research team at the Institute of Hydro-Engineering of the Polish Academy of Sciences (IBW PAN) undertook investigations sponsored by the Ministry of Science and Higher Education within the national project N N entitled Variability of hydrodynamic and lithodynamic processes in the coastal zone of the east part of the Gulf of Gdańsk. Described in the present paper, a part of these investigations comprising the theoretical analysis was concentrated on mathematical modelling. The computations concerned the following coastal physical processes occurring in the Gulf of Gdańsk: waves, wave-driven currents and longshore sediment transport. The field surveys have provided bottom sediment characteristics used in modelling of the sediment motion. The input wave conditions have been taken from the IBW PAN database comprising a long-term reconstruction of the deep-water wave climate in the Baltic Sea. Nearshore waves and currents have been modelled using bathymetric data provided by the Maritime Office in Gdynia the Inspectorate of Coastal Protection (the Polish coastal segment) and the Shirshov s Institute of Oceanology of the Russian Academy of Sciences Atlantic Branch in Kaliningrad (the Russian coastal segment). The bottom sediment sampling has been carried out on the beach and in the nearshore zone. The samples have been subject to sieve analysis. 2. Deep-Water Wave Climate and Energy For the needs of the present study, determination of the offshore wave climate has been based on the numerical prognostic wave model WAM4, in which the input is determined from meteorological (wind and air pressure) fields. A few years ago, the spectral wave model WAM4 was used under the HIPOCAS project (coordinated by GKSS, Germany), in which IBW PAN participated, for reconstruction of the long-term European wave climate in the period from 1958 to The reconstruction procedure is described in detail by Weisse et al (2009). In brief, the National Centres for Environmental Prediction National Centre for Atmospheric Research (NCEP NCAR) global reanalysis (Kalnay et al 1996), was applied in combination with the spectral nudging technique (von Storch et al 2000), as the forcing to the REgional Climate MOdel (REMO) which is based on the numerical weather prediction model EM of the German Weather Forecast Service (DWD), (Feser et al 2001). In this way, wind parameters (velocity and direction) at a height of 10 m above the sea and pressure fields were obtained. The results of the REMO model were next used in the WAM4 model. 142 R. Ostrowski, Z. Pruszak, M. Skaja, M. Szmytkiewicz Fig. 1. Locations of deep-water wave prognostic points in the east part of the Gulf of Gdańsk The WAM4 model is based on a so-called wave action balance equation and takes into account the energy transfer from wind to the sea, white-capping wave breaking, bottom friction and resonance interactions of wave components. For the Baltic waves reconstruction, the model resolution of the spatial grid was 5 5 (about 9 km). The model time step of the input wind data amounted to 1 hour. This input was then interpolated, yielding a computational resolution of 300 s. At each grid point for each hour of the 44 year long reconstruction period, the computational results comprised the following representative wave parameters: significant wave height, wave period and wave ray direction. Results of the wave climate reconstruction described above have been used in determination of the offshore wave parameters in the mean statistical year. The deep-water wave prognostic points have been chosen to represent wave conditions along the coastline of the east part of the Gulf of Gdańsk. In total, eight points have been selected: four located in Polish territorial waters and four others in the Russian coastal zone. The locations of these points are given in Fig. 1 while their coordinates, distances from the shoreline and corresponding water depths are given in Table 1. Variability of Hydrodynamic and Lithodynamic Coastal Processes Table 1. Geographical coordinates, distances from the shoreline and related water depths of deep-water points selected from long-term wave climate reconstruction data Approximate respective coastal Geographical coordinates Distance from Local location Latitude Longitude shoreline [m] depth [m] Jantar (PL) Kąty Rybackie (PL) Krynica Morska (PL) Piaski (PL) Russian part of the Vistula Spit (RU, KO) Baltiysk (RU, KO) Yantarnyy (RU, KO) Donskoye (RU, KO) On the basis of the reconstructed long-term wave data, so-called wave scenarios have been elaborated for all deep-water points. This means that at each point the wave parameters occurring in the mean statistical year have been determined. The ranges of wave height were assumed with resolution of 0.5 m. For each range of the wave height, the wave climate at each of the chosen deep-water location has been determined, represented by the significant wave height H s (mean of one third of the highest waves in a wave series), the wave period corresponding to the energy peak in the wave spectrum T p (a so-called wave peak period), the direction (azimuth) of wave propagation A z and duration of individual wave events. These wave conditions, together with coastal bathymetric data, were a basis for numerical modelling of wave transformation, wave-driven longshore currents and sediment transport on the cross-shore profiles at eight selected coastal locations (see first column in Table 1). Prior to this, however, the deep-water wave energy for all points was analysed. Conventionally, wave energy is calculated as E = 0.125ρgH 2 (where ρ and g denote water density and acceleration due to gravity, respectively). In the present study, two indicators of wave energy have been computed, namely the indicators of wave energy in the directions perpendicular (E per pendicular ) and parallel (E parallel ) to the coastline. Within these computations, the total wave energy indicator (H 2 s ) was projected on the above two directions. The applied formulas read, respectively: where: E per pendicular = 1 n n Hs 2 i cos θ i, (1) i E parallel = 1 n Hs 2 n i sin θ i, (2) i R. Ostrowski, Z. Pruszak, M. Skaja, M. Szmytkiewicz Donskoye (45 m) Yantarnyy (20 m) Baltiysk (40 m) Vistula Spit (RU) (20 m) Jantar (36 m) Kąty Rybackie (45 m) Krynica Morska (27 m) Piaski (50 m) Polish-Russian border Fig. 2. Deep-water wave energy indicators in directions perpendicular and parallel to the shoreline (values in brackets denote local water depths at the wave prognostic points) H si significant wave height in the one-hour period; θ i angle between the wave crest and the shoreline in the wave event; n number of 1-hour wave events in the 44-year long period of reconstruction (n = = ). In the computations, the deep-water waves directed seawards from the local shore have been skipped. Thus, in the direction perpendicular to the coastline only onshore wave energy has been obtained. In the direction parallel to the coastline, two components of wave energy have been calculated, yielding a certain resultant longshore wave energy at each location on the coastline. Results of computations of the wave energy indicators are depicted in Fig. 2. It can be concluded from Fig. 2 that the resultant (net) longshore wave energy is directed from Cape Taran to the Vistula River mouth everywhere except for Kąty Rybackie in Poland (root of the Vistula Spit). The biggest value has been obtained for Yantarnyy in Russia (0.80) and the smallest one for Krynica Morska in Poland (0.01). The cross-shore wave energy computations have yielded the maximum quantity at Variability of Hydrodynamic and Lithodynamic Coastal Processes Donskoye close to Cape Taran (1.48) and indicate a distinctly decreasing trend towards the Vistula River mouth, reaching a minimum at Jantar (0.48). The above results concerning cross-shore wave energy imply that the shores of the Sambian Peninsula are subject to significantly stronger wave impact than the shores of the Vistula Spit and the Vistula River mouth region. The extreme wave load at the shore has been obtained for the north part of the western coast of the Sambian Peninsula. Indeed, this coastal segment, much exposed to winds and waves coming from a wide sector W-N, is said to be frequently eroded by heavy storms. The computational findings concerning the wave energy component parallel to the coast suggest that the most intensive longshore impact of waves in the considered region takes place along the coast of the Sambian Peninsula, up to the Baltiysk Strait. This deep-water impact, however, cannot be directly related to longshore sediment transport since the motion of sediment in the coastal zone is driven by a complex influence of waves transformed on the cross-shore profile and nearshore wave-induced currents, including the longshore current. 3. Longshore Sediment Transport The wave-induced longshore currents, together with the wave-generated nearbed oscillatory flows, form the driving force of the longshore sediment transport. This coastal physical process has been modelled by the UNIBEST-LT numerical program. UNIBEST (UNIform BEach Sediment Transport) is a generic term for a software package that computes sediment transport along a uniform sandy coast and the coastal behaviour during human interference or storm. The software package UNIBEST consists of four separate modules; UNIBEST-LT, -CL, -TC, and -DE. UNIBEST-LT (Longshore Transport) can be used for the computation of net sand transport in the longshore direction and its cross-shore distribution. UNIBEST-LT supplies the boundary conditions for UNIBEST-CL (CoastLine dynamics), which can be used to assess coastline changes due to human influence (e.g. breakwaters, groins). UNIBEST-TC (Time-dependent Coastal profile model) can be used to assess coastal profile developments due to wave action. UNIBEST-DE can be used to compute dune erosion, and is quite similar to the TC module, but is especially intended to compute the effects of stormy episodes. UNIBEST-LT is an acronym for UNIform BEach Sediment Transport Longshore Transport. The module has been developed to compute tide- and wave-induced longshore currents and sediment transports on a beach of arbitrary profile. The surf zone dynamics is derived from a built-in random wave propagation and decay model, which transforms offshore wave data to the coast, taking into account the principal processes of linear refraction and non-linear dissipation by wave breaking and bottom friction. The longshore sediment transports and cross-shore distribution can be evaluated on the basis of different transport formulae (e.g. Bijker 1971, van Rijn 1993 and Bailard 1981). 146 R. Ostrowski, Z. Pruszak, M. Skaja, M. Szmytkiewicz One characteristic cross-shore profile, representative for the entire analysed coastal zone, is assumed in UNIBEST-LT. The longshore sediment transport rate is calculated for a variety of assumed angles between the characteristic profile and wave incidence. The computations comprise a series of wave events and yield the coefficients of approximate transport formula, being a function of wave-to-coastline angle. Within UNIBEST-CL the sediment transport variability along shore is modelled by using the shoreline angles at grid points and the coastline evolution is determined in each time step by the single line model. Major quantities and characteristics required in modelling by UNIBEST-LT are: (a) The initial bottom profile; (b) Time series of the offshore boundary conditions such as: water level, deep-water significant wave height, deep-water wave incidence angle, peak wave period, longshore tide-induced velocity. There are a variety of theoretical models describing the motion of sediment in the longshore direction. The most important are the approaches of Bailard (1981), Bijker (1971), CERC (Shore Protection Manual 1984) and van Rijn (1993). All of them are included in the UNIBEST-LT computational package (ver. 4.0) in use at IBW PAN. The present study is not devoted to tests of the models accuracy, applicability, limitations, drawbacks and advantages. On the basis of a long-term modelling experience, the model of van Rijn (1993) has been chosen as the most reliable tool in a class of engineering models. The computations have been carried out for 4 cross-shore profiles in Poland and 4 profiles in Russia (Kaliningrad Oblast). Locations and azimuths of these profiles, together with characteristic grain diameters, are shown in Table 2. The bathymetric cross-shore transects for the Polish coastal zone are shown in Fig. 3, while the profiles showing bathymetry on the Russian coast are plotted in Fig. 4. Figs. 3 and 4 show that there are 1 3 bars on each cross-shore transect. The mean inclination of all Polish cross-shore profiles, as well as the Russian profiles at Vistula Spit and Baltiysk, amounts to about 0.01 while the Russian profiles at Yantarnyy and Donskoye have steeper slopes, especially in the nearshore zone (500 m offshore) with inclinations reaching 0.02 m. It should be noted that a mildly sloped coastal seabed generally occurs at accumulative shores built of fine sand while a steeper nearshore sea bottom is usually observed at erosive coasts built of coarser sediments. For each cross-shore transect, waves resulting from individual deep-water wave climates (having parameters determined for the respective mean statistical years), have been numerically transformed to the shoreline, yie
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