APPLICATION OF A SCIENTIFICALLY-DRIVEN APPROACH FOR THE MANAGEMENT OF COASTAL EROSION ALONG THE HEL PENINSULA

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1 7 th ISE & 8 th HIC Chile, 2009 APPLICATION OF A SCIENTIFICALLY-DRIVEN APPROACH FOR THE MANAGEMENT OF COASTAL EROSION ALONG THE HEL PENINSULA WOJCIECH SULISZ Department of Wave Mechanics and Structural Dynamics, Institute of Hydroengineering, Polish Academy of Sciences, Kościerska 7, Gdańsk, Poland MACIEJ PAPROTA Department of Wave Mechanics and Structural Dynamics, Institute of Hydroengineering, Polish Academy of Sciences, Kościerska 7, Gdańsk, Poland MAŁGORZATA BIELECKA Department of Wave Mechanics and Structural Dynamics, Institute of Hydroengineering, Polish Academy of Sciences, Kościerska 7, Gdańsk, Poland The Hel Peninsula is an example of the area with several mutually related coastal zone management problems in Poland. Erosion has become a serious problem since the construction of a harbour westwards of the Peninsula in The site has been subject to a number of theoretical and field investigations, especially in the last two decades. Although some progress has been made, knowledge on sediment processes along the Hel Peninsula is still very limited. In this study a scientifically-driven approach is applied to investigate and evaluate processes responsible for the erosion problems along the coast and to support the management of coastal erosion. According to the approach, the Hel Peninsula site is treated as a coastal sediment cell. The coastal erosion is a result of a sediment imbalance in the cell induced by sediment transport processes. Based on available measurements the key concepts of the approach including coastal resilience as a strategic objective, a favourable sediment status as an operational objective, and a strategic sediment reservoir as a management goal, are applied to understand the specific character of the processes undergoing along the Hel Peninsula and to suggest solutions for local end users responsible for the management of the coastal zone. Keywords: coastal erosion, sediment transport, sediment cell, coastal zone management

2 INTRODUCTION Until the 17th century the Hel Peninsula was a chain of islands that formed a strip of land only during the summer. Now the peninsula is a sandy strip of land with the majority of the area lying below 2.5 m. It is 34 km long and its width varies from about 200 m in the narrowest part to over 3 km at the tip. This spectacular land constitutes a part of the coastal landscape park and is a famous recreational resort for the Polish and foreign tourists. A road and a railway run along the peninsula from the mainland to the town located at the furthest point, Hel, a popular tourist destination. The Hel Peninsula is an example of the area with several mutually related coastal zone management problems in Poland. Erosion has become a serious problem since the construction of a harbour westwards of the Peninsula in The harbour development has aggravated the situation. Since the Peninsula is very narrow it may be turned into an island by winter storms. Sand is supplied in large quantities to beaches to avoid breaching the Hel Peninsula. The site has been a subject to a number of theoretical and field investigations carried out at the Institute of Hydroengineering of the Polish Academy of Sciences (IBW PAN) in Gdansk, especially in the last two decades. Although some progress has been made, knowledge on sediment processes along the Hel Peninsula is still very limited. There are no field data on waves and currents and information about morphological processes is very limited. The amount of sediment supplied to the beaches is several times higher than a longshore sediment transport measured before the Wladyslawowo harbour was constructed and there is still a concern of breaching the Peninsula. It is planned to fill in this gap by measuring waves, modelling of short- and long-term beach profile changes, sand inventory and analysis of nourishment efficiency. The situation implies also a need to apply a scientifically-driven approach for the management of coastal erosion along the Peninsula. In this study a scientifically-driven approach that is based on concepts elaborated in the ongoing EU CONSCIENCE project is applied to the site of the Hel Peninsula. The approach is applied to investigate and evaluate processes responsible for the erosion problems along the coast and for the management of the coastal zone. According to the approach, the Hel Peninsula site is treated as a coastal sediment cell. The coastal erosion is a result of a sediment imbalance in the cell induced by the sediment transport processes. Based on available measurements the key concepts of the approach including coastal resilience as a strategic objective, a favourable sediment status as an operational objective and a strategic sediment reservoir as a management goal, are applied in the study to understand the specific character of the processes undergoing along the Hel Peninsula and to suggest solutions for local end users responsible for the management of the coastal zone.

3 COASTAL PROCESES AND PROBLEMS The main problem in the area is the coastal erosion occurring eastwards from the Wladyslawowo harbour. It has been triggered by the construction of the Wladyslawowo harbour, which started in 1936 and was completed in autumn At the end of the harbour construction the breakwaters were stretching 400 m offshore. Natural depth at the harbour entrance was 6 m. Before the harbour construction two longshore bars existed in the zone reaching 300 m offshore. The inner bar was usually situated some 150 m offshore. The outer one was greater and the average distance to that bar was m. Basing on observations, no particular local currents were detected in this area and only wave-driven currents were noticed. Distinct bottom movements occurred at depths up to 6m. The comparison of hydrographic maps showed that at greater depths the bed was either stable or its movements was insignificant (Szmytkiewicz, et al. 1998). Since the construction of the Wladyslawowo harbour significant coastal erosion occurred eastwards from the eastern breakwater and sand accumulation west of the western breakwater. These effects appeared rapidly. The construction of the harbour developed bed movements and its intensity was related to increasing breakwater lengths during the construction work. Shoals appeared along the whole surveyed area and further east. Outside western breakwater they formed a sand bank. According to the analysis of shoreline changes along the Hel Peninsula and along the whole Polish coast conducted by Basiński, et al. (1990), the breakwaters had a significant effect on shoreline in the vicinity of the harbour, especially, 2 km west and east from the harbour. Sediment transport in the area was assessed mainly on the basis of the longshore sediment transport. The transport rate for the period between spring 1936 and spring 1938 was calculated at 70,000 m 3 per year, upon the assumption that no by-passing occurred during that period (Adamski 1938). The longshore sediment transport, initially interrupted by the breakwaters construction, was restored due to beach accretion updrift of the breakwater. This fact was reported by many authors (Hueckel 1968, Mielczarski 1984, Szopowski 1958, Tubielewicz 1957, Zenkowicz 1962). In front of the breakwater the main bar appeared, which led to gradual shallowing of the harbour channel. Therefore already in 1948 dredging became indispensable. Despite intensive dredging, the depth at the harbour entrance decreased from 6 m reported in 1935 up to 4 5 m in 1975 (Szmytkiewicz, et al. 1998). The longshore sediment transport resulted in bed shallowing west of the harbor, where the isobath 7 m moved m offshore, and east of the harbour the shoreline retreated at a rate of 5 m per year (Szmytkiewicz, et al.1998). In order to protect the coast east from the harbour the artificial nourishment by direct dumping of sediment onto the beach has started since 1989 at several locations along the Hel Peninsula, including eastward vicinity of Wladyslawowo harbour. Besides the described above local erosion problems in vicinity of Wladyslawowo harbour, the storm surges are generally the crucial erosion factor for the whole Hel Peninsula (Basiński, et al. 1990). Another important element is decreasing sediment transport which is attributed to contraction of aeolian sediment transport path and thus to

4 the reduction of its intensity along the coast of Central Pomerania, where dunes are stabilised by afforestation (Mielczarski and Ostrowski 1989). The net sediment transport rates in the Władyslawowo region have been assessed since The transport rates were assessed either directly by computing volumes of sediment accumulated at the root of the western breakwater and dredged from the harbour channel, or by application of theoretical formulas. The assessed transport rates significantly varied depending on time and method of assessment (Table 1). Table 1. Assessment of net sediment transport rates in the Wladyslawowo region (Szmytkiewicz, et al. 1998) Source Active bed width Method of analysis Longshore sed. transp. rate 10 5 Dłuski (1938) ~400 m measurements of sediment accretion west of harbor m 3 /year 70 Słomianko (1960) up to h = 6 m as above 80 Słomianko (1960) up to h = 12 m Shizov s formula 178 Szawernowski the whole 120 (1965) nearshore zone Kowalski (1965) up to h = 6 m theoretical 92 considerations Kowalski (1965) the whole theoretical 120 nearshore zone considerations Semrau (1974) up to h = 8 m Present state... (1976) bathymetric changes and amount of dredged 128 sediment Janik (1978) Bijker 101 Mielczarski, et al. up to h = 10 m Bijker 192 (1981) Mielczarski, et al. up to h = 10 m Bijker plus corrections 115 (1982) stemming from other State of the art... (1986) methods assessment of sediment accretion west of harbour plus amount of dredged sediment Differences in the net sediment transport are significant and cannot be treated as reliable. The sediment deficit for the period of 1934 till 1971 was assessed as 0.189±0.142 million m 3 per year (Szmytkiewicz, et al. 1998). Another assessment carried out for the

5 period between 1960 and 1976 (Semrau 1985) resulted in 7 million m 3, i.e million m3 per year being twice as big as the previous estimate. The results of both studies differ significantly and pose serious uncertainties APPLICATION OF A SCIENTIFICALLY-DRIVEN APPROACH The uncertainties related with the differences in the net sediment transport assessed for the Hel Peninsula as well as a large scatter in the assessment of sediment deficit for the Peninsula imply a need to apply a scientifically-driven approach to investigate and evaluate processes responsible for the erosion problems along the coast and to support the management of the coastal zone. An approach that is developed in the EU CONSCIENCE research project has been selected. The project aims at implementing EUROSION concepts into measurable standards, which can be used and evaluated in practice for any field situation. According to the concept, coastal erosion is a result of a sediment imbalance occurring in a coastal sediment cell. The other key concepts are coastal resilience as a strategic objective, a favourable sediment status as an operational objective and a strategic sediment reservoir as a management goal. Suitable Coastal State Indicators (CSI s) are developed that can link scientific knowledge, measurements and modelling results to these management concepts. Accordingly, a scientifically-driven approach is applied to investigate and evaluate processes responsible for the erosion problems along the coast of the Hel Peninsula and to support the management of coastal erosion. In the first step, the application of the approach requires a selection of strategic and operational objectives. In the case of the Hel Peninsula the preservation of its coastal system has been selected as the strategic objective. This implies definition of two operational objectives, namely, maintaining the beach width and preserving breaching. In order to proceed with the application of the approach main natural processes affecting coastal erosion in the area should be indicated. The analysis indicates that the natural processes responsible for the erosion along the Hel Peninsula and affecting the management of the coast zone are longshore sediment transport and storm-induced erosion. These two processes have different time and spatial scales. The area affected by the longshore transport covers the whole coastline of the Hel Peninsula (34 km) and the time scale involved counts in decades. The storm-induced erosion is a short event that lasts several hours and affects certain parts of the Hel Peninsula coastline in the order of kilometers (Figure 1).

6 Figure 1. Hel Peninsula case study in the spatial temporal plane. Having defined the objectives and processes, the coastal cell boundaries can be established. According to the coastal sediment cell definition, it contains a complete cycle of sedimentation including sources, transport paths and sinks. The coastal sediment cell must include all coastal processes defined for the Hel Peninsula. Accordingly, the western cell boundary constitutes the breakwater of Wladyslawowo harbour that is impermeable for sand and a short section of the open sea that can take part in the transfer of sediment in the cell. Northern boundary of the coastal cell is an open sea boundary. The eastern boundary is located at the tip of the Hel Peninsula. The scheme of the Hel Peninsula coastal sediment cell is presented in the Figure 2. Figure 2. Sediment fluxes in the Hel Peninsula coastal cell. In the coastal sediment cell presented schematically in the Figure 2 natural processes as well as artificial nourishment are taken into account. The main cause of sand deficit in

7 the cell is a negative balance in the longshore sediment transport in the long-term temporal scale. A preliminary analysis indicates that the cross-shore sediment transport, which can be related to storm-induced erosion in the short-term temporal scale, is of secondary importance. An important process that has to be taken into consideration in the coastal cell is the artificial nourishment. This process requires determination of a strategic sediment reservoir for nourishment activities. According to the applied approach, the strategic sediment reservoirs are the sources of sediment from where sediment can be taken without endangering the natural balance and is required to achieve operational and strategic objectives. In the case of the Hel Peninsula one of strategic sediment reservoirs is the entrance channel to the Wladyslawowo harbour. To maintain navigational depths the channel must be constantly dredged. However, the amount of sediment is not sufficient to meet operational objectives. The analysis indicates that sand can be supplied from offshore areas. Moreover, under certain conditions, Puck Bay and Gdansk Bay may be considered as potential sources of sediment for the artificial nourishment of the Hel Peninsula. Suitable Coastal State Indicators must be defined in the applied approach to link scientific knowledge, measurements and modelling results to management concepts. An analysis of available reports regarding erosion problems indicates that there is a set of parameters, which can be applied in the management of the coastal zone for the Hel Peninsula. Threshold values for a required level of shore safety for T = 100 years have been proposed by Dubrawski and Zawadzka-Kahlau (2006). These values can be considered as preliminary Coastal State Indicators for the Peninsula. The values include beach width, beach height, dune width, maximum dune height, dune cross-section area and hinterland height. In order to better understand specific characters of the processes undergoing along the Hel Peninsula and to achieve the strategic objective more information and long-term data are necessary. In particular, long-term data regarding the rate of the sediment flux due to a longshore transport, cross-shore transport, and artificial nourishment are necessary. The set of long-term data can be obtained by the application of wave and morphodynamic models supported by available measurements and new field campaigns. These data and planned field measurements will be used to analyze long-term processes occurring in the coastal sediment cell. All these activities are necessary to identify a favourable sediment status that can promote coastal resilience and, in consequence, to achieve the strategic objective. SUMMARY The Hel Peninsula is an example of the area with several mutually related coastal zone management problems in Poland. Erosion has become a very serious problem since the construction of a harbour westwards of the Peninsula in The site has been subject to a number of theoretical and field investigations, especially in the last two decades. Although some progress has been made, knowledge on sediment processes

8 along the Hel Peninsula is still very limited and there is a concern that winter storms may breach the Peninsula. In this study a scientifically-driven approach is proposed to investigate and evaluate processes responsible for the erosion problems along the Hel Peninsula and to support the management of the coastal zone. According to the approach, the Peninsula is treated as a coastal sediment cell. Based on available measurements the key concepts of the approach including coastal resilience as a strategic objective, a favourable sediment status as an operational objective and a strategic sediment reservoir as a management goal, are applied to understand the specific character of the processes undergoing along the Hel Peninsula and to suggest solutions for local end users responsible for the management of the coast zone. The coastal sediment cell has been defined for the Hel Peninsula site. The processes responsible for the sediment imbalance in the coastal cell have been identified in both, spatial and temporal scales. In the case of the Hel Peninsula a longshore sediment transport has been indicated as a dominant process. Due to nourishment activities at the site, strategic sediment reservoirs have been proposed. Based on the present experience in dealing with coastal erosion problems preliminary Coastal State Indicators have been defined for the Hel Peninsula. The values include beach width, beach height, dune width, maximum dune height, dune crosssection area and hinterland height. In order to better understand specific characters of the processes undergoing along the Hel Peninsula wave and morphodynamic models will be applied to obtain long-term data to support the approach proposed in this study. These data and planned field measurements will be used to analyze long-term processes occurring in the coastal cell. All these activities are necessary to identify a favourable sediment status that can promote coastal resilience and, in consequence, to achieve the strategic objective. ACKNOWLEDGEMENTS The work described in this paper was supported by the European Community s Sixth Framework Programme through the grant to the budget of the CONSCIENCE project, contract REFERENCES [1] Adamski, Z Fishery harbour of Wielka Wieś. Polish Academy of Sciences Sea Research Committee, Studia i Materiały Oceanologiczne, No. 8, Wrocław- Warszawa-Kraków-Gdańsk, pp 5-35 (in Polish). [2] Basiński, T., A. Mielczarski and M. Szmytkiewicz Hel Peninsula protection concept. Report, Polish Academy of Sciences Institute of Hydro-Engineering, Gdańsk (in Polish).

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