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Long-term Coastline Evolution under Anthropogenic Impacts in the Bay of Varna (Bulgarian Black Sea Coast) part of Vignettes:Vignette Collection
The Bulgarian Black Sea coast is 412 km long (Stanchev, 2009) and embraces the territory between cape Sivriburun (the border with Romania) on the North and Rezovska River mouth (the border with Turkey) on the South (Fig. 1). The Bay of Varna is the second largest bay along the Bulgarian coast, located in its northern part between cape Sv. Georgi on the North and cape Galata on the South (Fig. 1). The bay was formed in the drowned mouth of the Provadijska River during the Holocene and was separated from the existing inland lake by a large sandy spit named Asparuhovska. The adjacent coastline is 17780 m long and has a general ESE orientation. Sandstones, marls and clays are outcropped at the coast around the Varna Bay, as the average rate of coastal erosion reaches 0.20 m/y (Peychev and Stancheva, 2009). Two wave energy fluxes with opposite directions determine sediment transport along the coast of the study area (Fig. 1): the first one named Evksinogradski, starts at cape Sv. Georgi with SW direction; and the second one, named Asparuhovski begins at cape Galata and westward directed to the bottom of the bay (Dachev and Cherneva, 1979). This way, two capes of Sv. Georgi and Galata are zones where fluxes diverge, while the western part of Varna Bay (Asparuhovo beach) is the zone of convergence. This is the possible reason for discharge of the sediment fluxes at the far end of the bay, which had favored the formation of the large sandy spit over the Holocene (Dachev, 2003). Presently, there are few sandy beaches in the Bay of Varna (Fig. 1): small Evksinograd beach; artificial Varna-groins beach, created between the fields of solid groins; Varna-central beach, formed in the re-entrant angle between port mole and the coastline; and the natural Asparuhovo beach (remain of past sandy spit). Case study The prevalence of the retreating cliff along the coast of the study area in the beginning of past century has enforced implementation of many hard stabilization structures (mostly dikes, groins and seawalls). These works have been particularly increasing over recent decades. The coastline retained its natural landscape until 1906, when a one km long mole of Varna Port was constructed, thus interrupting sediment supply to the Asparuhovo beach (Fig. 1). Thereby, the human impacts on the coast of Varna Bay started in the beginning of the past century with building of port mole and old navigational channel on the part of the large sandy spit. The created re-entrant angle on windward side of the mole had acted as a "trap" for the Evksinogradski long shore sediment flux. As a consequence, a new beach strip named Varna-central was formed, however the sand supply to the following in SW direction Asparuhovo beach has been obstructed. An enormous building of hard coastal structures was performed in 1980s: 3 km coastal dike and number of solid T-shaped groins and spurs. In 1976 a new navigational channel was dug to serve the deeper-draft generations of vessels on their way to the port of loading, located in the lake of Varna (Fig. 1). Various types of data were used to detect 100-year (1908-2007) coastline changes in the Bay of Varna: historical data from topographic map in scale 1:10 000, produced in 1908; and data from QuickBird`s satellite images (data acquisition May 2007), gained free from Google Earth (http://earth.google.com/). Detection of long-term coastline changes was supported by Geographic Information System (GIS), which allows combining historical datasets and modern satellite data. For the purpose of coastline segmentation a detailed field survey was carried out in May 2007, as coastline and port/coast-protection structures were measured with GPS "Garmin 12" (Stancheva et al., 2007). On the basis of data derived from historical map and GPS survey, the segmentation of both historical (1908) and modern (2007) coastline was performed, using geomorphologic and engineering criteria (Fig. 2 and Fig. 3). Various types of segments identified in Varna Bay were combined in two major groups: -natural coastal segments presented by sandy beaches and cliffs; -technogenous coastal segments presented by port and coast-protection structures. In the beginning of the past century (1908) a few maritime structures were built: the mole of Varna Port, old navigational channel and smaller port molе at cape Sahanlak in the northernmost part of the bay. By contrast, the coastline segmentation in 2007 shows a great increase of coast-protection structures, which have been constructed over the last few decades. Thus, during the GPS field survey in May 2007 a number of 27 technogenous segments with total length of 12655 m were identified at a 17780 m long study area. In this way long-term coastline changes in the Bay of Varna could be related to coastal defense and other maritime activities, furthermore at some sites the coastline was irreversibly modified. Through dike's construction the coastline was shifted seaward with 40 m and new land territory was created. Few years later the project for serial construction of solid groins was run, as well as beach filling and transverse sand bypassing were applied (Dachev, 2003). As a result, a few sandy beaches, generally named Varna-groins, were formed between the groins fields (Fig. 4). Since the beginning of the past century the Asparuhovo beach dynamic has mostly depended on built maritime structures in the Bay of Varna. Over the entire 100-year period the beach has been negatively affected by human activities, in particular by port functions. As mentioned above, the port mole has obstructed sediment supply to the beach, resulting in sand deficit at the adjacent underwater coastal slope. Two navigational channels have caused destruction of the large sandy spit and damages to the whole coastal ecosystem. Digging of deeper navigational channel in 1976 had very adverse impact on beach stability: its length was reduced with 800 m (Stancheva et al., 2008; Fig. 5). In addition, constantly performed dredging activities are acting as sand mining thus causing irreversible loss of sediments that could be used for replenishment of the beaches in the study area.
Flood-prone Low-lying Territories along the Bulgarian Black Sea Coast part of Vignettes:Vignette Collection
Over recent decades the increased frequency of natural hazards resulting from global climate change has become one of the most severe problems affecting sustainability of costal zones. In particular, low-lying areas which are strongly damaged by flooding and coastal erosion, pose the most serious consequences for the sustainability of coastal systems and public safety (Szlafsztein, 2005). Furthermore, these problems have been exacerbated due to rapidly growing coastal population pressure, leading to poorly planned development of the hazard-prone areas. Most vulnerable are those systems with highest sensitivity and lowest adaptability to climate change effects, including low-lying coastal areas, sandy coasts, dunes, marshlands, lagoons, firths, deltas, etc. (Klein and Nicholls, 1999; IPCC, 2007). An additional factor that contributes to vulnerability of coasts is a land subsidence, caused by tectonic vertical land movements. Such low-laying areas are also most attractive and productive environments (Nicholls, 2002) both for settlements and human activities, because they offer a great variety of possibilities for better livelihoods, easy access to the sea and recreation. These settings make even more important the efforts to identify coastal areas with high priority of risks to sea flooding. The Bulgarian Black Sea coast has a length of 412 km, as 58% of it is exposed to coastal erosion and landslide processes (Stanchev, 2009; Peychev and Stancheva, 2009). These natural processes have been aggravated by increasing anthropogenic impacts associated with accelerated urbanization and human influence along the coast. There are 14 Bulgarian Black Sea coastal municipalities which comprise about 5 % from the entire territory of the country and hosted about 9 % of the national population. Coastal units potentially vulnerable to the impacts of sea level rise along the Bulgarian coast are firths, lagoons, sandy beaches and dunes often being also under subsidence processes. The contemporary subsidence land movements constitute about 1/3 of the entire coast. The most intensive subsidence rates have been estimated at firths and lagoons at the South Bulgarian coast, as values vary from 0.8 mm/y to 3 mm/y at some firths, while in the region of the Burgas lakes this rate reaches even 5 mm/y (Keremedchiev and Stancheva, 2005). Firths and lagoons are typical of the Bulgarian coastal zone. Firths are old river valleys drowned during the Holocene. The number of firths and lagoons are 26 and 5 respectively (Dachev, 2000). Case study Long-term sea level changes along the Bulgarian Black Sea coast have been traced for more than 100-year period. Based on the records of two marine stations, located at towns of Varna and Burgas, a continuous sea level increase has been found, particularly over the last few decades. Natural and anthropogenic factors are pointed out as the main causes for accelerated sea-level rise. The natural factors contributing to sea level rise include changing river discharge into the Black Sea, rainfall-evaporation balance, water exchange through the straights linking the Black Sea to the Mediterranean (Dachev, 2000) and subsidence of the land. Anthropogenic factors affecting sea level changes are: urbanization, dam and reservoir buildings, groundwater mining, deforestation, etc. The mean values of average sea level rise for the Western part of the Black Sea vary between 1.5 mm/y and 3 mm/y (Pashova and Jovev, 2007). Although such rates are not dramatic for the Bulgarian coast there would be a case of sudden sea level rise under extreme storm conditions. In contrast to climate change-induced sea level rise, which can be predicted over a long-term scale (100 years), the extreme sea level increase associated with storm surges, tsunamis and rain-storms could have a short, but particular devastating impact on coastal areas. Significant coastal changes typically occur during such extreme events, causing more intense coastline retreat and beach erosion (Ashton et al., 2007; Boyd et al., 2002). As a consequence, functions and values of the coastal systems could be degraded, and public safety and economy could be affected (Szlafsztein, 2005). Coastal storms are extreme meteorological events that mainly occur along the Bulgarian Black Sea coast in winter with the strongest N and NE winds. Examples of such extreme events along the Bulgarian coast are: the storm happened in February 1979 accompanied by sea level increase; the storm in June 2006, also combined with intensive rains. Extreme wind waves are the main cause of flooding in low-lying coastal territories. These also cause increased salinity in most coastal firths and intrusion of seawaters into fresh groundwaters, the activation of erosion and landslides. The central parts of the Bulgarian Black Sea coast are mostly affected by salinization (Dachev, 2000). To indicate low-laying territories potentially vulnerable to flooding due to extreme sea level rise along the Bulgarian coast, 1:50 000 topographic maps were used (Palazov and Stanchev, 2007). Maps processing includes a series of steps: scanning, geo-referencing and digitizing with help of Geographic Information System (GIS) ArcInfo 9.2. The following approach was used: extreme sea level rise scenarios from 1 to 5 m with interval of 1 m were assumed on the basis of historical background and data collected over a 100-year period (Fig. 1). The performed analysis shows that 14 towns, 17 villages, 13 sea resorts and 7 small campsites would be potentially flooded by extreme sea level rise of 5 m. Number of affected coastal population counts almost 100 000 from nearly half a million residents at these sites, according to last Census data (NSI, 2001). Low-laying areas around Varna Bay (Fig. 2), Kamchia River resort, Burgas town, Sunny beach resort (Fig. 3 and 4), Pomorie town and coastal section between Albena resort and Kranevo village were identified as most vulnerable to inundation. The risk to flooding is higher in summer presumably due to greater numbers of tourists having their rest at the sea coast. In summary, about 20% (83 km) of the entire 412 km long Bulgarian coast are indicated as flood-prone territories or these are coastal units most vulnerable to inundation due to extreme sea level rise of 5 m. Total number of local coastal residents at these sites constitutes 6.93% of country's population.