Long-term Coastline Evolution under Anthropogenic Impacts in the Bay of Varna (Bulgarian Black Sea Coast)
Shortcut URL: http://serc.carleton.edu/31890
UTM coordinates and datum: North 4782639; East 574378, WGS_1984_UTM_Zone_35N
Climate Setting: Semi-Arid
Tectonic setting: Continental Arc
Type: Process, Chronology
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).
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.
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