water Article Impact of the Artificial Strait in the Vistula Spit on the Hydrodynamics of the Vistula Lagoon (Baltic Sea) Michał Szydłowski * , Tomasz Kolerski and Piotr Zima Faculty of Civil and Environmental Engineering, Gda´nskUniversity of Technology, Narutowicza 11/12, 80-233 Gda´nsk,Poland; [email protected] (T.K.); [email protected] (P.Z.) * Correspondence: [email protected]; Tel.: +48-58-347-1809 Received: 25 March 2019; Accepted: 8 May 2019; Published: 10 May 2019 Abstract: In the Vistula Lagoon, storm surges are induced by variable sea levels in the Gulf of Gda´nsk and wind action. The rising of the water level in the southern part of the basin, exceeding 1.0 m above mean sea level, can be dangerous for the lowland area of Zuławy˙ Elbl ˛askie,causing the inundation of the polders adjacent to the lagoon. One of the potential possibilities to limit the flood risk is to decrease the water level in the lagoon during strong storm surges by opening an artificial canal to join the lagoon with the Gulf of Gda´nsk.The decision to build a new strait in the Vistula Spit was made in 2017. In order to analyze the impact of the artificial connection between the sea and the lagoon during periods of high water stages in the southern part the lagoon, mathematical modelling of the hydrodynamics of the Vistula Lagoon is required. This paper presents the shallow water equations (SWEs) model adapted to simulate storm surges driven by the wind and sea tides, and the numerical results obtained for the present (without the new strait) and future (with the new strait) configuration of the Vistula Lagoon. Keywords: Vistula Lagoon; storm surges; mathematical modelling; flood risk; artificial strait 1. Introduction Coastal lagoons are defined in [1] as inland and shallow water bodies (depths not exceeding a few meters), separated from the ocean by a barrier. They are connected to the ocean by at least one restricted inlet. The orientation of the lagoon may vary, but the parallel to the shore is dominant. According to [2], coastal lagoons occur on about 12% of the length of the world’s coastline. The formation of lagoons could be caused by alluvial progradation, the transgression of barriers in response to a sea level rise, or be formed behind storm-build barriers. A good example of this last type of formation can be found on the southern shores of the Baltic Sea. These semi-closed water areas are very calm and have the same salinity as the open sea, making them ideal for aquaculture. Some lagoons have been extensively converted for brackish-water fish ponds (e.g., the lagoons south of Madras), shellfish aquaculture (i.e., lakes on Hokkaido Island [3]), or cage culture, which is widespread around the world [4]. Changing climatic conditions [5,6] over the next century are likely to increase marine influences in lagoons, by the action of wind and storm surges. The paper presents some possibility of flood relief by the example of the proposed cut channel on the Vistula Lagoon transboundary water body. The Vistula Lagoon is located on the southern coast of the Baltic Sea, in the east part of the Gulf of Gda´nsk (Figure1). The length of the lagoon is 90.7 km and its width varies from almost 6 km up to 13 km. The mean width is 8.9 km. The lagoon is a very shallow basin with a mean depth of only 2.75 m. It is separated from the Gulf of Gda´nsk by the Vistula Spit of a length of about 65 km. The only connection between the Vistula Lagoon and the Baltic Sea is through the Strait of Baltiysk, which is 2 km long, 440 m wide, and approximately 8.8 m deep. The Vistula Lagoon, which developed about 6000 years ago, was created by being cut off from the Baltic Sea by the Vistula Spit. Since the digging of the canal Water 2019, 11, 990; doi:10.3390/w11050990 www.mdpi.com/journal/water Water 2019, 11, 990 2 of 19 only connection between the Vistula Lagoon and the Baltic Sea is through the Strait of Baltiysk, which Water 2019, 11, 990 2 of 18 is 2 km long, 440 m wide, and approximately 8.8 m deep. The Vistula Lagoon, which developed about 6000 years ago, was created by being cut off from the Baltic Sea by the Vistula Spit. Since the digging of(artificial the canal strait) (artificial across thestrait) Vistula across Spit the in 1497Vistula in the Spit vicinity in 1497 of Baltiysk,in the vicinity the Vistula of Baltiysk, Lagoon the has Vistula been in Lagoonconstant has contact been with in constant the sea [ 7contact]. The lagoon with the represents sea [7]. aThe transboundary lagoon represents area divided a transboundary into two parts area by dividedthe state into border two between parts by Poland the state and border Russia. between The total Poland area of and the Russia. lagoon The measures total area 838 kmof the2, of lagoon which measures472.5 km2 838belongs km2,to of Russia. which 472.5 The shore km2 linebelongs is 270 to km Russia. long, The with shore a water line volume is 270 km of 2.3 long, km 3with[8]. a water volume of 2.3 km3 [8]. Figure 1. LocationLocation of of the the Vistula Vistula Lagoon. Unsteady flows flows in the Vistula Lagoon are usually caused by changes of the sea level in the Gulf of Gda Gda´nskandńsk and by wind action on the water surface of the lagoon. When When the the water stage of the Gulf of Gda Gda´nskrises,ńsk rises, a difference difference in sea level in relation to the lagoonlagoon appears. This This results results in in a strong water current directed into the lagoon. The The decrease decrease of of the the water water table table in in the Baltic Sea causes the lagoonlagoon water water to to flow flow into into the the sea, sea, creating creating a strong a strong current current in the in thestrait strait in the in opposite the opposite direction. direction. The 3 totalThe totalvolume volume of sea of seawater water inflowing inflowing into into the the lagoon lagoon equals equals about about 17 17 km km3 perper year. year. In In extreme 3 1 conditions, the water inflowinflow reaches 10,00010,000 mm3 s−-1. .Such Such a asituation situation can can occur occur several several times times per per year, year, mainly in autumn andand winter,winter, whenwhen storm storm surges surges in in the the sea sea reach reach a watera water level level of overof over 0.8 0.8 m a.s.l.m a.s.l. [9]. 3 1 [9].The The total total outflow outflow from allfrom the all rivers the enteringrivers entering into the into lagoon the is lagoon about 180is about m s− 180, and m has3 s-1 no, and significant has no significantinfluence on influence changes inon the changes water stagein the in water this basin. stage Except in this for basin. the water Except exchange for the through water exchange the Strait throughof Baltiysk, the theStrait hydrodynamic of Baltiysk, the conditions hydrodynamic of the lagoon conditions depend of the on thelagoon wind depend action. on In the the wind region action. of the 1 InVistula the region Lagoon, of the SW Vistula (from South-West Lagoon, SW direction) (from Sout windsh-West with direction) a velocity winds from 4 with to 6 ma velocity s− (wind from velocity 4 to 6measurements m s-1 (wind velocity at a height measurements of 10 m) prevailat a height [8]. of These 10 m) winds prevail can [8]. cause These a risewinds of can the cause water a level rise of in thethe water Gulf of level Gda´nskand in the Gulf in theof Gda Vistulańsk Lagoonand in the of upVistula to 0.8 Lagoon m over of the up mean to 0.8 sea m level. over However,the mean sea NE level.(from However, North-East NE direction) (from North-East winds in particular direction) cause winds a dangerous in particular water cause level a risedangerous in the southern water level part 1 riseof the in the Vistula southern Lagoon. part Forof the long Vistula periods Lagoon. (12 h Fo andr long more) periods of strong (12 h NE and winds more) (10 of mstrong s− and NE winds more), (10a rise m ofs-1 theand water more), level a rise exceeding of the water+1.0 level m and exceeding in extremes +1.0 reachingm and in +extremes1.5 m above reaching the mean +1.5 m sea above level can be observed [9]. The total tidal vertical range in the southern part of the lagoon is about 3.0 m (from –1.5 up to +1.5 m a.s.l.). Although NE winds are relatively rare, they can be very dangerous for Water 2019, 11, 990 3 of 19 Water 2019, 11, 990 3 of 19 Waterthe mean2019, 11sea, 990 level can be observed [9]. The total tidal vertical range in the southern part of the lagoon3 of 18 theis about mean 3.0 sea m level (from can –1.5 be upobserved to +1.5 [9].m a.s.l.). The total Alth tidalough vertical NE winds range are in relatively the southern rare, part they of can the be lagoon very isdangerous about 3.0 form (fromthe polders –1.5 up adjacent to +1.5 m to a.s.l.).