The paper presents the results of an analysis of the morphometric characteristics of scour hole located in the Strait of Baltiysk, which is the entrance to the Vistula Lagoon of the Baltic Sea. The temporal variation of main morphometric characteristics of the hole was analyzed for 9-year period (2008–2016) by year-to-year comparing the data of the digital relief model, changes of the contours of isobaths and displacement of the peaks in hole bathymetry. Specific attention was put the volume of deepest part of the hole – the volume below the isobaths of 21 m, 25 m, 27 m till maximum depths. It was found that, in general for the 9-year period (2008–2016), the volume of the hole increased, the average volume growth was 2450 m³/year. The growth was not monotonous, the deviation from the trend line was -9–6 %. The minimum volume was observed in the period of 2011–2013. The percentage changes in volume were up to 27 % in relation to the minimum volume found for the study period. Two peaks (‘northern’, the ‘near-mole’) and three erosive troughs (which volume gradually increased during studied period) were revealed in the morphometric structure of depression, These troughs elongated in different directions from the peaks, were marked, as the ‘northern’, the ‘near-mole’ and the ‘wraparound’ troughs. Synchronicity in the changes in the position of these two peaks was not observed, but on the whole both peaks were deepened approximately by 1 meter during the study period. Analysis of the dynamics of the depths along main longitudinal and transverse transects showed strong erosion of the base of the wall of the southern mole. The offshore slope of ‘northern’ troughs is conditionally stable, and the onshore slope has moved a little inside the strait for a distance of about 10 m. Analyzed information allows to put forward a hypothesis about the mechanisms of scour hole formation. It is assumed that it is supported by strong currents from the northern rumba due to the interaction of the alongshore current and the southern protective mole considerably protruding into the sea. The northern mole protruding into the sea reduces the cross-section of alongshore flow going from north to south. It is the reason of the acceleration of the flow near the end of the mole. This stream, interacting with the tip of the southern mole staying on its way, reflects from it, forms the central peak and the northern through, and also flow round the tip of the south mole from two sides, forming the outgoing arms of the ‘near-mole’ and the ‘wraparound’ troughs. The scour hole has not reached equilibrium and continues to increase in size. The shift of the onshore slope of the hole indicates a slow spreading of the hole deep into the Strait.

Zakirov Ruslan Bajaditovich, Postgraduate, Immanuel Kant Baltic Federal University, 14, A. Nevski st., Kaliningrad, 236041, Russian Federation, Senior Engineer, Shirshov Institute of Oceanology RAS, 36, Nakhimovski pr., Moscow, 117997, Russian Federation, tel.: (909)783-54-04, e-mail: kotruslan2@gmail.com 

Chubarenko Boris Valentinovich, Candidate of Sciences (Physical and Mathematical), Leading Researcher, Head Laboratory of Coastal Systems, Shirshov Institute of Oceanology RAS, 36, Nakhimovski pr., Moscow, 117997, Russian Federation, tel.: (906)239-10-32, e-mail: chuboris@mail.ru 

Sologub Sergej Petrovich, Head of Depth Control Group, FSUE “Rosmorport”, North-Western Basin Branch, 7, Pyetr Vyelikiy Quay, Kaliningrad, 236006, Russian Federation, tel.: (921)260-20-56, e-mail: sologub@portkld.ru 

Shusharin Aleksej Vital'evich, Chief Hydrograph, FSUE “Rosmorport”, North-Western Basin Branch, 7, Pyetr Vyelikiy Quay, Kaliningrad, 236006, Russian Federation, tel.: (4012)36-21-00, e-mail: a.shusharin@kld.rosmorport. ru

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