Volodymir Vorovka, Svitlana Vynokurova, AZBOS

Configuration of coasts of Molochnyi Liman and dynamics of changes of the coastline completely depends on relief of the left and right coasts. Their asymmetry is caused by river origin of the liman valley as a result of ingression of sea waters of a lower part of the Molochna River valley induced by epeirogenic depressions of land (Roslyi, 1990).

Sea waters filled the lowest part of the river valley – a low floodplain terrace 0.5-2 m high above water level in a low-water period. 1.5 m amplitude of height for a flat floodplain (now bottom of the liman) is significant and has an essential impact on the surface area of water during decrease of water level. Accumulation of liman deposits of Molochnyi Liman has lead to transformation of the river floodplain into the liman terrace.

An outer border of the left coast of a bowl of Molochnyi Liman, which also limits extension of the liman terrace during maximum water level, is a high river floodplain of the Molochna River (Nikitin, 1957). Its altitude is 3-3.5 m above water level in a low-water period, i.e. averagely 1.5-2.5 m exceeds floodplain level.  This terrace is represented by an abrasion ledge up to 2 m high, and also sites with a relatively smooth transition from a high floodplain to the first/second above-floodplain terraces which wedge out near Mordvynivka Village. Bedrock coast of the terrace is often accompanied with longshore accumulative depositions of liman origin (Dovgyi Island, Pidkova Island, Oleksandrivska Spit with a system of coastal capes, Kubek Peninsula), mouths of rivers (Dzhekelnia), small gullies (Oleksandrivska Bay), salt lakes in depressions (Molochne, Dunaivske). A considerable part of surface of the liman terrace of the left coast is much lower, compared to that of the right bank. This has an essential impact on width of the strip exposed during decrease of water level in the liman especially in the north-eastern part.

On the right coast there is observed a generally narrow strip of the liman terrace. Its width increases within concave sections of the coast and sharply reduces within ledges. It abruptly transit into the base of a loess accumulative valley, represented by a steep high erosion ledge formed by a river stream in geological former times. Compared to the left coast, the right coast has a relatively straightened configuration of the coastline. The liman coast is partially limited by precipitous sections of the bedrock coast (Bohatyrskyi ledge) and littoral accumulative walls of the floodplain of the Tashchenak River and Sheluihivske, Okhrimivske and Lymanske depressions (marshes). Most of right-bank area of the liman is related to abrasive-accumulated type with combination of steep slopes and accumulative coastal areas divided from the liman by sandy-shell walls. The strip exposed during decrease of water level is also relatively narrow that is explained by a lower hypsometric location of the right part of the liman bottom and also by a sharp transition from the liman terrace to a watershed loess plateau.

Accumulative formations in Molochnyi Liman are Oleksandrivska Spit with its coastal northern and southern ends, Kubek Peninsula which separates waters of Molochnyi Liman from Oleksandrivska Bay, Pidkova Island, Dovgyi Island, a number of small islands and spits of northern coast of Oleksandrivska Bay, right and left coast of the liman.

With 10-20 cm decrease of water level the area of accumulative formation logically expands, but only a bit, which is connected with characteristics of coastal bottom relief. Further general decrease of water level induces a sharp extension of the area of accumulative formations. A gradual character of transition from accumulative formations to the actual liman bottom also complicates calculations of dynamics of their area. It can be assumed that 10 cm decrease of water level provokes 15-20% extension of the area of accumulation formation at the expense of their underwater parts. Therefore, the total area of accumulative formations is exposed in case of 100 cm decrease of water level.

To analyze the dynamics of water surface area and accumulative formations of Molochnyi Liman there were used Landsat-7 satellite images freely distributed by the United States Geological Survey (USGS) through GLOVIS web service (USGS Global Visualization Viewer,http://glovis.usgs.gov) for 2005 (the most high-water year), 2009 and 2012. Since the main aim was to estimate the area of accumulative formations and exposed sites it was chosen the driest period of a year (July-September) when the areas of exposed sites are maximal. To analyze the images a combination of canals 753 was used.

In 2005, the most high-water year for the last decade, a greater part of large accumulative formations was covered with water and small formations were completely under water. When water level in Molochnyi Liman considerably drops the areas of accumulative formations sharply increase. Dynamics of areas for some key accumulative sites from 2005 to 2012 (with the low water level) is shown in Table and on Figure 1.

Table. Dynamics of areas of some accumulative sites of Molochnyi Liman
Coast site Area in 2005 (ha) Area in 2012 (ha) Increase of area, %
Peresyp Spit 1003.2 2464.6 245.7
Pidkova Island 30.8 238.0 772.7
Oleksandrivska Strilka Spit 156.4 1026.7 656.5
Dovgyi Island 1.4 12.9 921.4
Kubek Peninsula 20.0 104.4 522.0

Analysis of dynamics of the area of bottom exposure according to satellite images of different years during the last decade has shown considerable difference in values. Thus, in years with maximal water level in liman (August 2005) the surface water area was 22,049.9 ha, in September 2009 – 16,558.4 ha, and surface water area was recorded in July 2012  – 16,680.1 ha. Maximal difference between the surface areas for this period was 5,491.5 ha.

The widest exposed strips are observed along the eastern and southern coast, in particular in the north-eastern section of the coast between villages of Gyrsivka and Mordvynivka (including Dzhekelnianska Bay), in the south – along Peresyp Spit, and in the south-eastern part (Oleksandrivska Bay, Molochne Lake) due to a higher hypsometric location of bottom.

See map:

  • Figure 1. Dynamics of the surface water area and exposed areas in Molochnyi Liman in 2005-2012

A higher hypsometrical location of bottom of the northern-eastern part of the liman, conditioned by intensity of descending geological movements compared to central and southern parts of the liman and accumulative activity of the Molochna River for the period of liman existence, resulted in the width of exposed strip reaching 1,100 and 1,300 m from the border of maximum water's edge. A total exposed area together with exposed delta formations of the Molochna River and an exposed zone around Pidkova Island amounted 2,173.0 ha in July 2012.

A higher bottom along Peresyp is caused by accumulative activity of the sea at the phase of spit formation and connected with inwash of sandy-shell alluvium from the sea into waters of the bay of those times. The bottom also forms a wide exposed strip (800 to 2100 m from the border of maximum water's edge). A total area of bottom exposure in this site constituted 1,461.4 ha (until Kubek Peninsula).

Exposed strips of the south-eastern part of the liman is connected with a higher hypsometrical location of gully depressions of Oleksandrivska Bay, oval depressions of Molochne Lake and similar smaller lakes located to the north of Molochne. At the same time, bottom of Molochne Lake, in more high-water years for the liman, can be partially filled with water. In last years, in August the lake bottom remained dry.

The right coast of the liman is characterized by smaller areas of the exposed strip and its more uniform width. On the average, the exposed strip of the right liman coast was 250-300 m in September 2009, increasing within a lowland coast of the Tashchenak river valley, on a concave section of the coast near Lymanske Village and along the liman coast of Kyrylivka Village. In these zones the width of the exposed strip reached 800-1200 m.

Therefore, at the low water level in Molochny Liman (July 2012) a total exposed area reached 7,455.25 ha, and the area of water surface reduced to 16,680.1 ha, i.e by 24.4%.

It is natural that with decrease of water level in the liman the area of accumulative formation extends. In this process, essentially important are small depths, practically flat bottom with a smooth decrease of depth from the centre to coasts, and relatively sharp changes of depths in the coastal zone induced by a trough-like form of bottom.

The analysis of changes in areas of accumulative formations and dynamics of exposed areas of liman bottom allowed to reveal a relationship between water level in the liman and size of the exposed area. It gives a possibility of distant prognosis for the exposed area and new islands and has an important value in location of varied elements of flora and fauna.

References

  • Roslyi I.M. Geomorphology of the Ukrainian SSR. - Kyiv: Lybid, 1990
  • Nikitin V.N. On the geomorphology of the lower part of the Molochna River Basin / / Proceedings of the Kharkov State University named after A.M.Gorkyi, Volume 81. Transactions of Geographical Faculty, Volume 3, 1957. - P. 107-129