ИСТИНА

Reasons of ice dam phenomena in the Kuban river mouth and caused by them inundations are the autumn-winter flood regime of left-bank tributaries, unstable ice cover and presence of numerous obstacles to ice movement. It is possible to attribute meanders and narrow spot of a channel, knots of bifurcation, shallows to ice dams sites on the Kuban river delta. There are 20 bottom banks in the Kuban branch and about 25 in the Protoka branch. River mouth bars and artificial objects (in particular, bridges) have similar effect. For example, there are many mentions of ice dams and jams upstream railway and automobile bridges through the Protoka and the Kuban branches in reference books. It became one of the reasons of transfer of gauging stations Slavyansk and Troitskaya. Ice cover in river channel or at the nearshore zone can be as an obstacle to ice movement also. So, for example, it was in 1940, 1949, 1985 and 2002. On the whole there are about 7-10 ice dam sites on the Protoka branch and, probably, yet more on the Kuban branch. Last 15-20 years the situation with unobstructed movement of ice formations in deltaic branches has worsened. The reason is the stoppage of channel clearing and dredging works. Therefore after the disaster winter inundation in 2001-2002, besides embankment consolidation and heightening, the channel clearing and deepening, dredging works on river mouth bars have renewed also. However, according to expedition data for summer 2003, the reach of the Protoka branch between Grivenskaya and mouth cross-section is still heavily obstructed with fallen trees. The hydrological regime factors of formation of ice dams have undergone more essential changes. To a great extent it is connected to building of large hydraulic engineering objects in the river basin. So building of Krasnodar and other flood-control reservoirs has enabled to regulate winter flood of the Kuban River and its left-bank tributaries, which not only accompanies ice drift, but sometimes it leads to it. In case of ice dam formation large water discharges promote intensive rise of water levels in the river and branches and flooding of deltaic lands upstream ice dam. However, last years, due to the regional increase of precipitation quantity and decrease of construction safety the performance of this important problem has been complicated. The winter inundation in the Kuban river delta in 2001-2002 was confirmation to that. Besides, reservoirs have warming effect on river waters in the autumn-winter period. It is caused by decrease of intensity of cooling of water in a reservoir in comparison with a river conditions, and also evacuation of water in a tail-water pool through bottom outlets. Therefore the temperature of water, evacuated in tailrace channel, usually is higher than its natural values. In the tail-water of the Krasnodar reservoir (gauging station Krasnodar) water temperature has increased in autumn months, and also in December and January. As a result there was a February coldest month. To a certain extent the change of thermal regime of Kuban River was promoted by tendencies in air temperature fluctuations in its basin. According to data of weather-stations Armavir and Temryuk, from 1973 to 1986 on the middle and lower reaches of the river the intensive increase of January air temperature (t) was observed. It has averaged 0.3 and 0.5 °C/year accordingly. After this increase the fluctuations of t had already not a directed character, mainly with preservation of achieved before values. In March short-term warming were observed in middle and end of 1970th and more significant warming was last 15 years of XX century. In November, December and February the sequence of warm years doesn’t form increasing tendency (except for second half of 1990th for winter months) and the long-duration warm periods. On the contrary, the periods with rather low t are well distinguished. Modern region climatic changes either promoted to river water warming (January) or decrease of the warming (November) (tab. I). In other months the tendencies of t changes in different parts of the river basin either don’t coincide or absent. Therefore, the influence of the climatic factor is minimum. Downstream the Krasnodar dam the temperature of water is not reduced, and, on the contrary, increases on 0.2-0.8 °C (within the boundaries of city). The reason is «thermal pollution» by city sewage. Only outside the Krasnodar, in the direction of head of Kuban river delta (gauging station Tihovskiy), water temperature in autumn-winter months is a little reduced. It occurs due to heat exchange between river stream and atmosphere air, river bed, and also owing to slow speeds of current, assisting to this; dumps of cold waters from fields and from the Shapsugskoye reservoir. However, water temperatures don’t reach natural values. As a result, in all months of the cold season the increase of water temperature (after 1973) and reduction of number of occurrences with values of water temperature, which are close to 0 °С, are observed. The change of climatic conditions and winter thermal regime of the river, in turn, have led to change of characteristics of the ice phenomena in head and branches of the delta: 1) duration of the period with the ice phenomena and thickness of ice have decreased, 2) years with absence of ice cover began to be observed more often. Similar tendencies have been registered at the Kuban river nearshore zone too As a result, probability and duration of the phenomena of ice dams and jams in the Kuban river delta have decreased. However, the ice-clogging inundations still are observed here (average one time in eight years). In present days intensification of cyclonic activity during the winter period and the strong physical deterioration of the machinery and elements of hydraulic engineering and irrigational constructions promote to this. Therefore, in conditions of increased and badly predicted inflow of water during the winter period and deterioration of technical parameters of water reservoirs, they are compelled to dump larger volumes of water. In conditions of decrease of river channel capacity and in the presence of ice in branches or at the nearshore zone it increases probability of ice dam formation and inundations. For example, such situation was winter 2001-2002.