Условия формирования экстремально высоких половодий в бассейне Средней Десны (исторический обзор)

The Middle Desna basin is a region located in the center of the East European Plain. The catchment area is occupied mainly by agricultural land, with some areas covered by forests. The hydrological regime of the main river is characterized by spring flooding, periodically leading to floods. Extremely high floods threaten settlements in the middle Desna basin, including the cities of Bryansk and Trubchevsk, but the methodology for forecasting them does not provide an adequate assessment of the threats. One of the reasons for the complexity of forecasting is the variety of combinations of factors that provide very high water rises. Identification of such combinations is based on the historical approach, in particular, detailed study of the causes of each extreme flood; the priority of the chosen approach over statistical methods of research is justified. The greatest attention is paid to meteorological and climatic factors, which both rhythmically and progressively change in time and space. The article presents a retrospective analysis of meteorological and climatic factors that contributed to the emergence of extreme floods from the end of XIX to the beginning of XXI century. Their correlation with geologic-geomorphologic and anthropogenic factors is determined. Combinations of factors for each case of extreme high floods are considered and its possible mechanisms are substantiated. Years with high floods do not differ from average multiyear years in the peculiarities of the thermal regime, both immediately before the flood and in the preceding months. At the same time, peculiarities of the regime and amount of precipitation of the current and preceding year differ greatly - most noticeably winter precipitation (obviously) and, above all, rainfall during the flood period. The slow recession of floods due to the wide, strongly rough floodplain is superimposed on the abundant inflow of rainwater into the channel. Significant spring precipitation is not the exclusive factor in high floods. Extreme floods and floods close to them are distributed very unevenly in a number of years. There is no direct connection between the dynamics of flood heights and short-period climatic changes - very high spring water rises occur in different phases of fluctuations. In addition, very high floods are not the result of factors occurring during only one year. Meteorological conditions of previous years influence flood heights through groundwater flow. The above considerations may be useful in the development of conceptual models of extreme flood formation.