Impact of Land Drainage and Natural Factors on the Changes of the Hydrological Regime of the Tatula River

Abstract

Impact of the land drainage on the river catchment in Lithuania has so far been evaluated as of rather mixed character as the impact is rather complex and multifaceted. When drainage systems are installed, the hydrographic network of the territory changes and conditions of the formation of catchment may change as well. Implementing naturalisation processes at the regulated sectors of the river, there appear places where the limit of the flood hydraulic conductivity might reach the line of the overflow of the river in the valley. In the article, using the analytical, statistical and mathematical methods of modelling, the impact of land drainage and natural factors on the changes of hydrological regime of the Tatula river is analysed. Areas of the wet land in the basin of the Tatula river form 90 % of all the area. Analysis of the interchanges of excessively wet lands and drained area in 1960– 2009 shows that until 1961, such drained areas reached only 12 % of all the area of wet land, and in 2000 such area reached 94 %. When the biggest part of the basin area was drained, there appeared no essential annual or seasonal differences of the catchment changes. Using HEC–RAS programme for the modelling, it was determined that the coefficient of the hydraulic roughness in the Tatula river changes from 0.028 to 0.049. The water level in the river bed is in all the cases lower than that of the regulated river. Even when the spring debit is maximal, the water in the regulated bed of the river does not overflow. If the coefficient of the river roughness is made higher and reaches 0,160 the water in the bed overflows in all the regulated sectors of the Tatula river.