Impact of stream and floodplain modification on the flood effects.

The thesis deals with evaluation of the Volyňka river network modification on the basis of field survey, and identification of critical stream segments, which may influence course of flood events by means of GIS analysis. Two extreme floods took place in the Volyňka river basin in the years 2002 and 2009. The paper evaluates causes, course of events and effects of the floods. To get information about river network modification the HEM-F fiels survey methodology was used. The whole stream of Volyňka river was investigated from its spring to Volyně town, the tributaries in the upper part of the Volyňka river basin and the stream Spůlka. Intensity of stream modificatin and flood effects character were evaluated and critical sections were indentified on the basis of the mapping results. The potential flow obstacles are the most frequent type of critical modification of the stream. Especially the unsufficiently designed bridges, weirs and culverts cause the most intensive damage and geomophological effects of the flood. Individual kinds of obstacles are typical for certain sections of the river flow. Modifications limitating use of retention potential of the floodplain are often present in the central and lower parts of the Volyňka river basin. In several critical stream segments there were proposed..