Summary:

The reduction of flood effects may be attained by lowering the risk of flood. This often requires complex works not only on the assessment of conveyance of the open channel on the territory exposed of flood but also of the capacity of the upper part of the river channel. Great cost for decrease of flood damages can be avoided by improving the conveyance of the channel. This can be proceeded by assessment of the existing state of the open channel development. In result of assessment of damages caused by overflow from the the Czarna Woda channel it was established, that a water-muddy wave reached the center of the locality Łącko flowing over the whole width of the urbanized adjacent part of the valley. The aim of the studies was to point out the causes of flood threat to the locality Łącko. The state of development and conveyance of the Stream Czarna Woda, flowing across the upper districts and the center of Łącko was assessed. The Stream Czarna Woda flows in a trained open channel constructed by a stoneconcrete channel. The performed evaluation of conveyance capacity of cannel permits to identify sections of low conveyance of the stream. The performed measurements of the longitudinal slope of bed of the Stream Czarna Woda on the length of 460 m and in nine cross-sections have constituted the basis for calculations for reliable flow. This flow of probability 1%, is used to dimensioning of channel was calculated by use of the Carpathian Punzet’s formula. The reliable flow equals 94,32 m³·s^-1. In the studied section of the stone-concrete channel there are two bridges and one river barrage. The conveyance of the stream bed conditions of reliable flow trough these structures was determined. It was shown that in the stream bed in a section of length of 100 m located above the center of Łącko, overflow of water from the too shallow bed will occur. The bridges located at km 1+110 and km 1+554 do not cause the rise of water level at reliable flows. The river barrage, ensuring flow of reliable flow in the stream bed would require the elevation of side-walls by 0,56 m. It was stated that flood damages in the canter of Łącko are caused by overflow from stone-concrete channel funning from the river barrage to the new section of stone-concrete channel located in the center of the locality. Both the river barrage located at km 1+506 causing considerable damming up of the reliable flow and a too low embankment of the Stream Czarna Woda upstream of barrage are the causes of the existing flood threat.