Przebieg zjawisk fluwialnych w zimie na przykładzie małych rzek sudeckich
Abstract
Fluvial processes in winter, with examples from small Sudetic rivers
Abstract
The paper deals with freezing-up and thawing of river ice and snow and their influence upon the fluvial- and bank-forming processes, which modify the morphology of active river channels. Field examples come from small mountain rivers (Q mean less than 10 m3/s) of the drainage basins of the Upper Bóbr and Strzegomka Rivers in the Central Sudetes. In the area investigated ice rind is characteristic of early stages of freezing of tranquil--turbulent flows (Fr<0.15), especially under conditions of slight supercooling of superficial layers of water. Current ice and anchor ice, on the other hand, form in much more supercooled water and demand a very effective removal of heat of crystallization. Consequently, they grow particularly fast at times of strong icy winter winds, even if air temperature is only just below 0°C (for instance, -0.5 ÷ -2°C). At least during early stages of freezing both the varieties of ice display a very similar structure of typical frazil or spongy ice. The higher the Froude number the stronger the attachment of anchor ice to the river bed. In consequence, ice jams develop in small mountain rivers mostly during freezing-up (early freezing jams). They grow mostly at the expense of both current ice and anchor ice and tend to occur especially on riffles as well as within meander bends. Locally, they may result in stages higher than bankfull. Thawing of river snow and ice takes place so fast that the thalweg is usually free from ice before any substantial rise in river stage. Consequently, a mountain river breaks-up rarely and thawing jams of pressed ice occur rather sporadically. The most important changes in morphology of the active stream channels take place during winter thaws and in early spring thawing. This work covers mostly erosional processes including: 1) bank piping; 2) erosional effects of sliding masses of river snow and ice; 3) bank caving due to thaw waters and increased stream discharge, and 4) the influence of ice phenomena on channel pattern, but especially of meandering rivers. Bank piping as well as sculpturing in a bare fine-grained alluvia of deflation cavities (niveo-aeolian erosion) markedly increase the effectiveness of other erosional processes. Sliding of river snow and ice may result in stripping of turf covers and thus initiate fluvial erosion along what was originally an inactive scarp or bank. When affecting concave banks, this results in acceleration of meandering processes, while with the convex banks the reversal seems to be true. Thaw waters are very effective in eroding the river cutbanks. The process has much in common with thermal erosion known from cold regions of the Earth. The intensity of the erosion is markedly increased by the destructive influence of frost and ground ice on the structure of all alluvial deposits. Field evidence strongly suggests that they result in a considerable diminution in strength of cohesive fine-grained alluvia. Such deposits are eroded mostly in a zone limited by a mean low stage from the below and a maximum thawing stage from the above. It has been found in the field that the lateral winter-early spring migration of the river cutbanks may account for 60-90 per cent mean annual migration, especially in „dry” hydrologie years. Finally, it can be concluded that the most important effects of ice phenomena on the process of meandering seem to be rather retrogressive in character and lead to meander degeneration through chute cut-off.