Clogging processes in hyporheic interstices of an impounded river, the Danube at Vienna, Austria

Stream-aquifer interactions are influenced significantly by riverbed clogging processes. Detailed field observations have been made in the "Freudenau reservoir" of the Danube at Vienna. Different types of clogged layers have been observed. Multi-level-piezometers below the riverbed indicate that the overall clogging process consists of several clogging cycles of a few weeks each initiated by floods until a stable state is reached. Minor flood events cause a temporary increase in the leakage coefficient followed by a new decrease approaching the original level after a few weeks. Major flood events tended to add a sediment layer which re-initiated the clogging process leading over to a lower level of the leakage coefficient. The computation of the water balance of the two reservoirs of the New Danube indicated a reduction of the seepage rate by about 40% to 60% over a period of 4.5 years. Undisturbed riverbed sediment samples taken by an innovative freeze-panel-sampling method demonstrate that the depth of the clogged layer is about 2 cm for two of the clogging types (external clogging and armour layer clogging). Video techniques were used to identify the different types of clogged layers and their variability in time and space. Additionally, this technique facilitated observations of macrozoobenthos organism activities in the hyporheic interstices.