STREAM-AQUIFER INTERACTIONS: QUANTIFYING THE RIVERBED CLOGGING PROCESS AND THE SEEPAGE-RATES WITH THE HELP OF MULTI-LEVEL-PIEZOMETERS

摘要

Due to settling and straining of suspended and bed load sediment as water moves through the riverbed material a semipervious layer with a hydraulic conductivity less than that of the adjacent aquifer can be formed. These processes are commonly referred to as clogging and are investigated in the reservoir ?Freudenau“ created by the impoundment of the Danube River at Vienna, Austria. At the location ?Schulschiff“ the leakage coefficient of the clogged streambed was examined from March 1996 (when the Danube River was impounded by the hydropower dam ?Freudenau“) until June 1998. With the help of a one dimensional analytical groundwater model and pressure measurements under the riverbed with multi-levelpiezometers the spatial distribution at a given time and the time series at a given point of the seepage rates through the streambed were derived. The time series of the leakage coefficient shows the existence of quasi-stable states and also the effect of flood events in the whole clogging process. Smaller flood events cause an increase in the leakage coefficient but only for a short time and bigger flood events cause a new clogging process due to sedimentation. Therefore the whole clogging process so far consists of several individual clogging processes and each takes a few weeks until reaching a quasi-stable state. Due to the boundary conditions within the Danube River the spatial distribution of the seepage rates through the clogged streambed had maximum values at the left riverbank (of about 5 × 10~(-6) m s~(-1)) declining very sharply towards the right bank (impervious wall). At the end of the study period this spatial distribution was more uniform because of lower leakage values. The time series of the seepage rate at a given point within the cross-section ?Schulschiff“ showed seepage rates between 2 and 4 × 10~(-6) m s~(-1) with sharply increasing values during the flood events (of about 17 × 10~(-6) m s~(-1)). Additional impounding in November 1997 caused larger gradients through the clogged riverbed that were compensated by lower leakage values so that the seepage rates remained almost the same.