260
J. Fiirst and H. P. Nachtnebel
assumed to be spatially constant on the surface of the lake. Groundwater flow,
however, needs to be described with good spatial resolution along the banks of the
Alte Donau to account for locally different solute concentrations, passage times
and temperatures in the water entering or leaving the lake.
For this purpose, a two-dimensional finite element (FE) model of groundwater
flow was developed and calibrated (Diersch 1996). The model domain covers the
area between the River Danube, Alte Donau and its hinterland (Fig. 7). The banks
of the River Danube, the Neue Donau and Alte Donau are represented as firstorder boundary conditions for groundwater flow. The interesting results with
respect to the water balance of Alte Donau are the flow rates across the boundary
of Alte Donau. For selected time steps, flow vectors were calculated for each node
and then aggregated for characteristic sections of the boundary to display them as
time series of the water balance.
To identify changes in the water balance, the model was run with modified
boundary conditions, referring to the periods 1971-1973 and 1993-1996.
3.1 Water Balance Before Construction of the Flood Bypass
Channel (1971-1973)
These years before the construction of the flood bypass channel Neue Donau
represent conditions where the water level of the River Danube's main channel
was the dominating boundary condition for groundwater flow between the River
Danube and the Alte Donau.
Time series of aggregated groundwater flows for the main sections of Alte
Donau are displayed in Fig. 5. The most significant inflow is observed at the
upstream end. The right bank of the downstream part of the Alte Donau exhibits
inflow only during high water levels in the Danube and is essentially a region of
outflow on both banks. Outflows show slower seasonal fluctuations. They are
concentrated at the downstream end.
Flow directions. At high levels in the River Danube, inflow takes place along the
whole right bank of the Alte Donau, concentrated at the upstream end. Outflow is
mainly from the north and east bank to the hinterland. At mean and low levels,
inflow is still strong at the upstream end, but from the lower section, there is also
strong outflow from the right bank towards the River Danube. This seasonal
change of flow directions at the right bank of the lower section of the Alte Donau
is characteristic for the dynamics of groundwater flow (Fig. 7a,b).
Exchange rates and renewal times. The annual inflow and outflow of
groundwater would replace the average volume of the lake 1.2 to 1.5 times per
year. However, most of this recharge usually takes place within a short time in
spring, where large inflows coincide with very low water levels in the Alte Donau.
This means that in the period from spring to summer the lake was filled with
filtered, rather cold, groundwater. Starting at a volume of typically 1.6-2 million
J. Fiirst and H. P. Nachtnebel
assumed to be spatially constant on the surface of the lake. Groundwater flow,
however, needs to be described with good spatial resolution along the banks of the
Alte Donau to account for locally different solute concentrations, passage times
and temperatures in the water entering or leaving the lake.
For this purpose, a two-dimensional finite element (FE) model of groundwater
flow was developed and calibrated (Diersch 1996). The model domain covers the
area between the River Danube, Alte Donau and its hinterland (Fig. 7). The banks
of the River Danube, the Neue Donau and Alte Donau are represented as firstorder boundary conditions for groundwater flow. The interesting results with
respect to the water balance of Alte Donau are the flow rates across the boundary
of Alte Donau. For selected time steps, flow vectors were calculated for each node
and then aggregated for characteristic sections of the boundary to display them as
time series of the water balance.
To identify changes in the water balance, the model was run with modified
boundary conditions, referring to the periods 1971-1973 and 1993-1996.
3.1 Water Balance Before Construction of the Flood Bypass
Channel (1971-1973)
These years before the construction of the flood bypass channel Neue Donau
represent conditions where the water level of the River Danube's main channel
was the dominating boundary condition for groundwater flow between the River
Danube and the Alte Donau.
Time series of aggregated groundwater flows for the main sections of Alte
Donau are displayed in Fig. 5. The most significant inflow is observed at the
upstream end. The right bank of the downstream part of the Alte Donau exhibits
inflow only during high water levels in the Danube and is essentially a region of
outflow on both banks. Outflows show slower seasonal fluctuations. They are
concentrated at the downstream end.
Flow directions. At high levels in the River Danube, inflow takes place along the
whole right bank of the Alte Donau, concentrated at the upstream end. Outflow is
mainly from the north and east bank to the hinterland. At mean and low levels,
inflow is still strong at the upstream end, but from the lower section, there is also
strong outflow from the right bank towards the River Danube. This seasonal
change of flow directions at the right bank of the lower section of the Alte Donau
is characteristic for the dynamics of groundwater flow (Fig. 7a,b).
Exchange rates and renewal times. The annual inflow and outflow of
groundwater would replace the average volume of the lake 1.2 to 1.5 times per
year. However, most of this recharge usually takes place within a short time in
spring, where large inflows coincide with very low water levels in the Alte Donau.
This means that in the period from spring to summer the lake was filled with
filtered, rather cold, groundwater. Starting at a volume of typically 1.6-2 million