268
J. FUrst and H. P. NachtnebeI
management strategies therefore aims strongly at providing better conditions for
their growth.
The objectives for water management are identified as (l) to increase the
fluctuations of the water levels in Alte Donau, (2) to modify the seasonality such
that from a minimum in spring an optimum increase until summer is achieved,
making a large bottom area available for submersed macrophytes to grow, (3) to
fill up with bank-filtered groundwater only, and (4) to achieve a cost-efficient
solution.
Due to the situation in the centre of an urban area, several constraints need to
be considered. Groundwater in the neighbourhood is rather shallow and the
municipality will be made responsible for wet basements of houses. Facilities at
the Alte Donau have been adjusted to high water levels. The capacity of the well
field for abatement of the leaking waste dump site cannot cope with major drops
of water levels in Alte Donau, and the operating rule for the water levels of Neue
Donau, which is the boundary condition for groundwater flow, has a very strong
legal foundation. According to the above objectives and constraints, several
alternatives for future water management have been developed and their impact
evaluated by simulations in the groundwater model. Without construction of major
hydraulic structures, there are two options to lower the water table: option 1 is to
increase outflow in spring by surface withdrawal of water, and the second is to
reduce inflow from Neue Donau due to a modified boundary condition. In both
cases, the lake should be filled up exclusively by increased (due to the increased
hydraulic gradient) inflow of groundwater for the reasons outlined above.
The main advantage of the option 1 is that the drawdown phase can be
precisely controlled in its magnitude and time behaviour. An additional advantage
is that it does not interfere with the strong and complicated regulations for the
operation of Neue Donau and the hydropower plant Freudenau. The main
disadvantage is the risk of additional input of nutrients and contaminants due to
the increased hydraulic gradient. There is a conflict with the capacity of the well
field for abatement of the waste dump site. The fact that water table drawdown
coincides with increased inflow of groundwater, is somewhat unnatural.
Option 2 requires a new operating rule for the water table of Neue Donau. This
rule can be developed and its impact evaluated by scenario simulations in the
groundwater model. Due to delay between Neue Donau and Alte Donau, the effect
cannot be controlled as precisely as in option (1), because there is also an
influence from the groundwater situation in the hinterland and from precipitation.
The main modification is a lowering of the water table in winter and spring by 0.4
to 0.6 m, leaving the maximum in July unmodified. This increases the range of the
rising limb from spring to summer from 0.7 to 1.3 m. The direct effect in Alte
Donau is a minimum in late March, which is 0.4 m deeper, and a rising limb of 0.7
m, i.e. twice as much as under the current conditions.
Several scenarios with different modifications of the operating rule and
combinations with additional surface withdrawal have been simulated. Fig. 12, as
an example, visualises the effect of a scenario combining a modified operating
rule in the Neue Donau with additional surface withdrawal. It could be shown that
J. FUrst and H. P. NachtnebeI
management strategies therefore aims strongly at providing better conditions for
their growth.
The objectives for water management are identified as (l) to increase the
fluctuations of the water levels in Alte Donau, (2) to modify the seasonality such
that from a minimum in spring an optimum increase until summer is achieved,
making a large bottom area available for submersed macrophytes to grow, (3) to
fill up with bank-filtered groundwater only, and (4) to achieve a cost-efficient
solution.
Due to the situation in the centre of an urban area, several constraints need to
be considered. Groundwater in the neighbourhood is rather shallow and the
municipality will be made responsible for wet basements of houses. Facilities at
the Alte Donau have been adjusted to high water levels. The capacity of the well
field for abatement of the leaking waste dump site cannot cope with major drops
of water levels in Alte Donau, and the operating rule for the water levels of Neue
Donau, which is the boundary condition for groundwater flow, has a very strong
legal foundation. According to the above objectives and constraints, several
alternatives for future water management have been developed and their impact
evaluated by simulations in the groundwater model. Without construction of major
hydraulic structures, there are two options to lower the water table: option 1 is to
increase outflow in spring by surface withdrawal of water, and the second is to
reduce inflow from Neue Donau due to a modified boundary condition. In both
cases, the lake should be filled up exclusively by increased (due to the increased
hydraulic gradient) inflow of groundwater for the reasons outlined above.
The main advantage of the option 1 is that the drawdown phase can be
precisely controlled in its magnitude and time behaviour. An additional advantage
is that it does not interfere with the strong and complicated regulations for the
operation of Neue Donau and the hydropower plant Freudenau. The main
disadvantage is the risk of additional input of nutrients and contaminants due to
the increased hydraulic gradient. There is a conflict with the capacity of the well
field for abatement of the waste dump site. The fact that water table drawdown
coincides with increased inflow of groundwater, is somewhat unnatural.
Option 2 requires a new operating rule for the water table of Neue Donau. This
rule can be developed and its impact evaluated by scenario simulations in the
groundwater model. Due to delay between Neue Donau and Alte Donau, the effect
cannot be controlled as precisely as in option (1), because there is also an
influence from the groundwater situation in the hinterland and from precipitation.
The main modification is a lowering of the water table in winter and spring by 0.4
to 0.6 m, leaving the maximum in July unmodified. This increases the range of the
rising limb from spring to summer from 0.7 to 1.3 m. The direct effect in Alte
Donau is a minimum in late March, which is 0.4 m deeper, and a rising limb of 0.7
m, i.e. twice as much as under the current conditions.
Several scenarios with different modifications of the operating rule and
combinations with additional surface withdrawal have been simulated. Fig. 12, as
an example, visualises the effect of a scenario combining a modified operating
rule in the Neue Donau with additional surface withdrawal. It could be shown that