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Georg A. Janauer
Abstract
Vienna is an urban complex of about 1.5 milion people. The entire urban water ma-
nagement - drinking water supply, sewage collection and treatment, flood control and
the condition of the Danube River Wetlands - is highly functional. Internal co-opera-
tion between administration units concerned with different aspects of water manage-
ment provides the basis for successful problem-solving. Examples of the resilience of
aquatic systems, of rehabilitation after cyanobacterial impacts and for possible dan-
gers to the UNESCO biosphere reserve, show how ecohydrological practices make
ecosystem services available to the inhabitants of Vienna.
Key words: public administration, floodplain waters, river regulation, drinking water
supply, sewage treatment.
1. Introduction
Flood protection is a major goal of water
management when changing climate conditions
may result in increased, and more irregular, river
discharges. One strategy of mitigating flood
impacts is the use of existing floodplain areas as
retention basins and/or flood diversion channels.
Many river regulation schemes were introduced
in the 19th century, turning parts of the river cor-
ridor into detached floodplains with reduced
hydrological dynamics. As a consequence still
water conditions prevail there and a high level of
biodiversity in aquatic plant and animal life
developed. Flood peak diversion into such areas
will result in drastic changes of this ecologically
interesting and valuable situation. Conventional
solutions would put exclusive priority to solving
the flood risk and would accept the inevitable
loss in existing aquatic habitat quality. In case of
the Lobau Biosphere Reserve this cannot be the
best solution. Application of eco-hydrological
planning should close the gap between technical
and ecological priorities to sustain biodiversity
and ecosystem services without impairing the
flood protection aspect.
2. Water management practices in the
city of Vienna
Location, climate and general setting
Vienna is the capital and, simultaneously,
one of the nine federal provinces of Austria. The
area of the urban complex is c. 460 square kilo-
metres and about 1.5 million residents live with-
in the limits of the municipal district. The city is
located between the eastern limit of the Alpine
Range, formed by a high chain of hills, and a
Vol. 5
No 4, 281-286
2005
Aquatic habitats in Vienna (Austria)
- integrating ecology and urban water management
Aquatic Habitats
in Integrated Urban
Water Management
Department of Freshwater Ecology, University of Vienna,
Althanstrasse 14, A - 1090 Vienna, Austria.
e-mail: georg.janauer@univie.ac.at
G. A. Janauer
wide geological basin with level lower areas in
South and East directions. Atlantic and Mediter-
ranean weather systems cause warm and wet
weather periods, whereas continental influences
leads to dry and hot periods in summer and cold
periods in winter. Information on drinking water
supply, sewer system and sewage treatment,
aquatic habitats and the issues of river regulation,
flood control, and ecology, and on the administra-
tion is provided in highly informative and detailed
Internet homepages (http://www.wien-vienna,
http://www.wien.gv.at/wasserbau/stehende -
gewaesser/index.htm).
Drinking water supply
Potable water, of premium quality, originates
from two high alpine sources located more than
100 km distance from Vienna. Gravity-driven
mains were finished in 1873, and in 1910, respec-
tively, and still meet the needs of a 21st century
urban agglomeration (average daily consumption:
390 000 m3). The water is supplied directly to the
pipe system of the city (total length: 3273 km),
but mild chlorination is used as a prophylactic
treatment. Small quantities of drinking water
(sand filter, chlorination) are supplied to the west-
ern part of the city from a reservoir fed by a little
river. Shortage of drinking water (household, san-
itary use, gardening, most of industrial use) has
not been experienced for an extended period of
time, as revising the pipe systems and partial re-
lining of existing pipes in situ (done in part by
semi-autonomous robots) resulted in about 30%
increase in usable quantities. In case of shortage,
auxiliary well systems can be used, that either
take filtrate from the Danube River (three filtering
areas), or exploit a groundwater reservoir located
about 30 km from Vienna.
Sewer system and sewage treatment
Today 98% of Vienna and 62 000 PEs (per-
son equivalents) of smaller towns in the urban
agglomeration are directly connected to a gravity-
driven sewer system. The remaining waste water
is collected in, mainly encapsulated, septic tanks.
The length of the sewer system is 2158 km. The
biological sewage purification system, which is
located at the point of lowest elevation near the
Danube Canal, treats a capacity of 4 000 000 PEs
and includes phosphorus stripping and nitrogen
elimination (treatment efficiency >95%). Fifty %
of waste water is produced by households, the
other half is produced by trades, businesses and
industries. About 680 000 m3of wastewater are
treated daily. Strictly controlled laws oblige pro-
ducers of waste water containing dangerous or
toxic substances to pre-treat effluents within the
respective industrial complex ("at the source"
instead of "end of pipe" strategy). The combined
sewer system also receives rainwater and the
water from 18 little streams. A network of 20 rain
gauges and 40 discharge metering stations was
installed to allow for regulating internal locks in
the sewer system, thus retaining peak discharges
within the voluminous system of sewers and cav-
erns as to limiting the load at the treatment plant
to 18 m3s-1. This system ensures the treatment of
all water received by the sewer system irrespec-
tive of its source. Economic and structural opti-
misation of the treatment plant and more stable
conditions in the process are the beneficial results
of these measures. Surplus sludge is centrifuged
and burnt. Units which produce odours are cove-
red and the exhaust is cleaned with bio-filters.
Aquatic habitats: River regulation, flood
control, and ecology
In the mid 19th century the Danube River
still formed a complex of main channels, anasto-
mosing side channels, meandering smaller run-
ning waters and an uncounted number of flood-
plain water bodies, pools and wet spots in a river
corridor up to several kilometres wide. Following
extremely destructive floods, river regulation
started in 1875 and within the city limits a single
straightened main channel was constructed that
included an inundation area on the left bank. The
channel that leads to the historical city centre,
where a former harbour and business area were
located, was transformed to a canal with embank-
ments over most of its length. All other channels
and floodplain waters were either filled in to pro-
vide new development areas or were cut off to
become backwaters.
In the late 20th century, "total" flood protec-
tion (1000-years frequency, 14 000 m-3 s-1 dis-
charge) was achieved by excavating a c. 160 m
wide and 21 km long flood relief channel, the
"New Danube". It is located in the former inunda-
tion area. During periods of high discharge the
weirs of the New Danube are opened, but most of
the time still water conditions prevail. Between
the main channel and the "New Danube", the
Danube Island was created, which serves as the
main recreation area in this part of the city. With
the south-eastern city districts still vulnerable to
extreme floods, raising the protective dykes
became necessary. This was achieved in combina-
tion with the construction of the "Freudenau"
hydro-electric power plant, which turned the for-
mer straightened channel into an impoundment.
Man-made habitats along the left bank of the
impoundment were "eco-hydro-engineered" dur-
ing the construction of the power plant.
The "Old Danube" is a large oxbow that was
formed when the widest natural river channel was
separated from the regulated river. This oxbow
282
and all smaller former side channels, oxbows and
wetlands, as well as the numerous gravel and clay
pits excavated since the river regulation are fed by
groundwater. Therefore a wide range of aquatic
habitats of different size and type exists in Vien-
na, and the biodiversity of aquatic and wetland
plants is high. Parts of the water bodies and wet-
lands have been declared a UNESCO Biosphere
Reserve and an IUCN-recognised National Park.
All these water bodies serve as species refuges
since wetlands outside the present river corridor
hardly survived at all. Data describing the situa-
tion of these floodplain water bodies (physical
and chemical parameters, phytoplankton, etc.) are
abundant, but most of this material is not pub-
lished. Availability of non-processed, original
data may be insufficient in many cases.
Administration and water management
The urban water management is assigned to
several municipal administrative units. These
cover autonomously, but in co-operation when
needed, drinking water supply, waste (and rain)
water treatment, water engineering, and conserva-
tion just to mention the most important aspects.
Navigation and hydro-electric power production
are managed in co-operation with dedicated units
of Federal Ministries. Above all, the European
Water Framework Directive and other water-
related EU Directives form an umbrella under
which the future management of water bodies is
regulated and which compels the municipality to
comply with national legal derivatives worked out
by the respective Federal Ministry. The result of
the combined efforts is a situation for water bod-
ies and related biodiversity probably not found in
the world. Where flood protection and protection
of ground water resources interfere, or where dif-
ferent conservational strategies collide, such
problems have been solved by intensive interdis-
ciplinary discussion based on scientific research.
Water management in general takes account of
biodiversity and conservation aspects, but further
flood protection measures needed in the eastern-
most part of the city, will cause changes in the
composition of floodplain water body types and
related biota. Costs of water management directly
and indirectly influencing aquatic ecosystems in
Vienna have been high, but political will and
appreciation by the public back the opinion that
this financial effort is cost effective. So far water
management can cope with urban growth and an
increased water demand. It is a sustainable system
for an extended period of time, unless public and
political priorities change in the future.
Main features of sustainable urban water
management are the close interrelation of the
respective municipal administrative units, suffi-
cient funding, and - as a seemingly essential fea-
ture - the employment of academic biologists in
critical positions, especially within the technical
units and not only in the conservation department.
Environmental feasibility studies, the assessment
of environmental impact of urban development
and, of course, the implementation of the Water
Framework Directive with its obligation not to
cause the existing ecological status of water bod-
ies to deteriorate, are expensive, but efficient -
and in the long run usually cost effective - tools to
reach environmental and economic sustainability.
Policies must be based on solid scientific know-
ledge to obtain public support for the strategies
developed. Often these are not appreciated at the
beginning, e.g. modifications in fish stocking
practice during biomanipulation of eutrophic
waters, but the personal involvement of scientists
can provide the necessary ground on which to
build public awareness. Anything that transfers
technical needs and scientific knowledge to a
level of better understanding by the public will
help. Yet, there are gaps in the knowledge base
even in rather well studied systems like those in
Vienna. Seasonal and inter-annual fluctuation in
species composition, e.g. for macrophytes, and
the interrelation of fauna using the structure of the
aquatic plants as a habitat (e.g. effects of meso-
and micro-ecotones), are little understood and far
from being integrated into predictive models and
ecohydrological strategies (Janauer 2000;
Zalewski 2000; Zalewski et al. 1997). Further-
more, consensus on strategic environmental aims
may need lengthy preparative discussions, espe-
cially when political arguments may curtail tech-
nically and ecologically feasible solutions. The
basic requirements for ecohydrological solutions
and sustainable urban water management strate-
gies can be described, but in real-life practice
each case will call for individual solutions.
Results from man-made habitats in the Danube
impoundment, aspects of resilience of the aquatic
vegetation in the New Danube, and the possible
impact of river rehabilitation on the aquatic vege-
tation e.g. in Danube oxbows are presented as
case studies.
3. Case studies
'New Danube' - an example for resilience
The New Danube was constructed to serve
flood protection strategies worked out for the cen-
tral part of the City of Vienna (Michlmayr 1997).
An inlet weir and two more weirs downriver sta-
bilise the horizontal water level in two impound-
ments. During flood events the weirs are opened.
Today, seepage water from the "Freudenau"
hydro-electric power plant feeds the New Danube
via groundwater passage through the Danube
Aquatic habitats and UWM in Vienna 283
G. A. Janauer
Island (Janauer et al. 1985). The 21 km long canal
turned into a still water with semi-natural banks
and the adjacent island is a resort for recreation
and water sports. With respect to the latter use,
water quality is under constant control. The often
abundant growth of submerged macrophytes
keeps phosphorus loads at a low level and phyto-
plankton is of negligible importance. Therefore
ecohydrological techniques were applied to insure
the re-occurrence of macrophyte stands even after
flood impact. Table I shows the accumulated inter-
annual occurrence of aquatic species (Janauer,
Wychera 1999). The species showing the highest
rate of survival in the New Danube are most
important in river management (Krauze 2004).
After floods, the weirs are closed only when sus-
pended solids concentrations are decreasing, to
prevent too much siltation onto the surviving plant
stands. Re-growth of considerable plant mass usu-
ally started instantly after negligible flow was
again dominant (Janauer, Wychera 2000). This sit-
uation is paralleled in some floodplain lakes of the
Lower Rhine (van Geest 2005; van Geest et al.
2003). Two weeks after a flood passage, the nec-
essary hygienic status and turbidity level (mini-
mum Secchi depth transparency: 1.5 m) are
reached, to officially allow swimming and other
water sports again. Year-to-year estab-
lishment of the macrophytes is support-
ed by a water level change regime,
which was modelled to support and even
enhance aquatic plant growth. In this
regime the water level is regularly low-
ered in spring time and later raised to a
maximum which enlarges the water col-
umn that can be colonised by the macro-
phytes, which are the efficient competi-
tors of the phytoplankton. These man-
agement practices make ecosystem serv-
ices available to the public along the
whole length of the New Danube.
'Old Danube' - an example for
successful rehabilitation
The Old Danube oxbow (area:
1.6 km2, mean depth: 2.3 m, max. depth:
6.8 m, avg. volume: 3 710 600 m3) was
separated from the main channel during
the river regulation in the period 1870 to
1875. In a limnological context the Old
Danube represents a shallow lake. The
general condition was dominated, and
stabilised, by abundant submerged
aquatic vegetation, until a trophic deteri-
oration started at the end of 1980s. In
1993 and 1994 cyanobacterial blooms
caused intensive colouring of the water,
low transparency and resulted in an
almost total decline of the submerged
macrophyte vegetation. These conditions were
triggered by a combination of factors - reduced
water level dynamics, a 30 - 45 cm increase in
water level, changes in groundwater influx, nutri-
ent inputs from various sources, insufficient sewer
connections and leaking septic tanks, the intensive
recreational use of the oxbow (recreation-related
annual phosphorus input: 38 kg), by too high
numbers of waterfowl and an unsuitable structure
of the fish community.
The ecohydrological solution of this unplea-
sant situation was found in a multi-step strategy
(Ladinig 1998): Waste water effluents were kept
from the water body by installation of a sewer
system and installation of sealed septic tanks.
Phosphorus content in the Old Danube was
reduced by ferric chloride (in calcium-rich sus-
pension). De-nitrification was enforced by addi-
tion of calcium nitrate near the bottom. As a con-
sequence the algal biomass was reduced to a great
extent, and diatoms and green alga started to
dominate the phytoplankton spectrum. Several
times parts of the water volume were exchanged
with low nutrient water, which was taken from the
New Danube. Excess nutrient rich water was led
to the treatment plant via the public sewer system.
This afforded the construction of several conducts
284
Plant species years of
occurrence
Ceratophyllum demersum L. 11
Elodea nuttallii (Planchon) St.John 11
Myriophyllum spicatum L. 11
Potamogeton crispus L. 11
Potamogeton pectinatus L. 11
Potamogeton perfoliatus L. 11
Potamogeton lucens L. 10
Najas marina L. 9
Potamogeton mucronatus Schrader ex Sonder 7
Nitellopsis obtusa (Desv.) J.Groves 6
Polygonum amphibium L. 6
Schoenoplectus lacustris L. 6
Ranunculus circinatus Sibthorp 4
Sparganium emersum Rehmann 4
Chara sp. 3
Potamogeton pusillus L. sec. Dandy & Taylor 3
Zannichellia palustris L. 3
Alisma plantago-aquatica L. 2
Potamogeton trichoides Chamisso et Schlechtendal 2
Nitella mucronata (A. Braun) Miquel 1
Table I. Accumulated inter-annual occurrence of aquatic plant species
in the New Danube over an eleven year study period (adapted from
Janauer, Wychera 1999).
between the two water bodies, for no little cost. A
particular phosphorus-rich point source was sepa-
rated by an enclosure and a pumping system. Fish
ecological intervention enforced stocking with
predatory species which resulted in a recovery of
algae-feeding zooplankton. Waterfowl abundance
was reduced, in part by instructing the public
about the results of surplus feeding. The small
"Wasserpark" was separated with a gravel dam
and a special treatment was established there. Fol-
lowing several dedicated experiments, artificial
reduction of the water level in spring enabled
macrophytes to re-colonise deeper areas of the
oxbow. Finally transplants of different macro-
phyte species initiated a stronger development of
submerged water plants, which was sufficient to
control algal blooms. In 2003 the first complaints
about abundant, high growing macrophytes (e.g.
Myriophyllum spicatum) arose from the public
and a limited plant-harvesting program was
installed to promote shorter growing species like
Stoneworts. Whereas the recreational use of the
Old Danube was re-established in 1997, soon
after the chemical treatment, it took about ten
years to re-establish a more or less macrophyte
dominated state (Information: unpublished
reports, MA 45, Municipality of Vienna). Thus,
finding a balance between ecohydro-
logical control of phytoplankton by
macrophytes and a sensible water level
regime, and recreation needs which
result in local macrophyte removal will
be the most ambitious part of future
water management in the Old Danube.
UNESCO Biosphere Reserve
'Lobau'
The Lobau Biosphere Reserve
is an important ecological ensemble of
riparian forest and floodplain water
bodies. It is situated on the left bank of
the River Danube. Since the river regu-
lation in the late 19th century, this area
has been separated from the main
Danube channel by a protective dam,
which features a small opening at its
downstream end through which floods
can enter this area, and water can flow
out again when the flood peak has
passed this opening. In this semi-still
water situation, abundant aquatic vege-
tation developed: the Lobau Reserve is
an important species refuge. River re-
gulation, urban development and inten-
sive agriculture are the causes which
lead to a cultural landscape in which
practically all wetlands and small pools
were eliminated outside the main
dykes.
A "total" flood protection for Vienna needs
a larger cross section in the discharge channel
below the "Freudenau" Power Plant, but this
area is constrained by the dyke that separates the
Lobau from the main river. In case of the largest
calculated flood, an additional cross section for
up to 2000 m3s-1 is needed. Plans exist to build
weirs and overflow constructions at the upper
end of the Lobau area to divert flood water into
the wetland, and to open the downriver end of
the dam over a longer distance to lead the sur-
plus water back to the main channel, which is
much wider there. This would result in high
water flow velocities in the water bodies of the
Lobau Reserve. Anumber of GIS scenarios were
run to determine which aquatic plant species
would suffer most from such environmental
changes. Figure 1 shows the predicted effect on
aquatic plant species that existed in one of the
larger floodplain lakes during the study period. It
is clear that the ecological impact of water flow
by spates and/or floods diverted into a former
still water would be severe. Therefore studies
need to be initiated to show into which habitats -
natural or possibly constructed - the existing
valuable macrophytes can migrate, as to save
them from extinction.
Aquatic habitats and UWM in Vienna 285
Fig. 1. Predicted water flow impact on still water macrophyte vege-
tation due to flood diversion into the UNESCO "Lobau" Biosphere
Reserve - central section of the "Kühwörther Wasser" oxbow lake.
(Survey units for macrophyte assessment marked as lined areas).
All sensitive species will be eradicated in the central part of the
oxbow lake. In the near-bank parts of the cross section species insen-
sitive to rapid water flow will survive. The biomass of submerged
vegetation will be reduced to a considerable extent.
G. A. Janauer
4. Conclusions
Vienna is a city in a fortunate situation:
Water supply is sufficient to cover even consi-
derable increases in the future and excellent
water quality is provided from high alpine
sources. Losses in the pipe system have been
reduced to close to negligible amounts. The
sewer system covers the whole area of the city
and sewage treatment is at the top of modern
technology. Of course, development to the pres-
ent stage has a long history and high budgets
were needed to cover the respective cost. The
river Danube was regulated more than 100 years
ago, but still many floodplain water bodies of
considerable size exist. Of course, water tem-
perature is higher in the waterbodies which lack
surface flow today and turbidity is especially
low where dense stands of macrophytes deve-
loped. In the New Danube and in the Lobau
flood frequency parallels that of the main chan-
nel, but dynamics are much decreased in the
Biosphere Reserve, and no direct flood flow
exists in the detached Old Danube. Ground
water exchange rates are still high in the New
Danube and in the Lobau, and somewhat
reduced in the Old Danube. Thus, despite the
change, or loss (Old Danube), in natural surface
connection system dynamics in the secondary
water bodies are still highly influenced by the
discharge regime in the main channel. Today the
existence of, and the good water quality in these
waterbodies are both sustained by a combina-
tion of technical and ecohydrological interven-
tions, and by an ambitious management. The
different organisations of municipal administra-
tion co-operate on a broad base to achieve solu-
tions of problems that include the sustenance,
and even enhancement of ecological conditions,
in part by relying on ecosystem services of
especially of aquatic vegetation, the natural
competitor of phytoplankton. However, flood
threats still exist and ecohydrological planning
will be needed to provide space for flood dis-
charges and on the other hand save parts of the
aquatic vegetation from extinction. Achieving
these goals is based on a political will that is
friendly to ecological solutions, and a public
that is aware of ecologically sensible solutions,
and accepts the related cost.
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