4th ECRR Conference on
River Restoration
Italy, Venice S. Servolo Island
16-21 June 2008
MONITORING RESULTS OF REVITALISATION MEASURES
ON AN URBAN LOWLAND RIVER (LIESINGBACH,
VIENNA, AUSTRIA)
Panek K.1*, Korner I.2, Lang H.4, Markut T.4, Petz R.3, Petz W.3, Siegl W.1
1
ARGE Ökologie, Technisches Büro für Ökologie, Wien
AVL Arge Vegetationsökologie und Landschaftsplanung, Wien
3
Umweltgutachten Petz OEG, Neumarkt a. Wallersee, 4 Free consultants
* Corresponding author, e-mail: [email protected]
2
ABSTRACT
The Liesingbach, flowing through the south of Vienna, Austria, is an urban
stream that has been designated as a heavily modified water body mainly because
the river was channelised, its bed was hard and the water quality poor due to
considerable wastewater discharge. A study in 1999 before the restoration confirmed
the poor ecological status in terms of hydromorphology, aquatic biocoenosis,
riparian vegetation and water related terrestrial fauna. Until 2005, a 5,5 km long
reach close to the south-eastern city limit was revitalised with the intention to induce
an ecological development by improving the hydromorphological conditions.
However, the creation of a typical lowland river morphology was limited due to the
difficulties in acquiring adjoining premises.
The ecological monitoring commenced at the end of the year 2004 and ended in
2007. Investigated parameters were river morphology, sediment composition,
vegetation ecology, dragonflies, carabids, ciliates, macrozoobenthos and fish. This
showed that the morphological setting has dramatically improved resulting in an
increased variability in water depth, channel width and bank design. Wet and damp
sites with typical plant species developed. Riparian wood was planted in sections but
it still needs time to provide a considerable river shadowing. The species richness of
carabids increased distinctly reflecting an improvement in habitat heterogeneity. In
particular, ripicol carabids, which were rarely found before the revitalisation, appear
in considerable numbers now. Dragonflies were also nearly missing before, but 15
spp., including endangered and protected taxa, occur frequently now. At least 5 spp.
of these are considered autochthonous. While only some young specimens of four
fish species were found prior to the restoration, 16 species were detected afterwards
and the dominant taxa occurred with all age classes. Even some of the less frequent
species are now obviously spawning within the reconstructed river stretch.
Key words: River restoration, Liesingbach, monitoring, aquatic coenoses
Monitoring results of revitalisation measures on an urban lowland river
1. INTRODUCTION
The Liesingbach rises in the eastern foothills of the Austrian Alps, in the
Vienna woods, at an altitude of 520 m a.s.l., flowing east for almost 30 km
before discharging into the river Schwechat. The two major tributaries,
Reiche Liesing and Dürre Liesing, have geologically different catchments
and are therefore showing different flow characteristics. The Reiche Liesing
flows through flysch-sandstone with an impermeable underground causing
rapid and high flood peaks after intense rain. In contrast, the Dürre Liesing
has its source in the limestone alps, showing considerable infiltration to the
point of drying-up in low water periods (Fig. 1). Annual precipitation varies
between 800 mm in the headwaters and 550 mm in the downstream area.
After the confluence of both tributaries the Liesingbach enters the Vienna
basin, flowing through the south of the capital city. Within the city limits the
running length is 22,7 km. Of the total catchment area (112,4 km²), 45% are
situated in densely populated urban surroundings.
River management of the Liesingbach commenced many centuries ago.
First hydromorphological alterations took place in the early 13th century,
when several mills and sawmills were built. Due to a series of devastating
flood events at the beginning of the 20th century, when urban development
reached the plain tract, the idea for extensive river training came up. These
plans were finally carried out between 1947 and 1969. The predominantly
Figure 1 – The Liesingbach flowing through the south of Vienna (dotted line) in the
Vienna basin (yellow). Tributaries arise in the flysch-sandstone (blue) and limestone alps (orange). The red bracket indicates the revitalised river stretch.
Panek K., Korner I., Lang H., Markut T., Petz R., Petz W., Siegl W.
meandering river course with amplitudes up to 200 m was dramatically
straightened, the resulting increase of the bed slope was counterbalanced by
many ground sills with drops up to 3 m. The uniform cross section was
carried out in a double trapezoid form completely covered with cobblestones
in a concrete bed. Furthermore, sediment control dams severely limited the
natural sediment transport.
In addition, the immission load grew significantly over the years. Main
sources were about 380 surface water discharges, a poorly functioning
sewage plant and untreated effluents of hot springs. Not surprisingly, routine
examinations in the last decades of the past millennium confirmed a poor
ecological status and a nearly non-existing functional integrity of the aquatic
system within the hard constructed urban reach.
The implementation of the European Water Framework Directive into
national legislation gave rise to an interdisciplinary assessment of realistic
development objectives for an urban river like the Liesingbach.
Consequently, the Liesingbach was classified as a heavily modified water
body. To improve the ecological status of the Liesingbach, it was decided to
reduce the immissions significantly. The discharge of sulphureous hot spring
wastewater was stopped and also a small municipal sewage plant was shut
down while its wastewater was redirected to Viennas main clarification
plant. For this, a new main sewer had to be built following the river course.
These necessary construction works were used for the physical habitat
rehabilitation of 25% or 5,5 km of the channelised Liesingbach-stretch up to
the south-eastern city limit of Vienna. Main aims were the removal of the
hard bed and banks, demolision of ground sills and to enable a certain lateral
dynamic.
Comparative investigations before and after the implementation of the
revitalisation measures were carried out to prove the development of
instream and riparian habitats. The reference study was performed in 1999
while the subsequent monitoring ran from autumn 2004 to the end of 2007.
2. HYDROMORPHOLOGY
According to the evaluation scheme proposed by Werth (1987) and
Spiegler et al. (1989), the hydromorphological status in 1999 showed
adverse and unnatural conditions (structural status class IV). Beside
interrupted passability for sediment and fish, the hard construction caused
several deficits such as a straightened river course without bed sediments,
lacking variability in width and depth as well as missing riparian vegetation.
Surface water velocity was monotonous and rather high despite the reduced
bed slope due to ground sills.
In the course of the revitalisation measures, the hard construction was
removed from both the river bottom and banks. The river bed was stabilised
Monitoring results of revitalisation measures on an urban lowland river
by inserting coarse cobbles, which were covered with natural, site-specific
fine gravel. In some sections, an extension of the river course was made. A
meander-like bend and several expansions of the river bottom were built.
Unfortunately, a continuous widening of the river bed was not possible due
to a conflict of interests: The required space for roads, bicycle lanes,
promenades, bridges and new settlements often accounted for the
maintenance of the straight-line character. The river banks were enhanced by
planting groups of autochthonous trees and shrubs. These actions resulted in
a significantly improved hydromorphological setting with variable flow
velocities, heterogenous bed sediments, developing small islands and gravel
banks.
A preliminary assessment of the hydromorphological status indicated that
the habitat characteristics were at least one status class better than in 1999. It
is assumed that further improvements will take place, since the development
of river banks and slopes as well as the growth of the riparian vegetation is
still continuing. For reasons of flood control, however, the sediment is still
retained in the upper catchment. Nevertheless, gravel discharge is observed
in places because sediment dynamics within the revitalised river stretch were
initialised. This indicates, that the re-establishment of the natural passage of
sediment is quite essential to achieve a sustained revitalisation success.
3. VEGETATION ECOLOGY
As the succession in dynamic habitats along running waters proceeds
rapidly, vegetation surveys were taken twice a year. The monitoring program
was based on a system of long-term vegetation plots of 2 x 2 m, which are
subdivided into four smaller plots to estimate the proportional coverage of
plant species. The monitoring started in autumn 2004 on sandy, muddy and
gravel sediments of the riverbanks and the river bed and covered the entire
vegetation periods of 2005-2007.
A clear trend was already visible in 2006 on muddy sediments, where
pale smartweed (Persicaria lapathifolia), which dominated the riverbanks in
2004, decreased distinctly in favour of Canary grass (Phalaris arundinacea),
which became then the dominant species. This trend was slightly weakened
in 2007, because new sediment deposits occurred. The further development
of the muddy riverbanks is heading towards a bush-dominated riparian forest
consisting of different willow species (Salix spp.) when floodwaters do not
clear away the sediments again. The previous dynamics in some parts of the
Liesingbach were obviously not sufficient to maintain the habitat conditions
for a more species-rich stadium of succession. The development of the
vegetation towards an alluvial forest is responsible for the decrease in
species number per monitoring plot, e.g. the species number declined from
40 in 2005 to 20 in 2006 in selected plots, however, it increased slightly in
Panek K., Korner I., Lang H., Markut T., Petz R., Petz W., Siegl W.
2007. This shows that muddy riverbanks at the Liesingbach are subject to a
very rapid natural succession, leading to a dense and species-poor reed
vegetation. Similar observations were made in flood detention basins in
some rivers in the neighbourhood.
At the gravel banks, the association of the Rumici crispi - Agrostietum
stoloniferae represented the first step of succession, but was followed
already in 2006 by a more dense Polygono lapathifolii - Bidentetum. In
general, habitats with sandy and muddy sediments have already developed a
dense and species poor reed composed of Canary grass (Phalaris
arundinacea). Only at sites exposed to higher dynamics and with a higher
proportion of gravel, a species-rich and loosely covered vegetation is able to
withstand the overall succession trend. The vegetation cover of these habitats
differs only slightly from that in 2004 and still shows a Rumici crispi Agrostietum stoloniferae or a Polygono lapathifolii – Bidentetum
association, which are more diverse than the Canary grass reed. A small
portion of the gravel deposits of the Liesingbach do not reach above water
level (possibly due to an inadequate bed load) and bear no initial vegetation
yet.
On one gravel bank, the endangered water cress (Nasturtium officinale)
occurred in 2004. Subsequently, however, it was apparently washed away
and could not establish itself again in 2006 and 2007 due to high water
levels. In 2007, two other endangered species, the round-leaved cancer wort
(Kickxia spuria) and the brown galingale (Cyperus fuscus), were detected in
another plot on an approximately 40 m long gravel bank.
4. DRAGONFLIES
Dragonflies are particularly suitable for long-time studies of new inshore
waters because of their high mobility, size, noticeable color and colonisation
potential. These insects also play an essential role in the assessment of the
ecological integrity of aquatic systems and are reliable indicators of habitat
heterogeneity, connectivity aspects and ecological quality of the land-waterinterface. Only long-time studies will give information about succession
processes and other ecological issues (Chovanec & Waringer, 2005).
Between 2005 and 2007, the colonisation of the revitalised river stretch was
investigated. Four sites, each 100 m long, were periodically examined.
Adults, juvenils, exuviae and larvae of dragonflies and the reproduction
behavior were recorded.
In 1999 prior to the revitalisation, only one species, Calopteryx
splendens, was found at one sampling site at the Liesingbach. Colonisation
started shortly after completion of the construction works and total number
of species as well as number of endangered species were continously
increasing. In total, 20 species were identified including eight species
Monitoring results of revitalisation measures on an urban lowland river
classified “vulnerable” or “endangered” according to the Red List of Austria
(Raab et al. 2006). Total number of breeding, i.e. autochthonous, Odonata
was highest at the meander-like site, which was already completed in 2004.
Not surprisingly, species arrived first in areas with completed
revitalisation measures while the Odonata appearance was delayed in river
sections with a later completion date. Consequently, the similarity of the
fauna shows a gradient from the lowermost sampling site, where the
rehabilitation started, towards the uppermost sampling site. The shorter the
distance between the sampling areas the higher the similarity between their
species spectra. Furthermore, the steadiness, defined as an expression of
appearances, increased over the years at all sites. In 2007, 16 of 18 observed
species are classified as widespread because they occurred in at least three of
the four sites.
Within the few years, a typical “Gomphus - Calopteryx splendenscoenosis” (Jacobs, 1969) developed with the rheophilic representative
species Gomphus vulgatissimus and Onychogomphus forcipatus,
Platycnemis pennipes and Calopteryx splendens as well as the concomitant
species Ischnura elegans.
Odonata colonised the reconstructed part of the Liesingbach rather
quickly because of numerous potential sources in the neighbourhood and the
aquatic vegetation developed rapidly. Thus, a fairly stable community
occurred within the few years.
5. CARABIDS
Ground beetles (Coleoptera, Carabidae) are a very sensitive indicator
group (Günther & Assmann, 2005; Gerisch et al., 2006). They were
surveyed at various types of riverbanks (different slope angle, exposition,
shading, vegetation) along the revitalised Liesingbach in the years 20052007 using pitfall traps and compared to the carabid community prior to the
revitalisation (1999).
Activity abundance is 2.5-4.0 times higher than in 1999 and highest in
habitats with exposed river sediments and no mowing on banks near the
south-eastern city limit. In total, 107 carabid species were recorded after the
reconstruction. Annual species richness varied from 54 spp. in 2006 to 82
spp. in 2007 and is distinctly higher than before revitalisation measures (36
spp.). Also, the Shannon-Wiener diversity index increased slightly from 2.77
(1999) to 3.07 (2007). The species composition in recent findings differs
significantly from that in 1999 (Shannon Diversity t-test; PAST; at most
p=0.0109). A high proportion of sporadic abundant species (42-56%), a low
proportion of constant abundant species in each monitoring season (27%)
and a high proportion of macropterous individuals (currently ca. 85% vs.
35% in 1999) indicate dynamic recolonisation processes.
Panek K., Korner I., Lang H., Markut T., Petz R., Petz W., Siegl W.
The species composition of the ripicole carabid group as well as of all
carabids changed immediately after revitalisation. Most ripicole species were
found in 2007. Only Harpalus luteicornis (sporadic abundant) and the less
ripicole species Bembidion subcostatum (subdominant and dominant,
respectively) are found in every monitoring season. In general, the
development of the carabid community showed a decreasing abundance of
xerophilous open habitat species (Amara sp., Harpalus sp. and Anchomenus
dorsalis) and an increase of hygrophilic forest and/or ripicole species (e.g.
tribes Bembidiini, Trechini, Pterostichini, Agonom sp.).
These results indicate a dynamic and less constant carabid community
three years after reconstruction of the river and the riverbanks due to
changing habitat structures and their continuing epigaeic recolonisation.
6. CILIATES
The ciliate coenosis is a well-suited bioindicator for the water quality in
running waters. Thus, the ciliate fauna in the Liesingbach reflects the organic
pollution rather than structural improvements due to the revitalisation.
In 1999 prior to the rehabilitation, the saprobity index according to
Blatterer (1995) indicated β- to α-mesosaprobic (water quality class II-III)
and α-mesosaprobic (water quality class III) conditions, locally even a
tendency to polysaprobic conditions (e.g. sapropel was widespread). After
completion of the reconstruction works and the redirection of wastewater
discharges, water quality improved slightly, i.e. by about 0.2 units of the
saprobity index, indicating that a considerable organic load still remains.
The species richness increased slightly after the rehabilitation (119 spp. in
1999 vs. 150 spp. in 2005). However, the lower species number before the
reconstruction may be influenced by the rather high flow velocities then (up
to about 2 m/s) and the hard river bed, which often lacked any sediments. A
slight change occurred also in the species composition. For example,
Trochilioides recta, which feeds mainly on sulphur bacteria, was abundant in
1999 but disappeared after the redirection of the sulphureous wastewater. Some
other species, e.g. Stylonychia mytilus and Vorticella campanula, appeared and
occurred in high densities after 2004.
7. MACROZOOBENTHOS
Before revitalisation measures, habitats for benthic invertebrates were
rare. Only small areas of very unstable, accumulated sediments on the
cobbled riverbed and bunches of green algae (mainly Cladophora sp.)
inserting in cracks were inhabited. The coenosis showed a highly fluctuating
species composition. Oligochaets were distinctly dominant and represented
up to 90% of the total invertebrate fauna. Nais elinguis (Naididae), Tubifex
Monitoring results of revitalisation measures on an urban lowland river
tubifex (Tubificidae) and Lumbricillus rivalis (Enchytraeidae) appeared with
highest numbers indicating that organic pollution was high (water quality
class III, α-mesosaprobic).
The insertion of natural bed sediments was the main prerequisite for the
development of a stable macroinvertebrate fauna. In fact, shortly after
revitalisation measures the species number increased by 20-40%. Also,
species diversity (Shannon-Wiener-Index) considerably increased from
around 2,0 to 2,5-3,2. The latter findings result in part from increasing
proportions of Ephemeroptera, Plecoptera and Trichoptera species, which
reach up to 34% of the total species number in 2007, while the larger part
results from an abrupt rise in larvae of Hydropsyche sp. (H. angustipennis,
H. bulbifera, H. modesta). Prior to the revitalisation, these groups appeared
only sporadically with proportions well below 1%. On the other hand, the
percentage of Oligochaeta species decreased well below 50% in 2007.
Concomitant with a reduced wastewater discharge, the organic pollution
declined significantly resulting in water quality class II-III (β- to αmesosaprobic), which is now observed throughout the entire revitalised river
stretch.
Three years after recolonisation, some species typical for small lowland
rivers are still absent. The main reason for this is the spatial isolation of the
revitalised river course. The natural catchment area can hardly act as source,
since it is situated in higher faunal regions, and is separated by several
kilometres of city from the revitalised stretch. Other running waters with a
suitable stock of species are at least about 5 km distant, which is too far for
most of the volant adults.
8. FISH
Fish inhabit nearly all aquatic environments. Many species require certain
habitats and are bound to certain structures in a river, particularly for
spawning. This, their comparatively long life expectancy and the high
mobility qualify the ichthyocoenosis as a suitable indicator for the ecological
status of running waters.
Before the revitalisation in 1999, only a few individuals of four
euryoecous species (chub, Squalius cephalus; minnow, Phoxinus phoxinus;
stone loach, Barbatula barbatula; pumpkinseed sunfish, Lepomis gibbosus)
were found in the Liesingbach. After completion of the rehabilitation, on
average eight species occurred per sampling site. This increase in diversity is
also corroborated by the fact, that 16 different species were detected between
2005 and 2007. Now the dominant species are chub, gudgeon (Gobio gobio)
and stone loach. Their age structure shows stable populations with juvenile
age classes dominating. The barbel (Barbus barbus), the minnow, the
Prussian carp (Carassius gibelio) and Pseudorasbora parva (introduced
Panek K., Korner I., Lang H., Markut T., Petz R., Petz W., Siegl W.
species) occurred regularly in the revitalised stretches. Some of these species
are obviously even spawning here. Less frequent or rare were, e.g., brown
trout (Salmo trutta fario), common dace (Leuciscus leuciscus), Crucian carp
(Carassius carassius), roach (Rutilus rutilus), rudd (Scardinius
erythrophthalmus) and spirlin (Alburnoides bipunctatus). Some typical
lowland-river species are still absent. A main reason for this is a migration
barrier between the Liesingbach and the downstream systems of Schwechat
and Danube, so that a recolonisation from downstream is at least severely
limited.
Individual numbers and biomass varied considerably between sites and
years, but compared to the results before the revitalisation a distinct increase
occurred. Even though the recolonisation of the revitalised stretch started
immediately after the reconstruction works and was rather successful, it will
take some more time until a species-rich and stable fish community will
establish.
9. CONCLUSIONS
In the course of the reconstruction of a 5,5 km long stretch of a lowland
river, a comparative study of important abiotic and biotic parameters was
carried out. The removal of the monotonous, plain structures of the hard
river bed led to the enhancement of the land-water transition zone and
enabled certain lateral and longitudinal dynamics. Thus, the development of
habitats was initiated for the benefits of a rapid and extensive recolonisation
of many organisms. As accompanying measure, the immission load was
markedly reduced, which improved the water quality.
The newly built and developed structures were quickly colonised. Beside
ubiquitous organisms, faunal elements typical for lowland rivers and some
endangered species appeared in important numbers. The immigration of
additional species depended strongly on adequate sources and the capability
of species to cover longer distances.
The results indicate that even in an urban sourrounding with significant
spatial restrictions a revitalisation can be successful. Three years after
completion of the reconstruction works, the biocoenotic development is still
in progress.
ACKNOWLEDGEMENTS
This study was initiated by the municipality of Vienna, Dept. for Hydraulic
Engineering, and funded by the EU (LIFE-programme).
Monitoring results of revitalisation measures on an urban lowland river
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