Wetland Area And Construction Of A Power Station

Aus Stefan.Allerstorfer.at

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Transmitted by the Government of Austria

Prepared by Mr. Stefan Allerstorfer

NATION UNIES - COMMISSION ECONOMIQUE POUR L'EUROPE - Seminaire

UNITED NATIONS - ECONOMIC COMMISSION FOR EUROPE - Seminar

ОБЪЕДИНЕНЕННЪГЕ НАЦИИ - ЭКОНОМИЧЕСКАЯ КОММИСИЯ ДЛЯ ЕВРОПЫ - Семинар

Committee on water problems - Seminar on water managment systems

Bratislava, 15.-19.09.1986

Restricted WATER/SEM.13/R.45 23 June 1986


Flowing waters forming part of our landscape are decisive importance for the balance of natural resources, utilization of water resources, preservation of the alluvial wetland and for providing the population with recreation.

The regulation of the river Danube has resulted in a distinct degradiation of the river bottom since the turn of the century and, in this connection, also in a lowering of the Danube's water level and of the ground-water level in the wetland area. This degradiation is especially pronounced with regard to the river stretches Vienna to Hainburg and Zwentendorf to Greifenstein where it amounts to more than 1 metre. It causes a progressive deterioration of numerous sites in the alluvial wetland, more especially in the "Weichen Au" (with ist raw, coarse-grained soils).

Inhaltsverzeichnis

Measures in the river bank area

In the course of building measures required for the Danube power station Greifenstein, it was necesarry to erect in the backwater area situated on the left and right banks of the Danube levels free from flood spills. On the left bank, one levee extends from the power station Greifenstein to river location km 1972 (opposite the mouth of Perschling), a filly water-tight underground cutoff extending to river location km 1964 (Tulln).

Without accompany measures, the construction of the backwater levee on the left bank of the Danube would in the case of similar floods prevent the vital inundation of the wetland forest; in case of larger floods, the Danube would only be able to overflow on the left bank of the Danube downstream of Altenwörth and only adequatly inundate the northern parts of the alluvial wetland up to Stockerau/Korneuburg. The adjacent populated area on the right bank between the power station Greifenstein and the mouth of the Perschling will be completely free from floods; downstream from Zeiselmauer a limited backwater unundation from the tailwater will continue to occur in case of extremely high floods.

Measures in the northern adjacent area

The northern adjacent area with wetland forest area of about 10.000 ha extends from the river location km 1943 (downstream water of the power station Greifenstein) to the river location km 1980 (downstream water of power station Altenwörth) and is borderd in the north by the federal road B3.

There follows a report on the ecotechnical measures by the Österreichische Donaukraftwerke AG (company for Austrian power stations) with prevent a further lowering of the ground-water level, while also causing ground-water fluctuations to occur and making possible the frequent discharge of small and medium-sized floods of the Danube into the wetland area.

In co-operation with the authorities, water and landowners, inventories of wetland area were carried out. They included extensive investigations concerning ground-water level, thickness of aquifer, water quality and quantity; tributaries from adjacant areas; local plants and rush; forest production; depth of gravel, cover and consistency of alluvial meadow sand; sediment of suspend solids; soil erosion; the situation regarding drift flow and inundation in terms of reprentative water levels of the river Danube.

Water feeding and drainage system

Figure 1

By connecting existing old river courses by means of small drainage cuts and degradation of the river bottom, a run-through channel, i.e. the so-called "feeding water course", is constructed.

The feeding water course (figure 1), which has a lenght of about 40 km, runs from the "Rondellwasser" to the "Plackenwasser", continues through the gravel dredging pool upstream of the Tulln bridge and, by means of a drainage cut, under the road bridge crossing the embankments of the railway and of the understram gravel dredging pool in the "Hechtengraben", continouing to the "Krumpenwasser" and two old branches (Kierling and Stockerau) and finally leads to the confluence in the Danube at the river location km 1943,6.

In this connection, it is considered important to carry out only a small number of degradations of the river bottom. The route of the drainage outs is deliberately constucted in an irregular fashion so that a water cours with islands and shallow water zones, similar to that found under natural conditions, results. The maximum bottom width at drainage cuts and at degradations of the river bottom in the feeding water course amounts to 10 m.

Traversable reaches

Figure 2
Figure 3
Figure 4
Figure 5

In the feeding water course, the reaches are genereally constructed in those places where at present the main access roads on unspilled fords cross the channel, in order to adjust the water level in the feeding water course to the ground water level incline. 25 reaches and a pipe culvert are planned. The distance between the reaches ranges from 800 m to 3 km. Each of these reaches consists of a transverse embankment with a rectangular culvert and a fjord (figure 2).

The trafficable transverse embankment has a core of gravel, a downstream shell of riprap and flat slopes of gley sands is provided with a wide dam crest. The toe of slope is secured with riprap within the fluctuation limits of the mean water flow. By planting autochthonous plants and by eco-adaption of the slope to the existing bank, it will be possible to create vegetaion-backed structures (figure 3).

The ford on the dam crest, with ist connections to the rectangular culvert, is constructed as a riprap dam. It lies 30 cm lower than the trafficable concrete upper level of the rectangular culvert to make possible the drifting away of wood and floating matter from the inlet of the culvert (figure 4).

Irregular covers of alluvial sand on riprap-shelled slopes and tops of riprap covered by plants and grass will provide slope protection lost to view.

The rectangular culvert consisting of concret prefabs has a clear diameter of 2,50 m width and 2,30 m height. Within the framework of the project of DoKW (Danube power station company), the use of stop logs made of concrete will result in a water level equivalent to the present mean ground-water level. The slight difference between the headwater and tailwater makes possible a fish pass from one reach to the other. According to the requirements of the wetland forests (e.g. during dry periods), the land owners concerned may allow the water level of the feeding water course to be raised by about 0,50 m to 1,00 m by using wooden stop logs. In this way, an average depht of water table amounting to 2,50 m throughout the wetland area would be possible. It is specially important to make sure that the efforts to create optimum conditions for the wetland forest do not lead an impairment of other areas (water supply facilities, grassland and arable areas).

The construction of the rectangular culverts in the feeding water course was determined by several criteria. The top level of the rectangular culvert was chosen in such a way that the wetland area is flooded as evenly as possible. The bottom level may be reduced by about 40 cm. In most reaches it is thus possible to drive bulldozers directly to the channel bottom covered by silt in order to facilate the removal of any silt deposits. It is however expected that the channel bottom in the constantly wetted waterway will be kept open by erosion (figure 5).

The groundwater level is largely determined by the level of the receiving water in the surface water course. For this reason, keeping open of the bottom stratum in the feeding water course is an important precondition for the replenishment of ground-water in the wetland area.

The flooding-channel (feed-in section)

Figure 6
Figure 7

For the discharge of small and medium-sized floods of the Danube in the feeding water course, a flooding channel in the form of an overflow dam with a composite profile is constructed by the top-lowering of water levees between river location km 1976,25 and 1976,50. In the upstream part of the flooding channel, with a lenght of 30 m situated 179,50 m above sea level, even the frequently occuring small floods can flow over; the part situated downstream 181 m above sea level, with a lenght of about 150 m, is to serve for the influx of medium-sized floods. This does not, however, increase the discharge in the northern adjacant area in case of large floods (figure 6).

The overflow dam consists of a core gravel, a wide creast of riprap and slopes of riprap shell with an incline of 1:2. Gley sand covering the riprap shell as well as grass and plant cover will provide flood protection (lost to view) of the feed-in section. The mean water level on the "drainage cut Rondellwasser" on the land side of the overflow dam is about 50 cm higher than the raised mean water level of the Danube. In order to prevent water from infiltrating into the Danube, a pile sheeting screen is constructed for the entire lenght of the flooding-channel.

The influx into the lower flooding channel already takes place when the flow-rate of the Danube amounts to 3.100 m³/sec with a frequency of about 33 days per annum. On 23 days p.a. the influx will amount to some 5 m³/sec, on 10 days p.a. 10 m³/sec. If the influx exceeds 10 to 12 m³/sec, the water begins to flow over the fords at the reaches of the feeding water course. On 4 days p.a. there will be an influx of about 40 m³/sec, which result in an impoundage of the higher flooding level (figure 7).

If the water level rises even higher, the flood water will spill over the top line of the drainage cut. In this way it will be possible to promote annually accurring floods of large parts of the wetland area. At the same time, it seems safe to assume that these floods will not have an excessive duration, except in local low-lying areas.

Drainage cut "Rondellwasser"

The flood entrance is connected with the feeding water-course via the adjacent drainage cut "Rondellwasser", which is also provided with a composite profile. The mean water flow channel with a bottom level of 177,70 m above sea level and a minimum bottom width of 10 m is provided with an irregular profile and varying sole widths. The flood channel, with a bottom level of 179,50 m above sea level and a minimun width of 105 m, is ecologically adapted to the natural surroundings. It is sparsely afforested with black poplar and is to be managed as a meadow which also serves deer for browsing.

Minimum feed in the feeding water-course

The medium flow-rate results from the following inflows:

From the existing intake structure at the river location km 1979,7 about 1,5 m³/sec can flow into the Altenwörth branch and further, via the drainage cut "Rondellwasser", into the feeding water-course. The mean flow-rate of the tributaries into the system of old branches results in a total of 1,2 m³/sec. From the bank sections without underground cutoff, i.e. upstream from river location km 1964, a total of 2,5 m³/sec will infiltrate into the feeding water course. It is expected that the total of infiltration water and hillside seepage water will be sufficient for maintaining favourable ground water conditions in the wetland area. However, if additional water is required during dry periods, there will be the possibility of discharging a total of 10,2 m³/sec into the wetland area by using the three new intake structures constucted by the DoKW (Danube power staion company).

Ground-water

In a mathematical ground-water model, numerous forecasting calculations for ground-water levels with regard to the low and mean water flow on the Danube were carried out. Furthermore, the difference between the conditions before and after constuction of a power station and the dephts of water table were shown in plans. These plans show the desired raising of the ground-water level when this level is too low in wetland areas, without unfavourably affecting the adjacent zones.

Improvements and advantages

It is expected that these measures will lead to some important positive effects influencing the water balance in the wetland area:

  • Improvement of the water quality, which is partly very poor, in certain water courses by the discharge of water from the Danube and influx of ground-water and infiltrating water into the feeding water course to be established.
  • The characteristic effect of running water in the receiving course to be constructed will result in the flushing and cleansing of the bottom of the water course and possibly in the bottom strata being kept open.
  • The subsidiary channels flowing into the sides of the receiving water channel are generally only connected with the water-carrying channel on one side (impounded type). They are partly imponded by the reaches. Such subsidiary channels with tranquil flow not only serve as spawning grounds but also as a source of food, a shelter in case of floods, a refuge for animal species threatened with extinction, and as habitat for young fish.
  • By raising the water level in order to maintain a favourable depth of the ground-water table, large parts of the ground-water will lie above the gravle table. The capillary fringe will reach fine-grained soil and the roots will again be able to reach the ground-water level.
  • Preservation or creation of wetland biotopes by means of very flat embankment slopes. The wet biotopes provides good conditions for the minute world of micro-organisms. The natural result is the self-purification capacity of wetlands, which has been well known for decades.
  • More fluctuations and greater fluctuation limits of the mean water flow, including that of ground-water, will result in a certain aeration in the soil stratum.
  • If the water flow of the Danube exceeds the intake of the turbines (about 3.000 m³/sec) this automatically causes a discharge into the wetland area trough the flood channel at the river location km 1976,4.
  • During extremely dry years it will be possible to increase the withdrawal of water through the intake structures by raising the storage level at present approved according to the Water Law, thus additionally feeding the wetland area with water from the river Danube.
  • After introduction of impoundage, the observation of flood processes and the ground-water level will be continued in order to supplement further the water feeding and drainage system if necessary.

Siehe auch