## INFLUENCING OF HABITAT CONDITIONS BY MANIPULATION ON INTAKE STRUCTURE TO THE RIVER ARM SYSTEM AND THE OLD DANUBE RIVER-BED

#### Stefan NESTICKY

Forest Research Institute, Research Station Gabcikovo, 93005 Gabcikovo, SLOVAKIA

CONCLUSIONS By setting to operation the intake structure at Dobrohost and establishing culverts in the river arm system there were created conditions for preservation, and implementation of a optimal hydrological regime in the inundation area. It is possible to summarise principles of the manipulation order as follows:

- At the beginning and during a vegetation period it is to provide a water level in the river arm system in cascades at the barrier lines of compartments on the following altitudes: B - 120.50; E - 117.75; C - 119.10; F - 117.40; D - 118.50; G - 117.00.
- Generally it is not necessary to water a forest by the surface flooding. If it is necessary to simulate flood from other reasons, this should be done on the basis suitable for fishery, which is in agreement also with the needs of the forestry. Possible erosion in the inundation area is necessary to take into the consideration.
- In the autumn and in the winter period it is necessary to keep the water level in the river arm system at the barrier lines on the following altitudes: B - 119.80; E - 117.20; C - 118.80; F - 116.70; D - 118.00; G - 116.20

#### INTRODUCTION

Forest communities in inter-dike space of the Danube river have contrary to the other forest ecosystem a very dynamic development. A decisive importance is played by quality of deposits, a height of the ground water level during a vegetation season and a duration and height of floods. Decisive changes were made at the end of 19 century when protective anti-flooded system was built up consisting of earthy dikes and pumping stations. On the whole territory worsen conditions for autochtonous deciduous trees. These were gradually replaced by deciduous trees more pretentious to water, eventually suffering from long-lasting floods and wetting. Gradually there started to be formed a secondary forest ecosystem since the given time period, which is a result of anthropogene activity, in which a human factor will further play an important role.

The second qualitative break in the tree composition was recorded at the end of twenties, when domestic poplars were replaced to a prevailing degree by cultivar euro-american poplars. Without the change of the forest area extent, production increased almost twice during the last 40 years. Exploitation of wood increased from original 36 % to 94 % by silviculture of high-productive and resistant clones of poplars. We can again expect qualitative changes in this territory because of introducing a set of hydrotechnical measures at the beginning of May 1992.

#### FORESTS AT THE POWER CANAL FROM HRUSOV TO SAP

Forest communities in the Danube river inundation belong to the most productive ones in the middle Europe. The average yearly increment of wood can reach 15-30 m3 ha-1, which is four times more than the whole Slovak average. High production abilities are given by a tree composition, mineral richness of soils and a sufficient transport of water. The cultivated high-productive clones of poplars and willows are the most preferred. For their production they consumed 800 - 1,200 mm of water. An amount of precipitation in the vegetation season in this territory is on average 300 - 350 mm and in the last years it was still lower (in 1992 - 101 mm, 1993 - 266 mm). From the given facts results, that forest communities are up to 70 % supplied by the ground water, accessibility of which is for root system a limiting factor of its existence.

The character and dynamics of changes are necessary to monitor until the values characterising natural conditions of this territory are stabilised. This will last according to our estimate 4-5 years.

From the above-mentioned facts, it is clear, that a sufficient supply of the inundation territory by water is dependent, except discharge in the old river-bed, mainly on manipulation on the intake structure and on individual lines in the river arm system. The aim of the observation of influence is to obtain such data, on base of which it will be possible to elaborate an optimal, eventually compromise, manipulating system for all spheres of interest in the inundation area. To put together such a system it is inevitable to take into consideration also a technical solution of the compensation water-supplying structures.

#### BASIC DATA ON INTAKE STRUCTURE AT DOBROHOST SUPPLYING THE RIVER ARM SYSTEM

The intake structures providing supply of water to the left-side Danube inundation in the stretch of the power canal consists of the following main objects:

- intake structure at the by-pass canal,

- outlet channel and cascades,

- connection canal to the left-side arm system of the Danube river,

- measures of the left-side arm system of the Danube.

The hydrological regime of the old river bed was changed in the stretch from rkm 1811 up to the place of damming the Danube river at rkm 1851.75, turning aside part of its flow to the by-pass canal (power canal( with following utilisation of water for the production of electric energy by the hydropower station at Gabcikovo. Only sanitary discharge flows into the old river-bed. Except this into the old Danube flows discharge overtaking the capacity of the power station, and eventually discharge when passing ice-blocks from the reservoir. The level in the old river-bed is as a consequence of small discharge essentially lower so that it is impossible to water the arm system without additional measures. The old Danube river bed by its draining effect lowers the water level in the river arms to such a degree, that some stretches would remain empty without watering the river branches from the reservoir. This was done by the construction of objects for water supply to the left-side arm system and by the construction of dike lines in the inundation area.

#### EVALUATION OF UP TO PRESENT MANIPULATION WITH WATER ON THE INTAKE STRUCTURE AND IN THE ARM SYSTEM

The basis of preservation of the inundation territory character in the stretch of the power canal is a suitable depth of the groundwater level, which depends on the water level in the old river-bed and in the arm system. The water level in the arm system is dependent on discharge at the intake structure and on the way of damming of individual dike lines. Therefore we take these realities as a base for elaboration of a proposal of a temporary manipulating regime for the left-side inundation of the Danube river.

The collection of data was made with one-week step, if necessary also twice per week. The ground water level was measured in observation wells (drilled to depth of 6 m).

The distribution of monitoring points L3-12 is indicated on the map. Monitoring points L3 and L4 are not affected by manipulations in the river arm system. They are situated under the estuary of the Bakanske rameno arm at the old river-bed and they are influenced by the back water effect of the tail race canal. It is possible to create a suitable groundwater level from the point of view of the forest management, except to places localised in the near-bank belt of the old river-bed (approximately 50 - 200 m - monitoring plots L10 and L11), where the groundwater level decreases up to 3 m and is fully dependent on the water level in the old river-bed.

Suitability of hydrological conditions in the inundation area is testified also by height and thickness increments of trees (see Table), measured on the observation plots since 1990. 30 and registered trees, 1.3 m high, are measured in every monitoring area. The last measurements were made in January 1994. Above-normal high increments in areas L4 and L12 were caused by disappearing weaker individuals during the snow calamity in the winter period 1993/1994. The hydraulic interconnection between the groundwater level and the water levels in the river arm system in dependence on water levels in the old Danube river-bed and in dependence on the discharge via the intake structure from the reservoir is important from the standpoint of the operation of the intake structure. According to realised measurements it is possible to suppose, that a necessary groundwater level is possible to reach also at smaller discharges, by closing the gates of the culverts at the compartments lines. This fact is necessary experimentally to test and further optimise.

#### PRINCIPLES OF TEMPORARY MANAGEMENT INSTRUCTIONS FROM THE STANDPOINT OF FOREST MANAGEMENT

On the base of present observations it is possible to state, that at discharges between 15 to 50 m3 s-1 on the intake structure at Dobrohost, it is possible to preserve the ground water level at required standard. We recorded an unsuitable hydrological regime only in a near-bank belt of the old river (width about 50 to 200 m) in 1993-1994, whereas draining effect was manifested and the ground water level decreased there up to 3 m close to the Danube. Plots in this zone create approximately 20 % of the whole extent of inundation and in the case that no weirs are built in the old river-bed, it is necessary to calculate with the forest reconstruction. There was an unsuitable situation in 1993 from the standpoint of the depth of the groundwater level upwards the estuary of the Bakanske rameno arm in the region of that time non-finished dike lines H, I, J, where there was a decrease of 3 m during the vegetation period. After finishing the dikes in 1994 however groundwater level moved to a depth 1.5 - 2.0 m under the surface, which is from the standpoint of the forest management very suitable. It is possible to consider as a negative phenomenon an essential prolongation of the vegetation period, created as a consequence of permanently preserved high water level in the river arm system. This represents a threatens for annual shoots by freezing and following fungi illnesses. In order to avoid such a situation, it is necessary to lower the water level in the arms gradually from the half of August. It means a lowering of the operation water level in cascades by about 40 - 100 cm in autumn and winter period. Following increase of the water level is necessary in the beginning and during the vegetation period in the time of the maximal tree increment (May - half of August).

Principles of the management could be summarised as follows:

- At the beginning and during the vegetation period it is necessary to provide a water level in the arm system on the following altitudes: B -120.50; E -117.75; C - 119.10; F - 117.40; D - 118.50; G - 117.00.
- On the base of achieved results it is not necessary to flood forest stands by surface watering. If a simulation of flood is necessary with regard to needs of the other institutions it is necessary to stand by the principles of the management with regard to fishery, which is in compatibility with the forestry needs. In case that the requirement of floods is not covered with a natural high discharge in the Danube river and water does not bring sufficient quantity of deposits, it is necessary to consider a possible erosion in the arm system and to regulate the water flow velocity.
- In the autumn period the operational water level should be as follows: B - 119.80; E - 117.20; C - 118.80; F - 116.70; D - 118.00; G - 116.20

This low stand of water levels is necessary to preserve until the beginning of the vegetation period (approximately up till the end of February), in connection with a developing trend of the air temperature and the amount of precipitation. It is possible to suppose, that required water level in compartments is possible to reach also at lower discharges via the intake structure by manipulation on culverts. Further optimisation of water levels is necessary to determine on the base of exact observations.

Tab. 1. Basic characteristics of monitored plots

No. | Forest mana- gement unit | Stand | Tree composition | Set of forest types | Group of forest types | Soil type
--- | --- | --- | --- | --- | --- | ---
species | %
1 | Gabcikovo | 331 e | oak black locust | 78 22 | nutritious set "c" | UFrc-100% | cl FMm
2 | Gabcikovo | 330 b | cult. poplar "I 214" | 100 | nutritious set "c" | UFrc-100% | cl FMm
3 | Gabcikovo | 90 b | willow | 100 | nutritious set "c" | SAL-100% | cl FMm
4 | Gabcikovo | 84 g1 | cult poplar Robusta | 100 | nutritious set "c" | QFr-100% | cl FMm
5 | Gabcikovo | 63 g1 | cult poplar "I214" | 100 | nutritious set "c" | QFr-100% | cl FMm
6 | Gabcikovo | 79 p1 | cult poplar Robusta | 100 | nutritious set "c" | QFr-100% | cl FMm
7 | Samorin | 55 d2 | cult poplar Robusta | 100 | nutritious set "c" | QFr-100% | cl FMm
8 | Samorin | 49 r1 | cult poplar Robusta "I 214" | 100 | nutritious set "c" | QFr-100% | cl FMm
9 | Samorin | 53 h | cult poplar "I 214" | 100 | nutritious set "c" | UFrp-100% | cl FMm
10 | Samorin | 47 a | cult poplar "I 214" | 100 | nutritious set "c" | UFrp-100% | cl FMm
11 | Samorin | 46 a | cult poplar Robusta alder | 70 30 | nutritious set "c" | QFrp-100% | cl FMm
12 | Samorin | 33 f2 | cult poplar "I 214" | 100 | nutritious set "c" | UFrp-100% | cl FMm

Tab. 2 Average biometric values and increments in individual years

Year | OBSERVATION POINT
--- | ---
L1 | L2 | L3 | L4 | L5 | L6 | L7 | L8 | L9 | L10 | L11 | L12
d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h | d1,3 | h
1990 | 27.5 | 27.0 | 41.3 | 27.7 | 32.2 | 22.0 | 22.7 | 24.0 | 35.8 | 30.5 | 35.3 | 28.0 | 33.4 | 34.0 | 26.0 | 25.0 | 46.0 | 35.0 | 32.4 | 23.0 | 47.2 | 42.0 | 23.0 | 24.0
1991 | 27.7 | 27.5 | 42.4 | 28.1 | 33.7 | 23.3 | 24.6 | 24.6 | 38.4 | 31.3 | 36.8 | 29.0 | 32.7 | 34.5 | 27.2 | 26.3 | 47.7 | 36.3 | 35.0 | 24.0 | 48.0 | 43.3 | 23.5 | 24.4
1992 | 27.8 | 27.8 | 42.8 | 28.5 | 34.4 | 24.1 | 26.0 | 25.5 | 41.9 | 31.8 | 37.5 | 30.7 | 34.8 | 35.3 | 28.0 | 28.5 | 49.1 | 36.6 | 37.5 | 24.9 | 49.1 | 43.6 | 24.2 | 24.4
1993 | 28.5 | 28.2 | 44.2 | 29.3 | 37.2 | 24.9 | 29.0 | 28.7 | 42.8 | 32.7 | 39.6 | 32.3 | 35.8 | 36.0 | 29.7 | 29.8 | 49.7 | 37.0 | 39.0 | 26.2 | 49.8 | 43.8 | 28.2 | 24.4
| ANNUAL INCREMENT
1991 | 0.2 | 0.5 | 1.1 | 0.4 | 1.5 | 1.3 | 1.9 | 0.6 | 2.6 | 0.8 | 1.5 | 1.0 | 0.6 | 0.5 | 1.2 | 1.3 | 1.7 | 1.3 | 2.6 | 1.0 | 0.8 | 1.3 | 0.5 | 0.4
1992 | 0.1 | 0.03 | 0.4 | 0.4 | 0.7 | 0.8 | 1.4 | 0.5 | 3.5 | 0.5 | 0.7 | 1.7 | 2.1 | 0.8 | 0.8 | 2.2 | 1.4 | 0.3 | 2.5 | 0.9 | 1.1 | 0.3 | 0.7 | 0.0
1993 | 0.7 | 0.4 | 1.4 | 0.8 | 2.8 | 0.8 | 3.0 | 3.6 | 0.9 | 0.9 | 2.1 | 1.6 | 1.0 | 0.7 | 1.7 | 1.3 | 0.6 | 0.4 | 1.5 | 1.3 | 0.7 | 0.2 | 4.0 | 0.0

d1.3 - Average thickness (cm) of stems in height 1.3 m
h - average height (m)

Fig. 1.