Journal of Environmental Science for Sustainable Society, Vol. 1, 47-54, March 2007

RIVER BED EROSION ON THE HUNGARIAN SECTION OF THE

László GODA 1, Béla KALOCSA 2 and Enikő Anna TAMÁS 3

1 Dept. of Hydrology, Lower-Danube Environmental and Water Authority (Széchenyi u. 2/c. 6500 Baja, ) E-mail: [email protected] 2 Dept. of Informatics, Lower-Danube Environmental and Water Authority (Széchenyi u. 2/c. 6500 Baja, Hungary) E-mail: [email protected] 3 Assistant Professor, Dept. of Hydraulic engineering and Water management, EJC-TF, Baja (Bajcsy-Zs. E. u. 14. 6500 Baja, Hungary) E-mail: [email protected]

Investigations of the recent decades have revealed a significant lowering of the river bed of the Danube in Hungary. This phenomenon can be observed on a long stretch of the Hungarian Danube and it can be presumed that similar changes occur on other parts of the river beyond the borders. There are several methods to reveal and to measure the lowering of the channel. Long records of the water level are available for the trend- and homogeneity analyses. Results show that erosion processes started even at the beginning of the 20th century. In recent years results of bathymetry are available for the more detailed investigations of these changes. The main causes of the bed-lowering are river regulation activities, industrial dredging and decreasing of natural sediment load. Deepening of the river bed has unfavourable effects on the natural environment, on the navigation and also on the operation of man-made structures in the river. To avert these injurious effects a complex solution is needed, taking into consideration not just economical but ecological and social aspects as well.

Key Words : Danube, erosion, river regulation, navigation, sediment, bed load, dredging

1. BACKGROUND approx. 6500 m3/s − Some details about the Hungarian stretch: Several investigations of recent decades have - length: 417 km (from 1433 to 1850 river km) revealed a significant lowering of the river bed of the - there are just a few tributaries along the Danube in Hungary1,2,). This deepening of the Hungarian stretch. The growth of the mean channel is a result of erosion processes that affect not discharge is not more than 10-15 percent discrete sections only but long reaches of the river. between the entering and leaving sections. This study is devoted to the Hungarian section of the - typical discharges at Baja station: Danube but, considering the reasons, it can be Qmax: 8023 m3/s, Qmin: 680 m3/s, Qavg.: 2435 presumed that similar symptoms could be m3/s. experienced in other parts of the Danube. The whole catchment of the Danube is presented by Some facts about the Danube: Fig.1, the Hungarian section is given in Fig.2. − Total length: 2826 km. Classifying the Hungarian Danube, it belongs mainly − Source: Black Forest, Germany. Estuary (delta): to the middle course, partly to the lower course with a The Black Sea, Ukraine/Romania lowland type. The river bed is alluvial. In accordance − The total catchment area: 817 000 km2 with the lowland character the slope is mild (10-15 − 18 countries share the catchment. Though 5 of the cm/km). The river bed is wide, with a typical width of 18 countries have a very minor part. 400-500 m. The avearage depth for the mean − The average annual discarge at the delta: discharge is 5-6 m.

47 László GODA et al.

Fig.1 Geomorphologic map of the Danube catchment. The Hungarian section is highlighted..

A middle course type river usually has a balanced phenomenon, the arising problems and sediment transport and a balanced channel. The consequences. fact that this is not so nowadays for the Danube indicates that there were, or are, external effects disturbing the early balance. 2. DISCLOSING THE DEEPENING OF THE RIVER BED

(1) Hydrological-statistical methods Regular hydrological observations started in the second half of the 19th century in Hungary. It started first with observation of the water level at many stations along the Danube. Somewhat later, in the beginning of the 20th century, regular discharge measurements also began. Nowadays there are water level records available for more then 100 years, and discharge records for 80-90 years for many stations on the Danube. The trend analysis performed for the annual extremes and averages of these long records of water levels indicates a significant falling trend of the water levels. An example is shown in Fig.3. A similar symptom is demonstrated by the Kolmogorov-Smirnov type homogeneity test (Fig.4.). These investigations are also suitable to estimate the degree of deepening: taking the station Fig.2 Hungarian section of the Danube and the main gauges of the upper example (Baja) and focusing on the annual low waters our rough estimation can be 160 This paper intends to give an overview of the cm for the analysed 105 years. tools to disclose and measure the deepening of the Danube, to present the possible causes of this

48 RIVER BED EROSION ON THE HUNGARIAN SECTION OF THE DANUBE the flow regime, that is, the decrease of the natural water level Trend analysis Duna - Baja cm annual maxima water resources of the river. This question can be 1901 - 2006

900 answered by performing the same kind of analyses for the discharge records: the trend analysis shows 800 that a significant trend cannot be found in the 700

Annual means records of the discharges (Fig.5.). 600 500 discharge Trend analysis annual minima 400 m3/s annual maxima Duna- Baja 1930 - 2005 300 7000 200

100 6000

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 5000 Time [year]

Fig.3 Falling trend of the annual extremes and means of the 4000 Annual means water level (Baja station) 3000

annual minima The question can arise, if the observed change of 2000 the water levels indicates really the change of the 1000 river bed or this symptom is a result of a change in 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Time [year] Fig.5 Trend analysis of the discharges. There is no significant trend in the discharge records (Baja station).

Similar investigations have been performed for all the main stations on the Danube in Hungary and the results were more or less the same. These results are summarized in Fig.6 where the tendencies of the 15-year averages of the water levels are presented for the main stations along the river3).

Fig.4 Szmirnov-Kolmogorov-type homogeneity test for the

annual minima of water levels (Baja station).

0

-50 -100 -74 -70 -150 -81 -99 -123 -200 Deepening, cm -130 -151 -127 -250

-176 -156 -145 -181 -181 -216 Vác Komárom Ercsi Nagymaros Adony Paks 1901-15 Baja 1916-30 1931-45 Dunaújváros 1946-60 Dombori Dunaföldvár 1961-75 1976-90 Mohács 1991-2005

Fig.6 Decrease of the 15-year averages of the water levels between 1901-2005 at the gauging stations along the Danube in Hungary

49 László GODA et al. (2) Morphological methods 92 The morphological survey of the river bed (bathymetry) is a more 91 Changes of the river bed profile near Dunaföldvár ( 1559 + 400 river km ) direct and exact method to follow the 90 changes of the bottom. However, 89 adequate and sufficient results are 88 available only from the latest 1949 decades, following the appearance of 87 1975 the GPS, the echograph and GIS 86 1981 applications. Nowadays bathymetry 1998 is done every 5-6 years. Comparing 85 the result of more surveys, done in 84

Elevation above the sea level, m level, sea the above Elevation 2003 different periods for the same spot, 83 the changing of the river bed can be 82 directly analyzed. These comparing -300 -200 -100 0 100 200 300 results of a typical section (near Distance, m Dunaföldvár) are illustrated in Fig.7. The longitudinal profiles of the Fig.7 Changes of a profile of the river bed near station Dunaföldvár. bottom of the channel line are shown in Fig.8.

Fig.8 Difference map of a Danube section near Dunaföldvár from the surveys in1981 and 2003

3. CAUSES OF LOWERING OF THE RIVER BED

(1) River regulation During medium-flow regulation strongly River regulation works were started in the 19th meandering bends were cut, the horizontal century and are still going on along the Hungarian alignment of the river was fixed with bank Danube4). The aim of the early activities was flood protection structures and groynes, utilizing also the protection and to improve conditions for energy of the flow to form the bed. The aim of this navigation. In the course of high-flow regulation work was to speed up the travelling of floods and to lowland areas along the river were protected from help the movement of drifting ice. The improved inundations. The first dikes had a local role, later shape of the channel was favourable for navigation they were followed by higher levees, stretching for as well because the narrowed river bed resulted in a long distance along the river. deeper water for longer duration. However this Driven between the levees, floods are not able to transformation resulted in shortening of the river, spread – as they did in the natural, original state – increasing the slope and therefore increasing the and have a stronger effect on the bed forming sediment transportation capacity. Consequently the processes. original balance has been shifted towards sediment

50 RIVER BED EROSION ON THE HUNGARIAN SECTION OF THE DANUBE erosion. Narrowing of the river bed also increased flow velocities, involving the increase of the (2) Dredging sediment transportation capacity again. An old Gravel and sand, being dredged from the river, are map from the the turn of the 19th and 20th centuries valuable raw materials for construction industry. illustrates the result of the river regulation work Therefore – above the requirements of low-flow (Fig.9). regulation – huge masses of sediment were mined from the Danube between 1960 and 1990. According to well-founded estimations, about 70 million m3 of bed material was excavated from the Danube during the period mentioned2,5). Taking the affected stretch of the river and an average width of the bed into consideration, this removed mass would be enough to cause about 60 cm deepening of the bed. Of course the effect of dredging can not be projected onto the river so simply, but obviously the dredged holes are more or less filled up by the bed material of the upstream section, causing the deepening of the channel. Another unfavourable effect of dredging is that it breaks up the harder, „armoured” surface of the bed. The current erodes the upstream edge of the dredging hole and the looser layers below will also be accessible for the energy of the flow. Because of the problems mentioned above it was necessary to decrease the dredged amounts as small as possible. Nowadays industrial dredging is not allowed. Only river regulation and maintenance of the navigation way can be the reasons for dredging.

(3) Decrease of transported sediment Numerous barrages and reservoirs were built on the Danube and its tributaries in the second half of the 20th century6). Regarding the Hungarian section, first of all the barrages in Germany and Austria have an essential importance. The barrages on the Upper Danube are illustrated by a schematic longitudinal profile in Fig.10. The flow regime of the Danube was not considerably influenced by these structures, but the Fig.9 Cut-offs on the Danube-section upstream of Baja on flow conditions in the channel have essentially the turn of the 19th-20th centuries. (Nowadays the changed. The speed of the flow decreases Danube flows in the strait cuts while the meandering significantly in the backwater of a barrage and it bends became narrow dead branches.) causes the bed load to stop and the 30-40 % of the suspended load to settle. At the same time, Low-flow regulation is an indispensable auxiliary sediment transport capacity of the water increases element of medium-flow regulation. It on the downstream of the barrage causing an concentrates on the stretches of the river where increased erosion of the downstream channel medium-flow regulation was not enough to (unless the backwater of the next barrage on the develop the required profile, water is shallow and downstream reaches the section in question). fords obstruct the navigation. Additional regulation structures and direct dredging is applied on these stretches to get the required measures of the navigation profile. However, dredging helps the deepening processes, in more respects, as it is given below.

51 László GODA et al.

Fig.8 Barrages on the Upper-Danube (Germany, Austria, Slovakia).

However the amount of the eroded material does despite of the extensive regulation dredging, the not reach that of the sedimented masses. So the conditions of navigation did not improve result of the process is the decrease of sediment significantly in the last decades5). concentration, causing a higher sediment transport The navigational low water levels (94 % capacity on the Hungarian section7). probability, ice-free stages on the main gauges) are calculated by the Danube Comission from time to time. Fig.9 shows how the logitudinal profile 4. EFFECTS AND PROBLEMS plotted for the navigational low water levels follow the lowering of the river bed. Deepening of the river bed has unfavourable effects on natural environment, on navigation and (b) In 1983, during an extreme low water also on operation of man-made structures in the period, there were troubles with the cooling water river. Some typical examples are outlined below. of the Paks nuclear power plant, which is supplied gravitationally from the Danube (the pumps of the (a) To improve the conditions of navigation power plant hardly reached the water in the is an important aim of river regulation work. supplying channel). Later the reach of the river Medium-flow regulation activities have been more near Paks was controlled successfully with river or less finished by nowadays. Taking into regulation methods and similar problems have not consideration that the horizontal alignment of the arisen since then. Nevertheless, nowadays some Danube is practically stabilized – by cutting many deepening of the channel can be observed in the bends, building bank protection structures and vicinity of Paks as well. Results of numerical groynes – this work has been accomplished. modelling predict a continuation of this process for However, the bed of the river is still continuously the following decades. developing and forming. These changes are – at An other fact, calling for a special regard for this least partly – consequences of earlier interventions. area is that the fords downstream of Paks, acting as The most frequent navigation obstructions are bars, have a significant role keeping the low water fords, shallows and contractions of the navigation on the required level for the water intake. On the channel during low water periods. Fords are other hand, these fords represent awkward continuously building and forming in the changing, obstructions for navigation during low water unstable channel – calling for a permanent control periods8). of the responsible authorities. More studies, dealing with the analysis of low water periods and the efficiency of low water regulation prove that,

52 RIVER BED EROSION ON THE HUNGARIAN SECTION OF THE DANUBE

98

96 Navigation low-flow profiles on the Danube downstream of Budapest 94 Budapest 92 Ercsi 90 Adony

88 1966 Dunaújváros 86 Dombori 1976

Baja 1984

84 Dunaföldvár

Paks 1990 Mohács 82 2004 Elevation above sea level, m above sea level, Elevation 80 1440 1490 1540 1590 1640 Distance, river km

Fig.9 Navigation low flow profiles on the Danube downstream of Budapest.

(c) The Gemenc Forest (part of the Danube-Drava low in the river, pumping is necessary to lift the National Park, Hungary) is one of the substantial water into the channels. In this system, developed floodplain forests of Europe. Regarding the many primarily for gravitational operation, pumping wetlands, providing a home for rich flora and entails high economical burden. This problem fauna, Gemenc is irrecoverable natural value. The appeared one-two times annually in the recent water demand of the forest and the wetlands is years. supplied by the Danube, partly by subsurface feeding, partly through the net of oxbows and channels, sometimes inundating the whole area 5. CONSEQUENCES during floods. Forestry experts observed certain drying The lowering of the river bed of the Danube is a processes of the forests in the second half of the complex problem as it has been outlined above. It 80th-s. They noticed changes in the development of has far-reaching effects which require complex trees and also the replacement of autochton plant solutions, taking into consideration not just species. It was revealed by the detailed economical but ecological and social aspects as hydrological analysis of the water levels that the well. The most important aims are as follows: flood events reaching the oxbow lakes become rarer and their duration decreased too. Besides the - Improvement of navigation conditions regarding low water periods with longer duration are also European standards. unfavourable regarding the subsurface feeding. - Provision for the safe operation of water intakes. Some projects started after the investigations - Provision for the water supply of nature aiming at the improvement of water supply and conservation areas and other wetlands, ecological water retention in the oxbow lakes9). rehabilitation of oxbow lakes and other dead branches. However the effects of these interventions extend only to small areas and, regarding the whole of the Further requirements while achieving the forest, they can not be appreciated as overall and above-mentioned aims: accomplished solutions. - Improvement of water quality - Increasing of flood safety (d) The Danube has an essential role in the - Development of recreation and tourism. agricultural water supply along the river. Large areas are supplied by artificial, gravitationally In recent years there have been several studies operated channels. When the water level is very dealing with the deepening of the river bed, the

53 László GODA et al. problems arising from this and the possible harmonize the methodology applied in different solutions. Even nowadays there are several such counties along the Danube. projects underway searching for solutions. An essential question is whether traditional river regulation means are suitable to reach the above ACKNOWLEDGMENT: The authors thank to mentioned aims. Building barrages, that is to say the Environmental and Water Authorities at Győr, the canalization of the problematic sections of the Budapest and Baja and also to the VITUKI for Danube, could be an evident alternative for supplying us with the necessary data to achieve the regulation structures and dredging. For this, as has analyses presented in this study. already been mentioned, there are a number of examples on the Austrian and German strech of the Danube, where the effect of these structures can REFERENCES also be examined. But at present the damming of 1) Kalocsa, B., Zsuffa, I.: A Duna Magyar szakaszának the Danube is not supported by the Hungarian vízállás változásai (Water level changes of the Hungarian government. section of the Danube), HK (Journal of the Hungarian Hydrological Society) 1997/3-4., Budapest. The problem of the deepening of the river bed is 2) Goda, L. (snr.): A Duna gázlói Pozsony és Mohács között. closely connected with the question of sediment (Shallows of the River Danube between Pozsony/ transport. The studies dealing with sedimentation Bratislava and Mohács), VK (Hydraulic Engineering) bring our attention to the fact that the used data are 1995/1-2., Budapest. often incomplete and sometimes only estimates. 3) Tamás, E. A.: Navigare necesse est… avagy a Duna, mint főútvonal (Navigare necesse est … or, the Danube like a The reason for this is that the measurement of the highway), in press, 2006. sediment (sampling and laboratory analysis) is a 4) Stanciková, A: A Duna szabályozása (Regulation of the complex, time consuming and costly process. River Danube), VK (Hydraulic Engineering) 2001/3., Sampling requires a high precison on the part of the Budapest measuring staff because without this the value of 5) Goda, L (snr.): A Duna gázlói a folyam Szap - Mohács közötti szakaszán. (Shallows of the River Danube between the costly results will be doubtful. During floods Szap and Mohács) manuscript, 2006. sampling is difficult and sometimes inpossible. 6) Pasoi, I., et all.: Inventory of the main hydraulic structures Comparative analysis of the results of bilateral in the Danube Basin. The Danube and its catchment – A measurements of neighbouring counries show that hydrological monograph. Follow-up volume VIII/1. Bucharest, 2004. the results can be significantly different even if the 7) Rákóczi, L.: A Duna hordalékjárása (Sediment load measuring sections are relatively close to each conditions of River Danube), VK (Hydraulic Engineering) other. The reasons for this are to be found in 1993/2., Budapest different methods and equipment used. 8) Laczay, I.: A paksi Duna-szakasz mederváltozásának Despite of the problems we have to state that ellenőrzése (Review of the channel changes of the Danube reach of Paks), VK (Hydraulic Engineering) 1996/3-4., knowledge of the sediment stransport is essential to Budapest understand the morphological processes forming 9) Szlávik, L., Sziebert, J., Zellei, L., Zsuffa, I.: A the river bed. Nyéki-Holt-Duna rehabilitációja (Rehabilitation of the Consequently the improvement of the Nyéki Oxbow-lake of the River Danube) Vízügyi methodology seems to be necessary both for the Közlemények (Hydraulic Engineering), 1995/3-4., Budapest. suspended and also for the bed load. Not just measuring techniques but also data processing (Received February 21, 2007) methods should be improved. International cooperation would be highly desirable to

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