River Bed Erosion on the Hungarian Section of the Danube
Total Page：16
File Type：pdf, Size：1020Kb
Journal of Environmental Science for Sustainable Society, Vol. 1, 47-54, March 2007 RIVER BED EROSION ON THE HUNGARIAN SECTION OF THE DANUBE 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, Hungary) 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 Esztergom Nagymaros Adony Budapest 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.