PHYSICAL, CHEMICAL AND BIOLOGICAL ASPECTS OF HUMAN IMPACTS ON URBAN SOILS OF SZEGED (SE HUNGARY) Journal of Env. Geogr. Vol. III. No. 1-4. pp. 25-30 CHARACTERISTICS OF THE FLOW REGIME ALONG THE REGULATED TISZA RIVER REACH DOWNSTREAM OF TISZAFÜRED Bezdán, M. 1 1 VIZITERV Consult Kft, Budapest, Hungary Abstract In this paper, an attempt is presented at clearing the reasons of some uncommon flow phenomena observed over the Tisza reach down- stream of Tiszafüred, and at pointing to the practical significance of the results obtained. Over six hundred thousand daily stage data were selected from the more than six million (!) water level data, registered from 1876 to 2009 (on the gauges between Tiszafüred and Novi Bečej (Törökbecse)). Taking the data of ground water wells into considera- tion, the conclusions arrived at are believed to be of interest to theoreti- cal fluvial hydrologists, yet also to river engineers engaged in design- ing and operating (when needed) flood control schemes. Key words: low-water, high-water, ground water, hysteresis function, water level drop, backwater, barrage, permanence INTRODUCTION The comprehensive reclamation project conceived by Pál Vásárhelyi and implemented under the management of Károly Herrich has changed flow conditions of the River Tisza and those of the tributaries substantially. Flood peaks rose by 3 to 4, and low waters dropped by 2 to 3 metres in the bed, which was shortened by 32 % of the original length. The steeper slope has increased the aver- age flow velocities river = 1.26 times at medium stages. The coincidence pattern and extent of interaction with the tributaries have also undergone changes (Vágás 2007). Owing to drainage accelerated by roughly 26% in the main river and to the loss of storage space in the severed oxbows (Vázsonyi 1973), the spring and early summer flood waves travelled downriver faster and were followed by longer arid summers and longer low water periods. After up to 20 to 40 rainless days, most of the flow con- sists of groundwater drained by the river (Szalay 2000). Groundwater is depleted further by withdrawals for hu- man uses (Csatári et al. 2001, Völgyesi 2005, 2009), thus lowering the ground water table (Rakonczai 2006). The project has started hydrologic processes, which conflicted with traditional laws of fluvial engineering, and which failed to explain several of the phenomena and processes actually observed. These include the drop- ping low-water levels (Iványi 1948, Dunka et al. 1996, Konecsny 2010), the changes in the ground water table and the flow regime of rivers (Rónai 1985, Tóth 1995, Tóth – Almási 2001, Rakonczai 2001, Bozán – Körös- parti 2005, Völgyesi 2005, 2009, Pálfai 2005, 2011, Szalai – Lakatos 2007, Marton 2010), the rising high- water water levels and the effects of the barrages (Koncz 1999, Stegăroiu 1999, Schmutz et al. 1995, Giesecke – Mosonyi 2005). More recent studies have attributed these to changes in the state of the flood bed flanked by the levees (silting and vegetation growth) and in the channel geometry (Nagy et al. 2001, Schweitzer 2001, Gábris et al. 2002, Sándor – Kiss 2006). Without intend- ing to question the potential local significance of these effects, the hydrological phenomena dealt with below (viz., flattening flow profiles, effect of barrages and the actual hydrologic interactions between the recipient and the tributary rivers, as well as the ground water) are considered more general and substantial in nature. Flood waves starting form the Upstream Tisza have been observed repeatedly to overtake each other before entering the middle reach. Some of such flood waves have been observed more recently to split again on the lower reach. Backwater, respectively drawdown, by the major recipients has been offered as explanation for these phenomena (Vágás 1982). The flood waves enter- ing from the tributaries Maros and Hármas-Körös may coincide with rising or falling stages in the recipient Tisza River and raise, or lower the peaks of the latter. The actual stage in the Danube, the ultimate recipient, may also back up, or draw down the Tisza water level and even start a peak spreading upstream in the Tisza River. Such random flood waves may become percepti- ble as the cause of important changes in the flow regime in the Tisza River, owing to the extremely flat slope of the latter. The discharge measurements of growing fre- quency over the past decades have revealed that together with the gauge reading, the slope of the surface profile must also be allowed for when estimating the actual discharge (Dombrádi 2004). The 133 years long record of flows and floods in the River Tisza has revealed that the normal velocity and direction of flow differ markedly from the velocity and direction of the flow registered during the flood peaks (Vágás – Simády 1983). The flow velocity is determined normally by the discharge, bed- and surface slope condi- tions of the river in accordance with the conventional laws of hydraulics, whereas the velocity of the flood peaks depends highly on the actual backwater and/or drawdown caused by the tributaries, or the main recipient (the Dan- ube). The peak stage may spread upstream against the 26 Bezdán, M. JOEG III/1-4 direction of flow over long river sections. Flood waves starting over the upper reach have been observed repeat- edly to flatten over the middle, or lower reach, before entering the Danube. Although more in-depth studies on the aforementioned phenomena have been started but recently, their correlation seems obvious. The general opinion of Huszár (1985), Bogdánfy (1906), Erdős (1920), Tellyesniczky (1923), Korbély (1909), Iványi (1948), Lászlóffy (1982), Vágás and Simády (1983) was that not all flood waves travelling down the River Tisza, but only some of them peaked earlier at upper gauging stations than one of the down- stream ones, and ended “regularly” at the mouth to the Danube. An appreciable number of Tisza floods were affected on one of the middle or lower sections (not necessarily the same in every case) by backwater of a flood on the Danube or one of the Tisza tributaries – especially the rivers Maros or Körös Rivers, or draw- down in the case of their recession, so that the peak oc- curred earlier at the lower gauging station than at one of the upstream ones. A hydrological phenomenon observed long ago on the Tisza River is the hysteresis loop, viz., a curve dis- playing the discharge of a flood wave versus the corre- sponding stage (Bogdán-ffy 1906, Korbély 1937, Németh 1954). Its substantial feature is a peak stage residing over the same section for a longer period of time (several days). As a consequence of these irregularities the relationships based on steady flow velocity and sur- face slope are limited in their validity and usefulness. OBJECTIVES The aim was to identify and offer theoretical and practi- cal solutions to issues as yet not fully cleared in the Ti- sza River section downstream of Tiszafüred. Over six hundred thousand daily stage data were selected from the over six million water levels registered from 1876 to 2009 (on the gauges between Tiszafüred and Novi Bečej (Törökbecse). The more specific objective was to correlate stream- flow and progress of flood waves, to analyse their likeli- hood along the river section and in time, their statistical characteristics and to discover the hydrological and geo- graphical reasons leading to their occurrence in the Tisza section downstream of Tiszafüred and to identify those considered extraordinary relative to other rivers. An attempt was made at shedding new light on are- as of scientific interest believed to merit further study in order to improve flood control measures by drawing on lessons learned during floods of recent decades, and made possible by latest advances in computerised analy- sis. The impact of the tributaries and the main recipient – the Danube – on the Tisza was the most important ques- tion, which has not received adequate attention thus far. Surface profiles in river networks comprising tributaries and recipients are prominent areas in this respect. In such networks the distinction between the velocity of river flow and that of the flood peak is believed important enough to study more in depth. The phenomenon of flood peaks spreading opposite to the direction of flow is believed unique in global fluvial hydrology. Accelerated drainage in the wake of the Tisza Val- ley Reclamation Project has depleted the water resources also along the Tisza reach downstream of Tiszafüred. The extent thereof in periods of low precipitation was demonstrated on the basis of the data of ground water level detection that had started in the beginning of 1930’s. Weirs were studied for their impact on groundwa- ter, river flow and water regime. METHODS AND AREA OF ANALYSIS Tisza water levels registered daily after regulation from 1876 up to 2009 on the gauging stations between Tiszaf- üred and Novi Bečej (Törökbecse) were processed. Sur- face slopes were determined as the difference in water levels registered simultaneously on two adjacent stations and divided by the distance between the respective sta- tions. The surface slopes determined in this way were then evaluated statistically. The following river sections were studied: Tiszafüred, Taksony, Tiszabő, Szolnok, Martfű, Tiszaug, Mindszent, Csongrád, Algyő, Szeged in Hungary, and Novi Kneževac (Törökkanizsa), Senta (Zenta), Novi Bečej (Törökbecse) in Serbia. The gauge readings were divided into metre-ranges and the slopes obtained were entered in the appropriate range. The 133 years long record was also subdivided into periods. The daily surface profiles of the Tisza were plotted and the annual lowest-, mean- and highest stages were identified. Any change over time in these water levels was examined for events and interference with the life of the river, which have actually or potentially affected the flow regime. The annual duration of water levels below the gauge “0” and above the “600 cm mark” was noted, as well as the actual duration in the higher-than 600 cm range of the major flood waves. The flood waves travel- ling normally downriver, that is peaking successively at each gauging station and showing no backwater evi- dence were analysed. The temporal changes of the annu- al low level in the groundwater observation wells were compared to the low stages in the rivers. A few flood- loops were plotted making use of the scarce streamflow measurements and the corresponding gauge readings. For the same cases the conventional gauge relation curves were also drawn. JOEG III/1-4 Characteristics of the flow regime along the regulated Tisza river reach downstream of Tiszafüred 27 REVIEW OF THE FINDINGS By detailed scrutiny of the over 130 years long record of river stages in conjunction with over 80 years of groundwater readings, with due allowance for human interferences in the catchment, as well as for changes in rainfall pattern, the following conclusions have been arrived at: The major drop of low-water levels over the Mart- fű–Mindszent Tisza section is attributable to the substan- tial reduction of the runoff from, and exhaustion of, the storage opportunities in the Körös catchment. The major drop has taken place in two steps: In the 1841-1842 low- water period considered relevant for setting the elevation of the “0”point of the gauges, the combined flow in the three Körös branches was large enough to feed the Tisza section involved, so that the gauge readings were higher. Subsequent developments in the Körös catchment have depleted the low-water yield and thus reduced the dis- charge to the Tisza materially. The reduced low-water discharge from the Körös catchment has flattened the former bulge on the low-water surface profile on the affected section (Fig. 1). The low-water surface profiles react promptly to changes in the tributary discharges. Reduced inflow from the Körös catchment has reduced the low-water stream- flow in the recipient Tisza downstream of Csongrád. This, in turn, has allowed more pronounced effect of the backwater caused by discharge of the tributary Maros. As the consequence thereof, the surface slopes decreased from the Maros mouth upstream as far as Csongrád. On the other hand, upstream of the Körös mouth the surface slope became steeper again, because the low inflow from the Körös causes no backwater in the Tisza (Bezdán 2010a, 2010b). Completion of the Tiszalök Dam in 1957 and of the Eastern Main Canal in 1965 has relieved dry-weather water shortage in the region and raised low-water flows in the river section studied. Additional supplies to the region were channelled from the Kisköre Dam and the Western Main Canal. The low-water surface profiles became even smoother thanks to the backwater of the Novi Bečej Dam (1976). Flood waves travel downriver with different surface slopes. The average slopes on the various sections were calculated from widely scattered data. In terms of per- centages the differences are formidable and play a signif- icant role in changing the water levels. When compiling stage forecasts, local distribution of rainfalls must also be allowed for (Bezdán 2010a, 2010b). Except for river sections impounded also at low- water, surface slopes become steeper with falling depth. At mean water the surface slope is flatter than at low water. The cause of this phenomenon is that downstream of the mouth of a tributary, the discharge thereof is add- ed to that of the recipient Tisza, the water level in the latter rises with a steep surface slope. On the other hand, upstream of the mouth the flow in the recipient is backed up, the water level is raised, while the surface slope becomes flatter and may assume even negative values. Over the section affected peaking is delayed and an inverted flood loop occurs. Such reversed flood loops develop on river section(s) upstream of backwaters. Immediately downstream of tributary mouths normal flood loops may be expected. In cases where the back- water effect of the recipient, or a tributary, or a weir extends upstream beyond the mouth of a recipient, the sense of the flood loop downstream of the mouth will depend on the actual discharge of the tributary (Bezdán 1997, 1998, 1999, 2008, 2010a, 2010b). Fig. 1 Surface profiles of lowest low-waters in the years indicated 28 Bezdán, M. JOEG III/1-4 Over 70% of the Tisza flood waves were backed up by one of the tributaries or the Danube during the 1876 to 1975 period. More than 90% of the flood waves peak- ing with stages above the 600 cm range belonged to this category. Since the Novi Bečej Dam was commissioned in 1976, backwater has raised 80% of the flood waves and 95% of those with overbank stages. Conveyance of virtually all high floods is affected also by the fact that these are under the combined influence of two or more tributaries and since 1976 also of the Novi Bečej Dam. The most often backed up river section in the high-water ranges are those between Tiszaug and Algyő. Since the dams were commissioned, flood waves were backed up repeatedly along these sections. Flood waves peaking within the banks in the pre-dams period were backed up by the Danube beyond Szeged in 25% of the cases, this percentage having grown to 48% since the Novi Bečej Dam started operating. Of the overbank floods backed up beyond Szeged, 25% were registered before 1976 and 20% thereafter. This information is of paramount interest in forecasting and fighting floods. The number of flood waves peaking in “reversed order” decreases with dis- tance from the river causing them. The number of peaks attributable to the Danube backwater decreases with distance from the Tisza mouth, but owing to the clearly undistinguishable simultaneous influence of the Körös and Maros tributaries, the relationship is not a straight- forward one. The unbalanced distribution of the occur- rence of all “reversed” flood waves may be influenced by the possibility of interference between several gaug- ing stations (Bezdán 1999, 2008). Owing to backwater from the recipient, some flood peaks occur upstream of the mouth of a tributary. Thus 40% of the Tisza flood waves since 1876 and peaking at all gauging stations terminated upstream of Senta. This figure is 54% for flood waves peaking above the 600 cm mark. Since 1976, when the Novi Bečej Dam started operating, 58% of the flood waves terminated at one of the stations upstream of Senta, whereas formerly this figure was 28%. Of the flood waves peaking in the high- er-than 600 cm range those advancing farthest down- stream occurred most frequently between the stations Martfű and Szolnok during the 1876-1975 period. After 1976 most flood waves peaked latest along the Martfű, Tiszaug and Mindszent section. For the apparent “retar- dation” of the Tisza flood waves described by several authors, random changes of the actual hydrologic condi- tions in the catchment are believed responsible, rather than spontaneous, or artificial hydromorphological changes in the river bed. This information should be remembered when contemplating levee reinforcement projects (Bezdán 1999, 2002). During the 1976-2009 period, over the Tisza sec- tions influenced by dams and weirs the low-water sur- face slopes decreased, while their statistical scatter wid- ened. The high-water-slopes, on the other hand, became steeper with a narrower scattering range (Bezdán 2010a, 2010b). Downstream of the Kisköre Dam commissioned in 1973 the lowest low-water levels dropped until the year 2009 by more than 100 cm at Taskony, 50 cm at Ti- szabő, 20 cm at Szolnok and 10 cm at Martfű, Parallel thereto, the duration of low water levels has grown. Novi Bečej (Törökbecse) Dam commissioned in 1976 has created a backwater reach extending normally upstream to Csongrád (in the range of lowest low-waters up to Tiszaug) and raising the low-water level at Tiszaug by 55 cm, at Csongrád by 105 cm, at Mindszent by 150 cm, at Algyő by 170 cm, at Szeged by 200 cm, Novi Kneževac by 270 cm, at Senta by 300 cm, while at the dam itself by 385 cm on the average. This means that at lowest low-water the average readings on the same gaug- ing stations became -240, -135, -25, 50, 70, 140, 205 and 270 cm, respectively. The dam has thus actually elimi- nated low-waters in its vicinity, reducing flow velocities virtually to zero. Releases of low and medium flows are thus com- pletely controlled by the dam. This implies at the same time that the channel along the backwater reach has no influence any more on the flow regime and functions actually as an impounded system, which stores the water until it is released as scheduled by the dam operator (Bezdán 1994). Upstream of Kisköre Dam the lowest backwater level has increased by more than one metre since the nineties. Silting has raised the lowest backwater by 30- 50 cm also upstream of Novi Bečej Dam. The water levels at high flows are influenced by the dams and weirs, as well as by the discharges of tributaries. Kisköre Dam influences the flow regime by the water stored behind it in that any arriving flood wave spills over a raised water level rather than over a low one, changing the original conditions. Flood levels may then be raised substantially downstream as far as the southern national boundary, especially in then event of simultaneous flood discharges from the aforementioned two major tributar- ies (the Maros and the three Körös branches). At times of major flood waves, the backwater up- stream of the Novi Bečej Dam merges the former gauge relation curves as far upstream as Tiszaug. Traces of the historic gauge relations are difficult to recognise even at times of high Maros floods. This means that river flows are controlled by the Novi Bečej Dam also during high- water periods, in that releases depend on the conveying capacity of structure. The arriving flood flows are thus necessarily stored and raise the water level in the limited flood-bed volume enclosed by the flood levees up to the Kisköre Dam. The rate of release at the mouth depends JOEG III/1-4 Characteristics of the flow regime along the regulated Tisza river reach downstream of Tiszafüred 29 not necessarily on the rate of the original inflow, but on the conveying capacity and hydrologic state of the recip- ient streambed comprising the dam. During the pre-dam period the traditional gauge relation curves have clearly revealed any backwater and substituted the flood loops representing the stage-discharge relation and offered partial compensation for the scarcity of discharge meas- urements (Bezdán 1994). Besides backwater, dams are prone to raising also the groundwater table. Owing to the local topography and soil conditions, this impact is especially pronounced in the surroundings of the Novi Bečej Dam, where the groundwater never sinks below a certain elevation. Higher groundwater under large areas in low-water peri- ods has reduced the water absorbing capacity of the soil relative to the former, natural conditions. Infiltration from the storage ponds built in the elevated parts of the catchment has also raised the groundwater table. The resultant impact consists of shorter low water periods and longer duration of medium- and high waters. The lowest water tables sustained by the dams are also higher than those prior to the construction of the storage ponds, which prevent the groundwater from sinking below a certain level even in dry periods. Flood waves entering the Hungarian reaches from the headwaters encounter drastically changed conditions, in that backwater raises base water levels in the rivers possibly without higher streamflow rates controlled by dams. CONCLUSIONS Any statistics of water levels, sediment transport and channel morphology compiled on the Tisza River will be necessarily misleading without allowance for the fact that dams cause fundamental changes in hydrologic and sediment transport processes alike. This is why however long historical records and such created after the com- missioning of dams must be considered separately. Csongrád Dam postponed repeatedly, would be an important link in the canalised Tisza River. In a se- quence of dams in which the backwater of a downstream one extends to the tailwater of the one upstream, silting in the headwater and erosion in the tailwater are much less pronounced than in more loosely spaced schemes. Yet the river strives to attain equilibrium conditions so that high rates of sediment transport are attenuated grad- ually. According to experiences gained thus far, dams change the flow regime, raise the low-, medium- and high water levels, reduce flow velocity and cause silting. Over the Tisza reaches downstream of Kisköre Dam frequent and occasionally conspicuous natural and man- made backwaters (caused by the Körös and Maros tribu- taries, the Novi Bečej (Törökbecse) Dam and the Dan- ube) raise strong doubts as regards the effectiveness of emergency reservoirs upstream. Higher levels of safety against floods of higher peaks and longer durations would be achieved by raising and strengthening appro- priately the existing defences. Acknowledgement The author is grateful to László Márfai (OVK), Tiborné Göldner (OVK), Miklós Lázár (ATIKÖVIZIG), Brandenburg Róbert (ATIKÖVIZIG), László Márfai (jun.) 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