Integrated Flood Risk Analysis and Management Methodologies

Scenario Analysis THE IMPACT OF EXTREME PRECIPITATION PATTERNS ON THE FLOOD PEAKS ALONG THE TISZA RIVER Date March 2007

Report Number T22-07-03 Revision Number 1_3_P01

Task Leader VITUKI, Budapest

FLOODsite is co-funded by the European Community Sixth Framework Programme for European Research and Technological Development (2002-2006) FLOODsite is an Integrated Project in the Global Change and Eco-systems Sub-Priority Start date March 2004, duration 5 Years Document Dissemination Level PU Public PU PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services)

Co-ordinator: HR Wallingford, UK Project Contract No: GOCE-CT-2004-505420 Project website: www.floodsite.net

Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420

DOCUMENT INFORMATION

Scenario Analysis, The Impact of Extreme Precipitation Patterns on Title the Flood Peaks along the Tisza River Authors Balázs Gauzer, Gábor Bálint, Péter Bartha Péter Bakonyi, András Bárdossy, András Csík, Miklós Domokos, Contributors Zoltán Kling, József Szilágyi Distribution Public Document Reference T22-07-03

DOCUMENT HISTORY

Date Revision Prepared by Organisation Approved by Notes 22/03/07 1_1_P21 GAB VITUKI Initial draft 22/01/08 1_2_P21 GAB VITUKI Final 15/05/09 1_3_P01 J Rance HR Formatting for publication Wallingford 27/05/09 1_3_P01 Paul Samuels HR Allocated agains correct Milestone Wallingford M22.2 not D22.3

ACKNOWLEDGEMENT

The work described in this publication was supported by the European Community’s Sixth Framework Programme through the grant to the budget of the Integrated Project FLOODsite, Contract GOCE-CT- 2004-505420.

DISCLAIMER

This document reflects only the authors’ views and not those of the European Community. This work may rely on data from sources external to the members of the FLOODsite project Consortium. Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in this document is provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Community nor any member of the FLOODsite Consortium is liable for any use that may be made of the information.

© Members of the FLOODsite Consortium

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SUMMARY

In the course of the present work, it was investigated, what a hydrological situation would be created in the catchment area of the Tisza River by the occurrence of certain meteorological and hydrological scenarios, having real chances to happen. In the course of our investigations, from among the great Tisza floods experienced during the last decades, that of 1970 (differing in many aspects from all other floods), that of 1998 (triggered mainly by precipitations of high intensity) and that of 1999 (originating mostly from snowmelt) were selected for a more detailed analysis.

– As a first step, the mathematical models adopted for carrying out the simulation investigations were briefly presented.

– Thereafter, a brief description of the river system investigated followed and the floods occurring therein during the the last decades were surveyed.

– In the next chapter, the generating causes and the main features of the floods of 1970, 1998, and 1999 were analysed.

– Finally, the situations resulting, according to our calculations, from the various scenarios were presented in detail.

Prior to investigate the selected scenarios, it was intended to demonstrate the reliability of the simulation model to be adopted therefore by the computative reconstruction of the water level and discharge time series of the three selected flood waves. There was a very good agreement between the observed time series and their corresponding ones produced by adopting the simulation model, particularly concerning the peak water levels, most important for the present investigation. Thus the adoptability of the mathematical model was demonstrated. This report contributes to Milestone M22.2 of Task 22 of FLOODsite.

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Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420

CONTENTS

Document Information ii Document History ii Acknowledgement ii Disclaimer ii Summary iii Contents v

1. Introduction...... 1 1.1 General ...... 1 1.2 A brief presentation of the river system investigated, and the most important floods on the Tisza River during the last decades ...... 3 1.2.1 The Tisza Basin ...... 3 1.2.2 Hydrological features of the flood of 1970 ...... 6 1.2.3 Hydrological features of the flood of 1998 ...... 8 1.2.4 Hydrological features of the flood of 1999 ...... 10

2. Investigation of Weather Patterns Leading to Floods ...... 13 Type West (W) ...... 13 Type West with peripheric storm (Wp) ...... 14 Zonal type (Z)...... 14 Moving Mediterranean Cyclone (M)...... 14 Central Type (C)...... 14 Western Cyclone Type (CW) ...... 15 Type ‘Cold Cell’ (H) ...... 16

3. The VITUKI – NHFS hydrological modelling system ...... 23 3.1 The HOOLV Snowmelt Model ...... 23 3.1.1 The energy balance method and the temperature index method ...... 24 3.1.2 Calculating of the energy terms...... 25 3.2 The TAPI Rainfall-Runoff Model ...... 28 3.3 The Discrete Linear Cascade Model...... 30

4. Analysis of the Consequences of Hypothetical Meteorological and Hydrological Scenarios ...... 34 4.1 Overview of the flood waves investigated...... 34 4.2 Overview of the scenarios investigated ...... 36 4.2.1 Scenario 1: No levee failures upstream of Tiszabecs during the flood of 1998 ...... 37 4.2.2 Scenario 2: No levee failures along the Szamos River in 1970...... 37 4.2.3 Scenario 3: During the flood of 1970, no levee failures along the Szamos river, and the flood wave of the Szamos reaches the mouth 12 hours later than observed39 4.2.4 Scenario 4: The intensive precipitation activity preceding the flood wave of 1998 covers the whole sub-catchment of the Tiszabecs cross-section...... 40 4.2.5 Scenario 5 The intensive precipitation activity preceding the flood wave of 1998 covers, both the sub-catchments of the Upper Tisza river, and also that of the Szamos river...... 43 4.2.6 Scenario 6 The Upper Tisza flood wave of November 1998 coincides with the flood wave of the river network of the Bodrog River ...... 45 4.2.7 Scenario 7 The precipitation amount over the sub-catchment of the Tisza upstream of the Tokaj gauge exceeds that of in March 1999 ...... 48 4.2.8 Scenario 8 The intensive precipitation activity preceding the flood wave of November 1998 covers also catchments of the Körös and/or Maros rivers....54

5. Conclusions...... 60

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5.1 Review of Scenario Analysis...... 60 5.2 Basic results linked to different hydrometeorological situations...... 60 5.3 Basic results of the investigations ...... 61

6. References...... 63

Tables Table 1-1. Snow water equivalent of the snow-pack over the Tisza catchment at the beginning of large floods 2 Table 1-2. Highest observed flood crests (Hmax) before and after 1998 6 Table 2-1. Absolute and relative frequencies of flood inducing weather patterns 16 Table 2-2.. Occurrence of West (W) and Cyclonic ( C) patterns in 5-and 10-year periodst 1951–2000. 17 Table 3-1. Observed and simulated flood crests of the investigated flood waves 36 Table 3-2.. Flood crests of the 1970 flood and Scenario 2 38 Table 3-3... Flood crests of the 1970 flood, Scenario 3 39 Table 3-4. Flood crests of the 1998 flood, Scenario 4 41 Table 3-5. Az 1998. évi árhullám tetőző vízállásai néhány szelvényben, az 5. szcenárióban vázolt esetben 43 Table 3-6. Flood crests of floods in November 1998.and March 1999 in the Bodrog Basin 46 Table 3-7. Flood crests of the flood of 1998 and Scenario 6 46 Table 3-8.. Amounts of precipitation assumed for Scenario 7 48 Table 3-9.. . Flood crests of the 1999 flood and Scenario 7 48 Table 3-13. Flood crests of the 1998 flood and Scenario 8 56

Figures Figure 1-1 The Tisza Basin 5 Figure 1-2. The areal distribution of the precipitation on 12-13. May 1970 7 Figure 1-3. The areal distribution of the precipitation on 4-5. November 1998 9 Figure 1-4. Spatial distribution of surface water input values charactersticfor theperiod of snowmelt between 23 February and 12 March 1999 12 Figure 2-1 a-f. Flood inducing weather patterns:a - West (W); b - West with peripheric storm (Wp)15 c - Zonal (Z); d - Moving Mediterranean Cyclone (M); 15 e - Central Type (C); f - Western Cyclone (CW) 15 Figure 2-2. Monthly distribution of the frequency of weather patterns a - W, Wp and Z; 18 b - M, C, Cw and H 18 Figure 2-5. 19 Figure 2-6. 19 Figure 2-7. 19 Figure 2-8. 19 Figure 2-9. 19 Figure 2-10. 19 Figure 2-11. Normalized SLP anomalies corresponding to CP01 for the Tisza at Vásárosnamény (dashed lines – negative anomalies, and solid lines – positive anomalies). 20 Figure 2-12. The 100 largest observed floods of the Tisza at Vásárosnamény from the time period 1900-1999 with the corresponding CPs. 21 Figure 2-13. Annual frequencies of CP10 for the Tisza at Tivadar. 21 Figure 2-14. Annual frequencies of CP01 for the Tisza at Vásárosnamény. 22 Figure 4.1. Sequentiality of the basic layout of the simulation 23 Figure 3-4. Stage hydrograph of the 1999 flood atTokaj 35 Figure 3-5. Observed, adjusted for dike failures and simulated hydrographs of the 1998 flood 37 Figure 3-6. Stage hydrographs of the 1970 flood at Csenger, Scenario 2. 38 Figure 3-7.. Stage hydrographs of the 1970 flood at Vásárosnamény, Scenario 2. 39

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Figure 3-8. Stage hydrographs of the 1970 flood at Vásárosnamény, Scenario 2 and Scenario 3 40 Figure 3-9. Spatial distribution of precipitation pattern resulting the 1998 flood on Upper Tisza 41 Figure 3-10. Spatial distribution of precipitation pattern forScenario 4 41 Figure 3-11. Stage hydrographs of the 1998 flood at Tiszabecs, Scenario 4 42 Figure 3-12. Stage hydrographs of the 1998 flood at Tivadar, Scenario 4 42 Figure 3-12. Stage hydrographs of the 1998 flood at Vásárosmamény, Scenario 4 43 Figure 3-13. Spatial distribution of precipitation pattern resulting the 1998 flood on Upper Tisza 43 Figure 3-14. Spatial distribution of precipitation pattern forScenario 5 43 Figure 3-15. Az Stage hydrographs of the 1998 flood at Vásárosmamény, Scenario 5 44 Figure 3-16. Stage hydrographs of the 1998 flood at Záhony, Scenario 5 44 Figure 3-17. Stage hydrographs of the 1998 flood at Tokaj, Scenario 5 45 Figure 3-18. Spatial distribution of precipitation pattern resulting the 1998 flood on Upper Tisza 46 Figure 3-19. Spatial distribution of precipitation pattern forScenario 6 46 Figure 3-20. Stage hydrographs of the 1998 flood at Tokaj, Scenario 6 47 Figure 3-21. Stage hydrographs of the 1998 flood at Szolnok, Scenario 6 47 Figure 3-22. Stage hydrographs of the 1999 flood at Vásárosnamény, Scenario 7a 49 Figure 3-23. Stage hydrographs of the 1999 flood at Vásárosnamény, Scenario 7b 50 Figure 3-24. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7a 50 Figure 3-25. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7b 51 Figure 3-26. Stage hydrographs of the 1999 flood at Szolnok, Scenario 7a 51 Figure 3-27. Stage hydrographs of the 1999 flood at Vásárosnamény, Scenario 7c 52 Figure 3-29. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7d 53 Figure 3-30. Stage hydrographs of the 1999 flood at Szolnok, Scenario 7d 53 Table 3-11.. Flood crests of the November 1998 flood along the Tisza reach. Szolnok - Szeged and historical maxima observed before 1998 54 Table 3-12. Amounts of precipitation assumed for Scenario 8 over Körös and Maros catchments 55 Figure 3-31. Spatial distribution of precipitation pattern resulting the 1998 flood on Körös and Maros 55 Figure 3-32. Spatial distribution of precipitation pattern forScenario 8 55 Figure 3-33. Stage hydrographs of the 1998 flood at Mindszent, Scenario 8a 56 Figure 3-34. Stage hydrographs of the 1998 flood at Szeged, Scenario 8a 57 Figure 3-35. Stage hydrographs of the 1998 flood at Mindszent, Scenario 8b 57 Figure 3-36. Stage hydrographs of the 1998 flood at Szeged, Scenario 8c 58 Figure 3-37. Stage hydrographs of the 1998 flood at Szeged, Scenario 8d 58

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1. Introduction

1.1 General

Following the almost three decades’ time elapsing without any major event after the by now legendary great flood in the Tisza Valley of 1970, involving almost the whole river system and characterized in most sections of the latter by water levels surpassing all former records, during the last ten years there were as many as five outstanding flood waves in the Tisza River system (in the years 1998, 1999, 2000, 2001 and 2006) resulting along a number of various shorter or longer river stretches of the system in water levels surpassing all the previously observed maxima.

On the basis of a detailed analysis of the hydrometeorological scenarios leading to the various flood situations, one may conclude that ― although these flood peaks in a number of places substantially surpassed the former maximum values ― in most cases both the hydrometeorological scenario preceding the flood and that following it, were far from being the potentially worst ones. This fact, of course, is involving the sinister perception that there is a realistic chance for the future occurrence of flood waves characterized by even more extreme hydrological parameters than those observed in the past.

The results of the meteorological investigations presented in Chaper 2 indicated that the frequency of weather scenarios leading to flood waves is changing with time and that there is an increasing tendency of their absolute frequencies within the range of all typical weather situations. It is also obvious that in the lowland part of an extended catchment area like that of the Tisza Basin with its strongly ramified river system, flood waves propagate only very slowly, so that the genesis of the latter is predominantly determined by the weather conditions of longer periods, generally characterized by an accumulation of weather types. All the same, the attempts to maximize such weather type accumulations (Bodolainé 1983) did not provide acceptable results: since their complition, even more disadvantageous weather combinations were observed over the catchment. Therefore the hydrological investigations carried out in the framework of the present work were aiming at the determination and analysis of the potential changes caused by certain modifications of the flood scenarios already occurred in the past.

It was investigated, what would have been the effect exerted on the water levels of situations in which the meteorological and/or hydrological/flood defence conditions during and before the floods had been slighly more unfavourable than they were actually. Thus various hydrometeorological scenarios were generated by modifying one or more selected parameters of any of the selected actual (observed) meteorological and/or hydrological/flood defence situations and then the effect of such a modification was determined. According to the type of the modified hydrometeorological parameters, the scenarios generated may by classified into the following four types:

– On the upstream river stretches, the height of flood levees differs from the actual value, resulting in the non-occurrence of real dike failures (hydrological/flood defence scenario);

– Modification of the extension and the progress direction of the precipitation zone causing flood waves, i.e., of the areal distribution of precipitation as compared with the real one (meteorological scenario);

– Modification of the actual progress velocity of the flood-generating precipitation zone, leading to a modified timing of coincidence of flood waves of the various tributaries (meteorological scenario);

– Modification of the actual precipitation conditions of the various sub-basins of the river system (meteorological scenario);

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– In the following, when presenting the investigation carried out, examples will be shown for each of the scenario types listed above.

A survey of the flood waves of the last decade show that they can be classified, on the basis of their genesis and the manner of their propagation down, into two basic categories.

The first group includes those flood waves whose water level rising is basically triggered by an almost continuous rainfall of high intensity and long duration (lasting generally two or three days), while the snowmelt effect is practically negligible. In one part of the cases of this group, also a „preparative” precipitation takes place prior to the occurrence of the (principal) precipitation zone directly responsible for flood wave generation. This preparative precipitation moistens the soil and increases riverbed repletion prior to the time of the principal precipitation.

Since among the main sub-basins of the Tisza River Catchment it is the Transcarpathian areas of the Upper Tisza catchment where both the amount of precipitation and the frequency of precipitation activities of great intensity are the highest, the flood waves of this type in most cases start either from the Upper Tisza itself or, in varying degrees, from the catchments of its tributaries Szamos and Bodrog. These events may lead to very high water stages mainly along the Tisza stretches upstream Tokaj, due also to the fact that the flood waves of the Tisza, the Szamos and the Bodrog Rivers often coincide among themselves with very short time lags. Such flood waves, depending on the water volumes supplied by the Körös and Maros Rivers, generally flatten out along the middle and the downstream stretches of the Tisza, causing relatively lower water levels. From among the flood waves occurring during the last years, mainly those of 1998 and 2001 belong to this group.

The second group of flood waves , each of them generally composed by more than one wave, consists of those characterized by a lower degree of increase in water stage. In the generation of these flood waves the melting of great snow amounts accumulated on the area concerned also plays a substantial, and often basic role, besides the liquid precipitation fallig there on. The various subsequent flood waves generally „live their own lives” along the rapid stretches of the Uppermost Tisza (upstream Vásárosnamény), neither of them causing extremely high water stages as yet. However, more downstream, after Tokaj, but mostly along the Middle Tisza reach between Tiszapalkonya and Tiszaug, these initially individual flood waves pile up, creating one mighty, sluggish flood wave, resulting in very high water levels. From among the flood waves of the last years, firstly those of 1999 and 2000, but partly also that of 2006 belong to this group.

Table 1-1 contains the water equivalent stored in the snow-cover of the catchment area at the beginnings of the major floods on the Tisza River registered during the last decade. The data displayed in the table clearly illustrate the above statements.

Table 1-1. Snow water equivalent of the snow-pack over the Tisza catchment at the beginning of large floods

SNOW WATER EQUIVALENT TIME OF THE FLOOD OVER THE TISZA – SZEGED CATCHMENT [km3 ] November 1998 0,1 km3 March 1999 9,4 km3 April 2000 9,1 km3 March 2001 2,0 km3 April 2006 6,1 km3

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By taking into account the foregoing grouping of flood waves, the subjects of our following investigations will be, on one hand, the flood wave of 1970, and then those of 1998 and 1999, on the other, each of the latter ones representing one of the two groups described above.

In order to achieve these goals, a number of runoff simulation investigations were carried out, whose results will be presented later on. These simulation investigations were facilitated by the fact that the deterministic system modules used by the National Hydrographic Service of the Institute VITUKI (Budapest, Hungary) for forecasting the daily water stages and flow discharges of the rivers Danube, Dráva and Tisza, are suitable also for carrying out simulation investigations.The first application for simulation of this model system took place in 1992, during the filling up period of the impoundment of the Gabčίkovo Barrage on the Danube, for the determination of the low flow discharges along the middle reach of the Danube River. Further on, the following topics will be dealt with:

– The first part will include an outline of the description of the river system investigated, followed by a presentation of the floods taking place in the last decades, with a detailed analysis of the causes of the development and the principal characteristics of the floods of 1970, 1998, and 1999.

– In the second part, the results obtained by the investigations concerning the meteorological situations triggering the flood events will be dealt with.

– In the third part, the hydrological models applied for simulation will be presented.

– Finally, the scenarios investigated will be shown with the situations arising, according to our computations, as a consequence of their occurrence.

1.2 A brief presentation of the river system investigated, and the most important floods on the Tisza River during the last decades 1.2.1 The Tisza Basin With its catchment area of 157,000 km², the Tisza River is the greatest left-side tributary of the Danube. The uppermost part of the catchment is situated in the national areas of Slovakia, Ukraine and Romania, while most of its lowland part is divided between Hungary and Serbia. Although the Hungarian part of the catchment area is near to 48,000 km² and the lenght of the Tisza stretch itself crossing the country is 580 km, the runoff ratio the precipitation is rather low in the Hungarian part of the catchment, due to its predominantly lowland character. Thus the floods of the Tisza Valley are triggered by the precipitations reaching the upper parts of its catchment, situated in the neighbouring countries of Hungary. Due to the considerable slopes of its tributaries, sudden and vehement swellings take place along the latter, while the flood waves resulting from their accumulation on the Hungarian river stretches with low slopes propagate down very slowly and with very high water levels.

As for the river system concerned, it can be stated that the flow dischage of the Tisza River entering the national area of Hungary is composed by the discharges of four major and a number of minor rivers. The extension of the catchment area belonging to the Ukrainian/Hungarian border cross section Tiszabecs is 9,707 km². The four most important rivers within this sub-basin are the following: The Fekete and Fehér (Black and White) Tisza Rivers; The Visó River; The Iza River; The Nagyág River

Besides these four major rivers listed above, the discharge of the Tisza is being fed by a number of minor (mostly right-bank) tributaries, the most important ones being the Tarac and the Talabor. Just downstream the national border section, a right- and a left- side tributary join the Tisza River: the Borzsa and the Túr, whose discharges are insignificant during low flow periods, but may be considerable, particularly in the case of the Borzsa (reaching even several hundred m³/s) during flood periods. The most important tributary of the Upper Tisza, the Szamos River, joins its recipient at Vásárosnamény, arriving from Transylvania (in Romania), unifying along its upper stretch a number

T22_07_03_Scenario_Analysis_D22_3_V1_3_P01 15 May 2009 3 Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420 of branches, while the most important tributary of its middle and lower stretch is the Lápos River. The catchment area belonging the the Csenger gauging section of the Szamos River is 15,283 km² (thus surpassing by 50% that of the the Upper Tisza itself at their confluence). Its slope, however, is considerably lower than that of the latter.

The Kraszna River, joining the Tisza just downstream the mouth of the Szamos, plays a considerably less significant role.

The Bodrog River joins the Tisza at Tokaj. It collects with its extensive, fan-shaped river system the waters from the northern part of Transcarpathia and from Eastern Slovakia. To its gauging section Felsőberecki belongs a catchment area of 12,385 km², its resulting discharges being composed mainly by those of its tributaries in Trans-Carpathia (Latorca, Ung), and in a smaller degree by those originating from Slovakia (Laborc, Ondava, Tapoly).

Not far downstream from the Bodrog-mouth there is the Tisza Barrage Tiszalök whose backwater effect, during low flow periods, can be registered up to Záhony. The Sajó River, joining the Tisza downstream Tiszalök, and collecting the major part of its waters, together with its main tributary Hernád, from Slovakia, is characterized by a more tranquil flow regime than the so far mentioned other rivers of the Tisza system. Downstream from its mouth, at a distance of 85 km, there is the Kisköre Barrage, the second one on the Tisza River, which influences the water stages, depending on the actual replenishment of the riverbed, up to the Tiszalök Barrage. The small river Zagyva, joining the Tisza at Szolnok and collecting the major part of precipitation falling onto the Middle Hungarian Range, can have only a limited effect on the flow regime of the Tisza.

The Hármas-Körös River, reaching the Tisza just downstream Csongrád, collects the waters of Western Transylvania. The extension of its catchment area belonging to the gauging section Gyoma is 19,700 km². Its four main component branches are, in a North to South sequence, the Berettyó, the Sebes(Rapid)-, the Fekete (Black-) and the Fehér (White-) Körös. The flood waves of the Berettyó generally flatten out relatively soon whenever not coinciding with those of the Sebes-Körös, whose flood peaks, on the other hand, are considerably reduced by the effects of the storage reservoirs of huge capacities, situated in Transylvania (Romania). The catchment areas of the Fehér-, and particularly of the Fekete Körös have less elongated forms, so that the waters reach practically at the same time these recipients, occasionally resulting in most violent flood waves. Downstream the unification of the four branches, along the Hármas-Körös with its very low bottom slope, the water levels depend increasingly on the actual water stages in the main recipient, the Tisza-River. The flow regime of almost the whole Hungarian part of the Körös River System is influenced, during the major part of the year, by the barrages situated therein. The discharges of the Berettyó ― and in the low flow period also those of the Hármas-Körös ― are considerably influenced by the irrigation canals joining them (mainly the East Main Canal and the Hortobágy-Berettyó).

Just upstream Szeged the Maros River ― the third main great Transylvanian tributary, besides the Szamos and the Hármas-Körös ― joins the Tisza River. It collects the waters of Eastern, Central and Southern Transylvania. Its catchment area, belonging to the gauging section Makó, is rather considerable: 30,400 km². Its major Transylvanian tributaries are the Kis- and Nagy-Küküllő, the Aranyos and the Sztrigy Rivers. Before reaching the Romanian/Hungarian border, it makes a long way: the cross section of Gyulafehérvár (Alba Julia), which is the outlet of the upper part characterized by high runoff values, is in a distance of 350 km from the border.Along this long way, even the most violent flood waves of the upper stretches flatten out, so that the typical feature of the flow regime of the rather short Hungarian stretch of the river are its sluggishness and the prevailing backwater effect of the recipient Tisza River.

Along the low-sloped lowest stretch of the Tisza River, the water levels may occasionally (e.g. during the flood wave of 2006) be significantly influenced also by the water stages of its recipient, the Danube.

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The map of the whole catchment area of the Tisza River is displayed in Fig. 1-1.

Figure 1-1 The Tisza Basin

A great part of the Hungarian Lowland was regularly inundated, even during the 19th century, by the floods of the Tisza and its main tributaries. Flood protection works were started in 1846 and, with smaller and larger interruptions, the basis of today’s flood protection system was created by 1900. In the meantime, nevertheless, more than one devastating flood occurred, such as that of 1879, leading to the catastrophal inundation of the town Szeged.

There are relatively exact information about the „historical” floods of the Tisza. Before 1970, snowmelt used to be very intensive, which, superposed by a great amount of liquid precipitation, triggered the flood of March 1888, that of April 1932 (resulting in particularly high water stages along the river stretch between Szolnok and Szeged), as well as the flood wave registered in December 1947 in the Transcarpathian part of the catchment, leading to various levee ruptures along the Upper Tisza.

The complexity of the hydrological processes taking place in the river system of the Tisza is evidenced also by the fact that the so far highest water stages on the various stretches of the river were caused by different flood waves depending on the measure of the discharges of the tributaries and/or the manner of their coincidence.

Prior to the flood wave of November 1998, the highest water levels (Hmax) on the uppermost Tisza River stretch between Tiszabecs and Vásárosnamény had been observed during the flood of 1970. On the lower reach between Vásárosnamény and Tokaj, the highest stages were registered during the flood of March 1888, along most part of the reach between Tokaj and Szolnok in February 1979,

T22_07_03_Scenario_Analysis_D22_3_V1_3_P01 15 May 2009 5 Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420 while between Szolnok and Szeged again the flood of 1970 had caused the highest stages. Finally, along the lowest stretch of the Tisza, near to its mouth into the Danube, the so far highest water stages, due to the impounding effect of a great Danubian flood wave, had been measured in 1965.

As it is resulting from the above information, the Hmax values registered before 1998 used to be the highest peak water stages observed during more than one century. Due to a caprice of Nature’s forces ― and partly/sporadically also as a consequence of anthropogeneous effects ― these secular record values were surpassed (at some places significantly and not only once) almost along the whole Hungarian Tisza reach during the subsequent three and a half years, as it is shown in Table 1-2, displaying both the water level values observed before November 1998 and those measured since then, surpassing all former values, showing in bold printing the presently valid critical values. The Table also shows, that at present, the record values are those of 2001 upstream Záhony, those of 2000 between Záhony and Szolnok and those of 2006 on the lowest stretch of the Tisza River. The degree of surpassing the former maxima is particularly great ― more than 1 meter ― on the Central Tisza between Kisköre and Szolnok, while at Szolnok the record water stage significantly increased (by almost 0.7 m), even during this relatively short period, subsequently at two events!

Table 1-2. Highest observed flood crests (Hmax) before and after 1998

HIGHEST OBSERVED FLOOD CRESTS (Hmax) [cm] GAUGE before 1998 Nov 1998 Mar 1999 Apr 2000 Mar 2001 Apr 2006

Tiszabecs 680 708 732

Tivadar 865 958 1014

Vás.namény 912 923 941

Záhony 751 752

Tokaj 880 894 928

Kisköre 908 978 1030

Szolnok 909 974 1041

Csongrád 935 994 1033

Mindszent 982 1000 1062

Szeged 960 1009

1.2.2 Hydrological features of the flood of 1970 By the end of February 1970, a considerable amount of water, stored in the snow-cover, was accumulated in the catchment area of the Upper Tisza, due to the cold weather conditions, rich in precipitation. Most part of the snow melted during March, triggering a minor flood wave at the beginning of the month and a greater one at its end. As a consequence, riverbed fullness was high at the beginning of April. During March, there were no significant increases of water stages, but due to the almost continuously rainy weather ― only five days of the month remained without precipitation ―water stages practically were not able not to decrease. Therafter, in the second week of May, a precipitation of 40-50 mm reached most part of the catchment ― with near to uniform daily distribution ― keeping the uppermost soil layers humid.

The determinative part of the precipitation of high intensity triggering the flood wave fell between the evening of 12 May and the morning of the following day. At this moment, the riverbed repletion of the

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Tisza and the Szamos Rivers were the following:; Tisza Tiszabecs: 47%; Vásárosnamény: 67%; Záhony: 69%; Tokaj: 67%; Szamos Csenger: 51%;

As for the areal distribution of the precipitation, the significant volumes in first line fell in Transylvania and in some places of Transcarpathia, including the two focal areas of Máramaros in the North and the sub-basins of the Beszterce and the Nagy-Szamos Rivers in Transylvania. As a consequence, the major part of the water volume movinging down on the Upper Tisza was supplied by its left-side tributaries (Visó, Iza) , while on the tributary Szamos the absolute maximum flood wave was registered. In the catchment of the right-side tributary Bodrog there was a relatively smaller amount of precipitation. The areal distribution of the precipitation is shown in Fig. 1-2.

Figure 1-2. The areal distribution of the precipitation on 12-13. May 1970

In the surroundings of the precipitation focuses, i.e., on the rivers Visó, Iza and the Nagy-Szamos, the peaks occurred already around noon of 13 May, surpassing at all gauging stations the previous Hmax records. The flood crest of 648 cm at the gauging station Técső (by 26 cm lower than the former record Hmax) took place in the evening hours of 13 May. At Tiszabecs station, the peak stage of 680 cm (surpassing by 107 cm the former record Hmax) was read at 6 o’clock in the morning. The effect of the overtopping of the flood levee, occurring in the Ukraine, just upstream from the national border section, may have been rather limited.

After that, the flood wave became slower: the peak observed at the Tivadar gauging station at 8 o’clock a.m. of 15 May surpassed only by 17 cm the maximum value registered in 1947..Due to the extreme situation developed on the tributary Szamos, the peak water stage at Vásárosnamény station was read at the same time,. surpassing the former maximum Hmax ― in spite of the flood wave of the Szamos ― only by 12 cm.

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The water level of the Szamos River reached its maximum at 7 p.m. of 13 May, surpassing the previous Hmax value by 317 cm(!). At Szatmárnémeti station, the peak level of 810 cm was registered on the afternoon of 14 May, while that of 902 cm was read on the station Csenger at 20 and 22 o’clock on the evening of the same day. The latter value may have been slightly influenced by the dike failures occurring between 17 and 20 p.m. in the surroundings of Szatmárnémeti; notwithstanding they surpassed by 159 cm the maximum value as determined in 1888.

In Romania, there were 11 levee ruptures on the right-side of the Szamos River, in a total lenght of 1200 m. The peak discharge drained hereby was estimated to surpass 1000 m³/s. Altogether about 220 million m³ water was released in such a way during 4 days, inundating ― by following the slopes of the land surface ― the densely populated Romanian and Hungarian areas between the Tisza and the Szamos Rivers. Some of the prominent facilities (roads, railways) protected certain villages from the inundation, while other settlements were attached by the flood in a rapid and concentrated manner, thus increasing the extension of the destruction. The greatest damages were suffered by the villages Csengersima, Csegöld, Zsarolyán, Jánkmajtis, Nagyszekeres and Penyige. The flood wave, proceeding along the Hungarian stretch of the river, overtopped the levee crest ― in spite the 23 km long cofferdam erected within some hours’ time ― in the surroundings of Nábrád, on the right side, first on 15 May between 2 and 4 o’clock a.m. in two sections, and then to Tunyogmatolcs, on the left side in one section, the total lenght of levee failures on the Hungarian side amounting to ca. 480 m (Vίzgazdálkodás, 1971).

The order of magnitude of the levee ruptures taking place in the evening of 14 May and the early morning of 15 May, resp. the water volume released thereby, can be characterized by the fact that, due to them, the flood crest at Vásárosnamény occurred as early as on 15 May at 8 o’clock in the morning, i.e. in coincidence with the culmination at Tivadar, with the water stage of 912 cm ― i.e., considerably lower than that expectable without levee ruptures ― , this level surpassing only by 12 cm the Hmax value registered during the flood of 1888.

Downstream Vásárosnamény, on the Tisza stretch between that section and Tokaj, the flood records of 1888 were not yet surpassed: the rather protracted flood crest of 728 cm (occurring from 17 May, 16 o’clock p.m. until the noon of the following day) was lower by 23 cm, and that of 858 cm, observed in the Tokaj section, was lower by 14 cm than the record value Hmax.

In the same way, all the water stages read along the Tisza stretch downstream Tokaj, until Tiszafüred, during the flood event of May 1970, remained under the respective Hmax values. Between Tiszafüred and Szolnok they again surpassed by 9 and 15 cm the previous records. Downstream Szolnok, the water stages were again slightly lower (due to the relatively moderate discharge contribution of the Körös Rivers). More downstream, again flood crests equalling the previous Hmax records could be registered, due partly to the also considerable flood wave arriving on the tributary Maros and partly to the similarly high water stages in the Tisza's recipient, the Danube River.

1.2.3 Hydrological features of the flood of 1998 During September and October 1998, the precipitation volumes falling onto the whole Carpathian Basin surpassed by 100-200% the usual volumes. Although causing only minor flood waves, they resulted in high values of soil moisture and riverbed repletion. At the beginning of November, an extreme flood wave took place on the Upper Tisza, resulting partly from the contribution of the previous great flood wave, mostly triggered by the heavy precipitations starting on 29 October. This flood wave reached its crest at Tiszabecs gauge on 31 October with a water stage of 526 cm (surpassing by 26 cm the III grade of flood defence preparedness). As for the tributaries, also high water stages were observed on the Kraszna and Túr Rivers. As the resulting effect of all these, the flood wave culminated at Vásárosnamény on 1 November with 820 cm (by 92 lower than Hmax). Due to its limited volume, the flood wave flattened out relatively soon, resulting on the Tisza stretch downstream Záhony only in water stages more or less equalling the IInd grade of preparedness.

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The second flood wave, mostly triggered by the precipitation of 4 November, caught up with the first one. By the time of the precipitation event, already a considerable recession had taken place in the Tiszabecs gauging section, but still very high riverbed repletion values were typical along the Tisza stretches downstream Tiszabecs. The riverbed repletion values on 4 November were the following: ; Tisza Tiszabecs: 37%; Tivadar: 66%; Vásárosnamény: 79%; Záhony: 84%; Tokaj: 83%; Bodrog Felsőberecki 72%

As for the areal distribution of the precipitation, its overwhelming part fell in the Ukrainian part of the catchment area. As a consquence, the major part of the discharges conveyed by the Upper Tisza originated, besides the Fekete- (Black-) and the Fehér- (White-) Tisza, from the catchments of its right-side tributaries (Tarac, Talabor, Nagyág, Borzsa, etc.). All along these rivers, water levels closely approaching or else surpassing the previous Hmax values were observed, while the two large left-side tributaries, the Visó and Iza Rivers, arriving from Northern Transylvania, played only a minor role in generating the flood wave. In accordance with this, the major part of the discharges of the Bodrog River, originated from the eastern part of its catchment (i.e., the sub-catchments of the Rivers Ung and Latorca), while the contribution of the tributaries arriving from Slovakia (Laborc, Ondava, Tapoly) was not significant at all. Due to the moderate precipitations in Transylvania, no significant rises of water level were observed within the river system of the Szamos. The areal distribution of the precipitation can be seen in Fig.1-3.

Figure 1-3. The areal distribution of the precipitation on 4-5. November 1998

Along the Transcarpathian stretch of the Tisza, the flood crests surpassed everywhere the previous maxima. E.g., at Técső, on 5 November at 9 a.m., the water stage was 726 cm, surpassing by 52 cm the Hmax value observed in 1947. Typical for the vehemence of water stage rising is that by the evening of 4 November all contacts of the settlements situated in the Nagyág Valley with the outside world were cut. It was a particularly unfavourable circumstance that a unusually great part of the runoff

T22_07_03_Scenario_Analysis_D22_3_V1_3_P01 15 May 2009 9 Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420 arrived to the Tiszabecs section from the tributaries situated near to the national border, thus resulting also unusually limited flattenings of the waves upstream Hungary. Water stage culmination at Tiszabecs took place on 6 November at 2 a.m., when the value 708 cm (surpassing by 28 cm the maximum level observed during the flood of 1970) was observed. Both the crest value and its timing were significantly influenced by levee ruptures occurring upstream, in the Ukraine.

From the point of view of flood defence, the most critical hours were perhaps those following the culmination at Tiszabecs. Downstream Tiszabecs, in the surroundings of Milota and Tiszakóród, the overtopping of the levees could be prevented only by enormous efforts.

During the flood, water discharge measurements were carried out in the Tivadar gauging section of the Tisza. On the basis of the results obtained, and by taking into account the estimated contributions of the tributaries Túr and Borzsa, the culminating discharge at Tiszabecs may have been 3200-3300 m³/s.

In the Tivadar section of the Tisza, situated at ca. 40 km downstream Tiszabecs, water stage culmination took place on 6 November at 16-18 o’clock, i.e. 14-16 hours later than the culmination at Tiszabecs (delayed by levee ruptures in the Ukraine), with a maximum value of 958 cm, surpassing by 93 cm the Hmax value of 1970. The corresponding discharge is estimated, on the basis of measurements, to have been around 3500 m³/s.

The culmination at Vásárosnamény with 923 cm, surpassing by 11 cm the Hmax value of 1970, took place on 7 November, between 6 and 14 o’clock, with a discharge more or less equal to the 3500 m³/s value of Tivadar, indicating that the moderate discharge contribution of the Szamos River could hardly compensate the effect of flood wave flattening.

Since the tributaries Szamos and Kraszna brought only relatively small water volumes, the culminating water stage at Záhony, occurring between 22 o’clock of 8 November and 6 a.m. of 9 November, remained by 14 cm below the previous maximum value.

In the surroundings of Tokaj, at the confluence of Tisza and Bodrog, again a critical situation emerged. It is to say, another very significant flood wave moving down on the Bodrog (thanks to its tributaries Latorca and Ung), whose crest value at Bodrogszerdahely remained only by 67 cm below the previous maximum value. Due to the backwater effect of the recipient Tisza River, the culminating crest water levels on the Hungarian Bodrog stretch remained below the formerly recorded maxima only by 34 cm at Felsőberecki, and by 4 cm at Sárospatak. The culminating water stage at Tokaj, reflecting the joint effects of the confluent rivers Tisza and Bodrog, as observed on 11 November between 18 and 24 o’clock, was only by 8 cm lower then the previous Hmax. The corresponding discharge ― jointly resulting from the flattening of the flood wave of the Tisza and the discharge of 500-600 m³/s of the Bodrog ― may have been 3000 m³/s.

At Tokaj, the flood wave caught up with the previous one, having started some days earlier, and further on the two flood waves jointly propagated down as a unified wave. Between Tokaj and Szolnok, the water stages remained everywhere under the previous Hmax values, just strongly (by 8-38 cm) approaching the latter. Downstream Szolnok, mostly due to the relatively small discharges of the Körös Rivers, the flattening out of the flood wave was sped up, the culminating water stages remaining everywhere below the the level of the IIIrd grade of flood defence preparedness.

1.2.4 Hydrological features of the flood of 1999 The year 1998, extraordinarily rich in precipitation, was followed by a winter also with abundant precipitation. The major part of the snow fallen during December melted during the mite month of January, further increasing the ― because of the abundant rainfalls of the previous month ― anyway high degrees of soil moisture. Therafter, the highest volume of snow volume of the last decades was accumulated on both the mountainous and lowland areas of the Carpathian Basin. According to the computations of the National Hydrological Forecasting Service (VITUKI, Budapest, Hungary), the

T22_07_03_Scenario_Analysis_D22_3_V1_3_P01 15 May 2009 10 Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420 water volume accumulated in the snow-cover amounted in the catchment of the Upper Tisza, belonging to its Tiszabecs section, to 2.2 million m³, in the catchment of the Szamos River to 1.5 million m³ and ind that of the Bodrog River to about 1.8 million m³. The thaw, occurring almost simultaneously in the whole area at the end of February and the beginning of March, resulted in intensive snow melting, further increased by the otherwise non significant rainfall in the first days of March. On 8 May, a minor cold snap started, delaying the thawing in the lower altitudes and stopping it in the mountains. The cooler weather lasted untlil the last days of the month, when a warming up (stronger than the previous ones) gave the finishing stroke to the yet remaining snow-cover.

The highest snowmelt intensity occurred in the catchment of the Bodrog River, from which about 1 km³ water stored in the snow-cover disappeared during the period from 1 to 9 March. This amount corresponds in the sub-basin area belonging to the gauge Felsőberecki to a 81 mm thick water sheet, which otherwise would have been the result of a rainfall surpassing 100 mm during 8 days. This water volume could be drained by a discharge of about 1450 m³/s during 8 days. The cooling down of the air around the middle of the month could only decrease the intensity of snowmelt, so that during the last third of the month there remained scarcely any snow in the catchment.

In the higher altitudes of the catchment area of the Upper Tisza, the intensity of snowmelt was considerably lower in the first days of March, so that in the altitudes above 1000-1200 m a.s.l. snowmelt took place only during the springlike last days of March.

In the lower parts of the catchment area of the Szamos River, snowmelt started as early as in the last days of February, so that the temperature rising on 2 March found only a water resource already reduced by 20%. In the following days, until 9 March, the water volume stored in the snow-cover decreased by ca. 0.5 km³, thus the melting intensity surpassed the values experienced in the catchment of the Upper Tisza, without reaching, however, the extreme intensities characterizing the Bodrog catchment. The areal distribution of the surface water income values characterizing the melting period are displayed in Fig.1-4.

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Figure 1-4. Spatial distribution of surface water input values charactersticfor theperiod of snowmelt between 23 February and 12 March 1999

The most important feature of the development of meteorological conditions during March consists perhaps in the extremely low precipitation values in the last 20 days of the month. This fact may be considered a very favourable circumstance even when considering that March generally is anyway not one of the months particularly rich of precipitation within the three catchment areas concerned. During the month, precipitation in the catchments of the Upper Tisza and the Szamos was lower by 20-25% than the average, while average precipitation conditions prevailed in the Szamos River system. 60- 80% of the monthly precipitation amount fell during the first 10 days of the month. Hence the flood wave originated mostly from snowmelt, while the contribution of rainfall was only 10-15%.

Taking into consideration the foregoing statements, it is not surprising at all, that the major role in developing the flood wave of 1999 was played by the Bodrog River, the second being the Szamos River, while the Upper Tisza itself contributed thereto only a smaller water volume. In the Bodrog Catchment the rather rare situation arose in which all of its great tributaries (Latorca, Ung, Laborc and Ondava-Tapoly) supplied more or less the same volume of water, so that their individually not extreme discharges were able to jointly create a previously never experienced, great flood wave.

Due to the more moderate discharges of the Upper Tisza, no extremely high water levels were observed upstream the Szamos mouth. The peak of Tiszabecs remained by 4.5 m, that of Tivadar by 3.0 m below the respective Hmax values, the decrease between the two stages being partly a consequence of the effect of the Túr and Borzsa Rivers, and partly of the unusually low slope between Tivadar and Vásárosnamény, due to the considerable water amount arriving on the Szamos River. The water stage registered at Vásárosnamény, however, already surpassed the limit value prescribed for the rd III degree of flood defence preparedness, and was only by 1 m lower than the Hmax value measured in the autumn of 1998. The culminating discharge approached 3000 m³/s, its half originating from the Szamos Catchment.

Downstream the Szamos mouth, the flood wave first started to flatten out, resulting in gradually decreasing water stages, but from Tiszabercel the backwater effect of the Bodrog River could increasingly be noticed.

The coincidence of the flood waves of the Tisza and Bodrog at Tokaj resulted in water stages surpassing all former record values. Due to the delayed arrival of the flood wave of the Tisza, the water level surpassed „only” by 14 cm the former local maximum. The culminating discharge was about 2800 m³/s, including the contribution of near to 1000 m³/s of the Bodrog River.

Along the Tisza stretch between Tiszapalkonya and Szolnok, already containing in its riverbed the volume of the flood wave of the Sajó-Hernád system (culminating therin some days earlier, with local water stages above the IIIrd degree defence levels), and also showing the somewhat diminished effect of the timing difference prevailing at Tokaj, the previous Hmax values were surpassed by 40-70 cm. On the basis of the numerous discharge measurements carried out at Kisköre and Szolnok, the estimated maximum discharge at Kisköre was 2600 m³/s, that at Szolnok 2400 m³/s.

Downstream Szolnok, a rapid flattening out of the flood wave could be observed, mostly due to the relatively small water amounts arriving on the Körös Rivers. As a consequence, the peak measured at Tiszaug surpassed only by 1 cm the previous Hmax value.

During the flood period, 190 million m³ water was retained in the Slovak part of the Bodrog catchment, mostly in the storage reservoirs of Sirava and Nagydomása as well as in the emergency reservoir of Bős. Due to this storage activity, the peak water level may have been reduced by 30-50 cm on the Bodrog and by 15-25 cm on the Tisza.

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2. Investigation of Weather Patterns Leading to Floods

The bases of the flood defence system of Hungary were created as early as in the second half of the 19th century, when the Tisza River (previously causing significant flood damages) and its side branches were regulated, and also the flood protection facilities of the Danube Valley were established. When determining the water levels critical for planning these facilities, basically the previously observed highest water stages were taken into account. However, the latter values did change during the subsequent floods, thus eventually the critical levels also had to be adjusted continuously to the actual, higher water stages.

The impulses for a revision and scientifically based re-determination of the critical flood levels were provided by the damages caused by the Danube flood of 1965 and by the Tisza flood of 1970. The critical flood levels were determined by statistically processing the observed water stage data. In the course of this processing, however, it became obvious that the time series of water stages are significantly biased, due to the effects of levee ruptures occurred during the highest floods. At that time, there were no suitable mathematical models yet available for eliminating the effects of levee ruptures, so that only estimations based on hydrological considerations could be made, their limited reliabilty obviously affecting also the results of the subsequent statistical computations.

There were also other kinds of criticism concerning the adoption of the tools of mathematical statistics. On one hand, significant and continuous changes are taking place, due to the various human activities, in the catchment areas, both along the riverbeds and therein, so that the system is also continuously changing, exerting influence also on the water stages of the rivers. At the same time, a basic supposition for adopting the statistical methods was the stability of the system, the representativity and homogeneity of the data processed. The results obtained can be extrapolated to the future only if the system is constant, but this prerequisite is generally not granted.

The solution was looked for by analysing the totality of runoff processes, including the analysis of the meteorological (precipitation, snowmelting) situations triggering various runoff processes resp. generating flood waves.

A detailed investigation on the effects of flood-triggering meteorological processes was started around 1970 by E. Bodolai-Jakus (BJE), identifying the types of weather patterns triggering floods, determining the frequencies of occurrence of these patterns and the role of each of them in creating the various types of flood waves. When classifying weather patterns, the investigation was based on the actually observed precipitation cycles producing flood waves. Thus the flood wave periods were not connected to the types of a pre-existing catalogue, but inversely: the types of weather patterns providing large amounts of precipitation were identified for the catchment area of the Danube and Tisza, in accordance with the hydrological goal of the investigation. To put it more concretely, the quasi stabile objects were identified, for the theoretically significant period of classification, for about 800 rainy days triggering flood waves in the area investigated, on the 500 hPa absolute and the 500/1000 hPa relative topographic maps. The seven types of floodwave-triggering weather patterns were identified on the basis of the characteristic geographical positions of three objects: the near-to- surface centre of the cyclon, the depression- and the crest line of orographic maps.

Type West (W) The weather of this type is characterized by a very intensive current in W-NW direction. The cyclons activating the circulation are moving above the latitude 60º. The processes take place at a temperature above the normal one, associated, as shown in Fig.2.1., in the winter with a more humid, and in the summer with a dryer air than the average of 5 years. This distribution is a consequence of the fact, that in the case of this type the source of the air moisture is the Atlantic Ocean, from which in the winter relatively warmer, and in the summer relatively

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cooler air masses are being transported into the inner part of the continent. This type is particularly dangerous in the case of the flood waves occurring in winter and early spring, causing, besides the liquid precipitation volumes, also a sudden melting of the accumulated snow-cover.

Type West with peripheric storm (Wp) This type is characterized by a current in direction SW-WSWS. The steering cyclone is situated over the North Sea, while along its cool front ― in the foreground of the Alps and above the Lowland of the Po River ― there is a peripheric storm. This type is connected with temperatures even higher than with the foregoing type W. Its potential water resources are very great, its current system transporting the warm, humid air masses from above the Mediterranean Sea to the catchments. According to the map b of Figure 2.1, its humidity degree is considerably higher than the average both in winterr and in summer. It should particularly be noted that the area of great differences is concentrated to the eastern part of the Carpathian Basin in winter, and to its western part in summer. The current often is vertical to the ridges of the Northwestern Carpathians, resulting in a considerable additional orographic precipitation in the catchment areas of the Upper Tisza concerned. Summing up, the type Wp may be dangerous in any of the seasons of the year, involving great amounts of precipitation, since its processes of precipitation realisation are extremely intensive. Zonal type (Z) With this type, the direction of air current is SW-W in the near-to-the-soil layers and NW in the middle troposphere. One of its characteristics is that the temperature over Western Europe is higher than normal. During the winter the cold air replenishing the continent regresses to the east, due to the pushing forward of the warm air. In late winter and early spring, it always brings a considerable thaw in the Bavarian and Austrian sub- catchments of the Danube, while in the Carpathian Basin negative temperatures may still prevail for a certain time. The potential water resources of the zonal type are limited in spite of the over-average humidity of the atmosphere over Western Europe, as displayed in map c of Fig.2.1. All the same, the precipitation activity is weak since the humidity transport is realized on the nose of the anticyclone pushing forward from the Azores Islands, so that the upward directed currents can be effective only with in a narrow frontal zone. Moving Mediterranean Cyclone (M) This type is practically identical with the classic cyclone V b. It is passing in direction SW-NE through the Carpathian Basin. It comes into life as a peripheric storm of the basic eddy situated over Denmark. In the Bavarian and Austrian sub-catchments, the backside precipitation activity of the cyclone prevails. Consequently, a greater precipitation amount can here be expected only whenever the regressive occlusion is situated over the area mentioned. The Δwp field is rather important mostly for the Danube Catchment in summer, since this type did not lead so far to any summer flood wave in the Tisza Catchment, at least within the statistical sample investigated. On the other hand, it is the period between November and May, with a certain moisture excess (as indicated by the Δwp field of March in Fig. 2-1.d) which might be dangerous for the Tisza Catchment. This picture and even its absolute values are almost identical with the Δwp field of December. Central Type (C) It refers to such a cyclonic situation in which both the replenishment and the reduction of the cyclone coming into life within the Carpathian Basin or transferred thereto takes place in the same area. In the near-to-soil pressure field, this type is accompanied by a Skandinavian anticyclone, blocking the transference of the cyclone. The potential water resources of the centrum positions are higher than those of any other type. Similarly, the moisture excess of this type can surpass that of the other types also during the summer season, not to mention the

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nearly identical fields Δwp of March (Fig. 2-1.e) and November, whose moisture excess is higher than 6 mm in the whole Carpathian Basin.

Western Cyclone Type (CW) This type is determined by the fact that the central area of the near-to-soil cyclone of great extension is in Austria or Germany, while the cyclone of great altitude is situated over France. This type is dangerous principally in the Danube Catchment. The water resource of the warm, precipitation-effective sector is very large. This can be seen also in Fig 2-1.f, showing the great positive moisture excess of the summer season, transported to the Danube Catchment. The picture of March, on the other hand, indicates a moisture export directed to the Tisza Catchment.

Figure 2-1 a-f. Flood inducing weather patterns:a - West (W); b - West with peripheric storm (Wp)

c - Zonal (Z); d - Moving Mediterranean Cyclone (M);

e - Central Type (C); f - Western Cyclone (CW)

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Type ‘Cold Cell’ (H) During the occurrence of this type, the cold airdrop is situated over Germany, occasionally extending itself to the Northern Adriatic Sea. During such events, shallow regional eddies can be found from France, through the Po Lowland, up to the Carpathian Basin. The N-S oriented axis of the warm, moist air tongue almost completely coincides with the longitude 20º east. The occurrence of this type is rather rare. It occasionally plays a very effective role in the development of flood waves on the Tisza River. Its moisture excess is considerable during the spring season, when 55% of this type can be met in the Tisza Catchment. The occurrence of the type „cold airdrop” often creates meso-scale convective weather systems.

For determining the weather types triggering flood waves, Mrs.Bodolai-Jakus (BJE) compiled a catalogue of the meteorological situations for the period 1951-1970. Later on, she extended this catalogue for the period 1951-2000. On the basis of the latter, also the occurrence frequencies of the various meteorological types could be calculated.

Table 2-1 contains the absolute and relative frequencies of the occurrence of the various types. The absolute frequencies are listed in column 2 of the table, the relative frequencies in the columns 3, 4, 5, and 6, distinguishing in the latter the values referring to all days of the given period (18,236 and 10,958 days) and those referring only to the days of occurrence of the BJE-types (7,277 days). There are slight differences between the relative frequencies listed in columns 3 and 4, the greatest differences arising between the occurrence values of the types C and Cw, the occurrence of the C being by 2.2% lower and that of Cw by 1.2% higher in the 50 years long series. According to the latter series, the occurrence of BJE-types is by 2% lower among all days. When considering the occurrences referring to the sum of type-days, the change of the types C and Cw is also obvious in columns 5 and 6. As a matter of fact, a mutual change of positions takes place: while the frequency of type C is higher in the 30 years long series, Cw is more frequent in the 50 years long series.

Table 2-1. Absolute and relative frequencies of flood inducing weather patterns

Type Number of Percentage, relative to Percentage, relative to days total number of days %- classified number of days %- W 1328 7,3 7,7 18 18

Wp 1676 9,2 9,4 23 22 Z 709 3,9 4,3 10 10 M 771 4,2 4,5 11 11 C 1033 5,7 7,9 14 19

Cw 1336 7,3 6,1 18 15 H 424 2,3 2,0 6 5 Σ 7277 39,9 41,9

The timely variability of the frequencies of the weather types of BJE is already indicated in Table 2-1. The types mentioned can be categorized into two great groups: in that of West and that of Cyclonal character (in another approach: into the groups corresponding to the zonal and the meridional circulation). The absolute frequencies of the occurrence of the two groups within 5 of 10 years, is a clear evidence of temporal variability. These values are listed, for the 50 years long period between 1951 and 2000, in Table 2-2. The temporal variability of the occurrence of the type groups is expressed both within the given group and among all data. Within the group West and among the five

T22_07_03_Scenario_Analysis_D22_3_V1_3_P01 15 May 2009 16 Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420 years long periods the maximum number of cases was 458 (during1966-70) and their minimum was 309 (during 1991-1995). Within the Cyclonal group, the maximum was 482 cases (during 1966-70) and the minimum 292 cases (during 1971-75). At the same time, the period 1966-70 shows, for both groups, the absolute maximum of 5 years long periods within the 50 years. Note that within the decade 1961-70 also the frequency of precipitation-effective weather types was the highest. A comparison between the two type-groups shows, although only with a modest difference, the prevailing frequency of the West group, save for the 5 years long periods. The exception is the decade 1971-80, when the difference between the two type-groups (110 cases) is the highest. The prevalence of the cyclonic group arose during the 20 years between 1951 and1970.

Table 2-2.. Occurrence of West (W) and Cyclonic ( C) patterns in 5-and 10-year periodst 1951–2000. Típus csoport Σ Évek W C W,C 1951–1955 384 407 791 1956–1960 410 404 814 Σ 1951–60 794 811 1605

1961–1965 373 359 732 1966–1970 458 482 940 Σ 1961–70 831 841 1672

1971–1975 376 292 668 1976–1980 325 299 624 Σ 1971–80 701 591 1292

1981–1985 395 378 776 1986–1990 346 299 642 Σ 1981–90 741 677 1418

1991–1995 309 318 627 1995–2000 337 326 663 Σ 1991–2000 646 644 1290

The inter-annual distribution of the absolute frequencies of BJE-types is illustrated in the Figs. 2-2a and 2-2b on the basis of the catalogue referring to the 50 years between 1951 and 2000. The inter- annual graph of the types W, Wp and Z is characterized in Fig 2-2.a by maxima in summer and winter, and by minima in spring and autumn. It is worthwile to note, however, that while the types W and Z are observed more frequently in the winter months, Wp is rather a „summer” type (in these situations the great, meso-scale convective summer weather systems are most frequently observed). In Fig 2-2.b the spring and autumn variants of the cyclonal types can be seen. The peak values of occurrence are, subsequently, in April (M,C), in May (Cw) and June (H). The second maximum is, in the case of all the four types, in November. As for the type M, its November peak surpasses that of April. The inter- annual distributions derived from the 50 years long series show convincingly the synoptic climatological features thereof, concealing at the same time the temporal variability of the occurrences within the 50 years long period, that might be misleading also from the point of view of hydrological consequences.

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Figure 2-2. a. Figure 2-2.b.

Figure 2-2. Monthly distribution of the frequency of weather patterns a - W, Wp and Z;

b - M, C, Cw and H

The inter-annual distribution of the absolute frequencies of the seven weather types, compiled for each of the 5 decades separately, are displayed in Figs. 2-3 to 2-9, illustrating the temporal variability of the various types. One can see that there are more well-balanced types, such as W and Wp (Figs. 2-3 and 2-4), while others display significant differences between the various decades. The occurrence in April-October of type Z (Fig.2-5) was lower in 1981-2000 than in the foregoing three decades. Among the cyclonal types, it is particularly type C (Fig.2-7) whose frequency was lower in the last two decades than in the former three ones. At the same time, the frequency of type Cw (Fig. 2-8) increases significantly, likewise substituting type C. In the case of type M (Fig. 2-6), although there are great differences between the occurences of the various months, they are still less than in the case of the foregoing two types. Finally, as fore the rather rare type H (Fig.2-9), there are higher frequencies for the period April/August as computed for the last two decades.

Figure 2-3. Figure 2-4.

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Figure 2-5. Figure 2-6.

Figure 2-7. Figure 2-8.

Figure 2-9. Figure 2-10.

For summing up, it seems to be worthwile to survey the inter-annual distributions, referring to the full number of days of the 50 years long reference period, both of all BJE-types and their two (West and Cyclonal) type-groups (Fig. 2-10). These relative frequencies represent at the same time also the empirical probabilities of occurrence of the various types and groups. According to the totalizing curves of the two type-groups, the minimum probabilities of occurrence of the BJE-types belong to the months September and October. The tendencies of the type groups West and Cyclonal are contrary in April, indicating that the great precipitation events originate in the spring months from cyclonal situations. From the summarization according seasons of the relative frequencies one can conclude

T22_07_03_Scenario_Analysis_D22_3_V1_3_P01 15 May 2009 19 Scenario Analysis M22.2 Contract No:GOCE-CT-2004-505420 that the expectable probability of weather types triggering great precipitations only slightly changes from season to season: this expectable probability being in winter 9.8%, in spring 10.4%,in summer 10.5%, and 8.9% in autumn. Thus the probability of occurrence of great precipitation amounts practically does not depend on the season.

Similar results were obtained also by Bárdossy who investigated the weather stituations triggering flood waves in two sub-basins of the Upper Tisza. For both basins two critical flood producing CPs could be identified. For both the Tivadar and Vásárosnamény section of the Tisza the most critical CP caused more than 4 time higher discharge increments than a normal day in average. The obtained CPs do differ only slightly – with low pressure centres more to the east for the upper sub-catchment (Tivadar). Fig 2-11. shows the map of the anomalies corresponding to CP01 – the most flood-relevant CP for Vásárosnamény.

Figure 2-11. Normalized SLP anomalies corresponding to CP01 for the Tisza at Vásárosnamény (dashed lines – negative anomalies, and solid lines – positive anomalies).

The anomalies are well structured and show that a low pressure centre south-east of Poland is related to strong precipitation and subsequent possible floods of the upper Tisza basin. The occurrence of these patterns does not necessarily cause floods, but conversely most of the floods were caused by these patterns. Fig.2-12 shows the 100 biggest floods at Vásárosnamény for the period 1900-1999. The CPs occurring on the days before the peak, which are responsible for the increase of the discharge, were identified. Three classes were formed, CPs caused by CP01, CP05 and others. It is seen from Fig.2-3 that most of the floods were caused by CP01, some by CP05. The biggest flood, which does not correspond to CP01 or CP05, ranks 10 among the observed floods.

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4000

3500 /s) 3 3000

2500 CP01 Discharge (m 2000 CP05 Other 1500 1 10 100 1000 Return period (years)

Figure 2-12. The 100 largest observed floods of the Tisza at Vásárosnamény from the time period 1900-1999 with the corresponding CPs.

In order to investigate possible changes in the occurrence and magnitude of the floods, the frequency of the critical CPs was calculated for the observation periods. Fig.2-13 shows the frequencies of CP10 for Tivadar. A slight increase of the occurrence of this pattern can be seen on the figure.

0.16

0.12 y c n

ue 0.08 q e r F 0.04

0 1950 1960 1970 1980 1990 2000

Figure 2-13. Annual frequencies of CP10 for the Tisza at Tivadar.

For Vásárosnamény a longer series could be constructed. Fig.2-14 shows the frequencies of CP01 in the months of November to May. This time period was chosen as most of the floods occurred during these months – and CP01 was seldom related to floods in summer and autumn. The increase of the frequencies is significant for these patterns. Different statistical tests suggested in WMO (2000) were applied to investigate the stationarity of this series. All of them rejected the hypothesis of stationarity. The changes of the frequencies of the patterns can be a possible explanation for the increase of floods for the Tisza Basin.

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0.30

0.20 equency r F 0.10

0.00 1900 1920 1940 1960 1980 2000

Figure 2-14. Annual frequencies of CP01 for the Tisza at Vásárosnamény.

An investigation of the event series triggering flood waves resulted also in the statement that in great river systems whose catchment areas are larger than 100,000 km² and the total length of whose river network amounts to severel thousand km, significant flood waves can be produced only by precipitation events of several days’ duration. Typically, such precipitation events are not triggered by one solitary object, but by the accumulation of various types causing flood waves. Therefore, when striving at the determination of the possible maximum flood wave, it is not sufficient to carry out the maximization of the precipitation field of one selected flood-producing weather type, but also the question has to be answered, how these types can accumulate, leading to what a field of maximum precipitation and how long can last such a precipitation period. It was Mrs. E. Bodolai-Jakus who tried to identify such an event series after the Tisza flood of 1970. Due to the non-stationary behaviour of the occurrence of the types of meteorological events, however, not only the frequencies of dangerous weather situations increased during the last decade, but also the connected precipitation amounts seem to have become greater. Thus when estimating the expectable flood waves, one has to start from the data of flood waves observed in the past and from certain assumptions regarding the meteorological and hydrological conditions of flood wave generation.

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3. The VITUKI – NHFS hydrological modelling system

The NHFS GAPI/TAPI modeling system has been developed within the National Hydrological Forecasting Service, a unit of the VITUKI. The conceptual, partly physically-based GAPI model serves for simulations and forecasting of flow for medium and large drainage basins. The lumped system consists of sub-basins and flood routing sections (Figure 1.).

In the course of a decade of development and upgrading the forecasting package has grown into a complex tool containing snow accumulation and snowmelt, soil frost, effective rainfall, runoff and flood routing modules, extended with statistical error correction - continuous updating and hydraulic - 'empirical backwater effect' modules.

Figure 4.1. Sequentiality of the basic layout of the simulation

3.1 The HOOLV Snowmelt Model

Precipitation falling onto the land surface is one of the most important elements of the hydrological cycle, and it is the only input term of the water balance of the earth surface. In those areas of the Earth where a part of the annual precipitation falls in the form of snow the rhythm of the hydrological cycle, that is that of the water balance within the year, follows a pattern that deviates from that of the precipitation record. Precipitation falling in solid state of the matter enters the hydrological cycle with a time lag that might be as much as several months after the precipitation event. Therefore, instead of considering the observed values of precipitation when describing various elements of the hydrological cycle, it is more expedient to take the so-called "effective precipitation" into account. This is the fraction of precipitation which is present in the land surface in liquid state. Consequently the most important task of the various snow models is to transform the observed precipitation values into effective precipitation values.

The HOLV snowmelt model has a flexible structure, it's able to change its own structure in function of the data availability. In case of availability precipitation and air temperature data only temperature index method is used, when further data are accessible too (cloudiness, dew point, speed of wind), using of energy balance model is to be preferred.

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3.1.1 The energy balance method and the temperature index method

The energy balance of snowcover:

Eo-Ez = (1-A) Esw + Elw - Ee + Es + El + Ec + Eg where:

2 Eo - the total energy absorbed/emitted by the snow surface [J/m s] 2 Ez - the total flux of energy across the boundary between the snow and soil [J/m s] 2 Esw - the energy arriving onto the snow surface from short wave radiations [J/m s] A - the albedo of snow which expresses the fraction of the incoming short wave radiation, that will be reflected by the snow surface back to the atmosphere [J/m2s]. 2 Elw - the atmospheric long wave radiation [J/m s]. 2 Ee - the radiation emitted by the snow surface [J/m s]. Es - the sensible heat flux, caused by the temperature difference between the snow surface and in the atmosphere above the snow [J/m2s]. El - the latent heat flux, caused by the difference of vapour pressure at the snow surface and the atmosphere above the snow [J/m2s]. 2 Ec - the energy brought the snow surface by rainfall [J/m s].

In the practical cases the changes of energy of snowcover is expressed in terms of meltwater millimeter instead of the energy units.

If there are data available for calculation of the energy terms on the right-hand side of Eq.(1) energy balance method can be applied. In the most practical cases these terms can not be computed with acceptable accuracy. In these cases temperature index method can be used.

The basic equation of the temperature index method:

Eo-Ez = M = (Co+ Cp P ) ( T- To ) where:

M - the quantity of water melted/refreezed during the selected period of time [mm] o -1 Co - the temperature index [mm C ] o -1 Cp - coefficient for taking into account the effect of precipitation [ C ] P - the precipitation [mm] T - the air temperature [oC] o To - the threshold air temperature [ C] The temperature index is considered time-variable, as a consequence of seasonal changes of the solar radiation values corresponding to the same air temperature, and even of changes of the albedo of the snow surface.

When the precipitation falls in the form of snow, precipitation observation data are to be corrected as a function of speed of wind, with the following formula:

P = PoC;

C = 1.0+(u-1.0)Sc where:

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P - the corrected value of precipitation [mm] Po - the observed value of precipitation [mm] C - the calculated correction coefficient U - speed of wind [m3/s] Sc - the correction factor

The compression of snowcover is calculated as follows:

-kΔt -kΔt ρh,i = ρhmax (1-e )ρh,i-1e where:

3 ρh,i-1 - the density of snowcover in the (i-1)-th time step [gr/cm ] 3 ρh,i - the density of snowcover in the i-th time step [gr/cm ] 3 ρhmax - the obtainable naximum density of snowcover [gr/cm ] Δt - the time step k - rate of compression of snowcover

The intensity of the infiltration of effective precipitation into the soil, and thus the time variation of the rate of infiltration, depends basically on the frozen or not frozen state of soil and the depth of soil frost. Consequently a snow model can supply acceptable input to a precipitation-runoff model when it able to provide information if soil frost too. This process was accounted for in our model with the following expression:

TFi = TFi-1TFDH - αTFTl where:

TFi - the depth of soil frost in the i-th time step [cm] TFi-1 - the depth of soil frost in the (i-1)-th time step [cm] TFDH - the soil frost reduction coefficient corresponding to snow depth H. o αTF - the coefficient of soil frost [cm/ C] o Tl - the mean air temperature [ C]

3.1.2 Calculating of the energy terms

The short wave radiation can be calculated by the following formula:

Esw= E( asw + bsw Nf) where:

2 Esw - the short wave radiation arriving onto the snow surface [J/m s] E - the maximum possible radiation depending on the geographical altitude and on the season [J/m2s] Nf - fraction of the sky covered by clouds expressed as decimal fraction asw,bsw - empirical constants

In case of temporary lack of data, cloudiness is approximated in a function of the daily temperature fluctuation with the following formula:

Nf = af + bf (Tmax-Tmin)

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where Tmax and Tmin are the daily maximum and minimum air temperatures, respectively, and af , bf are empirical constants.

The albedo of the snowcover is mostly affected by the crystalline structure of the snow, which letter is changing in function of the melting process. The crystalline structure of the melting snow will be completely restructured, and this new structure remains unchanged after refreezing. This process is described as follows:

-ka√τ Ai = Aoe ; τ = ΣTmax where:

A - albedo of the snowcover in the i-th time step. Ao - the maximal albedo value of newly fallen snow τ - the sum of maximum positive temperature values observed since last snowfall [oC]. ka - empirical constant

Atmospheric radiation is calculated by the Stephan-Bolzmann law in the form of:

4 Elw = εaσTl where:

2 Elw - the atmospheric radiation [J/m s] εa - the radiation coefficient of the atmosphere σ - the Stephan-Boltzmann constant (5.735 10-8 J/m2K2s2) o Tl - the absolute temperature of the emitting object [ K].

Relationship of Brundt for calculation of εa:

εa=(alw+blw e ) where

e - the vapour pressure of the atmosphere [mbar] alw,blw - empirical constants

The value of vapour pressure of the atmosphere can be calculated from the value of dew point. In case of temporary lack of data, dew point can be estimated by the daily minimal air temperature. Atmospheric radiation becomes more intensive when the sky is cloudy, since clouds also emit radiation, This effect can be taken into account by modifying the radiation coefficient:

εa = εa (1 + 0.24 Nf)

To carry out the calculation knowledge of the temperature of the emitting object is also needed. This is given by the following relationship:

Tl=Ta+0.1(Nf ΔT) where:

o Tl - the temperature of the emitting object (clouds) [ C].

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o Ta - the air temperature [ C]. ΔT - the difference between cloud temperature and air temperature [oC].

Earth radiation can be calculated by the Stephan-Bolzmann law too. The temperature of the snow surface can be considered as that of the air temperature during the day, and the dew point during the night.

The sensible heat flux is usually calculated with the following expression:

Es = Dsu (Th-To) where:

2 Ee - the sensible heat flux [J/m s] 2o Ds - the coefficient of energy exchange [J/m C] u - the speed of wind at h m above the snow surface [m/s] o Th - the air temperature at h m above the snow surface [ C] o To - the temperature of the snow surface [ C]

The latent heat flux is decribed as

El = Dlu (eh-eo) where:

2 El - the latent heat flux [J/m s] 2o Dl - the coefficient of energy exchange [J/m C] u - the speed of wind at h m above the snow surface [m/s] eh - the atmospheric vapor pressure at h meter above snow surface [mbar] eo - the vapor pressure of the atmosphere at the snow surface [mbar] the energy provided by rainfall ca be described as

Ec = 4210 TpP where:

2 Ec - the energy input via rainfall into the snowcover [J/m s] o Tp - the temperature of rain [ C] P - the rainfall intensity [mm/s]

The temperature of rain can be approximated by that of the air.

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3.2 The TAPI Rainfall-Runoff Model

Runoff is one of the most complex processes within the hydrologic cycle. Certain part of the precipitation always evaporates or becomes intercepted by the vegetation canopy before it reaches the surface of the watershed. Interception may be significant, especially in summer over fully vegetated surfaces, thus it should not, in general, be neglected. Its magnitude is influenced mostly by canopy density and its wetness status. The former changes seasonally, the latter depends on prior precipitation events. The remaining part of precipitation reaching the ground collects in micro- and macro-depressions of the surface, or partly runs off over it. As time goes on during a precipitation event, ever more micro- depressions become filled and so an increasing portion of the catchment takes part in contributing to runoff. A significant portion of the water reaching the generally pervious surface of the watershed seeps into the soil. The rate of infiltration at a certain location of given geologic, soil, slope and vegetation characteristics, will predominantly be influenced by the moisture content of the topsoil, and so directly, by antecedent precipitation conditions. A significant part of the infiltrated water will contribute to interflow in the loose topsoil, driven mainly by topography. Interflow may be considerable especially in catchments with coniferous vegetation, where surface runoff may be negligible compared to the rate of interflow. Interflow is often considered as a cross-over between seepage and open-surface flow, and may be closer to the latter due to its generally high velocity and small residence time. Some of the infiltrated water reaches the deep soil where it may still contribute to stream flow as unsaturated flow, or may recharge the ground water which eventually supplies the stream as its base flow. The rate of change in the base-flow process is typically slow – especially for larger rivers - due to potentially significant underground storage and characteristic low flow velocities in porous media. This short description of runoff formation highlights the complex nature of the process, and points out the importance of the antecedent moisture status of the watershed besides the runoff-triggering precipitation event.

The current model’s name “TAPI” specifies the technique by which the actual soil moisture status is accounted for. The “API” part is an acronym for Antecedent Precipitation Index, while the “T” refers to the current method’s similarity to the Tank Model structure developed by M. Sugawara.

The runoff ratio (αt ) can be defined as

−QQ )( ∑ 0ii αt = ∑ui where:

3 -1 Qi [m s ] is the stream-flow rate at time i; 3 1 Q [m s- ] is the base-flow rate at time i; 0i 3 -1 ui [m s ] is precipitation rate at time i.

The above summations involve long time-series, thus the time-delay between precipitation and runoff is negligible. The base-flow rate can be obtained from multi-year stream flow records. It is possible to specify a summer base-flow rate, which may be several times larger than its otherwise regular value. This way base-flow contribution of glaciers in alpine watersheds (e.g. certain sub-catchments of the Upper Danube) can be accounted for. The ratio of surface, and shallow as well as deep subsurface flow changes seasonally and depends largely upon the moisture condition of the catchment, and so indirectly upon the antecedent precipitation condition.

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Typically, spatial distribution of the soil moisture status of the watershed is not known even for research catchments, and the few available point-measurements are seldom representative of the whole watershed. For operational rainfall-runoff models such information is therefore available only indirectly, through the use of antecedent precipitation indices. Naturally, the more time elapsed since the last precipitation event, the less is its ensuing effect on the actual moisture condition of the watershed. The Antecedent Precipitation Index (API) reflects this mechanism by employing weights in decreasing order as one goes back in time as multipliers of the observed precipitation values. The weight function may typically be linear, parabolic, or exponential in time. TAPI employs the following weighting

n −at =∑ −tii ePAPI t=0 where:

Pi is the precipitation sum at time i; n is the total number of values considered in the weighting process; a is a model parameter, setting the speed at which the weights decline backward in time.

The value of n is set by the term in the series that contributes less than 0.05 to the sum. This way one can say that the last n time-increments influence runoff of the catchment. Seasonally changing evaporation and interception losses are estimated by the following expressions

i = [ + ADV π ];)sin(1 5.304 − i A = 183 where:

th Vi [mm] is the loss at the i day of the year; D [mm] is the maximum value of the loss within the year.

After obtaining the value of API and Vi ,and calculating the value of effective precipitation as the difference between precipitation and losses, the ratio (α ) of surface and subsurface runoff can be 1 estimated as

CAPKUL α 1−= 1 CAPMAX if CAPMAX > API CAPKUL= CAPMAX− API

α 1 = 1 otherwise, where CAPMAX is the API value that belongs to a fully saturated soil. Once reaching this stage, all effective precipitation runs off on the surface.

When the top soil is frozen, the value of α is increased. A soil frozen to a depth of 5 cm results in 1 infiltration rates reduced by about 80%, while the same of 10 cm causes effective precipitation to form surface runoff entirely. A certain part (α ) of the infiltrated water will be lost for runoff, another portion (α ) of what is left b a will form subsurface runoff while the remaining becomes interflow. This way TAPI separates total runoff into four pathways:

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- surface runoff

= PQ αtifei α1

interflow

= PQ αtifki −α1 −αb −αa )1)(1)(1(

subsurface runoff

= PQ αtifai −α1 −α )1)(1( αab

- base flow, its estimation being explained above.

Once the different ratios are specified, the amounts must be estimated. Routing of water in all three pathways above is performed with the help of serially connected linear reservoirs (forming a cascade) in each path. This way one obtains three parallel cascades, with parameters ni and ki (i = 1,2,3) for each one, a discrete cascade model is employed. This algorithm can be found in the section describing flow routing. The following model parameters of TAPI need optimization: n , k , k , k , a, CAPMAX, α , and α . 1 1 2 3 a b

3.3 The Discrete Linear Cascade Model

The Discrete Linear Cascade Model (DLCM) developed by Szöllõsi-Nagy [1982] utilizing an approach similar to the one reported by Szolgay [1984] serves for the routing of flow components and channel routing. The model is based on the assumption that the watershed responds to precipitation as a series of n reservoirs or storage elements with the following properties valid for each element:

– change in water storage of the reservoir is equal to the difference of in- and outflowing water volumes during the specified interval – outflow is linearly proportional to the stored water in the element.

The above assumptions yield the following equations:

dS −= yu dt and: = KyS where:

S - is storage u - is inflow y - is outflow K - is a storage coefficient.

If K is a constant (time invariant case), the watershed unit-step response can be obtained from the solution of the following expression

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)t(dy K =+ )t(l)t(y dt

which is a first order, linear, inhomogeneous ordinary differential equation. When t<0, y(t)=0, since the watershed response cannot precede its cause, the unit-step function, 1(t). This is called the causality property, a prerequisite for physical systems. The step-response function, g(t), for t>0 becomes

g(t) = 1 - e-t/K

The system’s Green-function, h(t), also called impulse response or instantaneous unit hydrograph (IUH, which is the system’s response to an input in the form of a Dirac-delta function) can be obtained through the following relationships

l’(t) = δ(t)

g’(t) = h(t)

Thus, the IUH, as the result of differentiating g(t) becomes

1 )t(h = e− K/t 1 K

With the help of the above, the output of the system to any arbitrary input can be obtained via the following convolution integral t t d)(u)t(hd)t(u)(h)t(y τττ−=ττ−τ= 1 ∫∫ 1 0 0 where the commutative property of convolution was exploited. When the outflow of the linear storage element becomes the input to a second one, the two reservoirs form a cascade. IUH of the cascade can be obtained by convolving the IUH of the first reservoir with itself, since the second storage element is identical to the first (i.e. its storage coefficient is also k) one

t t 1 1 t d)t(h)(h)t(h =ττ−τ= τ− K/ dTe = te− K/t = e− K/t 2 11 ∫∫ K 2 2 0 0 K K

For n serially connected identical reservoirs one may write

t d)(h)t(h)t(h =τττ−= n ∫ 1 −1n 0 which becomes through successive convolution as

n−1 1 ⎛ t ⎞ 1 h (t) = ⎜ ⎟ e-t/K n K ⎝ K⎠ (n -1)!

The above equation is the IUH of a cascade of n linear, identical reservoirs, where n is integer. For such a cascade, Eqs. (2) and (3) can be replaced by

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d += )t(uG)t(xF)t(x dt and:

y(t) = H x(t) where:

u(.) is input to the first storage element x(.) is the so-called state vector, which contains the stored water volumes of each reservoir y(.) is output of the system.

The above continuous, time-invariant linear system can be characterized by the following three system matrices

k =− ji 1 []F = 1,-jik == 1,2,ji, ... n, k = j,i K 0 otherwise

G = [ 1, 0, . . . 0]T

H = [ 0, 0, . . . k]

The above model cannot be employed immediately for practical applications where measurements are typically obtained only at discrete time intervals; thus, necessitating discretization of the continuous system. An adequate discretization requires discrete coincidence, which means that the discrete model must give identical results to the continuous one, provided the inputs are the same in both cases. If the continuous signal is sampled at a constant dt intervals, the discrete model becomes

xt = Φ(Δt) xt-Δt + Γ(Δt) ut-Δt

Qt = Hxt where the n-by-n … state-transition matrix contains the following general expression at the ith row and jth column:

Δ )tk( −ji Δ− tk ,e = ji []ΔΦ )t( j,i = − )!ji( 0 < ji

The same for the input-transition matrix, F(dt) can be put as

⎡ −1i Δ )tk( j 1⎤ []−=ΔΓ ⎢e1)t( Δ− tk ⋅ ⎥ i ∑ !j k ⎣⎢ =0j ⎦⎥

Eqs. (9-10) provide an adequate discretization of Eqs. (4-8). The discrete model is called Discrete Linear Cascade Model (DLCM). Its parameters are: n and k. The present system formulation of the model results in a simple and fast algorithm of computations. The timing (tp) of the maximum value of the IUH can be obtained through temporal differentiation

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−1n ⎡ −2n −1n ⎤ d d 1 ⎛ t ⎞ 1 − K/t 1 1 ⎛ t ⎞ 1 − K/t ⎛ t ⎞ 1 − K/t n )t(h = ⎜ ⎟ e = ⎢ −1n ⎜ ⎟ e − ⎜ ⎟ ⎥ = 0e dt dt K ⎝ K ⎠ − )!1n( K − )!1n( ⎣⎢ ⎝ K ⎠ K ⎝ K ⎠ K ⎦⎥ which simplifies to t )1n( p =−− 0 k yielding

p = − ⋅ k)1n(t

The mean residence or storage delay time of the cascade of reservoirs results from the first moment of the IUH as = ⋅ knT which illustrates that the maximum outflow value will happen sooner than the storage delay time would suggest. The tp/T ratio is often used as an indicator for the skewness of the IUH.

Discharge hydrographs received are transformed to water levels by steady state rating curves. At lowland sections with very low slopes results are corrected by an elementary backwater module which utilises simplifications similar those suggested by Todini and Bossi [1986].

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4. Analysis of the Consequences of Hypothetical Meteorological and Hydrological Scenarios

So far, the most important hydrometeorological features of the investigated flood waves of the Tisza River were described. One of the ― somewhat alarming ― common properties of these flood waves is, that in the case of each of them, due to the actual allocation of the precipitation zone, always only a part of the catchment area concerned did play a really significant role, what is more, in the case of the flood of 1999, during the critical period even the precipitation amount was rather moderate. The question arise with reason, how would be the development of the flood situation during the occurrence of the large flood wave, if no mitigating effects could prevail.

In the following, various hydrometeorological/flood defence scenarios, hydrometeorologically more unfavourable than those observed in the past, but having realistic chances of occurrence, will be created in order to investigate the hydrological/flood defence situations resulting from their eventual realization. For comply with this task, a number of runoff simulation investigations must be carried out. However, prior to investigate the situations resulting from these scenarios, the suitability of the simulation models to be adopted has to be tested.

4.1 Overview of the flood waves investigated For checking the suitability of the simulation model, first the simulation (reconstruction) of the flood waves observed in the past was carried out (excluding the effects of the actual levee ruptures in the case of the 1970 and 1998 flood wave). As a result of these simulations, the most important characteristics of the flood waves, namely their peak water levels could be reproduced in each case with minor errors (of a few cm). For the computation, the water stages were determined by adopting the so-called rating curves including only two variables (water stage and discharge). As a consequence on the low-slope river stretches, there may have occurred certain underestimations during the recession periods.

In the Figs. 3-1 to 3-4, examples are shown for the simulation of the actually observed water level time series, while the observed and computed water levels are numerically compared in Table 3-1. Since the computations were carried out in time increments of 12 hours, in the following also the denomination „peak water level” will be used for the maxima of the readings at every 12 hours (in the morning or in the evening). These values may of course slightly differ ― particularly in the headwater river sections ― from the real peak values.

TISZA - VÁSÁROSNAMÉNY 1970 1000 900

800 measured 700 simulated 600 Hmax=900 cm 500 400 300 water level (cm) water level 200 100 time 0 3.16 3.26 4.5 4.15 4.25 5.5 5.15 5.25 6.4 6.14

Figure 3-1. Stage hydrograph of the 1970 flood at Vásárosnamény

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TISZA - SZEGED 1970 1000 900 measured 800 simulated Hmax=923 cm 700 600 500 400

water level (cm) water 300 200 100 time 0 3. 1. 3. 11. 3. 21. 3. 31. 4. 10. 4. 20. 4. 30. 5. 10. 5. 20. 5. 30. 6. 9. 6. 19. 6. 29. 7. 9. 7 Figure 3-2. Stage hydrograph of the 1970 flood at Sszeged

TISZA - TISZABECS 1998 800

700

600 m easured simulated 500 Hm ax=680 cm

400

300

water level (cm) 200

100

0

tim e -100

-200 9.1 9.11 9.21 10.1 10.11 10.21 10.31 11.10 11.20 11.30 Figure 3-3. Stage hydrograph of the 1998 flood at Tiszabecs

TISZA - TOKAJ 1999 1000

900 measured simulated 800 Hmax=880 cm 700

600

500

400

water level (cm) water 300

200 100 time

0 2. 20. 3. 2. 3. 12. 3. 22. 4. 1. 4. 11. 4. 21.

Figure 3-4. Stage hydrograph of the 1999 flood atTokaj

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Table 3-1. Observed and simulated flood crests of the investigated flood waves OBSERVED SIMULATED YEAR RIVER -STATION FLOOD CREST FLOOD CREST [cm] [cm] 1970. Tisza - Vásárosnamény 912 911

1970. Tisza - Szeged 960 961

1998. Tisza- Tiszabecs 700 724

1999. Tisza- Tokaj 894 893

Table 3-1 shows that the computations are able to reconstruct/follow up the hydrological processes with an acceptable accuracy. On this basis, it seems to be a well-funded assumption that these models will describe also the hydrological situations generated by the various hypothetical scenarios with an acceptable accuracy. (The causes of the exceptionally significant deviations will be dealt with later.)

In the following, the meteorological and hydrological/flood defence scenarios to be investigated will be surveyed.The bias caused by the simulation models are to be filtered out; the results of the various scenarios will be displayed not by the observed water levels, but jointly with the simulated values, only slightly differing therefrom.

4.2 Overview of the scenarios investigated In the course of our investigations, the consequences of the occurence of the following scenarios will be presented:

– Scenario 1: During the flood of 1998, no levee ruptures take place along the Ukrainian stretch of the Tisza River, upstream Tiszabecs

– Scenario 2: During the flood of 1970, no levee ruptures take place in the Romanian and Hungarian part of the catchment area of the Szamos River

– Scenario 3: During the flood of 1970, no levee ruptures take place in the Romanian and Hungarian part of the catchment area of the Szamos River,and the flood wave of the Szamos presents itself 12 hours later, than it happened in reality

– Scenario 4: The Transcarpathian meteorological situation preceding the flood wave of 1998 repeats itself, but the intensive precipitation activity extends itself also onto the catchment areas of the left-side tributaries (Visó, Iza) of the Upper Tisza River. In this hypothetical case, the precipitation amount falling onto the latter two sub-basins equals the amount directly triggering the flood wave of 1970

– Scenario 5: The Transcarpathian meteorological situation preceding the flood wave of 1998 repeats itself, but the intensive precipitation activity expands not only onto the left side tributaries of (Visó, Iza) of the Upper Tisza River, but also onto the river system of the Szamos. In this hypothetical case, the precipitation amount falling onto the catchment areas of the Visó, Iza and Szamos Rivers, equals the amount directly triggering the flood wave of 1970. The levee ruptures along the Szamos do not take place

– Scenario 6: The Transcarpathian meteorological situation preceding the flood wave of 1998 repeats itself, but the intensive precipitation activity extends itself also onto the catchment area

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of the Bodrog River. In this hypothetical situation, the flood wave of the Bodrog system will be that observed in 1999 (which is larger than the real wave of 1998)

– Scenario 7: During the flood wave of 1999, the precipitation falling onto the sub-catchment of the Tisza River upstream Tokaj will be greater than the really observed amount 4.2.1 Scenario 1: No levee failures upstream of Tiszabecs during the flood of 1998 The investigation of this scenario was carried out in two different ways, the first of them being the graphical correction of the series of water levels observed every two hours in at the Tiszabecs gauging station, and the second one adopting the simulation of the flood wave, by ignoring the effects of levee ruptures. In Fig 3-5, both the values estimated by graphical correction and those obtained by simulation for the period of culmination are presented. On this figure ― and on all further figures ― the present-day Hmax values are indicated.

TISZA - TISZABECS 800 750 700 measured 650 estimated 600 simulated Hmax=732 cm 550 500 450 400 Water level (cm) 350 300 250 time 200 05. 00:00 05. 12:00 06. 00:00 06. 12:00 07. 00:00 07. 12:00 08. 00:00

Figure 3-5. Observed, adjusted for dike failures and simulated hydrographs of the 1998 flood

It can be concluded that in the case of non-existing levee ruptures the peak water level at Tiszabecs would have been higher by 15-20 cm than the actual value of 708 cm, measured during the flood of 1998. Thus, the peak water stage resulting from this scenario would have been 725-730 cm. The provisional interruption of the swelling by levee ruptures and its subsequent continuation caused a delay of 4-6 hours of the culmination, hence the latter would have happened, without levee ruptures, already on 5 May at 22-24 h .

4.2.2 Scenario 2: No levee failures along the Szamos River in 1970 The flood wave rushing down on the Szamos River in 1970, destroyed the levee at 11 points, with a total rupture length of 1200 m, in the surruondings of Szatmárnémeti (Romania), on 14 May between 17 and 20 o’clock, the volume of the thus released water being about 220 million m³. Only a few hours later, on 15 May between 2 and 4 o’clock a.m., there were three more levee ruptures, with a total lenght of 480 m, along the Hungarian stretch of the Szamos River, in the surroundings of the settlements Nábrád and Tunyogmatolcs, the estimated volume of the released water being here 30-40 million m³.

The peak water levels of the Tisza, downstream Vásárosnamény, were significantly reduced by the enormous total water volume of 250-260 million m³ escaping through the 14 ruptures of the Szamos levees. According to our computations, the peak water levels listed in Table 3-2. could have been measured in the case of non-existing levee ruptures.

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It can be seen that the ruptures situated in the surroundings of Szatmárnémeti ― since they did happen just shortly before the culmination ― reduced only slightly the peak water stage at the Csenger section. At the Vásárosnamény section of the Tisza River, however, a significant effect of the ruptures could be registered, the peak level, arising by 24-36 hours earlier than without ruptures, was reduced by almost 1 meter, and even at 140 km downstream, at Tokaj, the reduction was still 0.5 m. According to our investigations, although the increase of the simulated water levels would not have influenced the peak level at Tivadar, it would have caused a deceleration of recession. The higher peak at Tokaj would have resulted in a similar increase of water levels also along the Hungarian stretch of the Bodrog River.

Table 3-2.. Flood crests of the 1970 flood and Scenario 2 SIMULATION BASED SIMULATION BASED SIMULATION BASED RIVER STATION ON OBSERVED ON DIKE FALURES ON SCENARIO 2 CRESTS [cm] [cm] [cm] Szamos Csenger 888 872 885

Tisza Vásárosnamény 912 911 1003

Tisza Záhony 728 732 797

Tisza Tokaj 857 852 906

The water level time series, obtained as the results of our computations, are presented in Fig. 3-6 for the gauging station Csenger on the Szamos, and in Fig.3-7 for the station Vásárosnamény on the Tisza River.

SZAMOS - CSENGER 1000

900 simulated 800 2. scenario Hmax=902 cm 700

600

500

400

water level (cm) 300

200

100 time 0 5. 2. 5. 6. 5. 10. 5. 14. 5. 18. 5. 22. 5. 26. 5. 30.

Figure 3-6. Stage hydrographs of the 1970 flood at Csenger, Scenario 2.

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TISZA - VÁSÁROSNAMÉNY 1100

1000 simulated 900 2. scenario 800 Hmax=941 cm 700 600 500 400

water (cm) level 300 200 100 time 0 5. 2. 5. 6. 5. 10. 5. 14. 5. 18. 5. 22. 5. 26. 5. 30.

Figure 3-7.. Stage hydrographs of the 1970 flood at Vásárosnamény, Scenario 2.

In terms of a summary, it can be concluded that the non-occurrence of the levee ruptures during the flood of 1970 in the surroundings of Szatmárnémeti would have caused only a slight rise in the peak water level at the gauge Csenger of the Szamos River.The occurrence of further levee failures along the Hungarian stretch of the Szamos can be assumed. Had the water arriving in the Szamos river reached without any loss the mouth of the river, it would have caused significant additional rises of the water levels (50-90 cm) along the Tisza stretch between Vásárosnamény and Tokaj.

4.2.3 Scenario 3: During the flood of 1970, no levee failures along the Szamos river, and the flood wave of the Szamos reaches the mouth 12 hours later than observed The time lag between the arrival of the flood waves of the Upper Tisza and the Szamos was one of the reasons for the relatively moderate peak water level on the gauge Vásárosnamény, during the flood of 1970. The following investigation has the goal to find out, how the the results of scenario 2 would have changed if the the flood wave of the Szamos River had arrived 12 hours later into the border section of that river. The peak water stages occurring in this case are listed in Table 3-3.

Table 3-3... Flood crests of the 1970 flood, Scenario 3 SIMULATION BASED ON SIMULATION BASED SIMULATION BASED RIVER STATION OBSERVED CRESTS ON SCENARIO 2 ON SCENARIO 3

[cm] [cm] [cm] Tisza Vásárosnamény 912 1003 1025

Tisza Záhony 728 797 802

Tisza Tokaj 857 906 906

It can be concluded that the postponed arrival of the flood wave of the Szamos would have caused a slight increase in the peak water level at Záhony and practically no sensitive effect onto that at Tokaj. These rather moderate changes would be due to the flattening of the flood wave arriving on the Tisza,

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In Fig.3-8, the water levels in the Vásárosnamény section of the Tisza River are displayed for the case of scenarios 2 and 3.

TISZA - VÁSÁROSNAMÉNY 1100 1000 2. scenario 900 3. scenario 800 700 600 500 400 water level(cm) 300 200 100 time 0 5. 2. 5. 6. 5. 10. 5. 14. 5. 18. 5. 22. 5. 26. 5. 30.

Figure 3-8. Stage hydrographs of the 1970 flood at Vásárosnamény, Scenario 2 and Scenario 3

The summarizing conclusion is that the postponement of the flood wave of 1970 of the Szamos River would have caused a significant additional rise only in the Vásárosnamény section of the Tisza River, due to the fast flattening of the flood wave moving in the latter. However, in the case of a flood wave of larger volume, meeting a more full riverbed of the Tisza (like that of 1998), also differences significantly surpassing those described may arise.

4.2.4 Scenario 4: The intensive precipitation activity preceding the flood wave of 1998 covers the whole sub-catchment of the Tiszabecs cross-section One of the most striking differences between the flood waves occurring in 1970 and 1998 on the Upper Tisza River was that while the flood waves of the two left-side tributaries, the Visó and the Iza, significantly contributed to the Tisza-flood of 1970, it were mostly the right-side tributaries that played a major role in the genesis of the flood of 1998. Thus, both flood waves surpassing all formerly observed peaks were generated in such a way that the really intensive precipitation activity took place only in one of the two major parts of the catchment area.

For our investigations the assumption was taken that the precipitation situation of November 1998 would repeat itself with the difference that also onto the catchment areas of the tributaries Visó and Iza (where in reality there had been also a limited amount of precipitation) would fall a significant amount of precipitation, equal to that preceding the flood of 1970.

The areal distributions of the real and hypothetical precipitation are presented in Figs. 3-9 and 3-10.

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Figure 3-9. Spatial distribution of precipitation Figure 3-10. Spatial distribution of precipitation pattern resulting the 1998 flood on pattern forScenario 4 Upper Tisza

The results of the computations are listed in Table 3-4, and the water level time series of the gauging stations Tiszabecs, Tivadar and Vásárosnamény are displayed in the Figs. 3-11 to 3-12. It can be seen that in the hypothetical case investigated the peak water level at Tiszabecs would have surpassed the observed one by more than 1 meter. The water levels would have overtopped the altitude of the levee crest on the Hungarian side of the Tisza River. At Tivadar, the water level would have surpassed by more than 20 cm the peak level of 2001, causing a levee break in the Tarpa section. According to the computations concerning the Tisza stretch downstream Tivadar, the peak at Vásárosnamény would have surpassed the Hmax level, at Záhony would have been practically equal to Hmax, and at Tokaj slightly lower than Hmax. The higher water levels would have resulted in a slightly slower movement of the flood wave, the relative time lag at Tokaj being estimated to be about 12 hours.

Table 3-4. Flood crests of the 1998 flood, Scenario 4 SIMULATION OBSERVED SIMULATION RIVER STATION BASED ON HISTORICAL BASED ON SCENARIO 4 MAXIMUM OBSERVED BEFORE 2006 CRESTS [cm] H [cm] max [cm] Tisza Tiszabecs 700 815 732

Tisza Tivadar 958 1040 1014

Tisza Vásárosnamény 923 962 941

Tisza Záhony 734 757 758

Tisza Tokaj 872 879 894

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TISZA - TISZABECS 900 800 simulated 700 4. scenario 600 Hmax=732 cm 500 400 300 200

water level(cm) 100 0

-100 time -200 10. 26. 10. 28. 10. 30. 11. 1. 11. 3. 11. 5. 11. 7. 11. 9. 11. 11. 11. 13. 11. 15.

Figure 3-11. Stage hydrographs of the 1998 flood at Tiszabecs, Scenario 4

TISZA -TIVADAR 1100 1000 900 simulated 800 4. scenario 700 Hmax=1014 cm 600 500 400 300 water level (cm) level water 200 100 0 time -100 10. 26. 10. 28. 10. 30. 11. 1. 11. 3. 11. 5. 11. 7. 11. 9. 11. 11. 11. 13. 11. 15.

Figure 3-12. Stage hydrographs of the 1998 flood at Tivadar, Scenario 4

TISZA - VÁSÁROSNAMÉNY 1100 1000 simulated 900 4. scenario 800 Hmax=941 cm 700 600 500 400

water level (cm) 300 200 100 time 0 10. 26. 10. 28. 10. 30. 11. 1. 11. 3. 11. 5. 11. 7. 11. 9. 11. 11. 11. 13. 11. 15.

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Figure 3-12. Stage hydrographs of the 1998 flood at Vásárosmamény, Scenario 4

For summing up, it can be concluded, that in the case of an expansion of the very intensive precipitation activity also onto the catchment areas of the tributaries Visó and Iza, the water levels resulting therefrom would have surpassed by 21-83 cm the maxima observed previously along the Upper Tisza River between Tiszabecs and Vásárosnamény, overtopping the present crest level of the flood levees along a significant lenght. Between Vásárosnamény and Tokaj, the water levels would have reached or at least strongly approached the previously observed maximum values.

4.2.5 Scenario 5 The intensive precipitation activity preceding the flood wave of 1998 covers, both the sub-catchments of the Upper Tisza river, and also that of the Szamos river The hypothetical case was investigated, when a great amount of precipitation was falling onto the whole catchment area of the Upper Tisza. In the following, the case will be dealt with, when the intensive precipitation activity reaches also the river system of the Szamos, the difference between the two scenarios consisting in the assumption, that in the Szamos Catchment not the (real) precipitation situation of 1998, but that of 1970 will be postulated. The real and the hypothetical precipitation distributions are presented in the Figs. 3-13 and 3-14.

Figure 3-13. Spatial distribution of precipitation Figure 3-14. Spatial distribution of precipitation pattern resulting the 1998 flood on pattern forScenario 5 Upper Tisza

The results of the computations are summarized in Table 3-5, and the water level time series of the Tisza reach between Vásárosnamény and Tokaj are shown in the Figs. 3-15 to 3-17.

Table 3-5. Az 1998. évi árhullám tetőző vízállásai néhány szelvényben, az 5. szcenárióban vázolt esetben SIMULATION OBSERVED RIVER STATION SIMULATION BASED ON HISTORICAL BASED ON SCENARIO 4 MAXIMUM OBSERVED BEFORE CRESTS [cm] 2006 [cm] Hmax [cm]

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Tisza Vásárosnamény 923 1136 941

Tisza Záhony 734 911 758

Tisza Tokaj 872 1000 894

TISZA - VÁSÁROSNAMÉNY 1200 1100 simulated 1000 5.scenario 900 Hmax =941 c m 800 700 600 500 400 water level(cm) 300 200 100 time 0 10. 26. 10. 28. 10. 30. 11. 1. 11. 3. 11. 5. 11. 7. 11. 9. 11. 11. 11. 13. 11. 15.

Figure 3-15. Az Stage hydrographs of the 1998 flood at Vásárosmamény, Scenario 5

TISZA - ZÁHONY 1000 900 simulated 800 5. scenario 700 Hmax=758 cm 600 500 400 300

water (cm) level 200 100 0 time -100 10. 26. 10. 28. 10. 30. 11. 1. 11. 3. 11. 5. 11. 7. 11. 9. 11. 11. 11. 13. 11. 15.

Figure 3-16. Stage hydrographs of the 1998 flood at Záhony, Scenario 5

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TISZA - TOKAJ 1100 1000 simulated 900 5. scenario Hmax=928 cm 800 700 600

water level 500 400 300 time 200 11. 1. 11. 3. 11. 5. 11. 7. 11. 9. 11. 11. 11. 13. 11. 15. 11. 17. 11. 19. 11. 21.

Figure 3-17. Stage hydrographs of the 1998 flood at Tokaj, Scenario 5

In the course of the computations, the effect of the levee ruptures in 1970 in the surroundings of Szatmárnémeti were neglected, thus assuming that the total water amount transported by the Szamos River would have reached its mouth, feeding the recipient Tisza River. Since the results obtained do not differ from those of scenario 4, they are not described here again. It can be seen, that in the present hypothetical case, water levels surpassing the levee crests or at least reaching them would occur, whose consequences should not even be estimated hereby.The levee overtopping would concern not only the line of the Tisza, but, due to the backwater effects, also a considerable part of its tributaries in Hungary.

In terms of a summary, it can be stated, that if the very intensive precipitation activity of 4 November 1998 had covered the whole catchemnt of the Upper Tisza River, and simultaneously the precipitation amount of May 1970 had fallen onto the Szamos Catchment, the water levels resulting therefrom would have surpassed by 106-195 cm the maxima observed along the Tisza reach beween Vásárosnamény and Tokaj. Such a hypothetical flood wave would result in levee crest overtoppings on the full lenght of the river system investigated, including both the Tisza reach between Tiszabecs and Tokaj, and the lowest reaches of its tributaries, resulting in a hardly manageable flood defence situation.

4.2.6 Scenario 6 The Upper Tisza flood wave of November 1998 coincides with the flood wave of the river network of the Bodrog River As already mentioned earlier, during the flood wave of November 1998 only the eastern part of the river system of the Bodrog River (including the catchments of its tributaries Ung and Latorca) were involved in the intensive precipitation activity, while no significant rises of water levels were observed on the Slovakian tributaries (Ondava, Tapoly, Laborc). During this event, the contribution of the Bodrog to the flood wave of the Tisza was not insignificant, but also not determinative, as against the event happened 4 months later, when the water amount arriving on the Bodrog was a determinative component of the flow regime of the Tisza reach downstream Tokaj. In Table 3-6, the peak water levels observed in November 1998 and March 1999 at some selected gauging stations are listed.

In the spring of 1999, the rather rare situation happened in the Bodrog system, that all four of its tributaries supplied more or less the same discharges (as a maximum, about 400-450 m³/s each of them). As a consequence, the resulting peak discharge on the near-to-mouth stretch of the Bodrog was almost by 50% higher than during the preceding autumn flood.

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In the following, we try to find out, what water levels could have been observed along the Tisza reach between Tokaj and Szolnok, had the flood wave of the Upper Tisza of November 1998 be joined by the discharge of the Bodrog observed in March 1999. The distribution of the real and the hypothetical precipitations are displayed in Figs. 3-18 and 3-19, the results are listed in Table 3-7.

Table 3-6. Flood crests of floods in November 1998.and March 1999 in the Bodrog Basin OBSERVED FLOOD OBSERVED FLOOD RIVER - CREST, NOVEMBER CREST, MARCH DIFFERENCE YEAR STATION 1998 1999 [cm] [cm] [cm] Latorica Chop 745 698 +47

Uh Lekarovce 1026 746 +280

Laborec Humenne 244 324 -80

Ondava-Topla Horovce 106 476 -370

Figure 3-18. Spatial distribution of precipitation Figure 3-19. Spatial distribution of precipitation pattern resulting the 1998 flood on pattern forScenario 6 Upper Tisza

Table 3-7. Flood crests of the flood of 1998 and Scenario 6 SIMULATION BASED ON SIMULATION BASED ON RIVER STATION SCENARIO 6 OBSERVED CRESTS

[cm] [cm] Bodrog Sárospatak 688 759

Tisza Tokaj 872 911

Tisza Tiszapalkonya 733 767

Tisza Kisköre alsó 897 927

Tisza Szolnok 906 934

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The joint effect of the extremely large water amount arriving on the Bodrog, along with the backwater effect exerted onto the Bodrog by the flood wave of the Tisza, would have resulted in the very significant rise by 71 cm of the water level observed at the gauging station Sárospatak on the Bodrog River. The rises at the Tisza sections downstream the Bodrog-mouth would have been, uniformly, between 37 and 42 cm. The peak water levels read both at Tokaj and Sárospatak would have surpassed the present-day Hmax values, observed in March 1999.

The water stages at the Tisza-gauges Tokaj and Szolnok are presented in the Figs. 3-20 and 3-21.

TISZA - TOKAJ 1000 900 simulated 800 6. scenario 700 Hmax=928 cm 600 500 400

water level (cm) 300 200

100 time 0 10. 19. 10. 27. 11. 4. 11. 12. 11. 20. 11. 28. 12. 6. 12. 14.

Figure 3-20. Stage hydrographs of the 1998 flood at Tokaj, Scenario 6

TISZA - SZOLNOK 1100 1000 simulated 900 6. scenario 800 Hmax=1041 cm 700 600 500 400 water level (cm) 300 200

100 time 0 10. 19. 10. 27. 11. 4. 11. 12. 11. 20. 11. 28. 12. 6. 12. 14.

Figure 3-21. Stage hydrographs of the 1998 flood at Szolnok, Scenario 6

Summarizing, it can be stated that the coincidence of the Upper Tisza flood wave of November 1998 and the extraordinary Bodrog flood wave of March 1999 would have resulted in previously never experienced high water stages both on the Bodrog and in the Tokaj section of the Tisza River. Downstream Tokaj, the water stages would have surpassed by 30-40 cm the values observed in November 1998, along the whole Tisza stretch between Tokaj and Szolnok.

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4.2.7 Scenario 7 The precipitation amount over the sub-catchment of the Tisza upstream of the Tokaj gauge exceeds that of in March 1999 In the framework of this scenario, five cases have been investigated.

In the first four cases (scenarios 7.a – 7.d), it was assumed that the precipitation conditions of the sub- catchment areas of the tributaries Szamos and Bodrog were identical with the actual/observed situation. Since in the catchment of the Upper Tisza, according to multi-annual observations, the precipitation in March is generally by 50% higher than in the two sub-catchments mentioned, and during the flood of March 1999 it was this area, generally most abound with preciptation, that contributed exceptionally less to the generation of the flood wave, the highest reality was attributed to the hypothetic assumption of a significant precipitation reaching this area. The case of a precipitation of two days duration was postulated, with an areal average of 15 mm on the first day, and with 20 mm on the second. This situation cannot be called an extraordinary one, since in 1999 there were 7 cases with at least as high precipitations.

The effects of these precipitation amounts were investigated in four different points of time, as shown in Table 3-8. Table 3-8.. Amounts of precipitation assumed for Scenario 7 SCENARIO UPPER TISZA SZAMOS BODROG PERIOD OF PRECIPITAION PERIOD OF PRECIPITAION PERIOD OF PRECIPITAION [dd.hh.mm-dd.hh.mm] [dd.hh.mm-dd.hh.mm] [dd.hh.mm-dd.hh.mm] 02.06:00-03.06:00 15 mm 7.a. observed observed 03.06:00-04.06:00 20 mm 05.06:00-06.06:00 15 mm 7.b. observed observed 06.06:00-07.06:00 20 mm 08.06:00-09.06:00 15 mm 7.c. observed observed 09.06:00-10.06:00 20 mm 11.06:00-12.06:00 15 mm 7.d. observed observed 12.06:00-13.06:00 20 mm 29.06:00-30.06:00 20 mm 29.06:00-30.06:00 15 mm 29.06:00-30.06:00 15 mm 7.e. 30.06:00-31.06:00 25 mm 30.06:00-31.06:00 20 mm 30.06:00-31.06:00 20 mm

In the fifth case it was investigated, what an effect would exert an even higher amount of precipitation ― occurring at the end of March during a repeated warming up of the air temperature, covering a larger area ― onto the flow regime of the Tisza reach between Tiszabecs and Szolnok. The assumption in this case was that during the two rainy days 20 and 25 mm would reach the Upper Tisza Catchment, and 15 and 20 mm the Szamos and Bodrog Catchments (Table 3-8).

The results of the computations are summed up in Table 3-9.

Table 3-9.. . Flood crests of the 1999 flood and Scenario 7 SIMULATION SCEN. SCEN. SCEN. SCEN. SCEN. SCEN. RIVER STATION BASED ON 7.A. 7.B. 7.C. 7.D. 7.E. 7.E. OBSERVED . . CRESTS [cm] [cm] [cm] [cm] [cm] [m3s-1] [cm] Tisza Tiszabecs 247 505 581 549 187 520 2200

Tisza Vásárosnam. 837 894 933 865 837 807 2500

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Tisza Záhony 649 717 725 699 649 611 2200

Bodrog Sárospatak 729 763 751 736 729 665 700

Tisza Tokaj 893 940 929 914 894 819 2400

Tisza Tiszapalkonya 780 823 808 794 781 678 2350

Tisza Kisköre alsó 978 1011 1003 998 984 864 2250

Tisza Szolnok 972 1001 993 990 878 869 2200

A comparison of the scenarios shows that the the precipitation observed on 5 and 6 March would have resulted in a greater flood wave than the same precipitation amount falling 3 days earlier. Presumably, this fact may have two different explanations:

– Since the melting process in the Upper Tisza Catchment started only on 3 March, only one part of the melting water and of the rainwater led to actual runoff, a considerable part of the liquid water being stored in the dry interior of the snow-cover.

– During the period of scenario 7.b, the measure of further storage keeps decreasing, consequently the major part of the generated melting water is running off upon the land surface, whose increased moisture is due both to the snowmelt activity of the last days and the ca. 12 mm actual precipitation reaching the catchment area of the Upper Tisza before 5 March. The water reaching the riverbeds is meeting a slighly increased fullness thereof.

At Tiszabecs, water level surpassing the IIIrd grade of flood defence preparedness would arise in both cases, while the scenario 7.b would led at Vásárosmnamény to a water stage equal to the value Hmax, observed during the flood wave of 1998. The significant difference expectable at Vásárosnamény would be the consequence also of the fact that in scenario 7.b the flood wave triggered by the supposed precipitation would optimally be superposed onto the actual flood wave, while in the case of scenario 7.a, the culmination would occur 24 hours prior to the actual point of time. In the Záhony section, the difference between the two cases is significantly smaller. The water stages at Vásárosnamény to be read in the cases of the two scenarios are presented in Figs. 3-22 and 3-23.

TISZA - VÁSÁROSNAMÉNY 1000 simulated 900 7.a. scenario 800 Hmax=941 cm 700 600 500 400 300 water level (cm) 200 100 0 time -100 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-22. Stage hydrographs of the 1999 flood at Vásárosnamény, Scenario 7a

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TISZA - VÁSÁROSNAMÉNY 1000 900 simulated 800 7.b. scenario Hmax=941cm 700 600 500 400 300 water level (cm) 200 100 0 time -100 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-23. Stage hydrographs of the 1999 flood at Vásárosnamény, Scenario 7b

Despite the larger flood wave resulting from scenario 7.b, the higher water levels of the Tisza River at Tokaj and then downstream until Szolnok are resulting from scenario 7.a.

It is to say, the impact of the time lag of the flood wave of the Tisza will be amplified, due to the very important role of the Bodrog River. Water stages at Tokaj are shown in Figs. 3-24 and 3-25. Along the mentioned stretch of the Tisza, scenario 7.a causes increases of water levels by 29-47 cm, and scenario 7.b., by 21-36 cm. In Fig. 3-26, the water stages of the Tisza are presented for the scenario 7.a.

TISZA - TOKAJ 1000

900 simulated 800 7.a. scenario 700 Hmax=928 cm 600

500

400

water level (cm) 300

200

100 time 0 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-24. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7a

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TISZA - TOKAJ 1000

900 simulated 800 7.b. scenario 700 Hmax=928 cm

600

500

400 water level (cm) 300

200

100 time

0 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-25. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7b

Scenario 7.c generates at Tiszabecs a flood wave somewhat lower than that preceding it by 3 days, due to the greater snow quantity due to the slightly cooler weather. The thus resulting flood wave does branch off from the original one at Vásárosnamény and slightly also at Tokaj, involving of course lower water levels than those of the original flood (Figs. 3-27 and 3-28). Since the flood wave triggered by the precipitation postulated in the scenario does gradually catch up with the original wave, also the differences between this and the former two cases are gradually diminishing.

TISZA - SZOLNOK 1100 1000 simulated 900 7.a. scenario 800 Hmax=1041 cm 700 600 500 400 water level (cm) 300 200 100 time 0 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-26. Stage hydrographs of the 1999 flood at Szolnok, Scenario 7a

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TISZA - VÁSÁROSNAMÉNY 1000 900 simulated 800 7.c. scenario 700 Hmax=941 cm 600 500 400 300 water level (cm) 200 100 0 time -100 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-27. Stage hydrographs of the 1999 flood at Vásárosnamény, Scenario 7c

TISZA - TOKAJ 1000 simulated 900 7.c. scenario 800 Hmax=928 cm 700 600 500 400

water level (cm) level water 300 200 100 time 0 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-28. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7c

Scenario 7.d, postulating the latest precipitation, falling simultaneously with the culminations along the Tisza stretch between Záhony and Dombrád, causes only an insignificant rise of about 120 cm at Tiszabecs (due to the temperatures becoming cooler in the meantime) and it can only slow down the velocity of recession along the Tisza reach until Tiszapalkonya. Downstream the latter gauging station, however, it catches up with the original flood wave, resulting in increases of a few centimeters of the peak water levels at Kisköre and Szolnok and a delay by about one day and a half of the recession process. Fig. 3-29 presents the water stages at Tokaj, and Fig. 3-30 those at Szolnok.

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TISZA - TOKAJ 1000 900 simulated 800 6.d. scenario Hmax=928 cm 700 600 500 400

water level (cm) 300 200

100 time 0 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-29. Stage hydrographs of the 1999 flood at Tokaj, Scenario 7d

TISZA - SZOLNOK 1100 1000 simulated 900 7.d. scenario 800 Hmax=1041 cm 700 600 500 400 water level (cm) 300 200

100 time 0 2. 20. 2. 24. 2. 28. 3. 4. 3. 8. 3. 12. 3. 16. 3. 20. 3. 24. 3. 28. 4. 1. 4. 5.

Figure 3-30. Stage hydrographs of the 1999 flood at Szolnok, Scenario 7d

The occurrence of the situation outlined in scenario 7.e, sharply differing from the previous scenarios, would generate water stages slightly surpassing the IIIrd degree level of flood defence preparedness on the the Tisza stretch between Tiszabecs and Vásárosnamény, and then ― after a temporary flattening ― again at Tokaj. The flood wave, superposing itself onto a gradually more and more full riverbed, is resulting, despite its flattening, in slightly increasing water stages, although the latter remain by ca. 1 meter still below the previous ones.

Downstream from Vásárosnamény, the water levels are being decreased by the not perfect co- operation between the flood waves originating from the three catchment areas.It is to say, if significant precipitation is falling simultaneously onto the catchment areas of the Upper Tisza, the Szamos and the Bodrog, then the flood wave generated on the Tisza generally reaches Vásárosnamény before that of the Szamos, while at Tokaj the flood wave of the Bodrog generally arrives earlier than that of the Tisza, rather slown down by that time.

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The time of the precipitation event postulated in scenario 7.e coincides with the recession branch of the main flood wave. The taking into account of the effect of more complex slope conditions than those of the previous cases, by adopting three-variate discharge rating curves being possible only with limited accuracy, the water stages resulting from the investigation of this scenario may be more vague than usual: a certain underestimation of the water stages might be supposed. Therefore, in the last column of Table 3-9 also the discharges obtained are listed. They indicate, anyhow, that the resulting water levels remain under those produced by the main flood wave.

Summarizing, it can be stated that the water stages of the flood wave observed on March 1999 on the Tisza River would significantly have been influenced even by a precipitation of moderate amount falling during the first half of March onto a minor part of the catchment area. The precipitation falling prior to the the culmination at Záhony on 11 March (scenarios 7.a-7.c) would have caused water stages surpassing by 20-50 cm the formerly observed maxima both on the whole Tisza stretch between Tokaj and Szolnok and on the Bodrog River. A precipitation event arriving later would have had only a slight impact on the Tisza stretch between Kisköre and Szolnok, due to the weather’s having become cooler in the meantime. A precipitation, falling at the end of March, more significant than any previously observed event, would again have resulted in very high water stages, which however would have remained below those of the main flood wave.

Thus the relative scarcity in precipitation of the first ten days of March has at any rate to be considered as an event of key importance.

4.2.8 Scenario 8 The intensive precipitation activity preceding the flood wave of November 1998 covers also catchments of the Körös and/or Maros rivers During the flood wave of November 1998, the water levels observed along the Tisza stretch between Tiszapalkonya and Szolnok remained only by 8-23 cm below the Hmax values in force at that time. Downstream Szolnok, however, a sudden droop of the degrees of riverbed repletion could be registered, as it is clearly shown in Table 3-11.

Table 3-11.. Flood crests of the November 1998 flood along the Tisza reach. Szolnok - Szeged and historical maxima observed before 1998

OBSERVED FLOOD Hmax (1998) DIFFERENCE RIVER STATION CRESTS, 1998 [cm] [cm] [cm] Tisza Szolnok 897 909 - 12

Tisza Tiszaug 752 843 - 89

Tisza Csongrád 780 850 -170

Tisza Mindszent 780 982 -202

Tisza Szeged 705 961 -256

As it is shown, while the water stage at Szolnok was yet near to the critical record value, the deviations downstream the Mindszent gauging station are over 2 metres. The explanation of this phenomenon is obvious: the drastic droop of water stages is due to the rather moderate discharges supplied at that time by the tributaries Körös and Maros.

In the following, the situation will be investigated, when during the occurrence of the flood wave of November 1998 the real hydrological situation of June 1970 would have prevailed in the Körös

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According to the results of previous investigations, the most dangerous hydrological situation on the lower stretch of the Tisza River ― resulting from the joint effects of coinciding flood waves on all rivers of the Tisza Valley ― can be expected when a great precipitation amount falls onto the catchments of the Körös and/or Maros Rivers with a time lag of two-three days after the culmination time at Tokaj station of the flood wave arriving there from the Upper Tisza. In scenario 8.d, this case was investigated.

The main characteristics of the hypothetical situations are summarized in Table 3-12. The areal distributions of the precipitation falling onto the Maros Catchment in May 1970, and that falling onto the Körös Catchment in June 1970, as well as the real precipitation situation preceding the flood of 1998, are presented in Figs. 3-31 and 3-32. The results obtained by simulation are listed in Table 3-13.

Table 3-12. Amounts of precipitation assumed for Scenario 8 over Körös and Maros catchments

PRECIPITATION KÖRÖS BASIN PRECIPITATION MAROS BASIN SCENARIOE REFERENCE REFERENCE PERIOD OF PERIOD OF RAINFALL RAINFALL RAINFALL RAINFALL AMOUNT AMOUNT June 1970. As in November 1998 November 1998 November 1998 8.a.

November 1998 As in November 1998 May 1970 November 1998 8.b.

June 1970 As in November 1998 May 1970 November 1998 8.c.

June 1970 1 day after the flood May 1970 1 day after the flood 8.d. crest reached Tokaj crest reached Tokaj in 1998 in 1998

Figure 3-31. Spatial distribution of precipitation Figure 3-32. Spatial distribution of precipitation pattern resulting the 1998 flood on pattern forScenario 8 Körös and Maros

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Table 3-13. Flood crests of the 1998 flood and Scenario 8 SIMULATION SCEN. SCEN. SCEN. SCEN. RIVER STATION BASED ON 8.A. 8.B. 8.C. 8.D. OBSERVED . CRESTS [cm] [cm] [cm] [cm] [cm] Tisza Mindszent 779 792 792 792 978

Tisza Szeged 707 732 836 919 999

By analysing scenario 8.a, it can be stated that a flood wave, identical with that of June 1970, passing the Körös Rivers, would have raised in November 1998 the peak water levels at Mindszent by 13, and at Szeged by 25 cm (Figs. 3-33 and 3-34). One can also see that the main impact of the flood wave of the Körös Rivers would not be the raise of the peak water stages but rather the prolongation of the duration of the near-to-peak levels. The relatively greater increase on the Szeged gauge on the Tisza would be a result of the „co-operataion” between the Körös flood and the otherwise insignificant flood wave of the Maros River.

TISZA - MINDSZENT 1998 1200 1100 Simulated 1000 8.a. scenario 900 Hmax=1062 cm 800 700 600 500 400 water level (cm) level water 300 200 100 time 0 9. 1. 9. 11. 9. 21. 10. 1. 10. 11. 10. 21. 10. 31. 11. 10. 11. 20. 11. 30. 12. 10.

Figure 3-33. Stage hydrographs of the 1998 flood at Mindszent, Scenario 8a

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TISZA - SZEGED 1998 1100 1000 simulated 900 8.a. scenario 800 Hmax=1009 cm 700 600 500 400 water level (cm) 300 200 time 100 0 9. 1. 9. 11. 9. 21. 10. 1. 10. 11. 10. 21. 10. 31. 11. 10. 11. 20. 11. 30. 12. 10.

Figure 3-34. Stage hydrographs of the 1998 flood at Szeged, Scenario 8a

Scenario 8.b, in which the flood wave of 1970 of the Maros ― and not of the Körös ― River joins the flood wave of the Tisza, would have only a minor impact on the peak water level observed at Mindszent, but a rather considerable raise over 1 meter therof at the Szeged gauge, due to the fact that the flood wave of the Maros would have contributed to the Tisza a water volume by 100-150% higher than that of the Körös System (Fig.3-35). According to scenario 8.c, the flood waves of the tributaries Körös and Maros reaching more or less simultaneously the Tisza, the water level to be read on the Szeged gauge would be very high, remaining only by less than 1m below the Hmax value registered in 2006. (Fig.3-36)

TISZA - SZEGED 1998 1100 1000 simulated 900 8.b. scenario 800 Hmax=1009 cm 700 600 500 400 water level (cm) 300 200 100 time 0 9. 1. 9. 11. 9. 21. 10. 1. 10. 11. 10. 21. 10. 31. 11. 10. 11. 20. 11. 30. 12. 10.

Figure 3-35. Stage hydrographs of the 1998 flood at Mindszent, Scenario 8b

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TISZA - SZEGED 1998 1100

1000 simulated 900 8.c. scenario 800 Hmax=1009 cm 700 600 500 400 water level (cm) 300 200 100 time 0 9. 1. 9. 11. 9. 21. 10. 1. 10. 11. 10. 21. 10. 31. 11. 10. 11. 20. 11. 30. 12. 10.

Figure 3-36. Stage hydrographs of the 1998 flood at Szeged, Scenario 8c

In scenario 8.d the case was investigated when the precipitation falling onto the Körös and Maros Catchment occurs during the possible worst period, so that the flood waves of these two tributaries and that of the Tisza can mutually strenghten each other. As it can be seen in Fig.3-37, this scenario would result in very high water levels both at the Mindszent and the Szeged gauging stations, the latter almost approaching the record value Hmax.

TISZA - SZEGED 1998 1100 1000

900 simulated 800 8.d. scenario 700 Hmax=1009 cm 600 500 400 water level (cm) 300 200 time 100 0 9. 1. 9. 16. 10. 1. 10. 16. 10. 31. 11. 15. 11. 30. 12. 15.

Figure 3-37. Stage hydrographs of the 1998 flood at Szeged, Scenario 8d

It should be noted, however, that the computation carried out in the domain of the very high water levels surpassing all previosly observed data, regularly involve considerable uncertainties, even further increased by the lack of discharge measurements in the Mindszent section.

For summing up, it can be stated that the flood wave of the Körös, arising simultaneously with the great precipitation falling in November 1998 onto the catchment area of the Upper Tisza, would only modestly raise the peak water levels at the gauges Mindszent and Szeged, due to the considerable time

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5. Conclusions

5.1 Review of Scenario Analysis In the course of the present work, it was investigated, what a hydrological situation would be created in the catchment area of the Tisza River by the occurrence of certain meteorological and hydrological scenarios, having real chances to happen. In the course of our investigations, from among the great Tisza floods experienced during the last decades, that of 1970 (differing in many aspects from all other floods), that of 1998 (triggered mainly by precipitations of high intensity) and that of 1999 (originating mostly from snowmelt) were selected for a more detailed analysis.

– As a first step, the mathematical models adopted for carrying out the simulation investigations were briefly presented.

– Thereafter, a brief description of the river system investigated followed and the floods occurring therein during the the last decades were surveyed.

– In the next chapter, the generating causes and the main features of the floods of 1970, 1998, and 1999 were analysed.

– Finally, the situations resulting, according to our calculations, from the various scenarios were presented in detail.

Prior to investigate the selected scenarios, it was intended to demonstrate the reliability of the simulation model to be adopted therefore by the computative reconstruction of the water level and discharge time series of the three selected flood waves. There was a very good agreement between the observed time series and their corresponding ones produced by adopting the simulation model, particularly concerning the peak water levels, most important for the present investigation. Thus the adoptability of the mathematical model was demonstrated.

5.2 Basic results linked to different hydrometeorological situations 1. Scenario 1 (no levee ruptures in the Ukraine during the flood of 1998). The analysis of this scenario proved that the levee ruptures taking place in 1998 in Transcarpathia had significant impacts only onto the Tisza stretch in the surroundings of Tiszabecs, decreasing there by 15-20 cm the peak water levels. Downstrem Tiszabecs, the impact of these levee ruptures was insignificant

2. . Scenario 2 (no levee ruptures along the Romanian and Hungarian stretches of the Szamos River during the flood of 1970). The investigation of this scenario showed that the levee ruptures occurring in 1970 along the Szamos River both in Romania (in the sourroundings of Szatmárnémeti) and in Hungary had a significant (50-90 cm) decreasing effect onto the peak water levels along the Tisza stretch between Vásárosnamény and Tokaj (as contrasted with the ruptures taking place in 1970 mostly in the narrow valleys on mountanious river stretches of Ukrainian Transcarpathia).

3. The occurrence of scenario 3 (arrival of the flood wave of 1970 of the Szamos River 12 hours later than in reality) would have had only minor increasing impacts on the peak water stages of Vásárosnamény and Záhony (due to the rapid flattening of the flood wave arriving from the Upper Tisza).

4. The occurrence of scenario 4 (the intensive precipitation activity preceding the flood wave of 1998 would extend itself also over the catchment areas of th left-side tributaries (Visó, Iza) of the Upper

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Tisza) would have resulted in very significant increasings (over 40 cm) of the water stages, as compared with the original observed values, along the Tisza until Vásárosnamény.

5. Scenario 5 (the intensive precipitation activity prior to the flood wave of 1998 covering the left- side tributaries of the Upper Tisza , extends itself also to the catchment area of the Szamos River). The realization of this scenario would result in water stages surpassing very significantly (by 106- 195 cm) the so far observed maximum values along the total lenght of the Tisza stretch between Vásárosnamény and Tokaj, including the lowest stretches of its tributaries, creating a rather awkwardly manageable flood defence situation.

6. Scenario 6 (the flood wave of 1998 of the Upper Tisza meets with that of the Bodrog River, the latter transporting the water amount as experienced in March 1999). The realization of this scenario would have caused both on the Bodrog River and in the Tokaj section of the Tisza water levels surpassing all formerly observed values, while along the whole Tisza stretch until Szolnok they would have surpassed also significantly, by 30-40 cm the peak stages measured in November 1998.

7. Scenario 7 (during the flood of 1999, a precipitation amount surpassing the actual one falls onto the catchment area of the Upper Tisza). In this case, even a not very significant precipitation falling during the first half of March would significantly have raised the peak water levels. Thus the relative scarcity of actual precipitation in the first 10 days of March has anyhow to be considered as an event of key importance.

8. Scenario 8 (the intensive precipitation activity preceding the flood wave of 1998 extends itself also to the catchment areas of the Körös Rivers and/or the Maros River). If the flood waves of the two tributaries are triggered by precipitation occurring simultaneously with that of the Upper Tisza, only the flood wave of the Maros would have a significant effect onto the hydrological situation of the Tisza downstream the Körös-mouth. If, however, the precipitation on these two sub- catchments occurs only some days after the culmination at Tokaj of the flood wave of the Upper Tisza, peak water stages around the so far observed maximum would arise at the Szeged gauging station of the Tisza.

9. The flood waves both of 1970 and 1998 were triggered by intensive precipitation activities covering only a part of the catchment area of the Tisza belonging to its Tiszabecs section. An analysis of these two flood waves call the attention to the significance of the precipitation conditions of the period preceding the occurrence of the floodwave-producing heavy rainfalls. This preparatory precipitations may also contribute to the arising of extremely high water levels, by creating both high soil moisture and higher initial riverbed repletion.

10. The circumstances of the genesis of the flood wave of 1999 call the attention to the water amount stored in the snow-cover, of higher importance than beleived earlier, and also to the possibility of the arising of a very great flood wave on the Tisza originating mostly from snowmelt.

5.3 Basic results of the investigations The most important conclusions gained by the analysis of the processes, can be summed up as follows:

1. For generating the flood waves of 1970 and 1998, intensive precipitation activities covering only a part of the Tisza catchment upstream Tiszabecs were sufficient. An analysis of the two flood waves called the attention to the significance of the precipition conditions during the period preceding the heavy rainfall triggering the flood wave itself. These preparatory precipitations may also contribute to the arising of extremely high water levels, by creating both high soil moisture and high riverbed repletion.

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2. The circumstances of the genesis of the floodwave of 1999 call the attention to the water amount stored in the snow-cover, of higher importance than beleived earlier, and also to the possibility of the arising of a very great flood wave on the Tisza originating mostly from snowmelt.

3. The analysis of the scenarios based on changes in the extension of the floodwave-triggering precipitation zone and in the direction of their movement indicates that even a moderate modification of the meteorological situation may result in a considerably more unfavourable hydrological situation, first of all along the upper stretch of the Tisza, upstream Vásárosnamény.

4. Changes in the progress velocity of the floodwave-triggering precipitation zone may significantly modify the water stages, first of all in the headwater parts of the catchments characterized by a rather violent runoff regime.

5. On the lower stretches of the Hungarian Tisza, the major potential danger may be represented by two precipitation zones, following each other with a time lag of 8-10 days, if the first one covers the sub-catchments of the Upper Tisza, the Szamos and the Bodrog, and the second those of the Körös Rivers and the Maros.

Thus the investigation just taken place calls the attention to the fact what extreme consequences may have even a slightly more unfavourble realization of the flood-triggering meteorological situations than those experienced in the past. When determining the critical levels of flood water stages and flood defence levees, also the runoff-producing processes and the conclusions drawn from their simulation have to be taken into account.

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6. References

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