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This article was previously published asBoucher (1972).

Often the occurrence of high intensity rainfall over relatively small areas leads to spectacular and devastating floods. The North African floods of September 1969 (Winstanley, 1970) may be cited as such a case. Floods may be due, however, to an accumulation of events over a relatively long period of time. This appeared to be true for the Rumanian flood disaster of May 1970 (Figures 1 and 2) [not reproduced here]. An attempt is here made to retrace the meteorological events that culminated in the worst floods to affect the Transylvanian Basin for many centuries.

The position of Rumania

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

As may be seen from the map (Figure 3), the Carpathian Mountains stretch in a great arc southeastwards through northern and central Rumania forming a barrier 1000m high with peaks reaching 2000m above sea level. At their southern end the mountains turn abruptly westwards forming the higher Transylvanian Alps (2000m) with peaks of 2500m, the only significant break in the mountain chain being the Red Tower Pass at 310m. Though the Apuseni Mountains rise to over 1800m they present no real barrier to low level air movement.

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Figure 3. Location map for places mentioned in the text.

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The effect of the mountains upon the weather is to provide easy access to low level airstreams moving from the west whilst hindering airstreams with an easterly or southerly component. This peculiar arrangement of topography has fairly frequently led to sharp contrasts in weather and air masses across the mountain divide. To the east, continental influences dominate especially in winter with the outflow from the Siberian High. To the west, Mediterranean and Atlantic influences are often reflected in the pattern of weather. It is not surprising that Rumania is no stranger to freak weather conditions. Severe droughts, snow storms, dust and thunderstorms accompanied by violent winds have all occurred in recent decades.

The drainage pattern

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

Two main rivers drain the Transylvanian Basin, the Someş flowing northwest to become the Tisza of the Hungarian Plains; and the Mureş (Maros) that flows west to the confluence with the Tisza at Szeged. The Tisza sub-sequently flows into the Danube 60km upstream from Belgrade. The volume of water flowing in these two rivers during May 1970 was so great that the natural flood storage reservoirs proved entirely inadequate to cope.

Other rivers, including the Danube, also overflowed and at one time both Braila and Galaţi were isolated. The hydrological regime of the Danube is complex and it was fortuitous that the left bank tributaries of the Danube staggered their flow of flood water into the main river otherwise the disaster would have been even worse.

Weather preceding May 1970

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

It appears that the weather of the previous seven months had a profound effect upon the conditions favouring floods in the Transylvanian Basin. Throughout the autumn until 25 November, the Medi-terranean remained virtually free of the disturbances that usually track eastwards at that time of the year. The general pattern of airflow at 500mb is indicated in Figure 4. The persistence of a warm anticyclone over or near Greece during the late autumn caused a marked positive temperature anomaly (3 degC). It is suggested that this reservoir of energy formed an important source of latent heat producing a persistent cloud cover over much of south-east Europe during December (Bodolai and Body 1971). Only six hours of sunshine were recorded at Budapest for the whole of December.

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Figure 4. Dominant 500-mb flow patterns in October–November 1969.

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It was not until January that depressions began to track north-east across Yugoslavia into Rumania.

The Rumanian winter and early spring of 1970

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

Cloudy, rather mild, rainy weather characterized the early months of 1970. The failure of the Siberian High to extend westwards meant that south-east Europe remained under the influence of air of Atlantic origin. Three types of circulation pattern tended to recur at 500mb from January to March. These are shown diagrammatically in Figure 5. In January, and again in March, a cut-off low was often present over Greece. A high zonal index during the first half of February induced a frontal zone to persist west-east across central Europe along which wave disturbances travelled at frequent intervals crossing Rumania. During the latter part of February, and again in March, a trough existed between 5° and 10°E resulting in moist air being advected northwards across Bulgaria and Rumania.

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Figure 5. Dominant 500-mb flow patterns in January–March 1970.

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Temperatures over eastern Hungary and Rumania fluctuated considerably as cold air succeeded bursts of moist warm air. These fluctuations continued until mid-May. The constant succession of fronts caused monthly mean values of precipitation to be exceeded nearly everywhere. Much of the precipitation fell as rain below 1000m so that early floods occurred on the Someş and Mureş rivers. The water levels had little chance of returning to normal after one flood before the next minor flood wave swept downstream. Table 1 summarizes the synoptic conditions that instigated six minor floods in the first three months of 1970.

Table 1. Resumé of synoptic conditions producing minor flood waves on the Upper Tisza river.Thumbnail image of

As would be expected the upper reaches of the Tisza responded more quickly to the daily rainfall pattern whilst at Szeged the individual flood pattern was partially masked by the cumulative effects of the flood waters.

Winter snowfall

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

A factor that aggravated the flood situation in the Someş and Mureş basins was the nature of the winter snowfall. Normally a general accumulation of snow through the winter together with an even spring temperature rise ensures a run-off with which the rivers can easily cope from April through to mid-summer. These conditions did not occur in 1970. Despite outbreaks of cold air, inter-vening mild south-westerlies prevented much accumulation of snow below 1000m. The maximum depth of snow over the eastern Hungarian Plain adjoining Rumania was about half the expected values (Table 2). In Transylvania snow depth was also below average despite higher precipitation. A high percentage run-off was combined with a low evaporation rate due to the generally cloudy weather. Hours of sunshine for the first three months were 40 per cent below expected values.

Table 2. Maximum depth of snow (cm) for selected stations in the Hungarian Plain, 1970.
 JanuaryFebruaryMarch
East Hungarian Plain   
 Nyiregyhaza13107
 Debrecen14107
 Békéscaba1210Nil
West Hungarian Plain   
 Budapest221620
 Baja452710

Spring weather 1970

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

Rainfall in April maintained the high river levels though without threat-ening any major flood. The alternation of cold and warm airstreams continued. At the end of April and beginning of May temperatures suddenly dropped to below freezing. This outbreak of cold polar air, caused by a vigorous depression moving north along the eastern limb of the Carpathian Mountains, was accompanied by heavy snowfalls above 1500m (120cm at Virful Omul in the Bucegi Mts). The snow did not begin to melt until 10 May, when once again warm air streamed across Transylvania causing a rapid thaw.

The mid-May floods

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

About 6 May, a mid-level trough wave developed south-eastwards from Greenland towards Spain. Unlike the previous trough that migrated east, this second one remained stationary. On the 9th the flow at 500mb bifurcated over Newfoundland and a cut-off low formed off north-west Spain with a closed circulation. From this low pressure centre a trough extended east across the Alps to Rumania. At the surface a complex system of fronts covered west and south Europe. By the 10th the main frontal activity had resolved itself into a zone between the Alps and Carpathians (Figure 6). A series of small depressions began moving east along this zone whilst a similar but less active system of fronts persisted across the North European Plain.

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Figure 6. Synoptic elements leading to the storms of 11–12 May 1970. Frontal positions at 00 gmt 11 May (based on Daily Weather Report of the Rumanian Meteorological Service). Heavy black line: track of depression.

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Meanwhile the persistence of the low over Iberia caused warm air to be advected north-east from the west Mediterranean. Trajectory studies show that the air at 500mb that reached Transylvania on the 11th (1200 gmt) must have been in the region of the Azores–Madeira on the 9th (Figure 6). A south-west jet stream was responsible for the transport of this air, reaching speeds of 45m sec–1 (90 knots) over southern Sardinia at 00 gmt on 11 May.

At 700mb the air flow appears to have taken a less direct route from the Atlantic. However, it is concluded that the mid-level airstream that arrived over Rumania during 11 May was of Atlantic origin and humid. At the same time the low over Iberia divided, a second centre becoming established at the eastern end of the trough over Central Europe. The zone of marked baroclinic flow moved east-north-east towards the Hungarian Plain, the main south-west-erly jet shifting east across Transylvania during the evening of the 11th. At the surface, a small depression had moved eastwards into the Hungarian Plain. Very warm, humid air was advected north from the Adriatic ahead of this centre.

Complex frontal troughs over Italy became associated with this thundery low as much cooler air flowed east across northern Italy and Austria. With light southerly winds over Rumania, temperature maxima of 25–30°C were recorded over practically the whole country on the 11th, even reaching 35°C at Videle, south-west of Bucharest. After midday, thunderstorms broke out over and to the west of the Transylvanian Alps and these subsequently drifted north-east into the U.S.S.R. The cooler air made a series of advances into the Transylvanian Basin during the morning of the 12th when there was a renewal of violent thunderstorm activity but over a much wider area (Figure 7).

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Figure 7. Rainfall dispersion (quintiles) for 124 gauges in Rumania, 30 April–15 May 1970.

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The nature of the storms

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

The character and type of storm varied both in time and space. Despite a rather chaotic pattern, certain distinct storm types were observed (Bodolai and Body, 1971).

Type 1. A general area of intense rain near the centre of the depression where individual storm cells were absent. Such a zone may well have existed over the upper basin of the Someş as the moist unstable southerly flow confronted the slopes of the Carpathians.

Type 2. Distinct storm lines mostly within the warmer air where the inflow/outflow pattern strongly suggests an overturning mechanism similar to that described by Pedgley (1965) and Browning (1968).

Type 3. Storm clusters of an unorganized character.

Type 4. Storms associated with the cold front where low level convergence was com-pensated by divergence in the upper air flow allowing the re-generation of storms.

Amounts of rainfall varied considerably. Many stations in the north-west received over 25mm, whilst Toplita and Bistrita recorded 86 and 89mm on the 12th. The general rainfall pattern is shown in Figure 8.

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Figure 8. Depth of precipitation, 11–14 May 1970.

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Subsequent rainfall

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

Further general rain from trailing fronts affected Rumania during 21–23 May and 31 May–2 June. The disastrously high water levels on the Someş – Tisza were maintained. Severe flooding was renewed as rain fell on six consecutive days in early June over north-eastern Hungary due to a stagnating depression. This final wet spell culminated in a second major flood wave on the Tisza caused by lines of thunderstorms within warm air during 9–12 June. In mid-June the circulation pattern changed and the long period of storm activity came to an end.

Final remarks

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References

Almost certainly this was the worst natural disaster to befall Rumania in recorded history. It is interesting to speculate about the effect that the unusual weather pattern in October 1969 may have had on the course of winter weather in the Mediterranean and south-east Europe. The long succession of depressions forming, passing through or stagnating over the Hungarian Plains and Transyl-vania along with the continual drawing in of warm, moist Mediterranean air from January to June caused excessive precipitation over Rumania and eastern Hungary. Melting snows and torrential thunderstorms were the immediate causes of the notable floods of 12 and 13 May.

References

  1. Top of page
  2. The position of Rumania
  3. The drainage pattern
  4. Weather preceding May 1970
  5. The Rumanian winter and early spring of 1970
  6. Winter snowfall
  7. Spring weather 1970
  8. The mid-May floods
  9. The nature of the storms
  10. Subsequent rainfall
  11. Final remarks
  12. References
  • Bodolai EJ, Body K. 1971. Synoptic and hydrometeorological description of the flood waves on the Tisza river in 1970. VIth Conference of the Danube countries on hydrological forecasting. Cent. Met. Inst., Publ., Budapest.
  • Boucher K. 1972. The Rumanian Flood Disaster of May 1970. Weather 27: 5562.
  • Browning KA. 1968. The organization of severe local storms. Weather 23(10): 429434.
  • Pedgley DE. 1965. The Dover storm of 14 October 1964. Weather 20(11): 351356.
  • Winstanley D. 1970. The North African flood disaster of September 1969. Weather 25(9): 390403.