Hydrology of and

The Maritsa/Meric River is the biggest river on the Balkan peninsular. Maritsa catchment is densely populated, highly industrialized and has intensive agriculture. The biggest cities are on the Bulgarian territory, with 650.000 citizens, and on the Turkish territory, with 231 000 citizens. The biggest tributaries are Tundzha and Rivers, joining Maritsa at Edirne. Within Maritza and Tundja basin, a significant number of reservoirs and cascades were constructed for irrigation purposes, and for Hydro electricity production.

The climatic and geographical characteristics of Maritsa and Tundja River Basins lead to specific run-off conditions: flash floods, high inter-annual variability, heavy soil erosion reducing the reservoirs' capacities through sedimentation, etc. The destructive forces of climatic hazards, manifesting themselves in the form of rainstorms, severe thunderstorms, intensive snowmelt, floods and droughts, appear to increase during recent years. After more than 20 years of relative minor floods during wet seasons, large floods started to occur more often since the end of the 90’s. Theses years of absence of large floods resulted in negligence of political action and financial investment for structural and non-structural flood mitigation measures and maintenance of the river bed and its embankments.

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The morphology of both river systems is somewhat similar: Maritza flows between Pazarjik to in a large flat plain where flood expansion would be very large if no dike contained the flows. However these dikes are not very well maintained out of the main cities, which actually protect them in a certain way ; In 2005 the large inundation upstream Plovdiv certainly prevented Maritza from overtopping the dikes in the city ! Downstream Parvomay, the relief becomes more hilly and the flood plains are more reduced in size until the Greek-Turkish borders where the plain becomes again wide and flat. On Maritza in , the two main risks concern Plovdiv and where thousand of people live in houses settled in flood prone areas behind dikes which may be overtopped or bypassed.

The most severe floods in the last years occurred in August 2005 along the Bulgarian part of Maritsa River and March 2006 along the Bulgarian part of Tundja River and the Turkish part of Maritsa. In November 2007 a large flood occurred too in the Bulgarian–Turkish border in Maritsa. The satellite image to the left shows the inundated areas during the flood of March 2006.

The Tundja Flood plain is also very large downstream Banya at the outflow of the Jrebchevo Dam but with a marked slope down to Mutchuritza confluence. From to the flood plain is very large and very flat and Tundja river, which has been elsewhere channelised, is let free to meander within large forested areas until Elhovo city. Downstream Elhovo the river goes sloppy again but through an unpopulated hilly relief which constraint its discharges in narrow cross sections down to the Turkish border. In the river gets again into a large and flat flood plain where it meanders (though it has been channelised) until Edirne where it joins Maritza. Yambol city is theoretically protected by high dikes which can handle 800 m3/s discharges. Elhovo downstream is much less protected and may be flooded in some areas with less than 200 m3/s. New settlements in flood prone areas have increased the socio-economic risks in both cities.

1 Some Historical Elements

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The following graph of flood records at show that before the 60’s - – the flood discharges look higher than after, when most dams and reservoir were constructed. The period between 60’s and 80’s is characterized by relatively high flood activities, but the higher discharges look lowering down. several flood events occurred, however with discharges above 1200 m3/s. After 1984 there is a period of about 14 consecutive years with hardly any significant flood events. Floods start again to show after 1998 with a climax period in 2005,2006 and 2007.

On Tundja river, the time series are less deep in time. So we cannot process the same analysis. The morphology of Tundja flood plain indicates that very large floods may have happened in the past along the river. In all cases, however, the construction of the Jrebchevo Dam in the early 1970s modified dramatically the river regime as it cut for Yambol city more than half of the watershed, and obviously the most rainy one. For Elhovo city the watershed reduction will amount to a bit less than a half .

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2 Meteorological Events Leading to Floods

Abundant precipitations are the primary event leading to floods. They either come in the form of rain over moist soil which runs off or come in the form of snow which may, in some cases, melt rapidly due to exceptional warming. In all cases heavy precipitations are associated with the development and evolution of Mediterranean cyclones in the vicinity of the Balkan Peninsula. Basically the Maritza basin faces four types of Floods:

 Winter floods (December to mid February): They are generally due to a single or more likely a series of Mediterranean cyclones propagating Eastward from across the Balkan peninsula. Such situations were the primary driving force is the dynamic of the athmosphere, produce intensive repetitive rain throughout the basin and result in large and long floods in the lower Maritza.  End of winter early spring floods: Their genesis is the same as above but with sudden air warming generally associated with large polar front waves generating meridional circulation. They are often worsened by the effect of snow melting due to a rapid advective warming over a well prepared land covered with a relatively thick fresh snow at low altitude.  Spring and early summer floods (later if rainy summers): Convective precipitation along the cold fronts of slowly moving cyclones (Eastward) re- alimented by soil moisture. These type of precipitations will touch the upper basins of both Tundja and Maritza and may produce large flash floods in these

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areas. Here the convection is the primary driving force for the formation of precipitations.  Fall season floods: They are generated by slowly moving cyclones, which centres follow the Balkan coasts then move eventually to the . Convective precipitations are alimented by the warm sea water from the Mediterranean then the black sea. The precipitations will touch mostly the lower parts of Maritza and Tundja.

Typical Winter flood:

A cut-off cyclonic system makes for a series of smaller Mediterranean cyclones which move through the country from the south to the north (type C2). There are a couple of days of strong precipitations Typical spring early summer situation: 09-22 May 1998

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The moisture comes either from the Aegean or the Black Sea. The mountainous regions are more likely to suffer heavy convective precipitations in late spring. In this occasion it is the region of Strandja, and Rodopi where the biggest precipitation amounts are registered on 11 May. The reason is the stationary paths of MCSs. Many of them end-up going through southeast Bulgaria. Precipitations as high as 70mm/24h are registered in the region of on 11.05 and as high as 80- 140mm on 11-12.05 in Rhodopi. A second system cyclonic develops to the west of the and then moves towards where it stays a couple of days around 17-19 May. It is the type of the summer flood systems and same dynamics apply though the registered precipitations are less important than those on 11-12 May.

2.1 Flood Seasonality

The following figure presents the monthly cumulative occurrence of floods on Maritza and Tundja. Only the annual maxima are considered. The annual distribution shows that winter floods (cold season, with or without snow melting) are largely predominant to convective types of floods, for moderate as well as for large floods. Note nonetheless that the worst floods during the last 8 years are very untypical: The most severe floods accured in August 2005 (upper part of Maritza) and Novembre 2007 (lower part of Maritza and Turkey): these months showed no occurrence of large floods before. One of the questions which may rise from these recent events is about the impact of the climate change on the intensity and seasonality of the floods.

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2.2 Flood Frequencies

We performed numerous statistical analysis on rains and discharges records at most of the hydro-meteorological stations. The data were provided mainly by NIMH. The technique is to fit the observed records along statistical laws which are known to well represent the probability of extreme hydrological events. We show hereafter some of the statistical adjustment we made with several of these statistical laws using dedicated software from HKV consultants. Flood probability of occurrence on Maritza

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Flood probability od occurrence on Tundja

The raw results from our statistical analysis have been then re-analysed in order to have a coherent set of values between the different stations, reflecting the physical processes and the behaviour of the different tributaries on which we performed also such analysis. More specifically we corrected some incoherences on discharges data probably due to some approximations in the used rating curves at stations, and we accounted for the dam and reservoirs influences on statistics. The very large number of reservoirs and dams, throughout the watershed, has modified the statistics of floods for the recent years as the most frequent floods are now much dampened by the water retention accumulated capacity within these reservoirs. However this dampening, which influences the statistics of floods up to a certain return period, would not hold for the most extreme floods (over 100 years return period) as most of the small retention dams would be overtopped for such extreme events. The final results of our analysis are:

Return Period 5y 10y 20y 50y 100y 1000y

Belovo (*) 100 150 180 250 400 700

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Pazardjik 200 300 450 650 900 1600

Plovdiv 400 600 750 950 1500 2400

Parvomay (*) 600 780 950 1200 1800 2700

Harmanli 950 1200 1400 1650 2400 3350

Svilengrad (*) 1000 1250 1480 1700 2540 3650

Return Period 5y 10y 20y 50y 100y 1000y

Banya (*) 50 75 100 120 130 1000

Yambol 125 150 200 250 300 800

Elhovo 150 170 200 240 270 600

Parvomay (*) 600 780 950 1200 1800 2700

The same type of analysis was also performed on the tributaries of both rivers Because of the variability of the basins in surface and shape and because of the too short period of data samples the statistical results were quite incoherent between each other. So we had to make a large cross coherence analysis between the basin Area, their morphology, the observed data and the statistical parameters by optimising the function:

f Qn = Cn x Km x Area

Where : Qn = discharge with n % probability ; (1/n = return period)

Q5 = 1/ 20 years ; Q1 = 1/100 year probability Q0,1 = 1/1000 year probability

Cn takes values depending on n only

Km takes values depending on the hypsometry of the basin only f : varies between [0,5 and 0,6] depending on land cover, hypsometry and shape…

We actually defined only two different classes of tributaries with two different values for Km and f

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Our final result for the tributaries are given in the following table : Station Name Area Q5% Q1% Q0,1% Sq km

71210 river Mati Vir – village 411 139 264 486 Mirovo

71380 r. Yadenica – village 138 72 137 252 Goliamo Belovo

71420 Chepinska river - Маrko 977 233 443 817 Nikolovo

71480 Toponitza river – village 947 229 435 802 Poibrene

71550 Luda Yana river – village 674 187 355 654 Sbor

71700 river Maritza - Belovo 750 199 378 697

72420 Parvenecka river – village 236 99 189 348 Hrabrino

72460 Chepelarska river – village 830 212 402 741 Bachkovo

73030 r.Chinardere - v.Dalbok izvor 130 70 132 243

73370 Banska river – village 333 122 232 428 Dobrich

73480 Sazliika river - Galabovo (F) 3121 255 485 893

73550 Harmanliska river - Harmanly 963 231 440 810

74200 r.Kalnica - v.Krumovo (F) 450 85 161 296

74440 r.Belenska - g.Chumerna 371 131 248 457

74500 Mochuritza river–village 1125 143 271 499

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Vodenichane (F)

74580 r.Sinapovska - v.Sinapovo (F) 868 123 234 431

(F) : Not steep Basin (others are considered as steep)

Source : http://maritsa.meteo.bg/apache2- default/maritsa/static/about.php?infoto=hydro

Technical Assistance for Flood Forecasting and Early Warning System under PHARE project "Capacity Improvement for Flood Forecasting in the BG- TR CBC Region".

This project is part of the more global European PHARE project "Capacity Improvement for Flood Forecasting in the Bulgarian-Turkey Cross Border Cooperation Region".

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