Exceptional floods in the Prut basin, Romania, in the context of heavy rains in the summer of 2010

Abstract

The year 2010 was characterized by devastating flooding in central and eastern Europe, including Romania, the Czech Republic, Slovakia, and Bosnia-Herzegovina. This study focuses on floods that occurred during the summer of 2010 in the Prut River basin, which has a high percentage of hydrotechnical infrastructure. Strong floods occurred in eastern Romania on the Prut River, which borders the Republic of Moldova and Ukraine, and the Siret River. Atmospheric instability from 21 June to 1 July 2010 caused remarkable amounts of rain, with rates of 51.2mm/50min and 42.0mm/30min. In the middle Prut basin, there are numerous ponds that help mitigate floods as well as provide water for animals, irrigation, and so forth. The peak discharge of the Prut River during the summer of 2010 was 2310m³s^-1 at the Radauti-Prut gauging station. High discharges were also recorded on downstream tributaries, including the Baseu, Jijia, and Miletin. High discharges downstream occurred because of water from the middle basin and the backwater from the Danube (a historic discharge of 16300m³s^-1). The floods that occurred in the Prut basin in the summer of 2010 could not be controlled completely because the discharges far exceeded foreseen values.

Nat. Hazards Earth Syst. Sci., 17, 381396, 2017 www.nat-hazards-earth-syst-sci.net/17/381/2017/ doi:10.5194/nhess-17-381-2017 Author(s) 2017. CC Attribution 3.0 License.

Gheorghe Romanescu and Cristian Constantin Stoleriu

Faculty of Geography and Geology, Department of Geography, Alexandru Ioan Cuza University of Iasi, Bd. Carol I, 20 A, 700505 Iasi, Romania

Correspondence to: Gheorghe Romanescu ([email protected])

Received: 30 August 2016 Discussion started: 12 September 2016

Revised: 4 January 2017 Accepted: 13 February 2017 Published: 10 March 2017

Abstract. The year 2010 was characterized by devastating ooding in central and eastern Europe, including Romania, the Czech Republic, Slovakia, and Bosnia-Herzegovina. This study focuses on oods that occurred during the summer of 2010 in the Prut River basin, which has a high percentage of hydrotechnical infrastructure. Strong oods occurred in eastern Romania on the Prut River, which borders the Republic of Moldova and Ukraine, and the Siret River. Atmospheric instability from 21 June to 1 July 2010 caused remarkable amounts of rain, with rates of 51.2 mm/50 min and42.0 mm/30 min. In the middle Prut basin, there are numerous ponds that help mitigate oods as well as provide water for animals, irrigation, and so forth. The peak discharge of the Prut River during the summer of 2010 was 2310 m3 s1 at the Radauti-Prut gauging station. High discharges were also recorded on downstream tributaries, including the Baseu, Jijia, and Miletin. High discharges downstream occurred because of water from the middle basin and the backwater from the Danube (a historic discharge of 16 300 m3 s1). The oods that occurred in the Prut basin in the summer of 2010 could not be controlled completely because the discharges far exceeded foreseen values.

1 Introduction

Catastrophic oods occurred during the summer of 2010 in central and eastern Europe. Strong ooding usually occurs at the end of spring and the beginning of summer. Among the most heavily affected countries were Poland, Romania, the Czech Republic, Austria, Germany, Slovakia, Hungary, Ukraine, Serbia, Slovenia, Croatia, Bosnia and Herzegov-

Exceptional oods in the Prut basin, Romania, in the context of heavy rains in the summer of 2010

ina, and Montenegro (Bissolli et al., 2011; Szalinska et al., 2014) (Fig. 1). The strongest oods from 2010 were registered in the Danube basin (see Table 1). For Romania, we underlined the oods from the basins of Prut, Siret, Moldova, and Bistrita rivers. The most devastating oods in Romania occurred in Moldavia (Prut, Siret) and Transylvania (Tisa, Somes, Tarnave, Olt). The most deaths were recorded in Poland (25), Romania (six on the Buhai River, a tributary of the Jijia), Slovakia (three), Serbia (two), Hungary (two), and the Czech Republic (two) (Romanescu and Stoleriu, 2013a, b).

Floods are one of the most important natural hazards in

Europe (Thieken et al., 2016) and on earth (Merz et al., 2010; Riegger et al., 2009). They generate major losses of human lives and also property damage (Wijkman and Timberlake, 1984). For this reason, they have been subject to intense research, and signicant funds have been allocated to mitigating or stopping them. According to Merz et al. (2010) the European Flood Directive on the assessment and management of ood risks (European Commission, 2007) requires developing management plans for areas with signicant ood risk (at a river basin scale), focusing on the reduction of the probability of ooding and on the potential consequences to human health, the environment and economic activity.(p. 511). Several studies investigated catastrophic oods or the oods that generated signicant damage. They focused on the statistical distribution of the maximum annual discharge using GEV and the links with the basin geology (Ahilan et al., 2012), climate change impacts on oods (Aleri et al., 2015; Detrembleurs et al., 2015; Schneider et al., 2013; Whiteld, 2012), disastrous effects on infrastructures such as transportation infrastructures and their interdependence

Published by Copernicus Publications on behalf of the European Geosciences Union.

382 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

Figure 1. The Danube catchment and the location of the most important oods that occurred from May to June 2010.

(Berariu et al., 2015), historical oods (Blschl et al., 2013;

Strupczewski et al., 2014; Vasileski and Radevski, 2014) and their links to heavy rainfall (Bostan et al., 2009; Diakakis, 2011; Prudhomme and Genevier, 2011; Rets, 2015), the public perception of ood risks (Brilly and Polic, 2005; Feldman et al., 2016; Rufat et al., 2015), land use changes and ooding (Cammerer et al., 2012), the evolution of natural risks (Hufschmidt et al., 2005), geomorphological effects of oods in riverbeds (Lichter and Klein, 2011; Lczy and Gyenizse, 2011; Lczy et al., 2009, 2014; Reza Ghanbarpour et al., 2014; Romanescu and Nicu, 2014), the spatial distribution of oods (de Moel et al., 2009; Parker and Fordham, 1996), and the interrelation between snow and ooding (Revuelto et al., 2013).

The Prut catchment basin spans three topographic levels: mountains, plateaus, and plains. The surface and underground water supply to the Prut varies by region and is extremely inuenced by climatic conditions. This study underscores the role played by local heavy rains in the occurrence of oods, as well as the importance of ponds, mainly the Stnca-Costesti reservoir, in the mitigation of backwaters. We also analyse the local contribution of each catchment basin on the right side of the Prut to the occurrence of the exceptional oods in the summer of 2010. Finally, we consider the upstream discharge and its inuence on the lower reaches of the Prut.

2 Study area

The Prut Rivers catchment is situated in the north-eastern

Danube basin. It is surrounded by several other catchments: the Tisa to the north-east (which spans Ukraine, Romania, and Hungary), the Siret to the west (which is partially in Ukraine), and the Dniestr (in the Republic of Moldova) to the north-east. The Prut catchment occupies eastern Romania and the western part of the Republic of Moldova (Fig. 2). The Prut River begins in the Carpathian Mountains in Ukraine and empties into the Danube near the city of Galati. The catchment measures 27 500 km2, of which 10 967 km2 lies in Romania (occupying approximately 4.6 % of the surface of Romania).

The Prut River is the second longest river in Romania, at 952.9 km in length. It is a cross-border river, with 31 km in Ukraine and 711 km in the Republic of Moldova. The mean altitude of the midstream sector of catchment area is 130 m, and for the downstream sector it is 2 m. The Prut has 248 tributaries. Its maximum width is 12 km (in the lower reaches, Brates Lake) and its average slope is 0.2 %. Its hydrographic network measures 11 000 km in total, of which 3000 km are permanent streams (33 %) and 8000 km are intermittent (67 %). The network has the highest density in Romania at0.41 km km2 (the average density is 0.33 km km2).

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G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania 383

Table 1. Overview of main ood events for the Danube River basin in 2010, as forecasted by EFAS and/or reported in international online news media (ICPDR, 2010).

From To River basin Country EFAS Date FAS Conrmed? Comment (dd.mm) (dd.mm) Affected Affected Alert sent? Alert sent

20.II 4.III Sava HR/RS Yes(Flood Watch)

24 Feb Yes Severe ooding in central and E Serbia, and in Sava and Morava river systems.

21.II 28.II Velika Morava RS Yes(Flood watch)

16 Feb Yes Severe ooding in eastern Serbia

16 Feb No (No reports found on online news media). Events to be conrmed by partners in next annual EFAS meeting

Feb Feb Koeroes RO/HU Yes

(Flood watch)

1.III 5.III Danube RO/BG Yes(Flood alert)

3 Mar Yes Severe ooding in S Romania and in NW and N Bulgaria.

March March Somes/Mures/Koeroes

RO/HU Yes

(Flood alert)

18 Mar No No reports found on online news media. Events to be conrmed by partners in next annual EFAS meeting

15.V 30.V Danube/Oder SK/PL/CZ/HU Yes(Flood alert)

12 May Yes Extensive ooding in central and eastern Europe, esp. Poland, Czech Republic, Slovakia, Hungary, and Serbia.

Late June July Siret/Prut/ Moldova/

Bistrita

RO/MD No Yes Severe ooding in NE Romania kills 25 people, also some in counties in Moldova.

15.VII 15.VII Prut/Olt RO Yes(Flood alert)

7 July Yes Maximum ood alert on Prut River in E Romania, along border with Moldova.

17.IX 19.IX Sava/Soca HR/SL Yes(Flood alert)

18 Sep Yes Severe ooding in Slovenia kills three people. Croatia also affected.

29 Nov Yes Severe ooding in Bosnia, Serbia, and Montenegro, with river Drina at highest level in 100 years.

Late Nov Early Dec Drina RS Yes

(Flood alert)

5 Dec Yes Heavy rain causes devastating ooding in the Balkans, esp.Bosnia and Herzegovina, Croatia, Montenegro, and Serbia.

9.XII 9.XII Tisza HU/RS No Yes Snowmelt and swollen rivers ood 3000 km2 of arable land, esp. near Szeged, on Tisza River in SE Hungary.

Dec Dec Koeroes HU/RO Yes

(Flood alert)

3.XII 8.XII Sava HR Yes(Flood alert)

3 Dec No No reports found on online news media. Event to be conrmed by local authorities in annual EFAS meeting

The Prut catchment is relatively symmetrical, but its largest proportion is in Romania. To the west, it has 27 tributaries, including the Poiana, Cornesti, Isnovat, Radauti, Volovat, Baseu, Jijia (with a discharge of 10 m3 s1, the most important), Mosna, Elan, Oancea, Branesti, and Chineja. The Jijia River is 275 km long, has a catchment area of 5757 km2 and an annual average ow of 14 m3 s1. Its most important

tributaries are Miletin, Sitna, and Bahlui. To the east, it has 32 tributaries, including the Telenaia, Larga, Vilia, Lopatnic, Racovetul, Ciugurlui, Kamenka, Garla Mare, Frasinul, and Mirnova (Romanescu et al., 2011a, b). The catchment basin has 225 small ponds, counting the Dracsani, which is the largest pond in Romania. Small ponds are used as drinking water for livestock or to irrigate rural subsistence households.

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384 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

Figure 2. Geographic position of the Prut catchment basin in Romania, Ukraine, and the Republic of Moldova, and distribution of the main gauging stations.

They usually belong to individual households. Large ponds, on the other hand, have multiple uses, such as ooding mitigation (such as Ezer dam, located in Jijia River basin, which was built to protect the town of Dorohoi from ood), irrigation, sh farming, etc. They were more effective over time because of their signicant surface and depth. Large ponds belong to rural or urban communities. The river also has 26 large ponds, of which the most important is the Stnca-Costesti reservoir, which has the largest water volume of the interior rivers in Romania (1400 million m3).

The topography of the Prut basin includes the Carpathians in the spring area and the Moldavian Plateau and the Romanian Plain near the river mouth. Arable land occupies 54.7 % of the Prut catchment, while forests occupy 21.4 %, perennial cultures occupy another 13.3 %, and the water surface occupies only 1.19 %. The mean annual temperature in the Prut catchment is 9 C, and the mean annual precipitation is 550 mm. The mean annual discharge increases downstream, varying from 82 m3 s1 at Radauti-Prut to 86.7 m3 s1 at

Ungheni to 93.8 m3 s1 at the Oancea gauging station situated near the mouth over the period 19502008.

Discharges in the downstream reaches of the Prut are controlled by the Stnca-Costesti reservoir. In the Romanian Register of Large Dams, the Stnca-Costesti Dam ranks 49th out of 246 dams in terms of height, but second in terms of active reservoir volume (1400 million m3, after the Iron Gates I, with a volume of 2100 million m3). It has a surface area of 5900 ha during a normal retention level (NRL). After con-

struction of the Stnca-Costesti reservoir, oods on the Romanian parts of the Prut diminished considerably. Because the Prut has higher banks in the Republic of Moldova, this area was not affected by dam construction. The reservoir was constructed with a mitigation level of 550 million m3, allowing the mitigation of a 1 % probability ood from 2940 to 700 m3 s1. The damming infrastructure constructed downstream from the hydrotechnical nodes prevents the ooding of approximately 100 000 ha of oodplain area (Romanescu et al., 2011a, b).

3 Methodology

Diverse methodology has been used to analyse exceptional oods. Hydrological data, including discharge and the water level, were obtained from the Prut-Barlad Water Basin Administration based in Iasi (a branch of the Romanian Waters National Administration). For catchment basins that did not have gauging stations or observation points, measurements were taken to estimate the discharge. Mathematical methods were used to reconstitute discharges and terrain measurements using land surveying equipment (Leica Total Station) to calculate the surface of the stream cross section. Most stations within the Romanian portion of the Prut catchment are automatic (Fig. 3). The recording and analysing methodology used is standard or slightly adapted to local conditions, e.g. the inuence of physicalgeographical parameters on run-off (Ali et al., 2012; Kappes et al., 2012;

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G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania 385

Figure 3. Main tributaries, reservoirs (left), and gauging stations (right) in the Prut River basin.

Kourgialas et al., 2012; Waylen and Laporte, 1999), the management of risk situations (Iosub et al., 2014; Delli-Priscoli and Stakhiv, 2015; Demeritt et al., 2013; Grobicki et al., 2015), the role of reservoirs in ood mitigating (Fu et al., 2014; Serban et al., 2004; Sorocovschi, 2011), the probability of ooding and the changes in the run-off regime (Hall et al., 2004, 2014; Jones, 2011; Seidu et al., 2012a, b; Wu et al., 2011), ood prevention (Hapuarachchi et al., 2011), run-off and stream ow indices (Nguimalet and Ndjendole, 2008), morphologic changes of riverbeds or lake basins (Rusnk and Lehotsky, 2014; Touchart et al., 2012; Verdu et al., 2014), etc. The cartographic basis used to map altitudes and slopes is Shuttle Radar Topography Mission (Global Land Cover Facility, 2016), at a 1 : 50 000 scale. The vector layers were projected within a geodatabase, using ArcGis 10.1. They include stream lines, subcatchment basins, and reservoirs and ponds polygons, as well as gauging station points. In order to generate the GIS layers, we applied the following methods: digitization, queries, conversion, geometries calculation (length, surface), and spatial modelling. Water levels and discharges data were processed and plotted on charts using Open Ofce software. We also used the Inkscape software to design the nal maps and images.

All areas with gauging stations had automatic rain gauges (Anghel et al., 2011; Tirnovan et al., 2014a, b) (Fig. 3, Table 2). The heavy rains that cause ooding are recorded hourly over the course of 24 h according to the Berg intensity scale (Berg et al., 2009). In the areas lacking gauging stations, data were collected from the closest meteorological stations, which are automatic and form part of the national monitoring system. The water level and discharge were

analysed throughout the entire ood period. For comparison, the mean monthly and annual data for the water level and discharge were also analysed. The processed data were portrayed as histograms that illustrate the evolution of water levels during the oods, including the CA (warning level), CI (ood level), and CP (danger level) ood threshold levels before and after the ood, the daily and monthly runoff, and the hourly variations of run-off during the backwater. For an exact assessment of the damage and the ooded surface area, observations and eld measurements were conducted on the major oodplains of the Volovat, Baseu, Jijia, Sitna, Miletin, Bahluet, Bahlui, Elan, and Chineja rivers (Romanescu et al., 2012; Romanescu and Stoleriu, 2013b; Mihu-Pintilie and Romanescu, 2011).

Nine gauging stations exist in Romanian sections of the

Prut River: Oroftiana (near the entry, only including water level measurements), Radauti-Prut, Stanca Aval (downstream), Ungheni, Prisacani, Drnceni, Falciu, Oancea, and Sivita (which is directly inuenced by the Danube, so no data were collected from this station) (Fig. 3, Table 2). The rst gauging station was installed at Ungheni in 1914, and the newest station is Sivita, which was installed in 1978. Much older water level and discharge data are available from stations in other places. The data on the deviation of rainfall quantities were obtained from the Climate Prediction Center NOAA and from the scientic literature (Hustiu, 2011).

Flood damage reports were collected from city halls in the

Prut catchment and the Inspectorate for emergencies in Botosani, Iasi, Vaslui, and Galati. In isolated areas, we conducted our own eld research. We note that some of the reports from city halls seem exaggerated.

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386 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

Table 2. Morphometric data for the gauging stations on the Prut River (Romania).

River length

Inauguration from the 0 m level of Gauging station year Geographic coordinates conuence Data on the catchment basin gauging station

mrBS

Latitude Longitude km Surface km2 Altitude m (Metres Black Sea)

Oroftiana 1976 48 11[prime]12[prime][prime] 26 21[prime]04[prime][prime] 714 8020 579 123.47 Radauti-Prut 1976 48 14[prime]55[prime][prime] 26 48[prime]14[prime][prime] 652 9074 529 101.87

Stanca Aval 1978 47 47[prime]00[prime][prime] 27 16[prime]00[prime][prime] 554 12 000 480 62.00 (downstream)

Ungheni 1914 47 11[prime]04[prime][prime] 27 48[prime]28[prime][prime] 387 15 620 361 31.41 Prisacani 1976 47 05[prime]19[prime][prime] 27 53[prime]38[prime][prime] 357 21 300 374 28.08

Drnceni 1915 46 48[prime]45[prime][prime] 28 08[prime]04[prime][prime] 284 22 367 310 18.65 Falciu 1927 46 18[prime]52[prime][prime] 28 09[prime]13[prime][prime] 212 25 095 290 10.04

Oancea 1928 45 53[prime]37[prime][prime] 28 03[prime]04[prime][prime] 88 26 874 279 6.30 Sivita 1978 45 37[prime]10[prime][prime] 28 05[prime]23[prime][prime] 30 27 268 275 1.66

4 Results

The majority of oods in Romania are inuenced by climate factors, manifesting at local and European levels (Andrei et al., 2011; Birsan, 2015; Birsan and Dumitrescu, 2014; Birsan et al., 2012; Chendes et al., 2015; Corduneanu et al., 2016). During the last decade of June (20 June 2010) and the end of July (30 July 2010), a baroclinic area was localized in northern Moldavia. This favoured the formation of a convergent area of humidity. In this case, a layer of humid, warm, and unstable air was installed between the surface and 2500 m in altitude. The high quantity of humidity originated from the Black Sea, situated 500 km away. The warm air was generated in the Russian Plain, overheated by a strong continentality climate. The cold air from the mid-troposphere, inducted by the cut-off nucleus that generated atmospheric instability, overlapped this structure of the low troposphere (Hustiu, 2011). The synoptic context was disturbed by local physicalgeographical factors, especially by the orography of the Eastern Carpathians, which led to extremely powerful heavy rains, e.g. 100200 mm in 24 h at the sources of Jijia (representing the amount that normally falls during June and July) or 4060 mm in 24 h at the Romanian frontier with Ukraine and the Republic of Moldova. The quantity of rainfall in 24 h were 23 higher than the normal values for this period (Hustiu, 2011) (Fig. 4).

There were six main extremely rainy periods in Romania, especially in the Moldavian hydrological basins (Prut and Siret): 2123, 2526, 2830 June, 34, 67 and 9 July. Rainfall quantities recorded in June were higher. The ash oods registered in northern Moldavia in 2829 June 2010 were generated by convective systems with slow spreading. Even if the rainfalls from 29 June were lower, the oods had devastating effects because they occurred in the context of the increasing water levels from 28 June 2010. The convection was organized by a mesocyclone extended over northern

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Figure 4. Cumulative precipitation for MayJuly (2010) interval, divided by normal precipitation Climate Prediction Center (source data from NOAA).

G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania 387

Figure 5. Cumulative precipitation amounts, in north-eastern part of Romania, from 21 to 27 June 2010 (left) and 28 June to 1 July 2010 (right).

Moldavia (the departments of Suceava and Botosani) (Hustiu, 2011).

Backwaters in the upper basins of the Prut and Siret (in north-eastern Romania) recorded during the summer of 2010 were caused by atmospheric instability from 21 June to 1 July 2010. At this time, the ood danger level (CP) was exceeded on the Prut and Jijia rivers. High amounts of rain fell during three periods: 2124, 2627, and 28 June 1 July 2010. Precipitation exceeding 100 mm was recorded from 21 to 24 June (105 mm, at the Oroftiana station) and from 28 June to 1 July 2010 (206 mm at Padureni and 110 mm at Pomarla on the Buhai River). Very high rainfall rates occurred within a brief time frame: 51.5 mm/50 min. was recorded at Oroftiana station on the Prut River and42.0 mm/30 min. at Padureni on the Buhai River (Romanescu and Stoleriu, 2013a, b; Tirnovan et al., 2014b) (Fig. 5).

Precipitation in the Carpathian Mountains in Ukraine initiated a series of oods in the upper Prut basin. Among the ve ood peaks recorded by the Cernauti gauging station, we noted one with a discharge of 2070 m3 s1 recorded on 9 July 2010 at 12:00 (UTC + 2). In comparison, an

other ood recorded in May did not have a very high discharge value (308 m3 s1). In the mountainous sector, the ood warning level (CA) was exceeded only twice, with water levels of 523 cm (+25 cm CA) and 645 cm (+145 cm CA)

(Fig. 6).

At the Oroftiana gauging station, where only the water levels are measured, the ood danger level (CP) was exceeded four times, with levels of 716 cm (+66 cm CP), 743 cm

(+93 cm CP), 736 cm (+86 cm CP), and 797 cm (+147 cm

CP, on 9 July 2010 at 12:00). The ood warning level (CA) was exceeded throughout the entire ooding period (May July 2010). In the month of May, the ood levels (CI) were not exceeded (Fig. 6). At the Oroftiana gauging station, one registered solely the water levels data. For all the other gaug-

Figure 6. Water levels and discharge on the Prut River at the gauging stations of Cernauti, Oroftiana, Radauti-Prut, Stanca Aval (downstream), Ungheni, Prisacani, Drnceni, Falciu, and Oancea during the summer of 2010.

ing stations the discharge data are being registered in addition to water level.

At the Radauti-Prut gauging station, three important peaks were recorded on 26, 29 June2 July 2010, and 1011 July 2010. A maximum discharge of 2310 m3 s1 was registered on 10 July 2010 at 9 pm. The ood danger level (CP) was exceeded four times, with water levels of 643 cm (+43 cm CP, on 25 June 2010), 685 cm (+85 cm

CP, on 29 June 2010), 721 cm (+121 cm CP, on 29 June

2 July 2010), and 744 cm (+144 cm CP, on 1011 July 2010)

(Fig. 6).

The Stanca Aval (downstream) gauging station is controlled by overow from the Stnca-Costesti reservoir. This control mitigates the ood hydrographs. The maximum dis-

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388 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

charge value at this station was 885 m3 s1 on 3 July 2010.

The ood level (CI) was exceeded from the beginning to the end of the ooding period. The ood danger level (CP) was exceeded from 1 to 13 July 2010, reaching a maximum water level of 460 cm (+85 cm CP, on 3 July 2010) (Fig. 6).

At the Ungheni gauging station, oods were recorded throughout the entire month of July. The maximum discharge was 673 m3 s1 on 8 July 2010. Flooding continued until5 August 2010. The ood danger level (CP) was exceeded during the 12-day period from 6 to 17 July 2010. The maximum water level was 661 cm (+1 cm CP) (Fig. 6).

Floods were also recorded throughout July at the Prisacani gauging station. The maximum discharge was 886 m3 s1 on9 July 2010. Flooding continued until 5 August 2010. The ood danger level (CP) was exceeded during the 16-day period from 4 to 19 July 2010. The maximum water level was 673 cm (+73 cm CP) (Fig. 6).

At the Drnceni gauging station, oods were recorded over a long period from the end of June until the beginning of August. The maximum discharge was 718 m3 s1 on17 July 2010. The ood danger level (CP) was reached or exceeded during the 18-day period from 4 to 22 July 2010. The maximum water level was 729 cm (+29 cm CP) (Fig. 6).

At the Falciu gauging station, oods occurred throughout July and during the rst half of August. The maximum discharge was 722 m3 s1 on 19 July 2010. The ood danger level (CP) was reached or exceeded during the 35-day period from 6 July to 2 August 2010. The maximum water level was 655 cm (+55 cm CP) (Fig. 6).

At the Oancea gauging station, two backwaters were recorded in July and August. The rst backwaters on19 July 2010 had a peak discharge of 697 m3 s1 and the second on 27 July 2010 had a peak discharge of 581 m3 s1.

Both backwaters exceeded the ood danger level (CP) throughout the month of July. The maximum water level of the rst backwater was 683 cm (+83 cm CP), and the max

imum for the second was 646 cm (+46 cm CP) (Fig. 6).

Backwaters were caused by increasing water level of Danube River, which inuences the measurements results at the gauging stations situated on the downstream sector of Prut River.

The western tributaries of the Prut (within the Moldavian Plain) are numerous, but they have only modest mean annual discharges. They are periodically affected by oods following heavy summer rains. At the Stefanesti gauging station, within the downstream sector of the Baseu River, oods were recorded from 1 to 4 July 2010. The maximum discharge was 107 m3 s1 on 6 July 2010. The ood level (CI)

was reached or exceeded for 2 days. The maximum level was 355 cm (+5 cm CI) (Fig. 7). The Stefanesti gauging station is

located in the downstream sector of the dam and it is directly inuenced by the discharge water from the Stnca-Costesti Lake (since 1978).

At the Padureni gauging station on the Buhai River, two backwaters were recorded in June and a secondary backwater in May. The maximum discharge was 470 m3 s1 on

Figure 7. Water levels and discharge on the main Prut tributaries during the summer of 2010: the Baseu, Buhai, Sitna, Miletin, Bahlui, Magura, and Bahluiet rivers.

28 June 2010. The ood danger level was exceeded during both backwaters, with water levels of 470 cm (+120 cm CP,

on 28 June 2010) and 440 cm (+90 cm CP, on 29 June 2010)

(Figs. 3, 7).

At the Todireni gauging station on the Sitna River (a tributary of the Jijia), oods occurred from 1 to 4 July 2010. The maximum discharge was 19 m3 s1 on 1, 2, and 4 July 2010.

The ood level (CI) was exceeded on 1 and 2 July 2010. The maximum water level was 387 cm on 1 July 2010. The ood warning level (CA) was exceeded on 4 July 2010 (Figs. 3, 7).

At the Nicolae Balcescu gauging station on the Miletin River (a tributary of the Jijia), oods were recorded from 26 to 29 June 2010. The maximum discharge was 60 m3 s1 on6 June 2010. The ood level (CI) was exceeded just once, on 28 June 2010. The maximum level was 444 cm (+22 cm

CI). The warning level (CA) was exceeded throughout the ooding period (Figs. 3, 7).

At the Sipote gauging station on the Miletin, four backwaters were recorded from 22 June to 2 July 2010. The maximum discharge was 45 m3 s1 on 29 June 2010. The ood level (CI) was exceeded from 29 to 30 June 2010. The maximum water level was 269 cm (+19 cm CI). The warn

ing level (CA) was exceeded throughout the ooding period (Figs. 3, 7).

At the Halceni gauging station on the Miletin, oods were recorded from 28 June to 5 July 2010. The maximum discharge was 32 m3 s1 on 12 July 2010. The ood danger level (CP) was exceeded during the peak discharge period, with a water level of 302 cm (+2 cm CP). The ood level (CI)

was exceeded throughout the ooding period (Figs. 3, 7).

At the Carjoaia gauging station on the Magura River (a tributary of the Bahlui), one major backwater was recorded.The maximum discharge was 73.5 m3 s1 on 28 June 2010.

The ood level (CI) was exceeded on 28 June 2010. The maximum water level was 280 cm (+90 cm CI) (Figs. 3, 7).

At the Targu Frumos gauging station on the Bahluet (a tributary of the Bahlui), one major backwater was recorded on

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G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania 389

Figure 8. Water levels and discharge on the Jijia River at the gauging stations of Dangeni, Todireni, Andrieseni, Victoria, and Chiperesti during the summer of 2010.

22 May 2010, with a maximum discharge of 48 m3 s1. The ood danger level (CP) was reached on the same day and the maximum water level was 250 cm (0 cm CP). The ood warning level (CA) was exceeded throughout the ooding period (Figs. 3, 7).

At the Harlau gauging station on the Bahlui (a tributary of the Jijia), successive and increasing backwaters were recorded from 22 May to 1 July 2010. The maximum discharge was 32 m3 s1 on 29 June 2010. The ood level (CI)

was exceeded throughout the ooding period. The maximum water level was 552 cm (+132 cm CI) (Figs. 3, 7).

At the Iasi gauging station on the Bahlui, oods occurred from 24 June to 4 July 2010. The maximum discharge was 44 m3 s1 on 1 July 2010. The ood warning level (CA) was exceeded throughout the ood. The maximum water level was 286 cm (+86 cm CA) (Figs. 3, 7).

At the Holboca gauging station on the Bahlui, oods were recorded from 29 June to 17 July 2010. The maximum discharge was 50 m3 s1 on 29 June 2010. The warning level (CA) was reached or exceeded throughout the ooding period. The maximum water level was 259 cm (+59 cm CA)

(Figs. 3, 7).

At the Dorohoi gauging station on the Jijia, several backwaters were recorded from 21 May to 7 July 2010. The maximum discharge was 119 m3 s1 on 29 June 2010. The ood danger level (CP) was exceeded from 29 to 30 June 2010. The maximum water level was 760 cm (+160 cm CP). The

ood warning level (CA) was exceeded throughout the ooding period (Figs. 3, 8).

At the Dangeni gauging station on the Jijia, several backwaters were recorded from 22 May to 28 July 2010. The maximum discharge was 116 m3 s1 on 1 July 2010. The ood level (CI) was exceeded from 30 June to 3 July 2010. The maximum water level was 578 cm (+108 cm CI). The

ood warning level (CA) was exceeded throughout the ooding period (Figs. 3, 8).

Figure 9. Water levels and discharge on the Danube at the Capitanie AFDJ gauging station in the summer of 2010.

At the Todireni gauging station on the Jijia, ooding occurred from 30 June to 6 July 2010. The maximum discharge was 104 cm on 1 July 2010. The ood levels (CI) were exceeded from 1 to 4 July 2010. The maximum water level was 417 cm (+47 cm CI). The ood warning level (CA) was ex

ceeded throughout the ooding period (Figs. 3, 8).

At the Andrieseni gauging station on the Jijia, ooding was recorded from 14 July 2010. The maximum discharge was 148 m3 s1 on 2 July 2010. The ood danger level (CP) was exceeded on 2 and 3 July 2010. The maximum water level was 461 cm (+11 cm CP). The ood warn

ing level (CA) was exceeded throughout the ooding period (Figs. 3, 8).

At the Chiperesti gauging station on the Jijia, successive and increasing backwaters were recorded from 1 to19 July 2010. The maximum discharge was 136 m3 s1 on6 July 2010. The ood warning level (CA) was exceeded throughout the ooding period. The maximum water level was 497 cm (+97 cm CA) (Figs. 3, 8).

At the Victoria gauging station on the Jijia, ooding occurred from 4 to 7 July 2010. The peak discharge was 100 m3 s1 on 5 July 2010. The ood warning level (CA)

was exceeded throughout the ooding period. The maximum water level was 485 cm (+35 cm CA) (Figs. 3, 8).

At the Capitanie AFDJ gauging station on the Danube, record oods occurred. The maximum discharge was 16 300 m3 s1 on 56 July 2010, which is a historic discharge for the Galati station. The ood level (CI) was exceeded from26 June to 14 July 2010 (Fig. 9).

5 Discussion

Cumulative heavy rains from 21 to 24, 26 to 27, and 28 June to 1 July 2010 caused water levels to exceed the ood danger level (CP) by 40150 cm on the Prut in the Oroftiana-Radauti-Prut sector and by 30150 cm in the upper basin of the Jijia. The ood level (CI) was exceeded by 80 110 cm in the middle basin of the Jijia and in its tributaries (Sitna, Miletin, and Buhai). Discharges within the lower Ji-

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390 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

Figure 10. Distribution of subbasins within the Jijia catchment and placement of the main ponds.

jia basin were controlled by upstream reservoirs and downstream polders in the lower reaches of the Jijia.

The Oroftiana gauging station only records water level measurements. The Radauti-Prut gauging station may be inuenced by the water stored in the Stnca-Costesti reservoir (which occurred during the historic ood of 2008) (Romanescu et al., 2011a, b). The Stanca downstream gauging station may be inuenced by overow from the Stnca-Costesti reservoir. The Oancea gauging station, situated near the mouth of the Prut, may be inuenced by waters from the Danube. The water level registered at the Radauti-Prut gauging station could have been inuenced by the backwaters caused by Stnca-Costesti Lake. The most obvious case of backwaters was registered during the 2008 historic ood.

High discharge and water levels of 2310 m3 s1 and 744 cm (+144 cm CP), respectively, were recorded at the

Radauti-Prut gauging station. The 2010 values are remarkable lower than the maximum values recorded in 2008 of 7140 m3 s1 and 1130 cm (+530 cm CP) (the highest value

for Romanian rivers). This value was recalculated after 2 years (through recomposed discharges), resulting in a discharge of 4240 m3 s1, which is the second highest value in

Romania (after the historic discharge of 4650 m3 s1 on the

Siret in 2005) (Romanescu et al., 2011a, b). The existence of ve backwater peaks (with the second and third backwaters being weaker) clearly indicates that they were caused by heavy rains in the Carpathian Mountains in Ukraine. A volume of 200400 mm of rainfall (i.e. 5080 % of the annual amount) was recorded between 1 May and 15 July 2010. Dur-

ing the ood that manifested in 2008, a historic discharge value was registered for Prut River, but the bypassed water volume was low (upstream of Stnca-Costesti Dam) because the ood duration was short. The 2010 ood registered lower maximum discharges compared to 2008, but it bypassed a larger water volume as the ood lasted longer. The ood hydrographs recorded at the Stanca Aval (downstream) gauging station features attened and relatively uniform backwaters, mostly in the central part of the river. This behaviour is due to the inuence of Stnca-Costesti reservoir, which signicantly reduced the maximum discharge at Stanca Aval (885 m3 s1) compared to the Radauti-Prut gauging station upstream of the reservoir. The water level was maintained within the upper limit recorded by longitudinal protection dams.

The Ungheni, Prisacani, Drnceni, and Falciu gauging stations had a attened and uniform backwater, which signies upstream control, including some of the tributaries. The ood danger level (CP) was exceeded by a few centimetres and the oodplain was partially ooded in these areas. The high discharges recorded at the Prisacani station occurred because of waters in the upper Prut basin, including controlled spills from the Stnca-Costesti reservoir. Downstream of the Prisacani station, the inuence of the Jijia becomes obvious: it increases the water level and lengthens the duration of oods.

Stronger oods within the middle reaches of the Prut occur because of its tributaries. Flooding on the Baseu, Sitna, Miletin, Jijia, Bahluet, and Bahlui rivers was strong, but it

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G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania 391

Figure 11. Polders on the Jijia and the oods recorded in the summer of 2010: storage of excess water (left) and its elimination (right).

Table 3. Values of CA, CI, and CP for the Oancea (Prut) and Galati (Danube) gauging stations.

CA CI CP Gauging station (Warning level) (Flood level) (Danger level)

Oancea (Prut) 440 550 600 Galati (Danube) 560 600 660

Table 4. Maximum water levels during ooding in the summer of 2010 for the Danube compared to values from other ood years.

River Gauging Maximum levels in the year (cm)

station 2010 2006 2005 1981 1970

Danube Galati 678 661 600 580 595

Isaccea 537 524 481 490 507 Tulcea 439 437 399 415 429

Table 5. Maximum discharges during ooding in the summer of 2010 for the Danube compared to the maximum values from 2006.

River Gauging Maximum discharges in station the year (m3 s1)

2010 2006

Danube Galati 16 300 14 220

Isaccea 16 240 14 325 Tulcea 6117 5768

the polders, which explains why high water levels persisted in the lower Prut for a long time (Fig. 11).

Discharge at the Oancea gauging station increased dramatically from 45 July 2010, coinciding with the increased discharge on the Danube at Galati. The backwater at Oancea was also enhanced by backwater from the Danube. The second backwater was caused by upstream contributions. The ood danger level (CP) at Oancea was exceeded by +83 cm

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Figure 12. Flooding of the sea cliff and the NAVROM headquarters in Galati.

was mitigated for the most part by the existence of ponds (Fig. 10). Therefore, the excess water entering Romania from Ukraine entered the Stnca-Costesti reservoir. The excess water downstream of the Stnca-Costesti reservoir came from tributaries. Discharge from the tributaries is controlled by hydrotechnical works within each tributarys catchment. The Jijia and Bahlui catchments are 80 % developed. The water levels downstream of these tributaries, in the lower reaches of the Prut, are mitigated by the extreme width of the Prut oodplain (the most important wetland of the interior Romanian rivers).

The system of polders in the lower reaches of the Jijia served as an effective trap for surplus water. High discharges on the Danube, which reached a historic maximum of 16 300 m3 s1 at Galati (5 July 2010), would have ooded the city centre without the precincts constructed on the Jijia that stopped a portion of the oodwaters. When the oods on the Danube ceased, the water was gradually eliminated from

392 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

Figure 13. The spider ow phenomenon in which the Buhai waters climbed the Ezer dam on the Jijia, in the area of conuence of the two rivers.

Table 6. Maximum water levels during ooding in the summer of 2010 compared to 2008 and 2005.

River Gauging Maximum Day Hour Difference Maximum Maximum station level from the three level 2008 level 2005 cm levels of cm cm danger cm

Prut Oroftiana 717 24.06 11 +67 CP 867 703

744 28.06 1112 +94 CP

737 1.07 04 +87 CP

797 9.07 1718 +147 CP

425 13.07 20 +75 CA

Prut Radauti-Prut 643 25.06 1819 +43 CP 1130 680

686 29.06 17 +86 CP

722 1.07 23 +122 CP

744 10.07 1920 +144 CP

Prut Stanca downstream 461 3.07 1522 +86 CP 512 331

Jijia Dorohoi 750 29.06 09 +150 CP 558 646

722 30.06 05 +122 CP

630 30.06 17 +30 CP

Jijia Dangeni 575 30.06 08 +105 CI 449 512

579 1.07 05 +109 CI

Jijia Todireni 417 1.07 08 +77 CI 123 420

Buhai Padureni 470 28.06 1920 +120 CP 292 Miletin Nicolae Balcescu 444 28.06 15 +24 CI 286 334

Miletin Sipote 226 27.06 12 +76 CA 198 236

269 29.06 18 +19 CI

Miletin Halceni 302 1.07 1518 +2 CP 226 238

Sitna Todireni 378 1.07 17 +28 CI

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G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania 393

(CP) during the rst backwater and by +46 cm (CP) during

the second backwater (Table 3). The discharge increase and the historic values registered were caused by several factors such as the water input from the upstream sector of Prut River and the water input added by the Danube backwaters.

The city of Galati is situated at the conuence of the Prut and the Danube rivers. Thus, water at the Oancea station may be inuenced by the Danube and the Prut. In the summer of 2010, the highest values of discharge and water level at Galati were recorded (Tables 4, 5). The control of ooding on the Prut meant that oodwaters in Galati reached the sector of banks where ood infrastructure had been developed (the sea cliff) as well as the lower areas of the city (Fig. 12).

Discharges and water levels in the middle sector of the Prut River (recorded at the Oroftiana, Radauti-Prut, and Stanca Aval stations) rank third in the hierarchy of oods (after 2008 and 2005). Values for the tributaries (particularly the Jijia, Buhai, Miletin, and Sitna) rank rst in the hierarchy of oods (Table 6).

The oods recorded in the summer of 2010 in the Buhai catchment (a tributary of the Jijia, which is a tributary of the Prut) caused backwaters to emerge at the mouth of the river.The manifestation of this backwater phenomenon is unique because the oodwaters of the Buhai River climbed the Ezer dam (on the Jijia River) and ooded its lacustrine cuvette.The phenomenon was named spider ow (Romanescu and Stoleriu, 2013a, b) (Fig. 13).

6 Conclusions

In the summer of 2010, large amount of precipitation occurred in central and eastern Europe. Heavy rains in northeastern Romania caused devastating oods in the Prut and Siret basins. Romania incurred huge economic damages.The ooding in 2010 was comparable with previous strong ood years in 2005, 2006, and 2008 in Romania. The greatest damage occurred in the middle Prut basin in the Jijia-Bahlui depression of the Moldavian Plain, where the largest arable area was destroyed.

Discharge in the downstream sector of the Prut was controlled by the Stnca-Costesti reservoir, which ranks second in Romania in terms of active reservoir volume (1400 million m3, after the Iron Gates I, with 2100 million m3). It has a surface area of 5900 ha for a NRL.Under normal circumstances, the Stnca-Costesti reservoir can retain enough water to control the downstream discharge and water level. The provision of an attenuation water volume (550 million m3) within the lake basin is efcient in retaining a 1 % probability ood (reducing it from 2940 to 700 m3 s1). Together with the embankments located on the dam downstream sector, it helps to prevent the ooding of 100 000 ha of meadow. At a normal retention level, Stnca-Costesti Lake has a total area of 5900 ha and a water volume of 1.4 billion m3.

Discharges downstream of the Stnca-Costesti reservoir are controlled by reservoirs and retention systems constructed on the main tributaries of the Prut. We emphasize that the Jijia and Bahlui catchments have hydrotechnical works on 80 % of their surface areas. The system of polders in the downstream sector of the Jijia River was used extensively to mitigate discharge and prevent the city of Galati from ooding (Galati is the largest Danubian port, situated at the conuence of the Prut and the Danube rivers).

The gauging stations in the lower sector of the Prut recorded high discharges and water levels because of excess water coming from upstream (the middle sector of the Prut). At the Oancea gauging station, however, which is situated near the discharge of the Prut into the Danube, there is a signicant backwater inuence. The Danube had historic discharge at Galati, which affected the water level at Oancea station on the Prut.

Floods during the summer of 2010, in north-eastern Romania, rank third among hydrological disasters in Romanian history after the oods of 2005 and 2008, which also occurred in the Siret and Prut catchments. The 2010 oods caused grave economic damage (almost EUR 1 billion in just the Prut catchment) and greatly affected agriculture. Furthermore, six people died in Dorohoi, on the Buhai River.

The 2010 oods caused a unique backwater phenomenon at the mouth of the Buhai River. Floodwaters from the Buhai climbed the Ezer dam (situated on the Jijia River) and ooded its lacustrine cuvette. The phenomenon was called spider ow. In order to avoid such phenomena it is necessary to increase the height of the overow structure.

Data availability. The water level and discharge data used in this research were provided by Administratia Bazinala Apa Prut-Barlad (ABA Prut Barlad). This data can be obtained with a formal request addressed to the institution mentioned above and, if accepted, it can be used only for the aim specied in the formal approach.

Competing interests. The authors declare that they have no conict of interest.

Acknowledgements. This work was supported by the Partnership in Priority Domains project PN-II-PT-PCCA-2013-4-2234 no. 314/2014 of the Romanian National Research Council, called Non-destructive approaches to complex archaeological sites. An integrated applied research model for cultural heritage management http://arheoinvest.uaic.ro/research/prospect/

Web End =http://arheoinvest.uaic.ro/research/prospect/ . The authors would like to express their gratitude to the employees of the Romanian Waters Agency Bucharest, Siret Water Administration Bacau, particularly to Jora Ionut, PhD, a hydrologist within this research and administration agency, who was kind enough to provide a signicant part of the data used in the present study.

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394 G. Romanescu and C. C. Stoleriu: Exceptional oods in the Prut basin, Romania

Edited by: M.-C. LlasatReviewed by: two anonymous referees

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