Extreme Hydrological Phenomena in the Barlad River Basin

Silvia CHELCEA, Alexandru PREDA, Adrian VLADUCU National Institute of Hydrology and Water Management Bucharest, ROMANIA

Abstract During the last decades extreme meteorological and hydrological phenomena became more frequent and more destructive in our country too. Among them, the ones that affect goods and human lives the most are: excessive rainfall that generate floods, and droughts (meteorological and hydrological) This paper presents special situations encountered in the evolution of water runoff in the Barlad River hydrographic basin (the largest tributary of the Siret River from the point of view of basin surface and length), situated in the Eastern part of Romania. Here, water courses with semi-permanent and temporary flow regime are predominant, and even the main drainage artery of the plateau, the Barlad River, has a semi-permanent flow regime due to small quantities of rainfall. On the other hand, the torrential regime of the Barlad River hydrographic basin, small longitudinal slopes, riverbed which is silted in many sectors, contributes to the flooding of large surfaces from its basin. Beside the knowledge of maximum and minimum runoff characteristics depending on the causal factors, the paper also pursues the discovery and pointing out the modification tendencies of river behavior in this hydrographic space, determined either by natural causes (general changes in climate and microclimate) or anthropic (deforestation, forestation, irrigation and drainage, damming of valleys and forming of reservoirs, dyke building and water transfer etc.).

Key Words: drought, floods, minimum runoff, maximum runoff

Basin Description The Barlad River basin lies in the western part of the country, entirely in the western part of the Moldavian Plateau (figure 1).

Figure 1 Geographical location of the Barlad catchement

The Barlad River, left bank tributary of the Siret River, is considered to be one of the large river in the country having a area of 7220 km2 and a medium altitude of 374 m. The whole Barlad river basin is situated in the Romanian province. The climate of the region is continental and is under the great influence of the Atlantic air masses that attenuate the continental nature of the climate, increasing its humidity. The small quantities of precipitation that fall in this basin determine a semi-permanent runoff regime. The relief of the Barlad river basin is modeled by its own system in sarmatian and Pliocene deposits with southern general inclination. This caused the development at North and South from the superior Barlad corridor, of a consistent network with southern movement direction. As a result, the Barlad River until the confluence with its tributary, the Rebricea River and downstream the Barlad town has subsequent character. The position of the Barlad River in the Barlad Plateau leads to a small variability with the altitude of the reception basins (figure 2).

Figure 2 Physic map of the Barlad catchment

In the superior course, subsequent, has a mean slope of around 3m/km, but downstream, until its mouth, it drops reaching mean values of 0.5m/km. The vegetation is characterized by forest (oak, durmast), which occupies the largest part of the plateau, and by the silvosteppe area, which developed on levigate chernozems. In the Barlad valley, the plateau, which was covered in the past by forest that were mostly cut down is characterized today by secondary pastures and crops. The hydrometric activity taking place in the Barlad catchment is characterized by two distinct periods determined by 1948 and 1952, weak until that moment (when only 3 hydrometric stations existed) and scientifically organized after that period. The three hydrometric stations with great functioning period (over 45 years) were all situated on the Barlad River. Currently there are 35 hydrometric stations. The geomorphological configuration of the Barlad River Basin leads to the development of reservoirs only on the tributaries, and not on the main course of the river. This is why the attenuation effect of these reservoirs do not fully protect against floods the flood plain of the Barlad River, where most of the economical objectives are. This is why the complex development solution pursued the increase in transport capacity of the riverbed, through regularizations and endykement of the Barlad River and its main tributaries: Vasluet, Crasna, Racova, Durduc and Tutova downstream the reservoirs.

Maximum Runoff The Barlad River is different from the other tributaries of the Siret River because of a torrential regime of the pericarpathian hills and a generally low runoff. Due to its relief with hill and plain areas, water runoff is characterized by a multiannual mean discharge of 10.3 m3/s respectively 1.52 l/s*km2 during 1950-2004. A special issue for this river is the maximum runoff and especially the maximum discharges due, on one hand to the attenuation phenomena which take place in the flood channel on the downstream sectors, and on the other hand, due to floods and high waters and even less ordinary succession in time. In this respect, we must notice that characteristic values that refer to the maximum runoff were produced both on the tributaries and on the main river in almost every year during 1969 – 1973 and even several times in the same year, which is rarely encountered in the usual succession of such events (table 1). Table 1 Recorded/reconstructed discharges at the hydrometrical stations from the Barlad River Basin. 2 3 River Hydrometric station F(km ) Year Qmax(m /s) Barlad Negresti 817 1969 301 1970 172 1971 212 1972 103 1973 135 19.06.1985 420 Barlad Vaslui 1540 13.07.1969 290 1970 186 1971 276 1972 152 1973 180 20.06.1985 410 Barlad Barlad 3952 15.07.1969 380 1970 265 1971 281 1972 214 1973 263 23.06.1985 430 Barlad Tecuci 6778 1969 268 1970 190 1971 185 1972 331 1973 322 24.06.1985 350 24.06.1999 382 Sacovat Sofronesti 299 19.06.1985 250 Vasluiet Moara Domneasca 497 20.08.1972 320 Berheci Feldioara 519 11.10.1972 94 Zeletin Galbeni 397 11.10.1972 122

The quasiperiodical occurrence of floods lead in time, especially after the floods in 1969 -1975, to the reorganization of development scheme of the hydrographical space through promoting and executing numerous hydrotechnical works and controlling the destructive effects of floods. The floods in the Barlad river basin are characteristic phenomena. This imposed the realization of complex development works of watercourses through regularization and endykement works, but also permanent reservoirs with attenuation trench and non – permanent reservoirs, only for the attenuation of floods. After the occurrence of the 1985 flood (the flood with the greatest discharge during the 1950 - 2004 period) on the Negresti - Vaslui sector, the dykes in that area were overflowed causing important damage. The dyke construction and regularization works on this sector were redimensioned at the importance class they were designed for. Currently the Barlad River, from Negresti until the confluence with the Siret River is regularized, as the lower courses of the direct tributaries. The greatest floods that occurred in the Barlad River Basin were during: 1969, 1970, 1971, 1972, 1973, 1974, 1979, 1985 and 1991 (table 1). These floods were caused by important precipitation quantities fallen quite in an uneven manner on the entire surface of the Barlad River Basin. Generally, the precipitations that determined the occurrence of floods with significant maximum discharges had a similar spatial variation, characterized by important quantities fallen in the northern part of the basin with a decrease towards its southern side. The time variation of the analyzed rainfalls was uneven, characterized by a central nucleus (which caused the floods) with a duration between 10 h (the July 1974 flood) and 30 hours (the June 1985 flood) (figure 3).

6 5 4 3 P(mm) 2 1 0 17VI 17VI 18VI 19VI Rain gauge - NEGRESTI

8 7 6 5 4

P(mm) 3 2 1 0 17VI 18VI 19VI Rain gauge - PLOPANA

Rain gauge - HUSI

Rain gauge - BARLAD

P(mm) 6

0 17VI 18VI 19VI Rain gauge - TECUCI Figure 3 Temporal distribution of precipitations fallen during 17-19.06.1985 at the meteorological stations from the Barlad River Basin Although the analyzed rainfalls have only one important nucleus, the floods caused on certain rivers within the Barlad River Basin, have several peaks, due to the way of their formation. In 1985, on 18-20 July, as a result of heavy precipitation fallen in the upper basin of the Barlad River, the flood (the maximum level was recorded at the Negresti hydrometric stationwas 978 cm on 19.06.1985 18.20 hours with a maximum discharge of 420 m3/s) caused the collapse of some dykes upstream the Vaslui town, in the Maraseni – Stefan cel Mare area. Because the upstream dykes broke, a great volume of water reached the flood channel, left and right bank, thus the maximum level recorded at the Vaslui hydrometric station wasn’t the correct one. The flood in June 1985 is the largest flood from the series of maximum discharges on the Barlad River Basin. From table 2 we can see that the maximum discharges recorded at the hydrometric stations on the Barlad River have exceeding probabilities of around 2% or even smaller.

Table 2 Maximum discharges with different exceeding probabilities 3 Statia Qmaxim p% (m /s) Raul hidrometrica F(km2) H(m) 0.1 0.5 1 2 5 10 Barlad Bacesti 138 292 426 303 250 200 137 94 Barlad Negresti 817 252 1013 688 545 435 262 160 Barlad Vaslui 1550 236 987 668 527 415 247 148 Barlad Barlad 4017 226 1042 727 589 480 309 204 Barlad Tecuci 6778 220 974 702 583 483 328 229

In figure 4 the rain isohyets are drawn from 17-19.06.1985 which produced the flood from 17 – 28.06.1985. The rain nucleus positioned in the north part of the Barlad River basin. At the same time it can be noticed that in the south part of the basin, the precipitation quantities were small.

Figure 4 Rain isohyets from 17-19.06.1985 in the Barlad River Basin In figure 5 there are presented the flood waves recorded at the hydrometric station on the Barlad River (Negresti, Vaslui, Barlad and Tecuci).

450

400

350

300

SH. Negresti 250 SH. Vaslui SH. Barlad 200 Q(mc/s) SH. Tecuci

150

100

0 I I I I V 9V 17VI 18VI 1 20VI 21V 22 23VI 24VI 25VI 26VI 27V 28VI

Figure 5 The flash flood form July 1985 at the hydrometric stations on the Barlad River

Minimum Runoff Rivers with a temporary runoff regime are characteristic to the plateau region of the Siret River basin, respectively the entire subbasin of the Barlad River at its important tributaries. Superficial runoff on these rivers takes place only during the melting of the snow or during long duration rainfalls (spring and autumn). The main collector, the Barlad River, has a temporary floe regime until the Negresti hydrometric post and semipermanent flow regime until the Barlad hydrometric post.

Figure 6 Depletion map in the Barlad river basin The minimum runoff of rivers occurs in summer-autumn periods, due to small precipitation quantities and high temperatures, and also during winter periods due to solid precipitations fallen in small quantities and low temperatures which produce the ice forming phenomenon in rivers. Based on the existent paper the depletion map (figure 6) was made showing the types of rivers from the point of view of the depletion phenomenon: - permanent rivers – the runoff (flow) of these rivers is permanent - semi-permanent rivers – the depletion phenomenon occurs once every 2 - 3 years or once several years - temporary rivers – the are depleted every year, during different periods; have superficial runoff only during precipitation with high intensity and long duration and also in spring during the period in which the snowmelt phenomenon occurs Climatic conditions in the Barlad plateau area are not favorable to the formation of the minimum runoff, but are not determined. Here annual mean precipitations vary between 400 and 500 mm and average air temperatures in summer months (July) oscillate between 20 and 23 degrees C, favoring the intense evapotranspiration process. The most important factor that determines the depletion phenomena is the geology of the terrain. The entire area is composed of Pliocene formations, mostly Levantine gravel with thicknesses of hundreds of meters, in which water infiltration is made easily, thus rivers in their path cannot intercept the level of the groundwater. The temporary rivers are: Sacovat, Racova, Tutova, Berheci, Zeletin, Tecucel, Geru, Suhu, Calnau. An important stage of the minimum runoff is the depletion of the rivers. The partial or total depletion phenomenon, on rivers or sectors of rivers, comes either during the cold period, either in the cold period of the year, when the link between the surface runoff and the underground runoff is interrupted. A first factor which determines the occurrence of the phenomenon is that the supply from the groundwater of the rivers isn’t uniform on the entire course of the Barlad River, because of the discontinuous character of these levels. As a result large discharge variations are recorded from one sector to another, variation that leads to the production of the depletion phenomenon, especially in the case of small rivers. The anthropic factor influences runoff thorough massive deforestation in that area. Strong soil erosion leads to the bridging of the riverbed with alluvia. This strong tendency correlated to the degradation of the water table, small quantities of precipitation and very high temperatures for a long time, favors "the depletion phenomenon", which is very frequent in the Barlad River Basin. For the determination of the minimum runoff, the minimum monthly insurance 80% and 90% discharges were statistically calculated, in natural regime, at the hydrometric stations on the Barlad River, taking into account the 1950 – 2004 period (table 3). These values were at the basis of the realization of certain synthesis relations of the specific mean monthly runoff with different morphometric elements of the river basin. Table 3 Minimum monthly mean discharges at the hydrometrical stations on the Barlad River 3 Hydrometric Qmediu lunar minim anual p% (m /s) River station F(km2) H(m) 80% 90% 95% 97% Barlad Bacesti 138 292 0.015 0.007 0.003 0.001 Barlad Negresti 817 252 0.068 0.035 0.016 0.007 Barlad Vaslui 1550 236 0.135 0.060 0.025 0.005 Barlad Barlad 4017 226 0.450 0.180 0.080 0.060 Barlad Tecuci 6778 220 1.05 0.620 0.350 0.250 After the analysis was made, we could see 1953 and 1954 were excessively droughty in the Barlad River Basin.

Conclusions The paper presents the extreme hydrological phenomena recorded on the Barlad River. Within the maximum runoff it is presented the largest flash flood occurred in the 1950-2004 recording period, and within the minimum runoff the presentation of the depletion phenomenon is emphasized, phenomenon which is often recorded even on the main course of the Barlad River. The occurrence of these extreme phenomena is due to the excessive temperate-continental climate, characterized by long periods of drought, interrupted by short interval of highly torrential rainfall.

References Mustatea, A., 2005: Viituri exceptionale pe teritoriul Romaniei. Bucuresti, Romania ***, 1967: Studii de hidrologie nr. XXII. Bucuresti, Romania Paduraru, A., 1978: Scurgerea minima pe raurile Romaniei. Bucuresti, Romania