Hydrological risk phenomena caused by rainfalls in the north-western part of Romania

Transcription

1 Hydrological risk phenomena caused by rainfalls in the north-western part of Romania V. Sorocovschil & G,Pandil ldepartment of Geography Babes-Bolyai University, Romania Abstract The surveyed territory occupies partially the mountainous region of the Carpathian Mountains, the range of sub-mountainous depressions, the hills from the Somes Plateau and the northern part of the Transylvania Plain. Most of its territory is drained by the Somes river. Among the extreme hydrological manifestations specific of the northwestern territory of Romania are the floods with various geneses (pluvial, pluvio-nival, and more rarely, nival) and the high mountainside run-offs, generated by torrential rainfalls and by sudden defrosting. The maximal risk factors are considered to be the spring-summer floods, when the natural conditions are favorable for the incidence of catastrophic deluges, The damage produced during this period are much more serious than those which occur during the period of vegetation. However, the convective or front rains may also generate exceptional floods during the summer months. The risks caused by the floods are characteristic of the areas exposed to the advection of oceanic air masses from the west, amplified by the orographic configuration. Such areas are more exposed to the high risk hydrological phenomena, In the hills/ uplands, with frail lithologic substratum, low forestation and steep slopes, the hydrological risk factors are generated by torrential rainfalls, which cause violent mountainside flows. The effects are more than often destructive, resulting in human casualties and material damage. The mountainside flows generate material damage in agriculture, 1 Precipitation as a risk factor The risk factors generated by excessive precipitation have a serious impact on the natural and socio-economic environment from this part of Romania.

2 90 Risk Analysis III Data analysis enabled the calculation of the periods with pluviometric surplus by means of the standard precipitation index (SPI), In order to obtain the value of precipitation excesslsurplus and the succession of the periods with pluviometric surplus, it was necessary to calculate the positive deviations of the extreme annual and seasonal amounts. Essential for the evaluation and prognosis of the hydro-climatic risk phenomena are the frequency and the succession of the pluviometric surplus periods. The risks caused by the pluviometric surplus become higher during the periods with several consecutive surplus years. The most frequent are the short pluviometric surplus periods, of two consecutive years. Such periods are absent on the territories protected by the mountains. There were surplus periods of three and four consecutive years generated partly by the intensity of the convective rains, produced by the circulation from the eastern sector. A close survey of the succession of pluviometric surplus periods demonstrates that there were few with a general character ( ), which proves the fact that they are included in the more or less widespread cyclone activity. Most of the pluviometric surplus periods have had a regional character. The surplus period was representative in this respect. The results of the assessment of the total number of periods with pluviometric surplus with different duration, calculated according to the SPI criteria, reveal that most of them (4-5) have been recorded in the eastern part of the territory. The hydrological risk phenomena, regional or general, are also generated by the isolated surplus years with significant positive deviations ( over 100 mm), Thus, the following were surplus years: 1970, in the center and east 1974, in the center and 1984, in the west and north, The total number of isolated surplus years was higher in the north and north-east (8-9 years), territories more affected by the western circulation, and less in the western region, protected by the mountains (5-6 years). The high proportion of precipitation surplus may be evaluated through various methods (Hellman, PSI, Emberger, Stewart, D.Jong, etc.). They are used to calculate the frequency of the pluviometric surplus time applied to annual, seasonal or monthly amounts of precipitation. The incidence of pluviometric surplus years, calculated according to PSI, is higher in the plateau (()-35~0), than in the extreme areas ( %0). The years with excess of precipitatioti The exceptionally rainy years have a very low incidence, being specific only to the central and eastern regions. The incidence increases from the exceptionally rainy years ( ) and the years with heavy precipitation ( )to the moderately rainy years (10-16?400and the less rainy years (10-18Yo). The chronological variation of the seasonal distribution of precipitation reflects the fact that the higher positive deviations are recorded in spring at the meteorological stations from the north-western and north-eastern areas ( mm), In the autumn, the positive deviations are lower than in the other seasons (90-120), Winter is the season with the lowest deviations ( mm). As in the case of the annual amounts of precipitation, the frequency and succession of seasonal pluviometric surplus were also determined, In the central

3 Risk Analysis III 91 part of the depression, the winters with pluviometric risk have the higher incidence (16-18Yo), as compared to the eastern extreme regions, where the highest incidence of pluviometric risk is in summer, In these areas, the regeneration of the fronts and the abrupt ascent of the air masses on the steep. slopes of the Carpathians, often determine significant amounts of precipitation In the western part, the maximal incidence of the springs with pluviometric risk is conditioned by the relatively slight advection of air masses coming from the Atlantic and the regeneration of the fronts. Autumn is the period with the lowest frequency of pluviometric risk, For an appropriate evaluation of the effects and a correct distribution of the hydrological risks, it important to know the year to year succession of the seasons with pluviometric surplus. In the north-eastern parts of the country, the winters with surplus can occur in a four-year succession. In spring and in summer, there may be a two-year or a three-year succession, while for autumn, only two- year successions are characteristic. To evaluate the effects of the pluviometric risk factors, we have to calculate the rate of the succession within the same year of the seasons with pluviometric risk (PSI >1). In this respect, it has been noticed that in the north-west, the incidence of two consecutive seasons with pluviometric risk is highest (12-16Yo). This is explained by the exposure of these territories to the humidity excess, which may last for a longer period of time, In the west, we notice a fi-equency of 2-4Y0. 2 Floods Table 1: Data referring to the hydrological stations surveyed River Length Surface Mean of Mean Hydrometric of basis Alt. slope M~an $ Station Course (km*) (m) ~, (m/km) (m /s) ) Nadas Aghiresu ,3 15 0,220 4:.1 Borsa Borsa Lonea Luna de Jos Gadalin Bontida Fizes Fizesu Gherlii Dipsa Chirales Meles Rusu de Jos , Ilisua C. Ciceului Salatruc Casei ,6 16 1, Poiana P. Blenchii Olpret Maia ,398 45,4 Almas Hida , Agrij Romanasi

4 92 Risk Analysis III Knowing the genesis and origin of the floods may contribute to the prevention and eradication of the economic, social and ecological effects caused by them, In order to explain the territorial and temporal differences between floods, data analysis and processing were carried out at 13 hydrological stations situated on the main local rivers (Table 1), 2.1 Genesis and incidence of floods In 50-55~0 of the surveyed cases, the pluvial floods, of high hydrological risk, are generated during the interval May-November. The next in order of their incidence, are the pluvio-nival floods, also of high risk, characteristic of winter and early spring, The richer discharges of the small hydrographic basins are generated by torrential rains, while in the larger basins, the discharges are generated by the long-term rain and by defrosting. 5- / A \ L..t. v~ 07 III III IV VVIWIVIIIXXXIXU... 1-w w a b I 20. b,\ B- t ) / \ 15- % 10. I II III IV VVIVIIVIJIIXXXI XII....@!e@Ckeuiui *i :.:.:.::.,,,,>,).:.:.:.!.,, m~ ~~~ti 07 III UINVVIVIIVIIIIX XXI XU w M-W = - -fi=w Chialq h&jc6 c Figure 1: The monthly frequency of the floods from the Cluj and Dej Hills (a), The Almas Depression (b), the Ciceu Hills and the Boiu Plateau (c) and the Transylvania Plain (d), d The various features of the flood are amplified by the characteristics of the basin, of the hydrographic system and of the river bed. Thus, with some of the

5 Risk Analysis III 93 long and narrow basins (Olpret, Luna, Salatruc, etc.), the climax of the flood is lower and weaker, The duration and dimension of the floods are also influenced by the capacity of the soil to retain water and by its feasibility to retain some amount of precipitation, In this respect, in the Almas, Ilisua and Poiana basins, the substratum rich in argil and the very limited forested surfaces contribute to the formation of rapid and high floods. The monthly flood incidence (Figure 1) records a maximum in March (19-28% of all the floods selected for our survey) on the rivers from the Cluj Hills, from Simisna-Garbou and on those from the central and north-eastern part of the Transylvania Plain (Fizes and Dipsa basins); In April, the maximum is reached on the rivers which flow in the Boiului Plateau and in the western parts of the Ciceu Hills. On the rivers west of the Transylvanian Plain and of the Almas Depression, the higher flood incidence is in May (20-26Yo) and on the rivers from the Agrij Depression, in June. The graphical representation of the monthly flood variation shows one minimum in autumn (September and November) and another in winter (February) when the hydrological risk is lower, too. The flood variation is influenced by the exposure of the basins to the advection of humid air masses from the west and by their mean altitude. Thus, for most of the hydrographic basins with a mean altitude of 450 m, the February floods have either a very low incidence or they are entirely absent. In what regards the seasonal incidence (Figure 2) of floods, a maximum is reached in spring (40-60Yo), while in summer, winter and autumn, the incidence is the lowest, FzesuG-d R6ud?b v% 6% 22% I 5YZ EEEiEIl E!zE, Ma CMesti acdu 1S% WY 24% 48% W% EIEzz EEE!21 Figure 2: Seasonal flood variation (I Winter, P Spring, V- Summer, T Autumn)

6 94 Risk Analysis III There exist, however, a few exceptions. The flood incidence cm some rivers from the Somes Plateau (Olpret), from the Transylvania Plain (Meles) and from the Ciceu Hills (Iliusa), whose basins have a favorable exposure to the advection of warm and humid masses of air from the west, is higher in winter than in summer. The high autumn flood incidence on the Fizes river is anthropic, because during this season the ponds are drained to gather the fish. 2.2 Characteristics The floods have several characteristics which can be determined through various methods. The extent of the damage caused by floods, for instance, depends on its duration, On most rivers, the floods with the highest incidence (Figure 3) last between 5.5 and 10 days (29-42% of the cases), An exception are the rivers with an incidence of floods which last 20 days or more (Fizes at Fizesu Gherlii, 39 /0, Borsa at Borsa, 29%), 15.5 to 20 days Dipsa at Chirales, 28%), or between 10.5 and 15 days (Salatruc at Casei, 40Yo), The floods with a duration less than 5 days, are characteristic of rivers such as Agrij, Almas and Nadas, They are determined by the torrential character of the rains, by the reduced capacity of the basins (Nadas at Aghires, 39.1 km2) and by the relatively steep slope (15 *). The territorial distinctions imposed by the character of the precipitation (duration, intensity) can also be traced when analyzing the growth duration of the floods, another element which plays an important role in flood prevention. Thus, rivers with similar basin dimensions may have different time of growth, depending mainly on the type of precipitation. This peculiarity can be best noticed in the Almas basin, where the frequency Tc<lday is higher due to the strong torrential character of the rains and to reduced forestation, The situation is similar on the Agrij river at Romanasi. On the Fizes, Tc<l day has a low incidence due to the regularizing role of the artificial ponds along its course.

7 Risk Analysis III 95 Another representative element in flood risk evaluation is the maximal discharge, which generates maximal flood. Multiannual maximal discharges varied between 34,2 m3/s (Gadalin at Bontida) and 220 m3/s (Agrij at Romanasi), depending on the climatic characteristics and the dimension of the basin. Maximal discharges with an approximate rate of incidence between 1 and 5~0 have been recorded in 1981, 1989 and 1998, when they produced catastrophic damage. If for the rivers from the Almas Depression, form the Cluj Hills and from the Transylvania Plain, the highest flood magnitude was recorded in 1989 and in 1998, similar values were recorded on the rivers from the Dej Hills, from Simisna-Garbou, Ciceu and from the Boiu Mare Purcaret Plateau in 1997, 1995 and Maximal discharges have a high incidence in March (34-40?40) on the rivers from the Cluj and Dej Hills and from the Fizes basin, whereas on the rivers from the north-eastern part of the Transylvania Plain, maximal flood incidence is recorded in April. In May (30 /0) and in June (3So/o), maximal flood incidence is recorded on the rivers from the Almas Depression. 2,3 Case study Three floods, which have been recorded lately in the north-western parts of Romania, made the object of a detailed analysis and were appreciated to have been characteristic of the entire territory surveyed. The first one was mixed, pluvio-nival, and lasted from December 23, 1995 until January 1, 1996, The rains which fell on a preexisting stratum of snow contributed to its melting, The quantity of precipitation, together with the water that resulted from the melting of the snow, did not exceed 54.4 mm (Almas at Hida) and mm (Ilisua at Cristestii Ciceului). Their values increased with the mean altitude of the hydrographic basin and with the exposure of the slope to the advection of warm and humid masses of air from the west. The discharge was high (25-45 mm) in the river basins from the Ciceu Hills and the Boiu Mare - Jugastreni Plateau, and was more intense on the foreign rivers. Also high discharge values were recorded on some of the local rivers - (35,7 m3/s on the Lonea at Lunca de Jos, 43.3 n-ills on the Poiana at Poiana Blenchii, 93.1 m3/s on the Ilisua at Cristestii Ciceului), corresponding to a rate of incidence of 20-22?40. The second flood, recorded between June 1998, was pluvial and was caused by high quantities of precipitation which varied between 33mm (Ilisua at Cristestii Ciceului) and 88.9 mm (Agrij at Romanasi). In the river basins from the Almas-Agirj Depression, from the Cluj and the Simisna-Garbou Hills, from the Transylvania Plain, the rainfalls exceeded 45 mm. Despite the high values of the generating precipitation and due to low flow coefficients, only less than 1/3 finally entered the discharge regime. The time of growth varied between 12 hours (Nadas at Aghires) and 42 hours (Almas at Hida). High values were recorded on the Rivers from the Trans ylvania Plain (35 hours at Bontida on the Gadalin, 40 hours on the Meles at Rusu de Jos). The maximal discharges varied between 9.20 m3/s (Nadas at Aghires) and 98.6 m3/s (Fizes at Fizesu Gherlii). In

8 96 Risk Analysis III Borsa and Lonea basins, the recorded discharges corresponded to the generation rates of 1l% (65.5 m3/s), respectively 13 YO(62.8 m3/s), The third flood analyzed here and which lasted from 8-15 March, 2000, was generated by the rains which fell on a soil already saturated with water from previous defrosting. The precipitation varied between 13 and 102 Q but higher values were recorded on the Ciceu Hills, on the Boiu Mare Jugastreni Plateau and in the north-eastern parts of the Transylvania Plain, According to the nature of the substratum the quantity of discharge varied between 2.3 mm (Gadalin at Bontida) and 47.2 mm (Poiana at Poiana Blenchii). The flow coefficients recorded high values in the Borsa (0.74) and Lonea (0.61) basins from the Cluj and Dej Hills, Meles (0.53) from the north-eastern parts of the Trans ylvania Plain, Salatuc (0.48) and Poiana (0,46) from the Boiu Mare Jugasteni Plateau. Table 2: Results of flood damage Damage.- 2 Terrain (ha) $ m.+ o ~ ~ B %] b o z.% 25 ~ ; j g # $ a $ # 3$ g ~ g & g & $j & 2 u & Q Tibles Ilisua Almas , Dipsa 8200 Meles 745 Ilisua 410 Gadalin 300 Fizes Salatruc Poiana Almas 65 Agrij Salauta Tibles 1 1 Dipsa Meles <9 6 Ilisua Lonea Salatruc 12 Olpret

9 Risk Analysis III 97 The time of growth kept within 44 hours on the Almas at Hida and 13 hours on the Fizes at Fizesu Gherlii (influenced by the presence of the ponds), The maximal discharges were very high on four rivers: Lonea (146 m3/s), Borsa (80 m3/s), Ilisua (160,3/s) and Salatruc (103 m3/s), these values corresponding to the rate of incidence of 2 8?4.. We also analyzed the damage (Table 2) produced by these three floods so as to be able to estimate the magnitude of the risk involved. Consequently, we evaluated the dimension of the flooded territory, the number of the affected localities and infrastructure. In the absence of the cost equivalents of the damage, we were unable to quantify the risks according to incidence of the maximal discharges and of the damages produced by it. 3 Conclusions In the north-western parts of Romania, flood formation has an apparently uniform character, however, at close look, there are significant territorial variations. This is influenced by the incidence in time and space of the precipitation, by altitude, by division, by the condition and orientation of the relief and sometimes, by the anthropic element. In most cases, the discharge regime is of the Transylvanian peri-carpathian type. On two narrow strips of land, it presents elements of transition towards the Western, respectively, Eastern peri-carpathian type. The maximal risk of flood incidence is in March and April and the minimal, in autumn (September and November) and winter (February). The highest flood discharges, with speeds of sometimes tens or hundreds m31s, generate social, economic and ecological damage. The only way to prevent these phenomena from taking place, is to build permanent and non-permanent accumulations in areas of increased flood risk both in precipitation and on the flow upstream the hydrological convergence, The data obtained from the quantification of the damage costs by the local administrations, together with our evaluation of the hydrological phenomena, will enable us to appreciate and chart the risks with accuracy. References Anitan, I., Maximaljlow in the hydrological basin Somes-Crasna, Ph.D. Thesis, Cluj-Napoca, 1974, Buta, I., The Somes Basin. A hydrological study. Ph.D. Thesis, Cluj-Napoca, 1967, Diaconu, C., Serban, P., Hydrological Syntheses and Regional Distribution, Editura Tehnica (Technical Printing House), Bucharest, So fronie, C., Hydrotechnical Structures in the Somes-Tisa Hydrographic Basin, Casa de Editura Gloria ( Gloria Publishing House), Cluj-Napoca, Sorocovschi, V., 13uz, V,, Mean Flow River Potential in the North-western Parts of Transylvanian Plateau. Analele Universitatii Oradea (Oradea University Annals), 1994.

10 98 Risk Analysis III Sorocovschi, V., Serban, Gh., Regional Flow Potential Contrasts in the Rivers from the Somes Plateau, Studia Review, 13abes-1301yai University, Geography XL, 1-2, Cluj-Napoca, 1995, Stanescu, V,, Al., Mustatea, A., Oberlin G,, Les trues de Roumanie: formation et particularities specijiques. Recontres Hydrologiques Frasnco-Roumaine, Montpelier, Selarescu, M,, Podani, M., Flood protection, Editura Tehnica (Technical Publishing House), Bucharest, Ujvari, L, Geography of the Romanian Rivers, Editura Stiintifica, Bucuresti, 1972.