Extreme Hydrological Phenomena in the Hydrological Basin of Upper Ialomita, During 2000-2005
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EXTREME HYDROLOGICAL PHENOMENA IN THE HYDROLOGICAL BASIN OF UPPER IALOMITA, DURING 2000-2005 Ovidiu Murărescu, Petre Breţcan “Valahia” University, Faculty of Human Sciences Târgovişte, Romania [email protected], [email protected] Abstract In the analysis of extreme hydrological phenomena, two distinct parameters are considered: on the one hand, the maximum flow and the high flows, which are more spectacular in point of the impact on settlements and anthropic activities, and on the other hand, the phenomena of minimum flow and drought, which are much less spectacular, but whose impact is much stronger. The maximum flow is characterized by the highest values (in percentage) during the different months of the year. Regarding the hydrographic basin of upper Ialomiţa river, where most of the hydrometric stations appeared only after 1975, we can highlight a series of territorial differences for this type of liquid flow, mainly generated by the conditions of the precipitations regime. The minimum flow represents a basic characteristic in the hydrological regime, being a parameter that is necessary in order to find out the river’s water reserves during critical periods with low flows, in order to satisfy the water demands. They appear during the periods with low quantities of precipitations or with no precipitations at all, when the alimentation takes place almost exclusively from underground resources. Keywords: hydrographic basin, maximum flow, flood, high flood, minimum flow, hydrological drought. 1 INTRODUCTION In the hydrographic basin of upper Ialomiţa the module flow increases from upstream to downstream, in direct relation with the increase of the number of tributaries and of the alimentation basin. Along the 71 km that it covers from its sources to Târgovişte, Ialomiţa River has a module flow that grows constantly from 1.15 m3/s, at the entrance in Bolboci lake, up to 7.97 m3/s at Târgovişte. Because of the hydrotechnical arrangements along it, there appear a series of modifications in the liquid flow regime, the affluent flow module for the accumulation lakes of Bolboci, Pucioasa, Doiceşti being much higher than the diffluent flow, which is dictated by the need of water of the socio-economic institutions situated downstream of these lakes. These lakes have as well the role of regulating the liquid flow throughout the year, in order to lessen the high flow waves or the drought phenomena that could appear. So, if the module affluent flow in Bolboci lake is of 1.15 m m3/s, while the diffluent one is of 0.81 m3/s, increasing to 6.88 m3/s in Moroeni, going down to 5.58 m3/s in Pucioasa and growing to 7.97 m3/s in Târgovişte. From the alpine area, Ialomiţa receives only one important affluent, Ialomicioara Leaotei, whose module flow is of 0.91 m3/s in Fieni. In the Subcarpathian area there are many tributaries of Ialomiţa’s, some of them reaching Ialomiţa River in this area, others in the southern plain zone. These rivers’ average module flows register values ranging from 0.17 m3/s (Slănic, in Gura Ocniţei) and 1,0 m3/s (Bizdidel, in Pucioasa). Depending on the climatic conditions, a series of extreme hydrological phenomena can develop, like the hydrological draught, or especially the floods and high floods, which occurred with a high frequency during the time period under analysis. 2 GEOGRAPHICAL SETTING The upper hydrographic basin of Ialomiţa River is situated in the center-south of Romania. The altitude ranges from 2505 m (Omu Peak – Bucegi Mountains), the maximum limit, to the minimal level of 291 m, in Târgovişte. The average of the surface situated above the sea level is of 1398 m, which is an important indicator if we consider the fact that vertical zonality is a main element of the matter and energy flow involved in the spatial evolution of the hydrographic system under analysis. Ialomiţa River, which has its spring in Bucegi Mountains, at over 2300 m, goes through the alpine and hilly area up to Târgovişte, covering a distance of 71 km (17.03% of its total length of 417 km), going down through a level difference of about 2010 m. It forms a basin whose surface is of 686 km2 (6.62% of the total basin surface of 10,350 km2). The river has a sinuosity coefficient of 1.88 for its entire course, with slightly lower values in the alpine and hilly area (between 1 and 1.50). The density of the hydrographic networks varies between 0.100 and 0.260 km/km2. At the contact between the hills and the plain, Ialomiţa has an average flow of 7.97 mc/s (11.61 l/s/km2). 3 DATA AND METHODS In the analysis of the liquid flow elements we used the statistic-mathematical series of data recorded at the following hydrometric stations and posts situated in the hydrographic basin of upper Ialomiţa: Table 1. Data concerning the hydrometric posts in the hydrographic basin of upper Ialomiţa No. River Hydrometric F med. L (km) Year of Year Measurements and observations made post (km2) height creation of end H Q R Ta Fi A (m) 1. Ialomiţa Bolboci 54 1680 10,75 1990 - - x - - - - 2. Ialomiţa Moroeni 264 585 29 1954 - x x - x x x 3. Ialomiţa Pucioasa 448 296 54 1976 - - x - - - - 4. Ialomiţa Târgovişte 686 296 71 1976 - x x x x x x 5. Ialomicioara Fieni 95 903 26 1975 - x x x x - x 6. Bizdidel Pucioasa 92 654 55 1976 - x x x x x - 7. Vulcana Şotânga 105 470 18 1983 - x x - - - - 8. Slănic Gura 57 260 25 1980 - x x - - - - Ocniţei 9. Cricovul Moreni 57 260 30 1980 - x x - - - - Dulce F – surface of the hydrographic basin (corresponding to the hydrometric post); L – river length (spring – hydrometric station); H med. – average height; Q – liquid flow; R – the flow of alluvial matter in suspension; Ta – water temperature; Fi - winter phenomena; A – physico-chemical analyses 4 RESULTS Extreme hydrological phenomena are determined by very large quantities of precipitations fallen in very short time intervals that generate high flows of great amplitude. They can also be caused by large and continuous quantities of precipitations recorded over a long time interval, corroborated to territorial differences on seasons and with favorable local factors. In the hydrographic basin under analysis, where most of the hydrometric posts were created after 1975, a series of territorial differences of the maximum liquid flow has been recorded, mainly generated by the conditions of the precipitations regime. So, the highest maximum flow recorded on Ialomiţa River so far was of 691 m3/s on June 19, 2001. High flows. In order to have a global image on the complexity of this phenomenon, we selected the characteristic high flows, with the highest water volumes. At the level of the entire geographic zone under analysis, during the interval 2000- 2005, significant high flows occurred. The history of this area recorded high flows before that as well, some of them catastrophic through their consequences, in the years 1971, 1972, 1974, 1975, 1979, 1980, 1983, 1985, 1988, 1991, 1993, 1997, 2001, 2005, which means that for every interval of 3-5 years, a large and a very large high flood occurs. The high floods can be grouped according to their size: - small high floods: 1969, 1970, 1988, under 100 m3/s: - average high floods; 1979 and 1991, under 200 m3/s; - large catastrophic high floods, in 1972, 1975, 1997, 2001, 2005, recording between 300-700 m3/s, on the large rivers, and 200-250 m3/s, on the tributaries. High floods occur because of abundant rains, or as the snow melts, or when these two phenomena combine, the most frequent being recorded in spring (30-50%) and in summer. High floods and floods, with an accuracy of 0.5-1%, are caused mainly by large quantities of precipitations with a significant degree of torrentiality, recorded after periods when the soil moisture had increased heavily and the water absorption capacity had become low. The propagation of high flood waves is strongly modified by the anthropic activities. Uncontrolled deforestations determined an increase of the speed of concentration of the flow, as well as an intensification of the erosion, of the transport and of the depositing of the alluvial deposits and an over-heightening of the riverbeds, which increases the risk of overflow. The high flood of June 2001 represented a peak, by its size and through the participation of the whole 686 km2 basin up to Târgovişte. It recorded the highest flows for all the hydrometric stations of Ialomiţa River, upstream of Târgovişte, higher than the ones of 1975. It was generated by a front of precipitations focused for 48 hours on Ialomiţa Valley, the abundant precipitations fallen in the alpine and Subcarpathian zone summing up between 125 and 175 l/m2. Another generating factor was the humectation of the basin, namely the quantitatively insignificant precipitations that prepared the basin. The high flood started to form in the evening of June 18, and in the morning of June 19 it was present on all the tributaries. Its growth evolution is spectacular; it started from values of 5-10 m3/s and in 15-20 hours it reaches values of 200 m3/s. So, on the night of June 19, values of 196 m3/s were reached on Ialomicioara River, in Fieni, 240 m3/s on Bizdidel River, in Pucioasa, 144 m3/s on Vulcana River, in Şotânga, and 373 m3/s in Moroeni and 691 m3/s in Târgovişte, on Ialomiţa River. The high flood began to decrease on June 21. The volume of the high flood was of 5 million m3/s on Bizdidel, 7-9 million m3/s on Vulcana and on Ialomicioara, 20 million m3/s in Moroeni and 27 million m3/s in Târgovişte, on Ialomiţa River.