Hydrological Risk Phenomena in the Basca Rozilia River Basin

Hydrological risk phenomena in the Basca Rozilia river basin Lidia Sǎlǎjan, Petre Stanciu The National Institute of Hidrology and Water Management, Romania Everything that mankind could not conquer or explain has been put on hazard. Hazard consists of two major components: one is related to the repeatability of a phenomenon and its main determining causes, the other one is related to its random character, having no connection with the past, component that give the hysteresis state of the studied system. In the first part of this presentation there are analyzed the main flood risk factors such as: general presentation of the river basin, the definition of flood, the main characteristics of a flood, the geographical position of the river basin and its main rivers, the human impact in the hydrological water flow regime. In the second part there are analyzed the three historical floods from 1975 and 2005 recorded on the Basca river at the Varlaam gauging station on the Basca Mare river and on the Basca Mica river, and the other one at the Basca Rozilia gauging station on the Basca river. After that, an evaluation of the negative effects are analyzed and actions to take in order to diminish the hydrological dangerous effects in this river basin are also suggested. The Basca Rozilia river basin Geographical location - is situated in the South – Easthern part of the country within the Curvature Carpathians Limit: -In the North – Western part is limited by Intorsurii Mountains ; -at North by Vrancei Mountains and Zabala River ; -at East with the Vrancei Mountains and the Sub-Carpathians ; -in South only with the Sub-Carpathians ; -and towards West by the Buzau River Morphometric characteristics The Basca Rozilia river basin is formed by the junction of the Basca Mare and Basca Mica rivers, this is the left tributary of Buzau River, having a length of 71,4 km and a surface of 776 square km. Basca Mare has its sources in the North of Lacauti at an altitude of 1776 m from Vrancei Mountains and Basca Mica under this peak, from where it lies toward South. The relief, the geological structure and the climate condition the natural shape of the hydrographic network with a permanent flow regime. The Basca Rozilia river basin- morphometric characteristics Length Square Main river Tributary (Km) (Kmp) Basca Mare 60.5 424 Holomul 7 20 Ghiurca Mare 8 23 Slobodul 5.82 10 Potacu 10 18.5 Basculita 10 28 Cernat 5.70 11 Milei 6 15.4 Ciresul 9 32 Saros 7 15.2 Darnaul Mare 10.2 21.5 Corongos 5.67 13.2 Basca Mica 43.9 235 Giurgiu 6.50 15.6 Sapte Izvoare 6.63 15.5 Brebu 5.86 11.2 Basca Unita 71.4 759 Paltinis 9 24 The Basca Rozilia river basin • In the Bâsca river basin, the hydrographic network is well organized and has a permanent flow regime. • The torrential organisms have developed on the slopes of the valleys in the depressions due to uncontrolled deforestations. • Within the mountain area of the internal and external flysch comprising the Bâsca Rozilia river basin, the prevailing rocks are: conglomerates, grit stones and limestone, favouring the infiltration through sreaming and superficial flow. • The steep slopes contributed to the organizing of the hydrographic network and the limestone rocks in the flysch area favoured the presence of flows with a semi-permanent flow regime, becoming dry every 2-3 years. The Bâsca Rozilia river basin is situated in the mountain area, with rural human settlements, such as scattered villages, situated on the river meadows and depressions. Thus, in the north-weastern part of the river basin, there is Comandău village (45059'19'' north latitude and 26027'52’’ east longitude; In 2002 Comandău counted a population of 1018 inhabitants. Gura-Teghii is a village in the Buzău county (45o20’ north latitude and 26o25’ east longitude), situated at the junction of Bâsca Mică and Bâsca Mare rivers, creating a beautiful valley, cut in the rocks, but with ideal sandy beaches. According to the population census, in 2002, it had a population of 3884 inhabitants. RISKS IN THE BÂSCA ROZILIA RIVER BASIN The risks are not limited in space and time, they can occur anytime, affecting all the natural, social and technical terrestrial geostructures at local, regional and global level; The drought and the floods represent important risk phenomena, being linked to the assurance calculation which represent the safety anticipation in that area. The risk elements can be found in the areas with different vulnerability degrees and comprise: “population, buildings, economic activities, public services, utilities, infrastructure which are prone to risks in a particular area” (Crozier, 1988). In the Bâsca river basin, the floods are the most frequent, causing major damage with serious consequences. They are mostly due to the global climate evolution and to the anthropogenic factor. Due to significant variations in time and space of the temperature and other climatic elements, the floods can occur anytime, but particularly during the interval April- September. The geological structure and the lithologic composition of the area in the mountain area, due to rocks impermeability, does not allow a massive soil infiltration, reducing to minimum the role of the underground drainage, but also represents a major vulnerability due to the big slope energy, which causes landslides and torrents. RISKS IN THE BÂSCA RIVER BASIN The vulnerability and resilience assessment represents a complex process, because it implies the presence of human communities at risk. In this area, there have been identified both slope and minor riverbed processes, especially in Gura Teghii and on the Păltiniş Valley, resulting from floods and the thaw-frost phenomenon. The villages gather on the valleys, mostly at the slope base, the main communication ways are not close to the sources, the most frequent being the forest roads. In this area, the population is scarce and old very poor. The population resilience analysis revealed that at the administrative units and domestic level, the incomes are insufficient for a decent living, the balance of these values being negative in most of the cases. The resilience represents the human capacity of recovering to the initial stage, after having suffered losses in a disaster. The causes of these risk phenomena in the Bâsca river basin are: the temperatures, the precipitation, the thaw-frost phenomena and the anthropogenic actions. The factors that determine the risk phenomena - Precipitation- The precipitation regime under the aspect of the annual amounts varying between 800 and 1200 mm in the mountain area represents a series of particularities connected to the general air mass circulation and the natural environment. The precipitation amounts lower from north to south, as a result of the wet western air masses, or the depression areas where larger amounts of precipitation are recorded due to the thermal advection. The average multiannual rainfall amount at the Lăcăuţi meteorological station over a 50-year period was of 65 mm, in 2005, the maximum precipitation amounts which produced floods over the interval May-September were of 120-220 mm at Lăcăuţi, and of 150-280 mm at Penteleu. At the Lăcăuţi meteorological station, these values are graphically represented by percentages. The fluctuation in 2005 of the monthly The percentage of the monthly rainfall amounts rainfall compared to an average year recorded in 2005 (multiannual mean monthly rainfall) at the compared to the( multiannual meam monthly Lacauti meteorological station ). values). Variation of rainfalls over the period 1971-1980 at the Lacauti meteorological station The factors that determine the risk phenomena -Temperatures- • The Bâsca river basin is located in a mountain climate with mean annual temperatures of 4-6o C. • On the basis of the statistical data analysed at two meteorological stations over the period 1961-1990, the minimum temperature in January if of -35,5oC (Intorsura Buzaului) and of -28,7oC (Lacauti). The maximum temperature at Întorsura Buzăului is of 34,2oC and at Lăcăuţi of 27,2oC both recorded in July. • The frost-thaw phenomenon occurs when the air tempertures drop below 0oC, the average date of the first day with frost is 1st of October in the mountain area, and the average date of the last day with frost is 1st of May. Distribution of mean monthly temperatures over the period 1971-1980 at the Lacauti meteorological station. Water shortage – major cause of environmental effects The water shortage is due to the lack of precipitation over a long period of time. This lack may produce hydrological and pedological effects. The tributaries of inferior order in the Bâsca Rozilia river basin have a semi-permanent flow regime. The water reserve shortage is characteristic in this basin and it produces during the winter, mostly in the period December- February, due to low temperatures. Sometimes, this deficit is not a consequence of the water shortage from the snow cover but the negative temperatures recorded during the winter, which do not allow release of water from the snow cover. Thus, the water is preserved and suddenly, in spring, together with the rainfalls, it brings its contribution to the occurrence of floods and high waters. The mean monthly multiannual flow From the hydrometric data analysis, it was concluded that the very low mean monthly multiannual discharges are in the intervals January– February and October–December. They will grow in March–September when flash floods may occur. Mean monthly multiannual flow ESTABLISHING OF THE MINIMUM, MEAN AND MAXIMUM YEAR OVER THE INTERVAL1973 – 2005 • Knowing of the maximum flow value is very important, taking into account the various negative effects produced by flash floods and high waters. The maximum flow usually occurs in spring when the fluvial organism supply is very rich (the melting snow layer combined with spring rainfalls).

Load more