 |  |
| Bildiri Özetleri | |
| Ana Sayfaya Dönüş |
| Back To The Main List |
|
A.Cioaca 1 , M.Dinu 2 1 Spiru Haret University, Bucharest 2 Romanian-American University, Bucharest Abstract In the last century, people became interested in investigating Dobrogea's soil and subsoil resources. As a result, this semiarid area, long deemed hostile to human settlement, started being populated so that more and more localities began cropping up. The increase of population was associated with the need for more terrain, which in turn prompted in-depth studies into the region's environment. The present approach tackles the problem of the relationship between the state of environment and the geomorphic processes peculiar to Dobrogea's semiarid space, with focus on the behaviour of surface formations to the impact of torrential rains. Although the region covers a fairly small area (10,400 km2), it nevertheless has a highly varied relief. The diversity of landforms is structure-controlled, petrographic conditions and successions of modelling systems stamping their mark in the course of time. What distinguishes Dobrogea's climate from other regions in Romania is the annual average temperature, somewhere over 110 , and the west-east - north-east decrease of precipitation from 450 mm/year to 350 mm/year. Regardless of lower intensities from west (7.5 mm/min) to east (2.5 mm/min), the risk for extreme climate phenomena to set in and affect the relief by torrential rain-induced processes in the valleys is by no means reduced. The final part of this study contains a map of rain-induced soil erosion risk in Dobrogea. Dobrogea, Brief Historical-Geographical Outline Surrounded by the Black Sea (Kara Deniz), in the east and the River Danube in the west and north, Dobrogea is quite a peninsular territory. The River's floodplain is dominated both by the Baragan Plain and the Delta's alluvial plain. The region is situated in the south-eastern part of Romania and covers 10,000 km2, that is 4.3 % of the country's surface-area. In Ancient Times, the first to colonise were the Greeks. Next came the Romans, who conquered and annexed it to the Empire by the name of Scitia Minor. After the Roman Legions left Dacia, the province remained a northern outpost of the Eastern Roman Empire (Byzantine Empire) and fell into decay. Eleventh-century documents speak of it as Paristrion. Between 1186 and the 14th century it was part of the Bulgarian Empire. In the 14th century, the Empire crumbled and this Principality, led by Dobrotich (from whom its name is derived), was temporarily annexed by the Romanian Principality of Walachia. For five centuries Dobrogea stood under the Ottoman Turks who changed the old place names eg. Callatis became Mangalia and Tomis, Kiustenge etc. On October 8/20, 1878 the province of Dobrogea was reunited with Romania. Although proofs of ancient and continous inhabitation from the Paleolithic (at Adam and La Izvor), the Neolithic (at Hamangia and Gumelnita) and the Bronze Age (Tracian population) do exist, the population began growing and increasing its pressure on the environment only in the Antiquity, after Milesian and Dorian Greeks settled at Tomis and Callatis, after the Romans conquered Dobrogea turning it into a Roman province (46BC) and especially after the Byzantine occupation that lasted up to the 10th century. The present geographical space between the Danube and the Black Sea is known as the Dobrogea Plateau, a name circulated in the 20th century due mainly to Bratescu's work, Die Dobrudgea (1909). The Current State of Dobrogea's Environment Geology : Much of Dobrogea's territory overlaps the Walachian Platform bordered by the Danube fault in the west, Sfântu Gheorghe fault in the north and a flexion in the east that extends into the Black Sea littoral platform. In the southern part, the province stretches out towards the Balkans (Bulgaria). The process of its formation was a stagewise event, beginning with the Lower Proterozoic during the Karelian orogenesis, followed by the Baikal orogenesis which shaped an exondated relief developed on granitic gneiss and crystalline schists in the south and green schists in the centre. In the Upper Proterozoic-Lower Paleozoic, the Hercynian domain emerged in the north, revealed by magmatites generating granite masses in Macin area. There were several synchronous sedimentation cycles in southern Dobrogea: Paleozoic (quartzites and clays); Jurassic-Barremian (carbonite deposits); Cretaceous (micro-conglomerates, sandstones and marl-limestones); Paleogene (numulites limestones); Miocene-Badenian (clays, limestone, marls); Sarmatian (lumashele limestones ), and finally Pliocene (marls, sands and Romanian lacustrine limestone). In the northern and central Dobrogea there are only two sedimentation cycles: Jurassic (limestone) and Cretaceous (littoral facies). That multitude of stages would explain the huge diversity of rocks: crystalline schists, clay -schists, quartzites, limestone, granitic magmatites, limestone marls, and Sarmatian lumaschele limestone. The wide expanses of surface loess and loessoid deposits (Cotet, Cioaca, Anton, 1968) form most of the parental material. The relief : As a result of a long subaerial evolution, the average altitude in Dobrogea is no higher than 125 m. However, in spite of it, altitudinal variation does exist and it reflects just the differences imposed by evolution stages and structural-petrographic conditions. Maximum altitudes occur in the north-west, along the Pricopanu Summit (Pietrosu, 426 m, Tutuiatul, 467 m, Moroianu, 428 m and Sacari, 406 m). Here are also summits and isolated peaks (over 300 m) and large plateau sloping down towards the Cernavoda - Constanta axis (under 100 m). The wide, flat southern interfluves mount up to 100-200 m. Since the relief in general is not very rough, reconstituting the levelled surfaces; ie. the Dobrogea peneplane (Cotet, 1968) interpreted as polygenetic pediplane (Posea, 1974), is not that difficult. Most geographers use to single out four levels: Greci (400 m), Niculitel (300 m), Tulcea (180-200 m), and the Quaternary (80-100 m). Our study addresses particularly glacis and pediments because these are the sites where torrential events take place, the floodplains being affected by fluvial processes. Reconstructing the terraces along valleysides is a more difficult matter because what has been left are largely fragments, or fossilised traces under colluvial-proluvial deposits. The Casimcea terraces (3 out of 5 of which only two for sure, the 2-5 m and the 10-15 m ones) and the 150-to-over-1,000 m wide floodplains, with 8-20 m-thick alluvial deposits, could be identified more easily. The structure-controlled relief shows a huge variety of forms: syncline valleys (Luncavita and Slava), anticline valleys (Valea Alba), depressions carved in anticlines (Megina and Boclugea), lithologic contact valleys (Peceneaga and Fântana Mare), tectonic depressions (Nalbant and Cerna-Mircea Voda). But our attention focused on the petrographic relief, particularly the one developed on loess and loessoid deposits, because in semiarid conditions gullying is more obvious there. The climate : The main climatic variables designating as Dobrogea semiarid region (Stanescu et al., 1993) are the thermal regime and the atmospheric precipitations conditional upon the radiative and dynamic factors of climate and topography. Since the sun shines for 2,200-2,500 hrs/year, the eastern part of Dobrogea registers the highest annual global solar radiation value in Romania, ie., over 132.5 kcal/cm2. Hence, the radiation balance during June and July is increasing, so that in midday Dobrogea's active surface benefits by over 0.78 cal/cm2/min, the highest value in this country. The air temperature averages over 110C/year towards the littoral area and the Danube floodplain, and no more than 100C and 110C in the north and centre. Precipitations have a similar distribution pattern : 375-400 mm/year towards the Danube and the Black Sea, and up to 450 mm/year or even over 500 mm/year in the north. There are 40 tropical days /year, and 220 frost-free days/year on average. Torrential rains (assessed by Hellman's criterion at 2.5-7.5 mm/min are absent in 85% of cases (years), and when they do fall, they cause havoc even on very mild sloping surfaces. Over the 1970-2000 interval, exceptional torrential events were locally recorded in the warm season as follows: July 1977, June, 1985, July 1993 and July 2001, enhancing sheet and linear erosion. As a result, important quantities of material were disloged and transported from these low-declivity surfaces. The waters : Deficitary water sheets lie at the basis of shifting deluvial deposits. In the floodplains, where alluvial deposits are thicker and groundwater structures are not very deep, they are closely relating to the other environmental factors (eg. in the Carasu Valley, Basarabeanu, 1973). The situation of deep water sheets occurring in limestone and other carbonate rocks is quite different. The surface net, supplied in proportion of 85% by rain water, has intermittent streams so that runoff from slopes, or sediments moved by floodwaves, are only occasionally evacuated. The eastern basins (Telita with Taita, and Slava with Hamangia, Nuntaşi, Casimcea and Mangalia) show frequently accumulations of alluvia in the floodplains, largely obturating the stream channel and forming a pile of sediments swiftly carried down by the floods and causing unpredictable changes in floodplain morphology. Therefore monitoring them becomes imperative. In the wake of exceptionally heavy rainfall, the rapid transport of eroded material from cultivated interfluves or from floodplains and terraces used as arable land, affects the tributary basins of the Danube, too (Topolog, Tibrin, Carasu, Rasova, Urluia, and Ceair). Exceptionally high floodwaters, with a great erosive power, carry important quantities of sediment load, the mass of fluid slush, posing a great threat to the human communities living on their banks. As a matter of fact, Dobrogea's rural population has long been aware of this aspect of intermittent watercources, naming it "seruri" (slimy stream). The vegetation : There are few forest-covered areas. Forest groves are seen only in the northern areas (Macin Mts, Tulcea Hills) and in the centre of Dobrogea (Babadag and Casimcea plateaus). They represent the mesophyllous belt with Mediterranean, Balkan, and Turanian-Caucasian elements, and the xerothermal belt (lime and oak). In the south-eastern part of the region the mesophyllous belt is represented by ash and hornbeam. In the sylvosteppe (over 100 m alt.), and steppe (under 100 m alt.) there grow couch grass feather grass and wormwood, but there are large expanses of non-agricultural areas, too. The soils : The soil cover belongs to the East-European Region, Danubian-Pontic-Province with excessive continental-temperate shades (Bucur, 1948). The majority of soils are molisols: cambic chernozems in the sylvosteppe; chocolate chernozems, and carbonated chernozems in the steppe, on the marginal glacis and on the low, flat interfluves, respectively; on the western and eastern parts of Dobrogea representative soils are kastanozems. Soils variously affected by podzolisation, more precisely the argillic brown soils, are found at depth of 250 m under the oak forests of the Macin Mountains, on Niculitel Plateau or Babadag Plateau. Intrazonal soils (erodisols and lithosols) have a local spread (Butnaru, 1962). They are specific to areas affected by torrential processes (sheet wash or rilling), and occupy the interfluves between the Telita and the Carasu valleys, a perimeter representing one of our case-studies. Dobrogea's soils are as a rule quite loose (0-053%), and have a moderate humus and nutrient content. Their vulnerability increases as we approach the littoral zone. Human pressure on the environment : The population registered a progressive increase from the Antiquity until the 10th century. When Dobrogea fell under Turkish rule (1417, Mohamed I) and throughout the centuries-long Ottoman occupation, the population decreased, but pressure on the environment did not diminish, on the contrary, the effects of the than massive afforestations are seen to this day. At present, Dobrogea's population numbers over one million inhabitants, the demographic trend indicating a slight decrease from 1,018,000 in 1992 to 1,014,000 in 1996. The rural population lives in 85 communes that englobe 306 villages, but it is unevenly spread in the territory, eg. 28% in Constanta County (southern Dobrogea) and 52% in Tulcea County (central Dobrogea). The average density of 100 inh/km2 drops in the central part (Casimcea Plateau) and rises on the southern coast where towndwellers are concentrated (Constanta 348,000 inhabitants, Mangalia 44,000 and Navodari 33,000). Soil Erosion Processes in Dobrogea Trigerred by Heavy Rainfalls : Geological built-up, together with relief, climate conditions and plant cover and human pressure not the least, are all favouring the development of a wide range of slope and channel geomorphic processes(Cioaca, 1996; Cioaca, Dinu, 1998; Dinu, Cioaca, 1997). Gullying and sheet wash are a frequent occurrence in much of the steppe and sylvosteppe used for hoeing cultures. The intensity of gullying-induced soil erosion in Dobrogea depends on the pressence of brittle deposits, steep slope sectors and moreover the torrential regime of the few precipitation falling there. Often enough, thermal convection would generate very intense local continental-type rains, associated with storms, lightning and hail, which in a short time-span pour down quantities several times the average of the respective month (Bogdan, 1995). One of the many cases of onset and intensification of gullying in the wake of torrential events studied by us illustrates this situation. On July 27, 1977, in a small area (cca 50 ha) belonging to the administrative territory of three communes (Ceamurlia, Stejaru and Beidaud), a five-hour rainfall discharged 108 mm of water, at an intensity of 0.2 mm/min in the first hour and 0.5 mm/min over the next four hours. As a result, nearly 350 tons of soil from the maize-planted interfluve were washed away, causing a loss of over 7 t/ha. The soil was transported through several ditches and 12 gullies whose drainage basins covered a total of 31 hectares. While the first part of the interval was dominated by high turbidity liquid flow, as the rain became more aggressive, more soil particles were entrained, the flow turning into a slimy mass which, reaching the slope foot, formed a 0.35 m-thick mud layer that covered people's courtyards and the streets of Beidaud village. Several wells from Hamangia Floodplain were mud-filled to the brim and no longer usable. The chance to assess right away the quantity of transported material and torrential rain variables made us extend the investigation to a wider area of the Dobrogea Plateau. Similar events associated with slimy mudflows, called "seruri" by the local Turkish-Tatar population, have been signalled to us in the course of time. In the last decade of the 20th century, a heavy and very aggressive hailstone-attached rain affected a strip of land stretching out between Valul lui Traian, Constanta and Agigea. The water quantities registered on that occasion (from 4.00 am to 12.00 am) were 100 mm at Valul lui Traian and Constanta, and 110 mm at Agigea. Noteworthy, the rain peak was reached between 7.00 am and 9.00 am, when over 80% of the total water quantity fell, associated with hailstones for over 35 minutes. Damage was done to the southwestern part of Constanta city (down-sagging on loessoid deposits in Bratianu district, affecting over 1,400 residences), the agricultural lands of Cumpana-Lazu-Agigea area, sinkings of the Danube-Black Sea Canal valleysides, and short fixed valleys forming on slopes and disloging over 300 tons of soil. In the conditions of an arid-like climate, particularly in the eastern part of Dobrogea, with annual precipitation means of under 400 mm, torrential rains may fall once in three years, and exceptional events occur once every ten years. Our research looked at soil quantities dislodged or lost in their wake, and at the relationship between this phenomenon and the periodicity of exceptionally heavy rain occurrences in the northern and central Dobrogea. The findings corroborated and completed the data reported by other researchers on the central and southern part of this territory (Bogdan, 1995; Cheval, 1997), revealing significant correspondences between the torrential regime of the June-August interval and the quantities of soil lost through heavy rainfall, especially on the side of the Dobrogea Plateau facing the littoral zone. Two areas, differing in point of soil erosion intensity, were found to be at risk from geomorphic processes (fig.1):
B. The central-western strip, contraining hill-like interfluves and that side of the Dobrogea Plateau facing the Danube, covering 2/3 of the region, is at moderate and low risk from heavy rain-induced processes (soil loss ca 33 t/ha/year).  References . Basarabeanu, N. (1973), Apele subterane din lunca vaii Carasu, An. Univ. Bucureşti, XXII, seria Geografie, p. 177-186, 2 fig. 1 tabel; . Bogdan, Octavia (1995), Un caz exceptional de grindina la Constanta (1 iulie 1992), S.C.G., XLII, p. 81 - 89. . Bucur, N. (1948), Cartarea agrogeologica şi agronomica. Reprezentarea cartografica a eroziunii solurilor, Anal. Inst.Agronom. . Butnaru, V. (1962), Contributii la studiul eroziunii solurilor, An. şt. Univ. "Al. I. Cuza", Iaşi, . Cheval, S. (1997), Variabilitatea şi tendinta de evolutie a precipitatiilor pe intervale agricole caracteristice la câteva statii meteo din sudul României, Geographica Timisiensis, VI, p. 47 - 57. . Cioaca, A. (1996), Evaluarea vulnerabilitatii terenurilor afectate de procese geomorfologice actuale,Analele Univ. "Ştefan cel Mare", Suceava, sect. Geogr.-geol.,V, 1997, pp 41-44, 1 fig. . Cioaca, A., Dinu, Mihaela (1998), Sensibilitatea reliefului deluros din judetul Argeş, ca urmare a schimbarii utilizarii terenurilor, An.Univ."Ştefan cel Mare", Suceava, VI, 29-40, 3 fig., abstr. . Cotet, 1968), Sur la peneplenization du Dobrougea, Cahier de geogr. Montreal, 6, Canada. . Cotet, P., Cioaca, A., Anton, O. (1968), Câteva profile în loessul din Dobrogea de Nord, Geografia Dobrogii, SSG filiala Constanta, p.162 - 169, resume, 4 fig; . Dinu, Mihaela, Cioaca, A. (1997), Precipitation - induced landslides in the Moldavian Plateau (1996/97), Rev.Roum. de Geogr., 41, Bucureşti, p. 69 - 80, 5 fig., 1 tab., abstr. . Posea, Gr. (1981), Pediments in Romania, RRG, Bucureşti. . Stanescu, V., Al., Adler, Mj., Cusursuz, B., Tuinea, P., Burcea, G., Ciuntu, A. (1993), Study of droughts in Romania for the assessement of aridissation and desertification trends, Romanian Journal of Hydrology and Water resources, 2. |