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| ŠUMARSKI LIST 13/2005 str. 104 <-- 104 --> PDF |
V. [vančević: BIOLOGICAL AND TECHNICAL REGULATION OF THE SEN.I TORRENT "TORRENTE´ Šumarski list - SUPLEMENT (2005), 91-109 INTRODUCTION The past disappearance of forests, particularly in the Croatian coastal region and on the islands, resulted in vast karst areas faced with a dramatic threat of complete destruction. Torrents are by far the most destructive agents, followed by different types of erosions. The local population, who sought their principal source of existence in forests, was largely responsible for these negative past trends. The environmental conditions and, indirectly, the life of the local population were severely deteriorated by the loss of forests and soil in karst areas. The authentic condition could only be restored with time-consuming and expensive biological- technical operations. The deeply cut valley of Senjska Draga with 33.80 km2 of a wider and 15.00 km2 of a narrower precipitation area is an outstanding example of such a process. The valley is situated between the mountain massifs of Velika Kapela and Velebit near the Town of Senj. There are big differences between the disappearance of forests and the related formation of karst, and their restoration to the original condition. Whereas the former processes evolve rapidly, the latter takes a long time and requires considerable financial means. Senjska Draga, the upper slope of the formerly highly turbulent Senj torrent, used to be covered with a dense forest until the second half of the 18th century, which marked the beginning of its rapid devastation. To curb this process, the military authorities of the time passed several legal regulations on the preservation and improvement of forests on karst, which also included Senjska Draga. The loss of the forest cover in Senjska Draga led to dangerous erosions and torrential sediments that constantly threatened the Town of Senj. In order to decline this threat, the torrent channel was relocated outside the town walls at the end of the 1811 century. This partial measure mitigated the risk for the Town of Senj but did not remove it completely. The Senj torrent and the erosions were finally regulated with combined biological-technical measures in its upper slope in the area of Senjska Draga at the turn of the 19lh century. The Senj Royal Inspectorate, the oldest forestry karst organisation in Croatia, had a very important role in carrying out biological and a part of technical operations. The remaining technical operations were undertaken by special government torrent services that employed foresters - torrent experts. The Senj torrent and the erosion in Senjska Draga were completely regulated with combined biological-technical measures. These measures also allowed the return of autochthonous vegetation, as well as increased the water capacity and the number of springs. The present forest cover in Senjska Draga has a primarily protective function, but also multitudinous non-commercial functions, which guarantee the preservation and improvement of the stability in the environment. The highly satisfactory results obtained from the Senj torrent regulations and erosion control in Senjska Draga may rightly be considered as one of the most successful pro jects of this kind in the entire Mediterranean area. GENERAL CHARACTERISTICS Position Senjska Draga, as a wider precipitation area of the formerly notorious Senj torrent "Torrentea", is a narrow, confined valley situated between the mountain massifs of Velika Kapela and Velebit at the 15lh meridian and the 45th parallel. From the Town of Senj inland- wise it forms a narrow valley with steep sides, and then broadens fan-like through numerous grooves and smaller coves towards the mentioned mountain massifs. The wider precipitation area of Senjska Draga takes up 33.80 km", while the narrower area covers 15.00 km . Senjska Draga may be considered a natural rarity, because this small area contains diverse geological substrates, soil types, climatic features and vegctational covers. Geological and pedological structure The largest part of the wider precipitation area is made up of Jurassic limestones (71 %), and a smaller part of impermeable Triassic rocks (29 %). The central part of Senjska Draga is composed of Triassic eruptive - amphibolic (green) schist bordered with Carnic rocks (clastites) from the bottom part in the narrow belt. Both groups of rocks are bordered with thinly layered dolomites and dolomite limestones of the Upper Triassic. The remaining, largest part of Senjska Draga is covered with alternating deposits of dolomites and the dominating Jurassic limestone. Conglomerates and differently sized stones of Pleistocene torrential origin occur in the belt of the former torrent. With regard to the geological substrate, soil, orographic features and other external particularities, Senjska Draga is exposed to strong erosions (Kovačević, 1981). Stable to minimally eroded terrain accounts for only 11 % of the total area of Senjska Draga, whereas the larger part is subject to dangerous erosion causing total degradation (64 %) and to intensive erosion (25 %). The average terrain slope gradient of Senjska Draga of 10 % is also responsible for distinct erosive processes, because any terrain with a slope higher than 2 % is erosion-prone. Naturally, stable terrains are also subject to "normal erosion", which removes to 1 nrVha of material annual |