Geomorfologia quantitativa e morfodinamica del territorio abruzzese: II – Il bacino idrografico del Fiume Tordino

Quantitative Geomorphology and morphodynamics of a the Abruzzo, Italy: II – The drainage basin of the River Tordino

Abstract

This second work carries on the publication of results obtained through a wide and articulate geothematic researches focussed on the contribution of quantitative geomorphology to the morphogenetic and morphostructural characterization of the Abruzzo region. Particularly, the Fiume Tordino drainage basin is here considered; it is located in the northernmost part of this region, in the area between the eastern border of the Apennine (Gran Sasso) and the Adriatic shore. Morphographic and morphometric analyses, supported by a basic geological study, allowed the morpho-structural assessment of the drainage basin and the typology and intensity of the acting denudational processes to be defined in a satisfactory way. The Fiume Tordino drainage basin is emplaced on foredeep terrigenous deposits, from Miocene to Pleistocene in age; locally the Mesozoic-Paleogenic carbonatic units of the «Abruzzo sedimentary basin» crop out. The study basin shows the effects of recent tectonic activity which controlled mainly the drainage network emplacement and evolution. The whole of the surveyed morphological evidence allowed the identification of some tectonic lines of regional importance NW-SE, NNW-SSE, NE-SW and ENE-WSW oriented which played a primary role in the definition of the drainage basin morphological aspect, The azimuths! distribution of each stream order, defined after Strahler, evidenced the control on the drainage network of tectonic lines N-S and E-W trending that the morphological evidence of tectonics failed in singling out; moreover this analysis made it possible to delineate a possible sequence of the tectonic line activities, In particular, the NE-SW, N-S and E-W lines seem to have acted in recent times as they clearly controlled the emplacement of the lower stream order and, therefore, the recent evolution of the drainage network. The study basin is affected by widespread denudational processes which are particularly intense in the upper and middle portions, because of the peculiar lithological and tectonic conditions. The tectonic control on denudation intensity is evident at the basin headwater where a strongly tectonized area is present in correspondence to the thrust of arenaceous lithologies on the pelitic-arenaceous ones, Moreover, the tectonic lines conditioning the main watercourses (Fiume Tordino and Fiume Vezzola) favour their deepening which, in its turn, is responsible for slope instability conditions. In the basin middle portion some areas affected by intense morphogenetic processes are discernable; particularly, large landslides and creep phenomena are present between Campli and Teramo, also due to the structural arrangement of this area, In the lower portion, from the Torrente Fiumicino junction to the Fiurne Tordino mouth, creep, sheet, rill and gully erosion prevail; badlands are often found where clay and marls crop out. All of these processes are widespread especially on the right slope of the main valley which is likely to have been uplifted as suggested by the morphological evidence for tectonics. The distribution and intensity of denudational processes within the study basin is confirmed also by the results of quantitative geomorphic analyses. Areas characterized by fast morphological evolution are drained by poorly organized networks and show high values of «Denudation index», calculated in indirect ways. Hypsometric analysis allowed a more precise characterization of the areas affected by intense morphogenesis; in fact a close relation has been found between the plano-altimetric configurations of the different partial basins and the type and intensity of denudational processes acting within the study basin.