Analisi attraverso metodologia GIS delle variazioni dei caratteri morfologico-sedimentari nella bassa valle del F. Sinni (Basilicata)

La nota descrive i risultati di uno studio, eseguito tramite strumenti GIS applicati ad un'analisi storico-cartografica, sulla dinamica fluviale del F. Sinni (Basilicata, versante ionico). Lo scopo è di definire le condizioni attuali dell'alveo e di caratterizzare il suo trend evolutivo, elementi considerati entrambi necessari ai fini di una corretta “gestione” del sistema fluviale (in termini di risorsa ambientale e territoriale) che tenga conto delle modificazioni prodotte dall'attività antropica e delle loro conseguenze sul sistema stesso. Lo studio si è così concentrato sull'analisi degli effetti prodotti da alcune opere idrauliche, quali in particolare la più grande diga in terra d'Europa (Monte Cotugno) e altre minori che hanno profondamente condizionato la dinamica fluviale del fiume Sinni.

The paper describes the results of a GIS implemented study of fluvial dynamics about the morphological, sedimentary and human characteristics of the River Sinni (Basilicata, Southern Italy). The aims are to characterize the present riverbed conditions and to define the evolutionary trend of the alluvial plain - riverbed system to assume criteria for a correct management of the fluvial system itself. The study was mainly focused on the determination of the impacts of some hydraulic works, first of all the Mt. Cotugno Dam, which is the biggest earth dam in Europe.
The River Sinni watershed measures 1427 km2 in area; the studied reach extends 36 km, from the Mt. Cotugno Dam to the river mouth in the Ionian Sea.
The study of morphological, sedimentary and human characteristics was primarily carried out by photointerpretation and by a field survey.
Moreover, the analysis of all the acquired historical documents (maps and aerial photographs) enabled us to measure the morphological planimetric variations of the riverbed occurred in the last 50 years.
The mentioned activities were made by the implementation of a geographic database, which is an useful tool to visualize, to manage and to analyse all the collected data.
The results show that the width of the active riverbed was progressively reduced (from 425 m in the 1954 to 110 m in the 2003); so the braiding index (from 3,1 m in the 1954 to 1,7 m in the 2003), while the extension of the vegetated sedimentary bodies increased (from 4,3 % in the 1954 to 37 % in the 2003) to the detriment of the ones devoid of vegetation. A qualitative analysis made on historical documents, anterior to the aerial photographs of the 1954, suggests that the changes started after the 1954.
Moreover we observed that such changes, pointed out by the historical analysis, didn't occur in the same way along the whole studied reach. In the first part, from the confluence with the major tributary (River Sarmento) to the check dam of S. Laura, the narrowing is substantially due to the reduction of the hydraulic discharge which caused a gradual shift from the sedimentary bodies, but there isn't evidence of vertical erosion or downcutting (which are indicators of the riverbed disequilibrium). This is happening because the sedimentary input of the River Sarmento combined with the flow modulation produced by the Mt. Cotugno Dam (which anyway causes the narrowing), are able to compensate the interruption of the sedimentary continuum due to the dam itself. Downstream S. Laura's check dam, until the river mouth in the Ionian Sea, the narrowing is connected with diffused processes of vertical erosion which induces a strong incision of the riverbed. In this reach we observed a morphological variation of the fluvial pattern: the riverbed passed from a multichannel braided type (comparable to the morphology of the upstream reach) to a single channel type with alternate bars (wandering).
The causes of the present situation, in addition to the mentioned hydraulic works, are the quarrying activity and works of flow channelization (levees and bank erosion protections).