The fluvial dynamics of the Arno River - 4. Map of the morphological-sedimentary and human characteristics of the Arno river bed and alluvial plain (Table 7 “MONTEVARCHI”).

The results of a survey of the morphological-sedimentary and human characteristics of the Arno River bed and plain of the upper Valdarno, including the confluence of the Ambra Creek and St. Giovanni Valdarno, are reported by means of a thematic map at 1:10,000 scale. In this stretch, the river flows in an almost straight line with alternate side bars, within a plain having an average width of 1 to 2 kilometers. The present planimetric form of the river bed derives from the number of training works and canalisations undertaken in this section from the beginning of the 17th century, and which have caused, as the most visible consequence on the morphology of the course, a noticeable reduction in the width of the bankfull channel (from 300-500 metres to the present 150 metres), and the disappearance of interlacement, which was very frequent before 1700. During recent decades, the morphological and sedimentary characteristics of the river have been strongly influenced by intense gravel extraction, and by the construction of the artificial lakes of Levane and La Penna immediately upstream. These factors have certainly influenced the sedimentary equilibrium of the river, causing or encouraging erosive phenomena: the river is affected by a notable lowering of its bed, while the planimetric activity is very limited since there is hardly an erosion of the banks. The sedimentary bodies have a low mobility and a tendency to remain "suspended" with respect to the present level of the low waters. The sedimentary characteristics of the river bed are also considered in this paper and are graphically documented: the river is gravelly, with the average diameter of the material being between -3 and -6 phi. The bank protection works grant the river bed an elevated planimetric stability. On the other hand, the lowering process of the bed has made consolidation work on existing bridges necessary. The construction of a dam by E.N.E.L. has contributed to an altimetric fixation of the river bed bottom. The main aquifers present in the alluvial plain are distributed in correspondance to the coarse flood material, especially along the ancient river bed. Usually, the groundwater is affected by a field of motion which converges towards the river and which seems to be fed by them, except for local and temporary inversions of the groundwater-river relationships, especially in those areas with a greater water withdrawal. As far as the risk of flooding is concerned, this paper is limited to reporting the situation after the flood of November 4, 1966 (the maximum flow was reached in Montevarchi: 2,580 m^3/sec). On this occasion however, the tributaries caused the most damage. The flooded area of 1966 can only give us an indication of the areas which are more at risk of flooding should the same conditions repeat themselves, even if one must consider that some of the tributaries have undergone works of reinforcement, bridling and raising of the embankments. Only a few points of localised flooding have remained more or less unchanged, for example around bridges.