B) THE CANEVARI PROJECT
The Commission preferred the Raffaele Canevari project to the less expensive one proposed by Possenti, the Commission's President. Possenti proposed to lower the main flooding levels by shortening the river route by means of "drizzagni" [straightenings] disregarding to eliminate the normal flooding of the lower town areas; this did not supply the same guarantees than the Canevari project that was based on the principle to contain the river stream between two continuous bank walls, high enough to exceed the 1870 flooding level (17.22 m at Ripetta), the highest of the last two centuries.
Therefore Canevari disregarded the maximum historical levels, as the 1598 one, considering them unrepeatable due to the changed conditions of the Tiber source field during the last centuries; the embankments were therefore designed 18.45 m high above the Ripetta zero level.
The project stated a constant width of 100 mt at the embankment foot and 110 m at the top; the wall slope, stated either vertical and at 45% sloping, was frequently modified during the erection.
Two platforms were foreseen at the wall foot to delimit the meagre riverbed: they were designed 8 m wide, passable and accessible by means of suitable stairs (see pict. C4).
The problem of sewers overflow in the lower areas of the town, that occurred in addition to the river flooding (see pict. C1), was faced and solved by means of two sewer headers, grounded at the embankments side (see pict. C4), to collect the sewers discharge conveying them downstream the town where the river level is lower.
On the top of the embankments, at both sides, two vehicles streets was foreseen, the so-called "Lungo Tevere" [along the Tiber], that still at the present are very important arteries of the urban traffic (see pict. C4).
The original project still stated to eliminate one of the two river branches at the Tiberina isle side (see pict. C3); this solution was definitively rejected by the Commission that finally saved the isle; the width of the left and right river branches at the isle was fixed at 60 and 70 m respectively.
In order to reduce the construction schedule and costs as the design calculations as the material selection were rather approximate (see pict. C6); the 8 m wide platforms foreseen in the project at wall foot were built only partially: as a consequence of this the wall foundations proved to be weakened and exposed to water erosion. The platforms were resumed only on 1901 by Cozza after the fall of the wall, undermined by water erosion, at the right bank facing the isle (1900).
The embankment construction, very criticized for the deep alteration of the characteristic and evocative tiberine landscape (see pict. C7), has anyway in short solved the flooding problem in Rome; however the combined effects of the riverbed regularization downstream the town and the reduction of the sediments transport due to the construction of hydroelectric plants upstream the town produced erosion and lowering of the riverbed, two effects that have still to be completely solved.