B) The Canevari project
C) The images
The exceptional flooding on December 1870 occurred just few months after the annexation of Rome to Italy Kingdom; of course this could not be allowed for the town that was becoming the Kingdom Capital. Immediately the Ministry of Public Works appointed a Commission to face and solve the problem.
"... esaminare sul luogo le condizioni del fiume Tevere e dei suoi principali affluenti; di studiare quali cause accidentali e permanenti determinano i disalveamenti del fiume a Roma; e finalmente di proporre come si possono risolvere, indicando i provvedimenti e quelle opere d'arte che valgano a migliorare il sistema del fiume per lo scopo suaccennato"
["... to examine on site the conditions of Tiber river and its main tributaries; to study which accidental and permanent causes determine the river to flood out the riverbed in Rome; and finally to propose how to solve them, suggesting the measures and restoration works suitable to improve the river system for the above mentioned scope"
This is the chronology of the main events:
|1870, dec. 29th||Exceptional flooding: 17.22 m at Ripetta.|
|Study Commission appointment.|
|1875, january||The Commission work has not yet produced any result.|
|1875, may 26th||Giuseppe Garibaldi, come back to Rome on January as a Member of Parliament, intervenes in the Assembly submitting a law proposal to consider all the works to preserve Rome from the flooding as a public benefit and showing his project for complete diversion of Tiber and its tributary Aniene out of Rome area.|
|1875, july 6th||Approval of the Law proposed by Garibaldi and financing of the study completion. The Commission starts again its works.|
|1875, sept. 23rd||Commission works completion: the preferred project is the ing. Canevari one that foresees, among other things, to eliminate the Tiberina isle by earth filling of the river left branch; the Garibaldi project is definitively rejected.|
|1875, nov. 29th||The High Council approves the Canevari project with some modifications and the prescriptions (11 points) of the works to be carried out among which, at point 5, the preservation of the Tiberina island.|
|1876, dec. 3rd||The Government, taking into consideration Commission decision, let out on contract the first lot of the works.|
|1900, dec. 2nd||Exceptional flooding of Tiber river: 16.17 m at Ripetta. The banks are near to be completed and the water flow is contained by the embankments; but, during the water ebb, a 125 m piece of bank at the right of the isle, between Garibaldi and Cestio bridges, falls down.|
|1900, dec. 15th||Appointment of an Investigation Commission. New proposals are discussed among which, again, the elimination of the Tiberina isle by earth filling the left branch.|
|1901, june 24th||The Ministry of Public Works definitively rejects the isle elimination.
The engineer Luigi Cozza states that the wrong water distribution in the branches at isle sides is the cause of the embankment erosion and falling down; he rebuilds the damaged part, reactivates the isle left branch, actually filled, and builds two "bridles" under the Cestio bridge side arches and the threshold under the central one, restoring the flow equilibrium between the two isle branches.
|1915, febr. 25th||Exceptional flooding of Tiber river: 16.08 m at Ripetta. No problem for the banks.|
|1926||Completion at Rome Town Council care of the last part of the embankments just below the Aventino hill. A plaque with verses from Virgilio's Eneide located on the left bank, in front of the Ripa Grande port, celebrates the event.|
|In a recent study have been calculated the equivalent hydrometer levels, referred at Ripetta, respectively before and after the embankments erection [P.Frosini]:|
The embankments erection clearly caused a lowering of the flooding levels referred at Ripetta but an increasing of the levels as measured downstream due to the obstructed river flooding dowstream St.Paolo, contained by the new banks. It is so explained that at Tiberina isle the plaque indicating the 1870 flooding level (17.22 at Ripetta) is located about 40 cm lower than the 1937 one (16.90 at Ripetta).
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.
C) THE IMAGES
C1 - Plan of the Rome areas flooded by the 1870 flooding (dwg by S.Pascolini; from M.Bencivenga and others - 1995); the picture shows the areas flooded either by river and sewer overflow
C5 - Demolitions for the new embankment building: a column is the only remain of a medieval building on the bank at the left of Tiberina isle, now "Lungotevere dei Pierleoni" (picture by E.R.Franz)
 Pietro Frosini "Il Tevere - Le inondazioni di Roma e i provvedimenti presi dal Governo Italiano per evitarle" - Roma - Accademia Nazionele dei Lincei - 1977
 M. Bencivenga, E. Di Loreto & L. Liperi "Il regime idrologico del Tevere, con particolare riguardo alle piene nella città di Roma" In: "La geologia di Roma. Il centro Storico". Memorie descrittive della Carta Geologica d’Italia, 50, 125-172 - 1995