A) The Tiber flow
B) The works on the right branch
C) The works on the left branch
D) The images of the construction
THE TIBER FLOW
The control of the Tiber flow in the two branches at the island sides has been always a problem difficult to be solved.
As detailed described in the monograph "Embankments", on the end of XVIII century, in order to control the river flow, it was proposed to eliminate the island by earth filling of the river left branch during the embankments erection works. Fortunately the island survived but the left branch continued to be earth filled due to the reduced water speed. This effect was increased by a design mistake during the embankments construction, as the right branch was wrongly designed wider than the left one (70m vs 60m); to achieve this the small lateral aches of Ponte Cestio were demolished (pict.A1) and rebuilt in the same width of the central one (the central arch itself was dismantled and rebuilt as it became precarious during the works). (pict.A2)
The unbalanced water sharing in the two river branches caused the left branch to be earth filled due to meagre flow and, in the right one, the collapse of part of the new embankment just built, eroded by the high flow (1900).
B) THE WORKS ON THE RIGHT BRANCH
The problem was faced and solved by the engineer Luigi Cozza that restored the balance in the two river branches reducing the flow in the right one by the construction of two "bridles" under the lateral arches of Ponte Cestio and a thresholds under the central one (pict.B1), the last also with the purpose to limit the drag effect of the riverbed.
The "bridles" are still visible in the original position, while in 1994 the threshold has been moved downstream (pict.B2) creating the characteristic waterfall (see D1).
C) THE WORKS ON THE LEFT BRANCH
A threshold was built also in the left branch, with the purpose to balance the water flows, upstream Ponte Fabricio near Ponte Garibaldi. (pict.C1; see also B1 and B2)
However the need to optimise the functionality of the thresholds for the different flow conditions of the Tiber that, in spite of its length, has a rather variable and torrential flow, brought to an overall design consisting in moving the right branch threshold downstream Ponte Cestio, as above mentioned, and replacing the fixed threshold on the left branch with a variable height one.
The works have been carried on in various steps during many years; in 2003 it has been undertaken the final phase with the realization of the mobile threshold and the conclusion of the project.
The thresholds are also essential to limit the erosion of the riverbed, but due to them the Tiber will never be navigable in the island area: this is the price to pay for the solving of the river hydraulic problems around the Tiber Island.
D) THE IMAGES OF THE CONSTRUCTION