The Tiber flow at the island - the mobile treshold on the left branch

A) The Tiber flow
B) The works on the right branch
C) The works on the left branch
D) The images of the construction
L'isola Tiberina
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 pict.A1 - Ponte Cestio before the demolitionby 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 pict.A2 - Ponte Cestio rebuilt with three arches of the same widththe 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).
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pict.B1 - The 'bridles' and the threshold under Ponte CestioThe 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 apict.B2 - The threshold moved dowstream Ponte Cestio 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).
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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)
pict.C1 - The threshold on the left branch of the river 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.
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D1 - The characteristic small waterfall on the right branch generated by the threshold downstream Ponte Cestio, more strong than usual due to the temporary closing of the other branch of the river (July 2003)
D2 - Closing of the left branch of the Tevere during the introductory works and the reinforcing of foundations (1998)
D3 - New closing of the Tiber left branch in July 2002 for the construction of the first section of the concrete seat that will accommodate the mobile stoplogs
D4 - The Tiber left branch is closed again for the last phase of the construction works (July 6th 2003)




D5 - At the beginning of July 2003 it has been completed the concrete seat that will accommodate the mobile stoplogs
D6 - At the beginning of August 2003 all the concrete works have been completed with the construction of the end shoulders; moreover the three mobile stoplogs have been already assembled in the final position
D7 - From this upstream view it is possible to see the square seats for the future intermediate pylons (see D12) and, on the left, the trench for accommodating the oil piping for stoplogs hydraulic control (August 8th 2003)
D8 - Detail of mobile stoplogs assembling with the temporary supports still in position (August 8th 2003)




D9 - On the middle of October 2003 the assembling of the mobile stoplogs and the relevant hydraulic pistons has been completed, so the temporary supports can be removed. Also the two pylons has been erected and one of them is visible behind the stoplog: the scope of the pylons, that have two lateral grooves as those provided in the concrete end shoulders, is to support three fixed stoplogs (see D16) to be inserted in the grooves in case it would be necessary to intercept the river flow in the left branch for mobile stoplogs maintenance. So in the future it will be no more necessary to block the river with an earth dam (October 14th 2003)
D10 - The stoplogs are now free to move
under action of the hydraulic pistons; the central stoplog (on the right) has been lowered in its concrete seat (October 14th 2003)
D11 - An overview of the whole work area; the first activation tests are in progress (October 14th 2003)
D12 - This detail photo of the right pylon allows to compare its dimensions to the worker height. On the back of the stoplogs are visible the square covers of the seats for the piston heads connections (October 14th 2003)




D13 - Through the trench in the left platform (see also D7) the oil pipes for pistons hydraulic activation, after rising the embankment wall, reach the control cabin located on the Lungotevere walkway (October 14th 2003)
D14 - The stoplogs control cabin is located on the walkway at Lungotevere de'Cenci at the beginning of Ponte Garibaldi (October 14th 2003)
D15 - Detail of the control cabin during the test phase: in the foreground the oil console and the oil pump motors (October 14th 2003)
D16 - The three fixed stoplogs foreseen to be inserted in the grooves of pylons and concrete end shoulders (see D9) to intercept the river flow in case of extraordinary maintenance
(October 24th 2003)




D17 - The works are by now completed and water starts to flow; it starts the removal of the earth dam and the restoration works (October 24th 2003)
D18 - On November 2nd 2003 the river flows again in the left branch; The works have been completed within the year as stated. The two pylons are the only visible part of the new mobile threshold
D19 - In this view from Ponte Garibaldi it is visible the threshold in operation during a normal flow situation  (November 2nd 2003)
D20 - On November 29th 2003 the Tiber level is high and the water completely submerges the pylons too

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