A) 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 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).