The River Arno Project
To save time and money, Colombino decided to build two shallower channels in place of the one deeper channel proposed by Leonardo. He hoped that the flow of water down the two trenches would erode the division between them. Despite misgivings about the plan, work began on the 20th of August, 1503. Within a month, the Florentine workers had constructed two channels, the larger of which was sixty feet wide and fourteen feet deep, while the smaller was fourty feet wide and fourteen feet deep as well. However, there was mounting pressure on Colombino to complete the diversion as soon as possible as war with Pisa was intensifying. Unfortunately, he had underestimated how many worker days would be needed and the difficulties of moving so much earth. This, combined with constant attacks by Pisan troops on his workers, as well as a chance flooding of the Arno led to the projects failure. In the end, the ditches were simply not deep enough to divert sufficient water away from its course and allow the erosion of the center mass. The flood destroyed much of the partially constructed dam and the Arno returned to its original course.
The failure of Columbino's attempted diversion can be seen in this video. Note that although some water flows down his channel, a great deal still flows towards Pisa, thus rendering the diversion pointless.