Floods, Rolling Stones and a Scour Hole at a Florentine Mill
Why the swollen stream runs straight, banks up a hill of gravel, then reverts to its old channel
At the 'little mill of Florence' Leonardo describes how the stream, which normally turns a right angle, runs straight in flood and strikes so powerfully that it rolls stones along and drops them at the mountain's edge. When the flood subsides the water cannot climb the gravel hill it has just built, so it turns back into its earlier channel, the one cut by the water spilling over the weir where it falls. The verso pairs this account with a pen sketch of the plunging water and the hollow it scours.
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In flood the stream runs straight and strikes hard
In its ordinary descent the water turns the right angle, but in floods it goes straight, and the impact is so powerful that, scouring, it drives the stones along with its course. The change from bend to straight line concentrates the erosive blow.
Stones rolled up and a hill of gravel left behind
The water rolls the stones up along the bank of the other stones and, following its leap out of its surface, leaves the pushed stones at the edge of the mountain. When the high water fails it cannot pass the gravel hill it has built.
The scoured hollow below the weir (pescaia)
Unable to cross the gravel, the water turns back into its first course, which was cut by the other water overflowing the weir (pescaia). That overflow forms the hollow in the very place where it falls.
Sketch of the plunge and its scour hollow
At the upper right a pen drawing shows water falling and swirling into a hatched scour hollow beside a small sluice-like structure, the deposited material sketched to one side.