Eddies on a riverbed and the sorting of gravel, sand and mud
How fallen bank-material splits the current and how the bed is stripped by fast shallow flow
This page studies eddies (retrosi) forming over a riverbed, showing how an uneven bottom sets up contrary motions between surface and bed. Leonardo traces the unevenness to the bending of banks and to material collapsed from them, then analyses how a lump of fallen earth at a splits the current into the eddy n and the onward flow to c. He notes that a swift, shallow current strips the bed down through coarse gravel, sand, mud and leaves. Two wavy-line eddy diagrams accompany the notes.
On this page
Contrary motions of water over an uneven bed
Water flowing over an uneven riverbed sets up contrary motions between the surface and the bottom. Leonardo attributes the unevenness of riverbeds to the bending of the banks and to material that has collapsed from the banks and settled at their feet.
Eddy n formed where earth falls into the current (a, b, c, n)
Earth fallen from the bank comes to rest underwater at a; the current b strikes it and divides, part turning back to form the eddy n and part passing on to c. The current b c keeps equal velocity on the side c, producing two contrary eddies.
Sorting of gravel, sand, mud and leaves by current speed
Where the water is shallow, the current runs as fast at the bottom as at the surface, so it lays bare the coarse gravel, then the sand, then the mud, then the leaves. Light material comes to rest wherever the flow slackens.