River waves striking eddies at a widened bank
With a plan for ordering the book on rivers and their obstacles
Leonardo argues that when a columnar wave strikes the eddies formed where one bank widens, those enclosed eddies narrow and gain great power to scour beneath the bank and ruin it. A passage headed 'Order of the book' lays out how the treatise should proceed: first what a river of even depth and slope does against obstacles, then obstacles in pairs, then matched obstacles on both banks, describing how the flows intersect midstream and act on the bed. A closing note observes that the swift, incident side of a wave is the end of its slow reflected motion, so the trough runs fast while the crest lags. Three figures in the right margin illustrate the waves produced by such obstacles.
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Enclosed eddies scour beneath a widened bank
Where one bank is widened, the columnar wave strikes the eddies formed there; being confined, these eddies narrow and gain great power. That concentrated force digs beneath the bank and undermines it until it collapses.
Order of the book: a program for studying river obstacles
Begin with what a river of equal depth and equal bed-slope does against obstacles of every kind. Then set the obstacles two by two, then face them with matching obstacles on the opposite bank, describing how the crossing flows interfere midstream and what each does to the bed by scouring and depositing.
Trough runs fast while the crest lags
The swift incident side of a wave is the end of the slow reflected motion. It follows that the motion of the wave's valley (trough) is fast and the crest of the wave is slow.