Why a constriction destroys a channel's bed and banks
A pen note on the fury of water forced through a narrows
A pen sketch at the top of the leaf shows the constricted stretch of a channel, and the long note below explains its destructive action. Where a channel of even slope, depth and width is narrowed, the water piles up behind the pinch, then rushes through with fury as it descends. Meeting the slower water below, it is checked, follows its line of descent to the bed, hollows it out, and with eddying circulations turns against the banks, undermining them until they collapse.
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The mechanism of scour at a narrows
Where the channel is constricted the water rises up behind the pinch, then passes through the narrows with fury because it descends. Finding the still water below, it is obstructed and follows the line of its descent to the bed, where it hollows out and, with eddying circulations, turns to the banks and undermines them until they fall, as shown in the drawing above.