A lab observation of this can be seen here . He doesn't mention if subcritical flow can become supercritical however.
Centrifigual force causes the flow to thin on the inner wall and thicken on the outer wall.
By mass conservation the flow accelerates on the inner wall. Chow p. 447 has a formula to find the depth and speed of fluid passing through the bend of a channel. The speed and depth across the channel depend on the radius of curvature of the bend and the channel width, as well as the specific energy E of the flow before it reaches the bend. I wanted to find out if subcritical flow could become supercritical going around the 32 degree bend of Cape Mendocino. For subcritical upstream flow and a wide channel, I chose 10 km as the radius of curvature of the Cape Mendocino bend and used Chow's formula.