FWIW One of my waterways was originally filled with 3 to 5 inch limestone rip rap. A heavy rain washed probably 20 yards of it and the clay underneath into the pond to create a "trench" 3 to 4 feet deep by 8 to 10 feet wide. I lined the new "trench" with 12 to 15 inch rip rap and the problem has not reoccurred even though we have had several heavy rains since. I ran the rip rap up the sides similar to what Snrub pointed out above and have had no problem with "flow around" erosion.

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