No Overflow

Chew Valley Lake, one of the largest reservoirs in the country, Mrs B has read somewhere, and it was a delight to walk around (in part) on a breezy day and sunny day.

This is the overflow, and it's dry.  But here's a question for any engineers in my audience: why do such channels always have ridges built into them?  Is it to slow the flow, to generate turbulence to reduce the growth of weed?