The "10 volts dry summer"? I think it means you can not charge a 12V battery at that time.
The "I'v seen it as high as 79 volts Dc after a big rain"? I think it means 79V open, and you would be fine with a 12V battery and 12V dump/diversion controller.
A 24V system may be better for half the year, but that is not important for the other half of the year.
Looks like it could be dammed up another foot?
Maybe restrict the wasted region around the prop?
The long way around. A 1930s design Luftwaffe plane used that style surface. Otto Skorzeny used one for something famous, which is why I remember it is not good. It was probably a Junkers, and the corrugation was probably in line with the flow. Wherever I saw it, it was serious enough to mention.
It would be much better if the pipe was smooth.
G-