storm inlet drain blocked

I assume that is a pic taken standing over the storm drain looking down into it. How interesting that someone put a sewer cleanout in there. Don't mess with that cleanout unless you want the problem to get worse real quick, I am sure it has nothing to do with handling rainwater.

Do you know where this is supposed to exit? Is it running underground to an outlet downhill somewhere? If you don't know where it goes to you probably shouldn't mess with this one.

If you know where the outlet is you MIGHT be able to put a pump in there and run a garden hose with a pressure nozzle on it down into there with the water on full and the pump keeping the drain empty and if there's no sharp bends you might be able to clear the blockage out if it's just mud. I would start with the exit end, though. If you cannot find it that doesn't mean it's not there, more likely it's a victim of years of neglect and is buried.

Sometimes these things ended up running into a sewer line or sometimes into an underground "french drain" To do this job right if you can't find the outflow you need a sewer endoscope and a sewer line locator the tooling alone is a couple grand.