This could be totally useless info, and if so, sorry for wasting your time:
We have one of those, metal, in our driveway, to drain the water away.
There is a metal mesh, kind of like window screening, but stiff metal, under a top metal grate.
Once or twice a year my husband takes off the grate, and empties what got through to the mesh, cleans off the mesh. He then cleans out the tray, but there never is much in there. Then he puts the mesh and grate back in the tray.
In the spring, a maple tree drops a lot of what maple trees drop in the spring, and then in the fall he cleans out the maple seeds.
He's been doing this for 35 years, no problems yet.
Sounds like you need a mesh layer.