My keeping our drainage ditches clean is our best water removal feature for deluge or continuous rainfall times. The taper is from a foot to about 4ft deep ditch feeding into the underground cement pipe. An interesting thing we learned is that drain pipes with that much water going in, need venting! Without air they do not drain so fast, we get flooding. We luckily had steel covers over joints where 18in cement pipe changed direction. So we are able to lift the heavy cover for air, prop it open on pieces of firewood and no more flooding! Water usually leaves very fast, even in large quantities, since our ditch is fed from along the road and fields across the road thru the 30 inch tube under the road, all downhill to us, then off to the river further downhill.
With cover closed causing flooding, the flow is EXTREMELY strong, even if only 3 to 4 inches deep. Any deeper is dangerous to try getting thru. I leave the one cover open all winter and spring for surprise rainfall, prevent any flooding. We also vented drain tile we put in across the fields and they do seem to help with faster field runoff.
Venting is something to consider if you plan new drainage. Who would have thought it?! Need for venting was interesting to learn for us! This is why manholes have holes in them. Not always enough of vent for air supply though, since one joint cover here is an actual manhole we salvaged. We do keep the second cover open for that reason, to get more air in with the water flow.