How to Improve Drainage:

Remove the obstacles. Callan found this clip at Geo Slice. I found another copy (that I could embed) at YouTube. A commenter at Geo Slice left a link to an extended (and higher quality) clip at Truveo. (You'll be forced to sit through a 15-second commercial with that one.)

This is a cool depiction of the power of running water, but my favorite part (at about 2:50 in the above clip) is when the whole culvert rears up out of the ground like Shai-Hulud in Dune. In the extended version, some of the later, post-flood, scenes give you a much better sense of scale than you get in the (sensibly) farther-removed shots taken during the event. It's not nice to fool Mother Nature, but unless you really know what you're doing, it's downright dangerous to fool with her.