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1.
An urban waterfall is a natural waterfall located in an urban area that has some man-made changes done to it, such as enclosing the natural creek above or below the falls into a storm sewer or box culvert or the construction of a bridge or road over the creek above the falls. The actual waterfall itself normally has not changed, just its natural surroundings have been changed by man.
Dundas Falls located in Hamilton Canada is an urban waterfall because Spencer Creek above the falls has been enclosed into a large storm sewer pipe as shown on this link:
http://www.conservationhamilton.ca/parks/visit/dundasfalls.asp
(copy and paste)
by Mr. Hamilton Waterfall March 27, 2009
 
2.
A constant flow of rainwater caused by overflowing spouting and/or guttering.

most commonly seen in schools and blocked gutters.
-"What a skody school"
-"I know their such a beautiful freak of nature"
-"what are?"
-"urban waterfalls"
by SAMTHEMAN177 June 14, 2010