Flood control channel

The peak flood level occurs as a very steep, short spike; a quick spurt of water.

Straight, clear, smooth concrete-walled channels speed up flow, and are therefore likely to make flooding downstream worse.

[4][5][6][better source needed] Flood control channels are not to be confused with watercourses which are simply confined between levees.

They often contain grade control sills or weirs to prevent erosion and maintain a level streambed.

By definition, flood control channels range from the size of a street gutter to a few hundred or even a few thousand feet wide in some rare cases.

Really bad floods are caused by really brief spikes of river level. Channellization in concrete chutes speeds the water up and makes the flood peak higher, while slowing the water down spreads the flow out over time and blunts the flood peak.