Flood control channel

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Image:Tujunga Wash.JPG|Tujunga Wash is an example of a concrete flood control channel. The concrete has previously failed during floods.:File:Los Angeles River - flood of 1938 - confluence of Tujunga Wash and LA River (SPCOL27).jpg

File:Pool River, Riverview Walk - geograph.org.uk - 3147150.jpg|In contrast, a channelized part of the River Ravensbourne, with deliberate gravel shoals and plantings in its bed to make the flow meander and slow it down. This is cheaper than demolishing the concrete (see images below). The adjacent park also acts as a floodplain.{{Cite web |last= |first= |date=23 September 2008 |title=The River Restoration Centre Case Study Series: River Ravensbourne, Ladywell Fields (QUERCUS) |url=https://www.therrc.co.uk/case_studies/ladywell_fields.pdf |archive-url= |archive-date= |access-date=9 February 2021 |website=The River Restoration Centre}}{{Cite web |last= |first= |date=7 February 2017 |title=London Rivers Week: River Ravensbourne, Ladywell Fields, Lewis ham |url=http://www.thames21.org.uk/wp-content/uploads/2016/06/River-Ravensbourne-Ladywell-Fields-Lewisham-1.pdf |archive-url= |archive-date= |access-date=2021-02-09 |website=Thames 21}}

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Flood control channels are large and empty basins where surface water can flow through but is not retained (except during flooding), or dry channels that run below the street levels of some larger cities, so that if a flash flood occurs the excess water can drain out along these channels into a river or other bodies of water. Flood channels are sometimes built on the former courses of natural waterways as a way to reduce flooding.

Channelization of this sort was commonly done in the 1960s, but is now often being undone, with "rechannelization" through meandering, vegetated, porous paths. This is because channellizing the flow in a concrete chute often made flooding worse.

File:Environmental assessment- Rehabilitation of flood control works, Meadowhurst Levee, Benewah County, Idaho - USACE-p16021coll7-10091 (page 7 crop to flood level graph).jpg

Water levels during a flood tend to rise, then fall, exponentially. The peak flood level occurs as a very steep, short spike; a quick spurt of water. Anything that slows the surface runoff (marshes, meanders, vegetation, porous materials, turbulent flow, the river spreading over a floodplain) will slow some of the flow more than other parts, spreading the flow over time and blunting the spike. Even slightly blunting the spike significantly decreases the peak flood level. Generally, the higher the peak flood level, the more flood damage is done. Straight, clear, smooth concrete-walled channels speed up flow, and are therefore likely to make flooding downstream worse. Modern flood control seeks to "slow the flow", and deliberately flood some low-lying areas, ideally vegetated, to act as sponges, letting them drain again as the floodwaters go down.{{cite news |last1=Bolstad |first1=Erika |title=Irony: Levees Could Make River Flooding Worse |url=https://www.scientificamerican.com/article/irony-levees-could-make-river-flooding-worse/ |access-date=26 October 2024 |work=Scientific American |agency=E&E News |language=en}}{{cite magazine |last1=Grabar |first1=Henry |title=And the Waters Will Prevail |url=https://getpocket.com/explore/item/and-the-waters-will-prevail |website=Pocket |date=30 August 2018 |access-date=26 October 2024 |language=en}}{{cite web |last1=Boddington |first1=Andy |title=Slowing the flow: managing water to reduce flooding |url=https://www.cpre.org.uk/opinions/slowing-the-flow-floods-in-england/ |website=CPRE |access-date=26 October 2024}}{{better citation needed|date=October 2024}}

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File:Ladywell Fields - geograph.org.uk - 227032.jpg|Before. Note chimney in this picture and next.

File:Ladywell Fields after development - geograph.org.uk - 691859.jpg|Immediately after re-meandering.

File:Lewisham, Ladywell Fields - geograph.org.uk - 1331455.jpg|A year later, when the banks have renaturalized somewhat.

File:Ravensbourne diversion channel in Ladywell Fields - geograph.org.uk - 2712275.jpg|Three years after re-meandering.

|caption=Another part of the River Ravensbourne of the same river have been removed from the concrete and re-meandered. Park use more than doubled and most people went from feeling unsafe there to feeling safe.{{Cite web |last= |first= |date=23 September 2008 |title=The River Restoration Centre Case Study Series: River Ravensbourne, Ladywell Fields (QUERCUS) |url=https://www.therrc.co.uk/case_studies/ladywell_fields.pdf |archive-url= |archive-date= |access-date=9 February 2021 |website=The River Restoration Centre}}{{Cite web |last= |first= |date=7 February 2017 |title=London Rivers Week: River Ravensbourne, Ladywell Fields, Lewis ham |url=http://www.thames21.org.uk/wp-content/uploads/2016/06/River-Ravensbourne-Ladywell-Fields-Lewisham-1.pdf |archive-url= |archive-date= |access-date=2021-02-09 |website=Thames 21}}}}

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File:Flood storage pond at Garshake - geograph.org.uk - 5911841.jpg|A more modern flood control measure, a flood storage pond, intended to soak up flood water and release it slowly.

File:Flood storage pond at Garshake - geograph.org.uk - 4281582.jpg|The same pond, full of the water it is detaining, during a flood.}}