Flood control channel

  • Tujunga Wash is an example of a concrete flood control channel. The concrete has previously failed during floods.[1]
    Tujunga Wash is an example of a concrete flood control channel. The concrete has previously failed during floods.[1]
  • 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.[2][3]
    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.[2][3]

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.[4][5][6]

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.

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.[4][5][6][better source needed]

  • Before. Note chimney in this picture and next.
    Before. Note chimney in this picture and next.
  • Immediately after re-meandering.
    Immediately after re-meandering.
  • A year later, when the banks have renaturalized somewhat.
    A year later, when the banks have renaturalized somewhat.
  • Three years after re-meandering.
    Three years after re-meandering.
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.[7][8]
  • A more modern flood control measure, a flood storage pond, intended to soak up flood water and release it slowly.
    A more modern flood control measure, a flood storage pond, intended to soak up flood water and release it slowly.
  • The same pond, full of the water it is detaining, during a flood.
    The same pond, full of the water it is detaining, during a flood.
  1. ^ File:Los Angeles River - flood of 1938 - confluence of Tujunga Wash and LA River (SPCOL27).jpg
  2. ^ "The River Restoration Centre Case Study Series: River Ravensbourne, Ladywell Fields (QUERCUS)" (PDF). The River Restoration Centre. 23 September 2008. Retrieved 9 February 2021.
  3. ^ "London Rivers Week: River Ravensbourne, Ladywell Fields, Lewis ham" (PDF). Thames 21. 7 February 2017. Retrieved 2021-02-09.
  4. ^ a b Bolstad, Erika. "Irony: Levees Could Make River Flooding Worse". Scientific American. E&E News. Retrieved 26 October 2024.
  5. ^ a b Grabar, Henry. "And the Waters Will Prevail". Pocket. Retrieved 26 October 2024.
  6. ^ a b Boddington, Andy. "Slowing the flow: managing water to reduce flooding". CPRE. Retrieved 26 October 2024.
  7. ^ "The River Restoration Centre Case Study Series: River Ravensbourne, Ladywell Fields (QUERCUS)" (PDF). The River Restoration Centre. 23 September 2008. Retrieved 9 February 2021.
  8. ^ "London Rivers Week: River Ravensbourne, Ladywell Fields, Lewis ham" (PDF). Thames 21. 7 February 2017. Retrieved 2021-02-09.