Bypass and Diversion Channels

Bypass channels divert river flows at a point upstream of an area requiring protection. These diverted flows can be discharged back to the same river, herein referred to as a bypass channel, or into another natural drainage system nearby, herein referred to as a diversion channel. Flows into bypass and diversion channels are regulated by gates. Functioning of a bypass channel depends mainly on its location, length, carrying capacity and inlet characteristics.

Bypass and diversion channles may have the following environmental impacts:

Flow regime:

  • Increased flooding downstream, as waters are rushed through the bypass channel, leading to faster travel times
  • Little impact if the bypass channel is used only during flooding for bypassing
  • Reduced river flow, stage and velocity in the bypassed reach if the water diverts flows permanently into the bypass channel

Sediment load and channel structure:

  • Possible aggradation in the bypassed reach, if the bypass takes only flood water but does not allow for intake of its share of bed load into the bypass channel

Habitat, biodiversity and natural resources:

  • With reduced flows in the main stem of the river, streamside vegetation can encroach into the river channel, thereby changing its physical and biological character

Possible Mitigation Measures

  • Managed flow by design or operation to attain a new dynamic equilibrium under the altered flow and sediment regimes
  • A bypass channel can be planned in conjunction with a detention basin downstream of the bypass channel, in case the altered flow largely increases flooding downstream

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