Increasing Floodplain Conveyance

Potential permits required: CWA 404, Threatened & Endangered Species, NHPA, Floodplain

OUTSIDE HELP:

Increasing floodplain conveyance spreads flood flows out across the floodplain, reducing the amount and speed of water flowing through the main stream channel. As a result, the flood has less destructive power when it comes into contact with streambanks and structures like bridges or buildings. Benefits include:

• Reduces main channel velocity and risk of erosion
• Provides greater riparian habitat
• Allows for storage of sediment
• Allows activation of overflow channels

Discussions with outside resources about floodplain conveyance should include topics such as the extent of the floodplain and your property boundaries, as well as what the proposed floodplain will look like.

As the engineers work on the design, they should be able to tell you how much soil is being moved and where. Try to include other strategies with this project. These could include:

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Willow stake planting

In many stream systems, the stream is confined to its main channel and it is unable to reach its floodplain. Where the situation allows, it is beneficial to re-shape the channel so that flood waters can flow into the floodplain.

Increasing the floodplain conveyance is changing the floodplain, and therefore will require engineering and floodplain permitting. It is paramount to ensure the main channel carries enough flow to provide a healthy sustainable system, while not conveying so much that erosion and damages are caused during large events.

Increasing floodplain conveyance could also include installing floodplain culverts at a crossing. This allows the water on the floodplain to stay on the floodplain by passing through additional culverts as opposed to the entire flood wave being forced through a single opening.

Conveyance: The amount of water moved through an area.