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Raise the Channel or Lower the Floodplain? Methods for Floodplain Reconnection

Brett Long
Biohabitats, Inc.
Baltimore, MD

Floodplain reconnection offers many benefits including reduction of erosive forces on stream channels, temporary storage of floodwaters, and rehydration of adjacent riparian areas. Many stream restoration projects seek to capitalize on these floodplain reconnection benefits by reconnecting the floodplain using two primary approaches: raising the existing channel elevation or excavating the floodplain adjacent to the channel at a lower elevation.

Raising the channel has advantages and disadvantages depending on existing site opportunities and constraints. Advantages include relatively lower costs due to lower excavation volumes and less impact to existing riparian vegetation.  Disadvantages include temporary disruption of existing channel substrate/biota and increased flood elevations.  This approach is advantageous where increased flood elevation is allowed and/or impacts to high quality riparian vegetation is of significant concern.

Excavation of a floodplain at lower elevation adjacent to the existing channel typically has opposing advantages and disadvantages compared to the raising the channel approach.  This approach is typically advantageous where significant increases in flood elevation are not allowed and/or existing riparian vegetation is low quality.

Other consideration when choosing the approach for floodplain reconnection include the presence of existing of FEMA floodplain/floodway designations on the project reach, excavated sediment disposal locations, existing floodplain wetlands, and the potential presence of threatened/endangered species.

Two examples of projects of the floodplain restoration approaches included the Stream Restoration of Unnamed Tributary to the West Branch of Herbert Run at the University of Maryland Baltimore County and River Valley Ranch Stream Restoration in Carroll County Maryland. The UMBC project is an example of raising the channel approach using excavated fill and riffle grade control structures to reconnect the incised stream channel to its adjacent floodplain.  At the UMBC site, increases in the floodplain elevation were allowed as there were no occupied structures in the floodplain impact area and adjacent riparian vegetation was poor quality.  Representing the floodplain lowering approach, the River Valley Ranch project was on a reach of the Gunpowder River that had FEMA regulated floodplains/floodways and occupied structures located along the project reach, so floodplain excavation was deemed the preferred restoration approach at this location.