Fluvial Criteria in the FAHP

“All permanent stream crossing replacements must provide for a fully functional floodplain”

The OTIA III Fluvial Performance Standard

Allow normative physical processes within the stream-floodplain

corridor.

“Normative Functions”• Connectivity with the floodplain:–Normal flow paths and depths during

common flood events– Allow some lateral movement of the

channel

• Maintain sediment transport and debris movement for common events.

• Important habitat features (spawning beds, refuges) preserved.

Span the Functional Floodplain

• What is the Functional Floodplain?

• Rosgen’s “flood prone area” used to define the “Functional Floodplain”

The Functional Floodplain• 2.2 times the Bankfull Width if the

Entrenchment Ratio is > 2.2or

• The width of the Floodprone area if the Entrenchment Ratio is <2.2

Fully Functional Floodplains Made Easy

• No Piers? The opening needs to be 1.5 x Active Channel width

• Piers? Then the opening needs to be 2.2 x the Active Channel width

These are the widths for permit compliance.

Hydraulic considerations may result in wider

openings.

Incised stream channel

More openingsInstall relief conduits, as necessary, within existing road fill at potential

flood flow pathways

Remove remnant piers and fill

Protecting Abutments: Staying outside the scour

prism

What if the “Fluvial Design Criteria” isn’t practical

?

Sometimes the FPS just doesn’t make sense

Hillslope constrained

Built environment

Estuarine or Large River Delta

Coordinate with NMFS!

What Triggers Mitigation?Encroachment into the General

Scour Prism

Off-setting Scour Prism Encroachment

Remove artificial obstructions in the channel

Old Bridge Supports Old Wooden Piles

Off-setting Scour Prism Encroachment

Remove Floodplain Fill and Constraints

Other Fill

Approach Fills

Dikes and Berms

Off-setting Scour Prism Encroachment

Remove Useless or Harmful Fill and Bank

Protection