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 > 2.2 or 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 openings Install 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 SupportsOld 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