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Fluvial Criteria in the FAHP

Fluvial Criteria in the FAHP. “ All permanent stream crossing replacements must provide for a fully functional floodplain”. Nat. Weather Service photo. The OTIA III Fluvial Performance Standard. Allow normative physical processes within the stream-floodplain corridor. .

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Fluvial Criteria in the FAHP

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  1. Fluvial Criteria in the FAHP

  2. “All permanent stream crossing replacements must provide for a fully functional floodplain” Nat. Weather Service photo

  3. The OTIA III Fluvial Performance Standard Allow normative physical processes within the stream-floodplain corridor.

  4. “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 movmement for common events. Important habitat features (spawning beds, refuges) preserved.

  5. Span the Functional Floodplain What is the Functional Floodplain? Rosgen’s “flood prone area” used to define the “Functional Floodplain”

  6. 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

  7. Incised stream channel

  8. 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.

  9. More openings Install relief conduits, as necessary, within existing road fill at potential flood flow pathways

  10. Remove remnant piers and fill

  11. Protecting Abutments: Staying outside the scour prism

  12. One way to protect an abutment and stay out of the scour prism

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

  14. Sometimes the FPS just doesn’t make sense Hillslope constrained

  15. Built environment

  16. Estuary or Large River Delta

  17. Coordinate with NMFS!

  18. What Triggers Mitigation?Encroachment into the General Scour Prism

  19. Off-setting Scour Prism Encroachment Remove artificial obstructions in the channel Old Bridge Supports Old Wooden Piles

  20. Off-setting Scour Prism Encroachment Remove Floodplain Fill and Constraints Approach Fills Dikes and Berms

  21. Off-setting Scour Prism Encroachment Remove Useless or Harmful Fill and Bank Protection

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