1. Fluvial Criteria in the FAHP

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

3. The OTIA III Fluvial Performance Standard
Allow normative physical processes within the stream-floodplain corridor.
A dual approach here: spanning the ffp is assumed to allow for natural channel processes.
Other element included in the fps – don’t induce downstream erosion, design to minimize debris jams
Riprap in PBO, but not specifically in the fps 3

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.
Try to avoid scour under the bridge, so no contraction scour for most events (up to the 10 year) 4

5. Span the Functional Floodplain
What is the Functional Floodplain?
Rosgen’s “flood prone area” used to define the “Functional Floodplain”
The alternative, included in the FPS but mostly as an option, is to focus on the sediment transport through the bridge opening (the incipient motion thing). The floodplain criteria explicitly includes wiggle room for some channel movement.
Rosgen selection: has a quantitative definition of an incised/unincised channel, relatively easy to determine, accepted by resource agencies 5

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
Having a functional floodplain wide enough to not be “incised” would provide adequate scope for normal stream and floodplain processes.
Streams with some degree of incision do not need the extra width – no appreciable benefit to having a wider bridge opening 6

7. 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.

8. Incised stream channel

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

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

14. Sometimes it just doesn’t make sense
Hillslope constrained

16. Built environment

17. Estuarine or Large River Delta
Will need approval from NMFS.

18. Coordinate with NMFS!

19. What Triggers Mitigation? Encroachment into the General Scour Prism
Offset volume requires approval.
Basic Goals
Approval needed

20. Off-setting for Scour Prism Encroachment
Artificial Floodplain fill
Old Wooden Piles
Berm Removal
Old Bridge Supports
Off-setting for Scour Prism Encroachment
Concrete Chunks
Unnecessary riprap