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. 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

“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

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

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

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 it just doesn’t make sense Hillslope constrained

Built environment

Estuarine or Large River Delta Will need approval from NMFS.

Coordinate with NMFS!

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

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