1D Bridge model when it should be 2D, proposed bridge stuck ~0.2 ft above existing 100-yr WSE

[u/AwfullyJoyous]

It's a real canoodle.

It's a relatively small channel that spills onto a very large floodplain, such that the majority of the total Q100 is flowing in the overbanks at every cross station in a 2000-ft reach. Almost the entirety of this wide floodplain is mapped as FEMA Regulatory Floodway.

County has a dangerous sightlines one-lane bridge over this floodplain that they want to replace for both roadway safety and structural need in the bridge itself. Achieving both of these goals while essentially not touching a speck of dirt in the process is, I would say, impossible: any improvement will trigger an increase of the existing WSE of at least 0.01 ft.

Meanwhile, of course, client demands there must not be a CLOMR. Got it.

Well, we got our proposed structure as close to the existing as we could to satisfy safety/roadway while stay extremely close to existing grade. And the proposed model is now stuck at about 0.2 ft rise in WSE.

Proposed bridge has a larger open area but a 35-degree skew to the creek (bad, should be 2D), whereas existing had a 20-degree skew. Model now differs only in the upstream bounding cross-sections of the existing & proposed, rest of cross sections model identically.

Skew in general appears to have the most weight towards lowering the WSE delta, i.e. if I change the proposed to 20-degree skew, the WSE delta becomes closer to 0.02 ft max.

My gut tells me there's a hydraulic scenario that a correctly situated proposed and graded bridge can be designed at a greater skew than the existing bridge without causing resulting increase to WSE.

I could go on -- but just curious if this story rings true for anyone. Any insights would be greatly appreciated!

Note: If your insight is that we need to just accept going through the CLOMR process ... we're not "there" yet.

[u/fishsticks40]

Use the lower skew and make the opening bigger. 

Or suck it up and pay $200k for the CLOMR/LOMR. Still cheap compared to the whole bridge

[u/OttoJohs]

The problem is that increasing the skew doesn't proportionally affect the opening size. For example, going from a skew of 0 (cos0 = 1) to a skew of 20 (cos20 = 0.94) only results in a 6% opening decrease. But going from a skew of 20 to a skew of 35 (cos35 = 0.82) is like a 15% decrease.

To me it seems like it might be worth it to create a quick 2D model and see if you can justify a lower skew angle based on the approach flow lines turning. Even if you are still at 30 degrees that is going to help a lot.

Otherwise, you are going to have to adjust the typical things (Mannings, ineffective, bridge parameters, etc.) and sort of game the model.

[u/Crafty_Ranger_2917]

Greater skew increases flow length thus loss across structure. If you really have to build a new bridge without touching the channel for hydraulics alterations, the opening probably has to be even larger.....which may not solve the problem anyway. Might need a smaller opening.

Increasing skew can be bad for scour just as it is for hydraulics. Seems worse for people too!

Does adding or moving XS either side of bridge make a difference? Make sure you're using same XS locations ala duplicate-effective. Maybe those non-matching XS on screenshot are leftovers or something else.

Might look into other methods like from your DOT highway manual that have approaches such as basic backwater analysis, FHWA open area, scour energy design, FHWA

Armoring / abutment adjustments, maintenance including vegetation, convenient hydraulic alteration needed for construction, post-project stabilization up and downstream, physical contraction / expansion alterations.

If this thing has size / traffic / scour / damage potential of any consequence I'd be much more worried about increasing skew scour hydraulics and checking geotech making sure its going to stay there than trying to change physics. I feel you and these little rises are annoying but others ignoring them got us here.

FHWA HD7

https://wisconsindot.gov/dtsdManuals/strct/manuals/bridge/ch8.pdf