Pedestrian Bridges in Floodway 1

[ArkiePE]

My firm has landed some work with a municipality to design some pedestrian trails throughout the city. In many areas, the trail follows natural streams and there are several stream crossings. For each stream crossing, we are required to produce a "no rise" study showing a 100-yr water surface elevation increase of no more than 0.1'. This has proved to be problematic. Due to the topography, spanning the floodway for a 10' wide pedestrian trail is not feasable. This leaves 2 options: build a steel truss bridge subject to flooding, or build a shallow bridge with no handrails. Any pedestrian bridge located in the floodway must not be more than 2.5' in elevation (from the flowline of the stream to the walking surface) without handrails for safety reasons. I can't meet the "no rise" requirements on any bridge with handrails or trusses because these would block the flow. The low-water bridges (2.5' from flowline) are so shallow that base flow, blockage, and frequent flooding become an issue. Also, we are trying to span the full length of most of the streams to avoid permitting difficulties with the corps of engineers, DEQ, and US Fish & Wildlife. Has anyone else run into this problem?

 

[Ryb01]

Completing a hydraulic analysis for small pedestrian bridges seems excessive to me, especially for bridges only 10' wide. We've completed several designs and permit applications for steel and wood truss bridges for golf courses and muncipal trail crossings. I've been given the following general "guidelines" from agency personnel;

1)The bridge is to be located at a relatively stable channel reach, from a fluvial geomorphologic perspective (i.e. not at the outside of a meander belt or an otherwise actively eroding location).

2)The bridge abutments are to be set well back from the channel edges to protect the supports from future channel widening or migration. A general rule of thumb for abutment spacing is 1.5-2.0x the bankful width.

3)The cross sectional area of the bridge structure should be minimized to the extent possible to ensure that the structure will not result in significant backwater effects during severe storm events.

4) The bridge approaches should be as close as possible to existing grades to minimize fill in the flood plain and backwater effects associated with an approach embankment.

5) The distance between the bridge low chord and the normal water level should be maximized to the extent possible to facilitate debris passage and improve connectivity with the flood plain during severe storm events.

Again these guidelines have only been provided for pedestrian crossings only. These guidelines have served us well and have not had issues to date.

It's also a good idea to show a net gain environmental in the area of the proposed crossing with soft armouring techniques and planting vegetation native to channel corridors.

It may be worth putting in a call to the corps to get a better idea of what they require for such small crossings.

Hope this helps.

 

[ACtrafficengr]

Unless it is a rustic wilderness trail and not an accessible route, don't you need handrails to comply with ADA? Perhaps you should go back to the client and suggest their project scope is unworkable.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[ArkiePE]

Yes I agree with the above guidelines. I think we will be meeting with the city engineer to discuss the 0.1' rule. This requirement is pushing us into a corner to design something that is neither functional nor attractive. It will probably be on a case-by-case basis. Installing a potential blockage in a dense residential area is different than a wide open pasture. I guess the only other solution is to provide a LOMR and submit to FEMA, but the engineering fees and review fees would cost more than the bridge itself!

 

[TerryScan]

You mention the stream cannot be spanned. We did a pedestrian bridge for a greenway in our city. I looked for an image of it, and the best I came up with was a video:

http://preview.tinyurl.com/38wlu4m

The bridge is at time 0:22. The bridge has piling supports. I believe the corp required the supports to be protecected from ice/debris with the pile clusters up&dowstream of the supports. The corp also required navigational lights on the piles even though navigability ends at a spillway only a few hundred feet upstream.

 

[gbam]

How wide are these streams? Have you considered prefabbed bridges, "Stead-Fast", or precast low span culverts, "Con-Span or ConArch"? These have been used often in our area with and without piers. We typically use these to span the 404 limit but I have spec'd out Con-Span culverts to span 100-Year floodplains for the "no-Rise".

 

[beej67]

Hate to say it, but this sounds quite a bit to me like the engineer pulled that "0.1 foot no rise" criteria out of thin air, and didn't realize what it was going to entail. If you're working for the municipality, and the municipality is also your reviewer, then go haggle with the municipal engineer about it. Your meeting will be much more productive if you invite the Parks people and the Purse people to the meeting as well, so they can both see the additional cost and/or additional ugliness that the no-rise criteria is giving you.

That said .. 0.1 foot of rise is quite a bit of leeway for a "no-rise" criteria. Quite often I've seen 0.01 feet of rise as a limit. Couldn't hurt to check your RAS file for misplaced decimal points or other type-os before you go too far down the "argue with the client" rabbit hole.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[eea]

0.1' of max rise 500' to 1000' upstream is pretty common around here. Not sure what a concession with the City Engineer is worth once it gets to the Corps or DNR through the Joint Application process.

ArkiePE, have you modeled overbank culverts or additional spans? How did you determine the 100yr flow? I've had FIRM maps that grossly overstated the flow compared to basin hydrology modeling and site specific survey data. I had three Park District structures that would have been twice as long if I only used FIRM mapping for the 100 yr event.

 

[francesca]

In Indiana it's 0.14 ft at the limit of the property line. However, if you're not adding fill but merely putting in a structure that allows water to flow through, under and over it (like the bridge in the video posted above), it would be hard to create 0.1 ft of rise.