Shallow Ford Stream Crossing 3

[bws2]

Hi,
Could anyone recommend a reference (or any ideas) for a shallow ford (low flow)stream crossing for a walking trail - concrete or otherwise?

I have seen a couple of NRCS details which utilize small stone held in place by large boulders down stream, but I don't feel that these will be permanent enough for our application.

Thank You

 

[PELS]

I once had to design a replacement for an AC bike path that crossed a creek that was dry in the summer but carried heavy flows with high velocities in the winter and early spring. The local Parks & Rec district originally had the AC path continue through the creek, but it got washed out every winter.

So we recommended that concrete be used through the stream portion. Due to the velocities the concrete path required a vertical stem wall on the upstream side to help resist the stream force to overturn the path. That was 20 years ago and the path is still functioning today. The concrete made for an expensive bike path, but it was offset by the reduced annual maintenance issues.

Here's some additional info on stream crossings. There is a small detail shown as well:

http://www.fhwa.dot.gov/Environment/fspubs/07232806/page11.htm

Good luck.

 

[cvg]

the cutoff wall should also be constructed on the downstream side of the crossing which arguably may be more important than the upstream side

 

[gbam]

cvg hit a good point. to mitigate for head-cutting and undermining of the crossing a cut-off wall should be placed on the downstream end of the crossing. In general I use a 4-ft deep cut-off wall downstream and a 2-ft deep wall upstream. These walls can be constructed wil concrete or gabions. If gabions are used be sure to size the rock for your design velocity.

 

[GoldDredger]

How about using CMP culvert or two? Do you have an idea of what sort of flows your can expect when you say low flow? You could have a little concrete flared wingwall on the upstream end.

How seasonal is this flow? Is it after every rain or does it take a good sized event to experience flows? Do you live in snow country?

If it is really seasonal, and not much flow, you might even use a small rectangular channel across the sidewalk, with a prefab grate on top. Small flows can go through it, larger flows can overtop it.

 

[cvg]

if this is just a trail crossing, I would hesitate before putting in a culvert. a culvert will cause ponding upstream and possibly scour downstream. This will have some environmental impacts as well as affecting sediment transport. It would still require the cutoff walls and concrete paving as previously recommended as it will overtop in larger flows. It will also require placing fill in the river which will possibly require the army corps to get involved issuing a 404 permit.

 

[bws2]

Thank you for your responses,
The stream we are crossing is in a FEMA regulated floodplain and drains a fairly large area. When I said low flow, that probably was not the correct term. Any time it rains the crossing will be impassable.

There is rock roughly 4-5' below the streambed. Based on what I am reading here and elsewhere, I am thinking of proposing a concrete crossing and keying a cutoff wall on both the upstream and downsteam end into the rock. The area between the cutoff walls and under the concrete crossing would be filled by washed stone.

Are there other issues I should be looking at? - And is this overkill for a walking trail?

 

[GoldDredger]

Not what I originally envisioned. Not only is it a waterway but it sounds like a wetland area too.

I would look into the requirements for a 404 permit as indicated by CVG above. The link below give you an idea of when a 404 is or isn't required. It's an interesting read, which this post has inspired me to look into as well.

Probably worth while to at least contact the Army Corps and document it. They might tell you a 404 isn't needed as its 'only a walking trail'.

If it drains a fairly large area, you will need to quantify the amount of storm flow. I think 300 acres and less you can use the rational formula. Are you planning on designing to the 100 year storm for the peak flow through?

How about a prefabed walking bridge or something like that. Hard for me to say, I just don't have a good feel of what the area looks like and what you need to do.

http://www.epa.gov/compliance/resou...ectors/constructmyer/myer1c_dredgeandfill.pdf

 

[CarlB]

Yeah some of this sounds like way overkill to me, for a walking trail. Your first boulder idea seemed reasonable. Are the velocities high at flood stage?

How about a "landing mat" or other metal grating, staked down?

http://www.usace.army.mil/publications/eng-pamphlets/ep870-1-42/c-3-4.pdf

 

[civilman72]

If you truly have a low profile crossing, a cheap solution that I have seen used when walking trails cross a drainage includes wood planks (walking surface) placed on railroad ties. It's kind of like a real cheap bridge, but you can make the opening underneath the planks as wide as you want. So you can calculate the anticipated design flows, and generally can make the width of the crossing sufficient to handle these flows.

The ACE in my region has actually encouraged these types of stream xings for trails for a few reasons. One, the timbers do not create a concern with water quality, but pouring concrete next to a sensitive stream crossing does concern them, particularly during construction. Second, the actual "impact" to the stream bed is limited to only the area under the railroad ties, which again would be less than a concrete structure.

Sorry, can't offer any specs, but volunteer trail builders have been installing these successfully over the last ten years, so I'm guessing there's not much to specs for this type of construction.

 

[bws2]

CarlB,
After taking a closer look at it, and doing some more reading, I think you are right. We are going back to the original thought utilizing stone. We have done some velocity calculations just assuming bank full and it is somewhere in the range of 3 fps. I think the stone is a reasonable solution.

Thanks for all the great ideas.