70-foot Seasonal Bridge
May 16, 2016 2:26 PM   Subscribe

I'm looking for ideas on how to construct a seasonal bridge across a 70-foot wide river using man power only. The bridge would be built at the beginning of summer and dismantled at the end of summer to be used again the next year.

A summer camp I work with has to put in a seasonal bridge every year. The current steel bridge requires serious heavy equipment to install and remove, and that costs ~$10k per year. The total camp budget is $50-60k per year, so this is a major expense. I'm looking for other bridge ideas that could be installed and removed with manpower alone. The current bridge can support full size vehicles, but the new bridge would only have to support an ATV (and preferably horses).

The river is ~70 feet wide at the beginning of summer. We are not allowed to install any permanent structures; the river is federally designated as a Wild and Scenic River. My only idea so far is a floating pontoon bridge constructed in sections, with aluminum decking, and anchored by some cables to boulders upstream. I found a place that could make custom pontoons from polypropylene. I checked, and such a bridge would be acceptable to the Dept. of Fish and Game.

Do you have any experience with pontoon bridges or other removable bridges? Thank you!
posted by huckit to Travel & Transportation (16 answers total) 1 user marked this as a favorite
 
Would it be possible to find a decommissioned pontoon bridge from a military outlet?
posted by banannafish at 2:30 PM on May 16, 2016 [1 favorite]


I doubt you can do this any cheaper and meet your permit requirment and be safe. Pontoon bridges aren't suitable for horses for starters and there are a LOT of liability and permitting issue you seem unaware of. Just the fact you are posting here concerns me.

I apologize if you are aware but your post appears very naive to me. I have a lot of experience putting in temporary and permanent access and working with engineering firms and permitting agencies.

You're not going to get a better deal than $10k/year to cross a 70 wide river. Especially when it provides tried vehicle access (including emergency vehicles and fire trucks?) and is grandfathered in on a W&S river. That's incredibly cheap.
posted by fshgrl at 2:38 PM on May 16, 2016 [22 favorites]


there are a LOT of liability and permitting issue you seem unaware of

Indeed. If it's a navigable water, you'll need to make sure you don't block navigation, which you would do with a pontoon bridge.

The state Dept of Fish and Game is not your only regulator. For one thing, W&S rivers are regulated by a federal agency -- either USFS or NPS, in most cases. Like so. I wonder about your current removable bridge, but perhaps it's grandfathered in or allowed because it's only seasonal?

Anything going into the water would require coordination with the Army Corps of Engineers and the state water quality control board. And of course a bridge over a navigable water requires a permit from the Coast Guard.
posted by suelac at 2:56 PM on May 16, 2016 [4 favorites]


In sum, you need an environmental planner. I would keep what you have, because it'll likely cost you far more than $10K just to get the permits to replace it...
posted by suelac at 2:58 PM on May 16, 2016 [4 favorites]


This outfit sells a nifty-looking modular pontoon system that probably would work. I don't know about horses, they might get spooked. They thing that would concern me, and what we can't tell you from what we know, is how this would behave in moving water rather than the still water illustrated at the link. If it is a pretty lazy wild and scenic river, maybe. But if it's pretty wild, there's no way any kind of pontoon will work.
posted by beagle at 3:27 PM on May 16, 2016


I would start by checking with your insurance company to see what they would allow you to use and how much any options they would give you would increase your insurance rates. As well as looking into what the permitting issues will be.
posted by Candleman at 4:09 PM on May 16, 2016


If horses/small vehicles are going over, do NOT get a pontoon bridge. Seriously, there was one out at my cousin's lake and even though it was rather big and sturdy looking, someone ended up going over it on an atv and the entire thing separated and the atv went into the lake. Pontoons are basically only good for foot traffic for a short distance. Also, anything floating is a death trap when horses are involved, the sheer topheavy weight of a horse will mean if it spooks(which is inevitable at some point) you'll have a bridge swaying wildly with an animal and people on it.


Honestly I don't know if you'll be able to get anything safe for under 10k. This is something you need to be asking to parks officials/insurance providers.
posted by InkDrinker at 4:41 PM on May 16, 2016 [1 favorite]


Also, any other users of the river are not going to be thrilled that you have just added a pontoon bridge that they now have to portage around...are you willing to add the nessesary infrastructure to deal with this? Does the river flood? Will the bridge you build be able to deal with a couple trees smacking into it?
posted by rockindata at 6:17 PM on May 16, 2016


Thanks, everyone. Some clarification. There is already a bridge that gets put in and taken out every summer. We own it. It supports ambulances and fire trucks. The river does flood, but only in the winter (the Eel River in Northern California). The costs for installation and removal are ~$10k per year. We have permission to drive a large excavator through the river twice. They drive across, pick up one end of the bridge, drag it back across and fill in around the landings with rock and sand. I am considering avoiding installation costs by changing to a smaller bridge that can be installed by hand.

Permitting is not my role, but we do have permits in place and would just be changing the type of bridge. There's no navigation on the river except tubing, and the current bridge already forces people to get out and walk maybe 20 feet.

I'm focused on technical ideas and concerns. So no horses on a pontoon bridge. Maybe no ATV's, either. We could consider foot traffic only. That might require an ATV on either side for evacuations. Maybe a barge concept would make sense.

Thanks again for your comments.
posted by huckit at 6:37 PM on May 16, 2016


How much traffic crosses this river? If not too much, perhaps a cable ferry would be one possible solution? I imagine you'd need to train people to operate it, but that might be simple to do. In the off season you could (maybe) just let the cable go slack and sink to the bottom, and haul the ferry out. At the beginning of the season, do the reverse. Note that I have no idea if this would be feasible or even legal in your case, but it sounds plausible to me.

Honestly though, a 70 foot span is pretty big for any kind of bridge that you expect to be able to regularly put together and take apart using only people power. All the ideas I can think of (other than pontoons) involve building some kind of serious permanent anchors on each shore and then running cables in between. You would send a team of people across with a pilot cable and a turnbuckle, which they would then attach to the anchor on their side and use to haul the bridge across. More turnbuckles would be employed to tension the entire bridge such that it formed a sort of taut rope bridge type affair made out of steel wire. I have no idea if such a thing exists commercially, this is just a concept in my head. And I can't imagine it would be suitable for even light vehicle traffic, I'm talking foot traffic only here.

If you want vehicles, you need some kind of rigid structure. I very much doubt you're going to get that for less than what you're paying now, or in a format that can be erected and dismantled by hand.
posted by Anticipation Of A New Lover's Arrival, The at 7:23 PM on May 16, 2016 [1 favorite]


Permitting is not my role, but we do have permits in place and would just be changing the type of bridge.

You would need new permits and lots of them. Also the emergency vehicles thing is probably an insurance issue but you can ask and see if it is.
posted by fshgrl at 7:41 PM on May 16, 2016 [1 favorite]


Some concerns:

Do you have the manpower budget to assemble and disassemble any replacement bridge? Something like a pontoon bridge requires a significant number of man hours to assemble. Much of it fairly hard labour in dangerous conditions (over the flowing water). This work may have to be supervised by a credentialed person of some sort either by the government or your insurance company. If it takes a five person crew a week to install or breakdown the bridge you might not save any money.

To put it another way your current bridge could probably be hand (or portable electric) winched into place if you constructed a set of dollies. Having moved single wides hundreds of feet with nothing but dollies, jacks and hand winches I bet no one wants to do that sort of work by hand. Let alone the shovel work to build the landings.

Any multipart bridge is going to have wear components so you'll need a budget to account for replacement. Depending on the fasteners/joiners used they might also need replacement every year if they are of a torque to yield type.

As an aside, unless your camp is of the everyone in tents variety, not having the ability for tradespeople to drive to your camp is a serious hassle. Replacing anything with significant mass like a commercial stove or large HVAC unit is complicated by having to load and unload twice (truck to boat then boat to truck). Even getting food supplies to the camp becomes more difficult and moving fuel if you have generators or liquid/gas fueled light/heat/cooking is even worse. Having to stretcher an unconscious person over a pedestrian bridge complicates things.

All of these things can be managed but they all take time that could otherwise be spent with the campers. I worked F&B receiving at a ski hill where one of the corporate restaurants was only accessible by ski lift or snowcat in the winter and it was a serious pain in the ass that significantly limited the services provided compared to our base operations. You might find that much of the 10K saved is eaten up over the course of the summer by additional or redirected labour costs.
posted by Mitheral at 8:00 PM on May 16, 2016 [2 favorites]


You might find that much of the 10K saved is eaten up over the course of the summer by additional or redirected labour costs.

Came here to post something to this effect. Any time there's a place you need to get gear in and out of, that you couldn't haul yourself, that you can't just drive a truck up to... it gets STUPIDLY expensive.

Even if the new bridge cost $0, you might be spending a significant amount of money on getting someone to say, load their gear on a barge and come up to do work on your equipment/infrastructure/buildings.

My dad used to work with this kind of stuff, and hire out barges so contractors could work on places only accessible by water. It got REALLY expensive fast. Especially when it involved moving them from wherever they were stored up the canal/river to the worksite. Think long and hard about what you need or could need out there that can't be walked across a small bridge.

And that isn't even getting in to the permiting part. Even from my limited knowledge relayed from my dad, this bridge sounds like the kind of thing that would be hideously expensive or even possible to get permitted now, a lot like docks in certain desirable/scenic areas. You're probably getting a SMOKING deal here at 10k/year.
posted by emptythought at 10:03 PM on May 16, 2016


This guy spec'd out the materials cost of an 80ft cable suspension bridge at around $2500 in 2013. There are lots of foot bridges like this around here.
posted by maggieb at 10:54 PM on May 16, 2016 [2 favorites]


I apologize if you are aware but your post appears very naive to me. I have a lot of experience putting in temporary and permanent access and working with engineering firms and permitting agencies.

You're not going to get a better deal than $10k/year to cross a 70 wide river. Especially when it provides tried vehicle access (including emergency vehicles and fire trucks?) and is grandfathered in on a W&S river. That's incredibly cheap.


I have similar experience and I agree. Just the mobilization/demobilization plus hourly operation of the excavator for the bridge installation could easily run more than what you are paying currently, never mind the time and hassle of dealing with the permitting agencies.

Around here we are often required to spec temporary bridges that can carry fire equipment in case of forest fires (meaning both width and weight requirements). Whether or not that is the case where you are will depend on what you are doing across the river, the current fire risk status, and your state fire regulations. Research these rules before changing bridges, and be aware that requirements can change at the whim of regulators or as the result of increased fire risk.

I do wonder if it might pencil out to get quotes for a permanent bridge installation (presumably elevated and/or wider than your current bridge). It's a lot of money up front, but then you are free of the yearly cost of the temporary bridge.
posted by Dip Flash at 11:05 PM on May 16, 2016 [1 favorite]


Dip Flash the Eel runs almost dry in the summer, you can wade it many places if you don't mind the toxic algae. But in winter it's a different story. They recorded nearly 1,000,000 cfs once back in the 60s or 70s. Largest ever riverine discharge in CA. It probably averages 20-30k cfs in the rainy season.

The design flows and seismic requirements would make a permanent bridge a multi million dollar endeavour.
posted by fshgrl at 1:46 PM on May 17, 2016 [1 favorite]


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