Multiple culverts run in parallel?

[MartyM]

I have a seasonal run off ditch that crosses my drive, it is dry most of the time. It really is the perfect natural creek crossing in that it has fairly even approach from both sides and the creek bed it self is a smooth slab of limestone about 12 feet wide and 7 or 8 ft from bank to bank.  This is northern Arkansas Ozarks so the ground is very rocky and mud is rarely a concern.  My problem is the banks are steep enough, and so close together that my trailer drags for about 3 feet when crossing.  (my truck starts climbing out while the trailer is still descending.)   I want to put in a culvert to raise the roadway up off the creek bed at least 2 feet.. More would be nice but also more work.  To move the volume of water I have seen run through there I think I need about a 3 ft wide culvert.

If I put in 4 15 inch culverts right next to each other it would move about the same volume of water as a 3 ft culvert with out squeezing it into a 3 ft channel.  Now for my question. Would running 4 culverts so close together compromise the strength of the culverts?  There would be very little dirt fill between each of the culverts, so would I end up crushing them when I drive over? 

Any thoughts would be appreciated.

[Redoverfarm]

From what I have seen and been told that a culvert will only carry 1/2 of it's volume of water.  So you are not diverting as much water as you would expect.  I would elect to install the 36"  culvert as it would more likely cause less problems.  With multiple culverts there is more of a tendency for trash to get caught on the inlet end.  You will need to rip/rap the culvert inlet end wing walls with rock to keep it from washing out.  If you taper the inlet wall toward the pipe it will not tend to wash out the banks or dam portion.  Both of which need stone.

[MountainDon]

Is it possible to cut the banks down to a more gentle approach and departure angle?

[MartyM]

Thanks for the quick reply Red and Don.   

   Unfortunately I cant change the approach on either side without dynamite       followed by having to  make  a switch back farther on up the hill.   The entrance side is a  shared drive that follows the creek up hill,  turning on to my property it drops straight down to the creek bed. About an 8 ft drop in 25 ft then it starts the climb to my slice of heaven. Not quite as steep but very rocky.  I mean BIG mostly flat  slabs of more limestone.

As for the fill around the culvert or culverts I got rocks out the wazzoo!  (Not as painful as it sounds really)     Around 40 years ago the previous owner had the 10 acre  plot bulldozed to push all the loose rocks down towards the bottom of the land.  So I have about a 400 yard long 3 ft high 10 or 12 ft wide pile sitting on my lower lot line. I had thought of the trash being a bigger problem with the smaller culverts, but it seems to me that the up hill side  erosion would be less of a problem with a row of culverts the width of the creek bed. There would be less bottle neck effect. Wouldn't a row of 4 culverts ruffly the same width of the existing creek bed allow for a smother flow with less water hitting the wall before entering the culvert?  That is assuming I can put them in a row without worry of crushing them.

[Redoverfarm]

Marty you mentioned that the stream is dry for the majority of the time so I would assume that when you have a hard rain that you probably get plenty of water where you normally would no have any. Here they are sort of like flash floods which bring everything down stream that is not attached somehow.  The larger diameter will let more trash flow through and will be less likely to wash out where a lower profile will cause the water to back up and eventually go over the top and wash out your fill and erode.

If you have an abundance of stone you have the ideal material.  Lay your pipe, rack/stack larger stones at a 30-45 deg angle from the pipe mouth to the embankment.  Start your stone at the pipe where the stone surface(up stream) is about even with the pipe or a little past. Add your fill as you progress to the top with the laid stones.  You will also need to use some larger stones at the  trailing end to hold the fill.  You will not get any errosion as the water will leave as soon as it exits the pipe.

We have several small mountain streams like yours that are normally dry but become major rivers when it rains carring small limbs, leaves and other vegatation.  The most successful bridges and culverts are large diameters and even then sometimes they have to be cleared of the trash. 

If your pocketbook will afford it I would opt for two 36" culverts which will almost bridge the complete area and not require as much fill and allow a larger volume of water to pass through.

[Don_P]

But, fill over the culvert is what gives it its' strength. If a culvert is not buried deeply enough the cover material does not form an arch. A larger culvert requires more fill over it's already taller profile. We have a 5' pipe across the creek at the entrance to the neighborhood I'm working in. They didn't have enough cover over it and many of us  brought in things like cranes and concrete, tractor trailers and prefabricated foundations. The neighborhood engineer flagged me down at the entrance one day and we looked through the culvert, which had 2 deep wheel tracks crushing into it. The capacity was shot and they ended up replacing them, this time with alot more road over top.

My own creek has a 3' culvert, the soil and water agent checked the watershed and vegetation and specced a single 36" or two 30" culverts. I had the room for the 36 and then had the dozer flatten an old pickup bed I had and we spanned over the culvert with that as well.

But, little pipes clog up and are about impossible to keep clear. The guy below me sized his himself, put in a pair of 24's, there is alot more acreage between me and him. I watched them get cut out twice before he wised up and upsized.

[MartyM]

Red,  It sounds like you have been on site and seen my creek in action.     On the one hand I don't want to put some thing in that will at best be a constant headache.  On the other hand  2 36" culverts really add up.  I feel pretty confident that Noah would have to be involved if 2 36' pipes were over whelmed.   If this was my only possible driveway obstruction I wouldn't hesitate to put in the best possible solution.  How ever about a mile down the County road from my drive, between me and the highway sits Bear Creek and it has a low water bridge. With a decent rain I will be stuck waiting for the water level to drop any way.   

Don P,  The crushing of the pipes in parallel  was my original concern. I can't see any thing larger than a power Co. line truck needing to use my crossing. Possibly a cement truck if I could some how talk the driver in to attempting the climb up to my build site. For now I don't see any need for that much concrete.  I am guessing here but I thought I read some where that 1 ft of fill on top of the pipe was a minimum.  Does the fill amount vary with the pipe diameter and expected load?   

[Don_P]

I hoped to talk to my engineer friend but haven't run into him... so I googled and it sounds about the same from memory. For <36" pipe at least a foot of cover, if greater than 3' then the cover should equal 1/3 of the pipe diameter.