</font><font color="blue" class="small">( he needs to either dig down to install the culvert )</font>
Not always an ideal situation. If you have the exit of the culver underneath the grade of the rest of the trough, you will start eroding that area of the trough... Besides.. if you have a large dip in the middle of the flow.. you will eventually have a swimmin pool... it will probably start holding water and collect sediment at the lowest point. The sediment collection will also be bad at the pipe exit where there is a grade change due to restriction.
Out of all the culvert ideas here, I like the small pipes the best, obviously, and the half burried large ID pipe.. if you can get sufficient materil over the top of it for load bearing issues.
As for the small pipes.. i can't tell you how manu municipal WRA havedrainage eareas that use 3" and 4" pipe in some areas before hitting storm drains...
</font><font color="blue" class="small">( Having tried to clean out 12" corrugated metal culverts .......Not everyone has access to the equipment you mention to clean out culverts -- especially out in the middle of the woods... )</font>
Before we had that machine, we used the '15' shovel' All the local contractors in our area had a long shovel. It was nothing more than a shovel head welded to a long pipe. I personally cleaned ( helped clean) man culverts like that when i started working. It wasn't a savory job.. but for young laborors in college.. it paid for the books and tuition...
A small timber bridge is sounding better and better.
As for fish.. I believe he was going to gravel it over completely.. so i doubt there is a concern for fish.
Now I remember why I said i was never going to give engineering advice over this site again... reminds me of an old thread.
Ok.. scratch all my previous message in this post.. my new advice is to contact a civil engineer that is licensed in your local area, have them inspect and survey the site, and design a drainage structure that will be load bearing.. ( box culvert is the ticket.. reinforced concrete.. ) get the structure specially precast and delivered, and either install it yourself, or get a contractor to drop it in. In most cases you will need a NPDES permit, and also a permit from your local water management district, plus a building permit. I figre about 20 grand ought to cover it.
That's the end of my advice..
Soundguy
Soundguy