Step 4 The Deck

Another big consideration is how to secure the top deck. Once your culverts are secure, you'll want at least 3 feet of fill over the top of them, which will allow water to back up some and pressure-flow through your culverts which will in turn decrease the frequency at which your low-water crossing is overtopped. You could build this fill out of well-compacted dirt, gravel, or bull-rock, or even concrete the whole volume. My experience has been that dirt is just fine as long as the culverts aren't undermined. If you build good headwalls, with a deep toe wall, the culvert fill can be just dirt.

If you want to concrete the drive across the top, treat it just like a standard driveway. However, for added security against undercutting, you should turn down the edges of the deck to earth-form another 3' deep toe wall. Like this:



If you're only going to concrete the driveway in the vicinity of the low-water crossing, then concrete about 20 feet past the high-bank line of the creek, and about 2 feet vertically above the high bank. That way, any washout will be well away from your expensive culverts, where it's easier to repair.

Step 5 Erosion control

One last thing to consider is that exposed soil especially sandy soils are very easily eroded. The quickest way to have your project go to hell in a hand-basket is to leave exposed/bare soils unvegitated. Your single best line of defense against erosion is to get grass growing. Give serious consideration to sodding the areas between your deck and the culvert headwalls. Even better install a turf reinforcement mat. What you should definitely NOT do is just say "grass will grow in on its own eventually". Those are the famous last words of hundreds of contractors building creek crossings whose recently-completed culverts are now located miles downstream.

If you can't sod it, at least seed it and water it. Use whatever seed your local supplier recommends for that time of year.

This low-water crossing will by definition have some high-flow velocities, especially in the channel just downstream of the culverts. We normally call for 6"-12" diameter bull rock just downstream of the culverts, which will slow the water down. I'd say you should go about 1 linear feet of stream for each inch of diameter of your culverts. You'll want to cover the entire flowline of the creek, and 2-3 feet up the banks as well.

Lastly: This thing's going to take maintenance. When dirt erodes off of your toe-walls (and it will!) then you need to add dirt back. If a big storm blows all your bull rock downstream, you need to replace it. If a harsh summer kills your vegetation, you need to replant.

water turkey said:

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If your impact is less than .10 acre (most likely) and it is not a wetland, you are authorized under Nationwide permit #14, with no Pre Construction Notification required.

Go forth and conquer.

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Just saw this post. This is where most of my projects fall. I can confirm that this is the correct answer.

Ornlu said:

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Step 4 The Deck

Another big consideration is how to secure the top deck. Once your culverts are secure, you'll want at least 3 feet of fill over the top of them, which will allow water to back up some and pressure-flow through your culverts which will in turn decrease the frequency at which your low-water crossing is overtopped. You could build this fill out of well-compacted dirt, gravel, or bull-rock, or even concrete the whole volume. My experience has been that dirt is just fine as long as the culverts aren't undermined. If you build good headwalls, with a deep toe wall, the culvert fill can be just dirt.

If you want to concrete the drive across the top, treat it just like a standard driveway. However, for added security against undercutting, you should turn down the edges of the deck to earth-form another 3' deep toe wall. Like this:



If you're only going to concrete the driveway in the vicinity of the low-water crossing, then concrete about 20 feet past the high-bank line of the creek, and about 2 feet vertically above the high bank. That way, any washout will be well away from your expensive culverts, where it's easier to repair.

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Just curious, if you go with fill over the Culverts and then concrete the entire top part (above the fill) so that the entire low water crossing is concrete covered, this should then lower maintenance need and during bigger rains it just flows over the top... we had a company put one of these on our property and they concreted the entire structure so that water flows through the culverts durIng normal or small rains and then over the top during bigger rains. I guess you still could have wash out on the ends where the concrete stops ? Just wanted your input on this design ... seems it will last a long time ?

Yes, concreting the whole top is a good idea. They do that in state parks.
You'll still have to make sure that the toewalls don't get undercut, so it's not completely maintenance free.