Culvert Issues, help!

[paulsharvey]

The 2 or 3 4" pipes is a bad idea, dont do it.

Im assuming the RCP you have in place is 15 or 18" round RCP. Thats like 20x the capacity of a few 4" pipes.

 

[paulsharvey]

Once you add the bag headwall; you need to add addional fill on both sides. It looks like even 5 ft away, your approaches are maybe half way up the pipe. Ideally, the pipe invert sits at the flow line of the creek, then atleast 12" of coverage over the top of the pipe. Headwalls up to the driving surface, extending atleast 18" min on both sides of the pipe.

It looks like a swampy area right now; but if you have a situation where the water raises 24" in an hour, we really need to start thinking 36"+ on the pipe. Swampy areas generally dont get that flash flood type water movement, but I dont know your area

 

[paulsharvey]

On pipe sizing... So, if set at the flow line, in normal weather, you want no more than about 1/4 of the pipe diameter flowing water; that gives you about a good bit of added surge capacity for storm events. If the pipe routinely flows at 50% diameter or more, you have nothing left for storm events/surge.

 

[paulsharvey]

So, I went back and looked at the pictures a bit closer; but correct me if wrong.

So, the upstream end does have a cast in place headwall, the pipe is in the center of a stream, but the 30 or so feet left/right is flood plain of the creek? With that, the goal, im guessing with the pipe elevation is to have a year long pond across the flood plain, up stream of the pipe, and the pipe is meant as a control structure. Looking at the weeds/grass that appear pushed over, during heavy rain events, the entire flood plain is flowing.

So; if that is right, we mostly need to focus on 2 things. #1 is a Whole lot more fill: compacted and then the slopes stabalized with grass. #2 the pipe is serving as a bleed down, but we need a true over flow.

So, typically, you would have a small bleed down orifice, and then a much larger overflow. I would build the entire bankment up, probably 24-30", with well packed material; and pick one area, about 12 ft long, off set from the pipe, and leave it about 12" lower then the general embankment. That serves as the true over flow. It will need armored; with rip rap, soil cement bags, crushed ruble, geo rid, fabriform or similar, because that water, when it reaches over flow point, will have velocity.

 

[RayIN]

Present culvert is undersized IMO. A larger diameter will allow it to be lowered to allow water flow while maintaining the height of the banks.
That said, a drive-through will be better, if made correctly.

 

[HawkinsHollow]

There is a cast in place headwall on the upstream side of the pipe it could probably be wider. At high water it is going around it. Yes, there should be more fill over the pipe, but fill is hard to get to this location. This whole areas is under 2 or 3 feet of water when the creek overtops the banks. I think as it is getting to that point this somewhat dry creek becomes an overflow and moves quite a bit of water. This is when the damage is happening. I am chalking it up as a failed attempt and will be making a crossing there when I have time. Plan on using the rubble from the pipe to armor the crossing.

 

[Todd727]

It’s pretty simple, like @turkish said the pipe isn’t big enough. Either a bigger pipe or build a wet crossing.

Or bridge.

OP, go look at the "Trailer as a Bridge" discussion.

 

[repete]

Your culvert needs to be able to handle the highest flow rate with no water higher than the INSIDE edge of the culvert AND the downstream side must never be higher than the inside of the pipe.
Once your water level exceeds the culvert interior wall height you are in trouble.
When the culvert is half full, is the stream level the same 5' from either end of the culvert?

 

[dodge man]

A little off topic but culverts hardly ever flow full. They are governed by what is inlet controlled. A headwall that is beveled helps. Anything that helps funnel the water in helps. It’s not uncommon for a culvert that is full at the upstream end to be flowing half full on the downstream end. There is also a design parameter called allowable headwater. If you have a high bank and the water can backup that increases the flow because of some pressure. In the OP’s case there would be almost no allowable headwater since it just overtops his crossing.

 

[paulsharvey]

How soft is the creek bed? Can you just drive through the creek at low water, or us the whole flood plain swampy? To do a proper, 365 day crossing; i think you would need to build the embankment up about 3-4 ft, and have double or even triple 24" pipes.