Cost to put in a 36" concrete culvert about 30 feet long?

A corrugated steel culvert is about $2300-2400 for 36”x30’. Concrete is a lot more expensive.

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In what way is the steel superior to concrete other than price?

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IMO it's not. You have to watch what crosses over the steel ones to much weight and they can collapse.

4600

I deal with drainage materials all of the time. Where are you located? For installation prices, the rule of thumb is take the per foot price of the pipe and multiply it by 3. This is IF it is being done correctly. If this a ditch crossing in the back of some hunting land, it can be less.

For context, LADOTD requires 12” of cover or dirt over the top of any accepted pipe. However, you do need to watch out for rutting. Just use any material you dig out to mound it over the pipe…if it is needed.

RCP is not readily available and heavy. If you have not so good soils, RCP can sink or settle, leading to joints separating and ultimately failure of your crossing. If installed properly, it will probably last the longest.

CMP will be very close in price to RCP, maybe a bit cheaper. Positive is that it is lighter and can be purchased in one 30’ joint. No joints. No joints = nothing to fail. Depending upon the acidity of your soils, it can rot.

HDPE will be a bit cheaper than the other options, though not by much. It can be rolled into the ditch if you dig out the ditch to the proper level. You will have to purchase two 20’ joints so it may have a total price that is higher than the others. No rotting. If you choose to DIY, this is 100% the way to go.

Hope this helps.

I'll try to give more details without giving too many details.

There's a pond/lake. The existing spillway is a 36" concrete pipe completely horizontal to the plunge pool, and is completely satisfactory for normal conditions.

There is an emergency spillway in the case the principal spillway becomes clogged or otherwise can't handle a 100 year weather event, such as a tropical storm that comes over and stalls, dumping an inch an hour for days at a time. This emergency spillway is a glorified ditch lined in rip-rap that would route excess water around the dam to the creek's flood plain.

According to the state, even this is not adequate.

The fear according to the state is that if both of these spillways can't handle a major event, the dam could be overtopped and fail, releasing all of the water in the lake in a catastrophic event.

The drainage basin is 93 acres, according to the state. The surface area of the lake is 4.5 acres, according to the state.

Based on runoff calculations in a 100 year rain event, we'd need to keep up with something like 113 cubic feet of water discharge, per second. Based on internet calculations, our existing principal spillway would absolutely not be able to hand that. I don't know how well the emergency rip rap ditch would handle the flow, I don't know how to calculate that.

My idea is to put in a 3rd spillway that is the emergency emergency spillway, above the water line, on the dam itself, going through the dam about a foot or two above the waterline... and have it pitched 2 feet from end to end, as online caluclations show this exponentially increases potential discharge rate.

So, if the primary spillway can't keep up, and the emergency rip rap ditch can't keep up, here's a 3rd emergency spillway that will discharge more than the 117 cubic ft per second that would be needed, all by itself. Even if a total blockage of the primary spillway occurs, and a pirate ship washed up blocking the rip rap spill way, this third spillway would still be able to discharge the amount of water necessary for a 100 yr rain event.

So I'm trying to figure out how much that would cost. The state's recommendations for things they would like done are in the six figures.

I would just make my emergency spillway larger rather than adding a third outlet.

Use double corrugated. Will work perfect. Just back fill with #57 clean Limestone. I’ve installed many culverts in my days. Concrete cracks and galvanized rusts.

Let me make sure I have my head screwed on straight here. I design dams, but am not going to design your dam.

So we have an existing dam with a principal spillway and an emergency spillway. Principal spillway is a 36" diameter RCP that is horizontal at its discharge. I am assuming there is an elbow where it goes vertical. The Emergency spillway is a rip rap lined channel that I am assuming starts at a higher elevation than the intake for the principal spillway.

The state says the existing spillways are insufficient and you either need to add or expand your spillways to keep a breach from happening.

There are a couple of ways to attack the problem. To do everything by the book you would need to first do a breach analysis, this will be $60k and just tell you how much water is needed to cause issues downstream and what issues those would be in case of a breach of your dam. From there you can properly size your spillways.

There are tables that tell you flow capacities of pipes and ditches. How many acre-feet of water would your dam hold at full pool?

Regardless of everything above, you aren't going to properly solve your issue without digging up and replacing part of the dam. Why is the state involved to begin with if you weren't already changing the dam?

Sounds like it’s a volume issue not weight above…you’d be better off using pvc.

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Based on runoff calculations in a 100 year rain event, we'd need to keep up with something like 113 cubic feet of water discharge, per second.

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So, if the primary spillway can't keep up, and the emergency rip rap ditch can't keep up, here's a 3rd emergency spillway that will discharge more than the 117 cubic ft per second that would be needed

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Why overbuild so much here? This is all for 100 year events right? I'm not saying do the minimum, but you could build something well that's smaller and fit your needs perfectly well.

Or do one that's smaller and have it not just be for emergencies but used routinely?