Timbered Geometry Dam

I was going to post this on Daron Williams thread regarding building a Beaver Dam, but that would almost suggest that what he was doing was wrong. That is not the case at all, but on my own farm, eventually I want to put in a pond. Because the waterway already enters a very narrow channel, simply creating an 8-10 foot wide dam would back up a lot of water. At first I thought about doing that with an earthen dam, but is there a better, cheaper, faster way? After a little thought, I think there is. I call it a Timbered Geometry Dam that I devised.

This is the Dam Site I would like to install my Timbered Geometry Dam. (It is in one of my sheep pastures)

The first shows the outlet channel, and the second shows where the water will back up. This area accepts water off 25 acres of open land, with percipitation at 57 inches per year, at 136 rain days per year.

The dam I devised really has two elements, the truss framing and the sheathing.

The overall structure of the dam would be made up of framing lumber. The dimensions would vary, dictated by the height of the dam. If the height of the dam was going to be only 4 feet height, 2x6 lumber would work, but if it was going to be 12 feet high, then 2x12's would have to be used. Either way, the truss would be built just like a house, only preferably with rot resistant wood like White Cedar or Hackmatack. Preferrably steel would be used for attachment points, but plywood could also be used.

The keyway is simply vertical framing with sheathing on the face so that as the water in the pond builds up, it does not undermine the dam by hydraulic pressure and blow out. The sheathing can be wood lumber, or steel roofing.

The trusses should be no more then 1 foot on center, but they could also be doubled up, or laminated for additional strength. Like a house, they should be spaced by cross bracing and purlings that the front sheathing is attached to.

The sheathing can be boards that have been caulked, plywood with the edges sealed, or my preferred method; steel roofing as it gives long life, would be water tight, and covers a big expanse cheaply. This is simply attached to the purlings with self-tapping screws. Obviously at this point, each flank of the dam is sealed to prevent scouring, and the water begins to back up.

The the dam works all by its geometry. The dam is an equilateral triangle, so as the water begins to back up on the dam, it puts weight forward trying to tip the dam over. But because the front of the dam extends backwards into the water, the same amount of weight is pressing it down. With both forces equalized, the water rises until it splashes over the dam. Ideally a section in the center of the dam is lowered by a bit to make a sluiceway for a little extra freeboard in heavy rain events.

But overall it would be an effective, low cost and easy dam to build.

Oh...one other point, this design can be scaled up or down. Because of the equilateral triangle design, whether the dam is a foot high, or 12 feet high, the forces and weight of the water holds the dam in place. Only the framing lumber needs to be sized up so it does not buckle.

Also, because of the truss construction, it would be a modular dam, meaning a person could just add more and more trusses (or sections) to the dam to make it. By that I mean a person could make a 4 foot high dam premade at their house, then install sections of the dam side by side to span the width they need. They could also make the dam straight or in a semicircle.

Travis

Happy New Year!


The wood dam looks interesting. Seems like something worth pursuing. A couple thoughts come.

First and the one I'm least detailed about is the undermining. Not sure about anything here - I don't have any PE creds. It seems to me that weight of the structure may have a bearing on how well it "sticks" in place. Also how well it resists lifting. I _think_ the wood structure may be considerably lighter than most dams. Then there's the question of soil permeability in the immediate dam area - how much "ground water" would you create with the pond, moving how fast, how deep? Would that impact the footings of dam significantly?

Second what is the life cycle of the dam? How does it evolve over 1,2,5,10 years with possible silt,  erosion, wood decay? Since the life cycle is probably much shorter than that of a concrete dam, maybe it should be a very specific part of the design.

Third, what are the failure modes? Well designed engineering fails gracefully. Ie., it fails slowly, with multiple layered warnings and in manageable ways and locations. Because there are doubtless properties not to far downstream and possibly even work areas, habitations and such, this might be the most important consideration.

Oh, and fourth, how much would depend on fasteners? Metal fasteners don't have the great a working life in water. Fresh is much better than salt, but seems like it might need planning for.


Cheers,
Rufus

Rufus Laggren wrote:Travis

Happy New Year!


The wood dam looks interesting. Seems like something worth pursuing. A couple thoughts come.

First and the one I'm least detailed about is the undermining. Not sure about anything here - I don't have any PE creds. It seems to me that weight of the structure may have a bearing on how well it "sticks" in place. Also how well it resists lifting. I _think_ the wood structure may be considerably lighter than most dams. Then there's the question of soil permeability in the immediate dam area - how much "ground water" would you create with the pond, moving how fast, how deep? Would that impact the footings of dam significantly?

Second what is the life cycle of the dam? How does it evolve over 1,2,5,10 years with possible silt,  erosion, wood decay? Since the life cycle is probably much shorter than that of a concrete dam, maybe it should be a very specific part of the design.

Third, what are the failure modes? Well designed engineering fails gracefully. Ie., it fails slowly, with multiple layered warnings and in manageable ways and locations. Because there are doubtless properties not to far downstream and possibly even work areas, habitations and such, this might be the most important consideration.

Oh, and fourth, how much would depend on fasteners? Metal fasteners don't have the great a working life in water. Fresh is much better than salt, but seems like it might need planning for.


Cheers,
Rufus

All valid points.

I was thinking about the scouring and undermining of the dam this morning, and realized I had put the key in the wrong spot. The key should actually go on the leading edge of the dam to prevent water from slipping under the dam instead of being in the middle of it. That would also act like a post, where the friction of the earth on the key would hold the dam down. The weight of the dam itself however has no bearing in the matter because of its shape. It is essentially a wedge in a waterway, and wedges exert great pressure on things, just as a stone mason can tap in a feather (wedge) and split a rock, or a logger can drive in a plastic wedge to tip a massive tree over. The dam is using the weight of the water to hold the wedge (dam) firmly in place. The greater the volume of water, the greater the pressure holding it in place.

The longevity of the dam is problematic to say the least, at least in terms of a "green solution". I use truss construction on every building I build here because it is fast, strong and cheap. To make the best connections however, the framing angle should be dissected, then plywood nailed to each side. Ideally this connection would be pressure treated framing lumber, with pressure treated plywood, with galvanized nails. That would be a pretty long-lived dam...but is hardly "green". Cedar could be substituted for the framing lumber, but boards cannot be used for gusset material because it cracks when nailed. Really plywood or steel needs to be used. Steel would work, but would be expensive.

I am not sure about failure. Honestly, it is so rural here that on my farm failure would not have an impact on anything, but I recognize that such a thing is just me. However, I am not sure my dam is any different then any other dam though. In some ways I think it would be better in a failure. If there was enough hydraulic movement to cause an earthen dam to fail, a torrent of water would be let loose down stream, just as any dam would: concrete, steel, earthen...wood. But if there was failure due to age, the wooden structure would mostly not fail all at once, but slowly collapse like how a mine collapses over time, or an old building from snow loads. As it did, the water level in the pond would just go down slowly, and the water released, would be partial, and not all at once. Again this is no different then any other dam that may, or may not collapse partially or fully depending upon circumstances.

Sounds like you have a safe test area. Sounds like maybe try it out? Possibly start low, 3-4' and see what happens. 50% of the knowledge and gain for (probably) 25% of the cost.

Thinking about it, resistance to the water weight still looks really iffy. Consider concrete  forms and the need for whalers - fluid weight applies a _lot_ of horizontal force. I don't think a sloped face will change that force much - just maybe allow more down force. Which may not be enough to hold the thing in place w/out multiple _structural_, not hydraulic, keys. The more I think about the more it looks like "low" and "short term" may be the defining characteristics. And that would point to the question of how much benefit such a thing would provide you. Viable life span figures pretty important when trying to price out the ROI.

Another thought: If a small dam would cause enough sediment buildup quick enough, maybe it would effectively "grow" and maintain some effect even after it begins to fail.

Here are two links I dug out from another life. They are just random pages, examples - neither addresses the question here. But I think the sites themselves can be mined for good info on this type of project so I pass them along.

https://www.eng-tips.com/viewthread.cfm?qid=162089
https://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html

Rufus

Rufus Laggren wrote:Sounds like you have a safe test area. Sounds like maybe try it out? Possibly start low, 3-4' and see what happens. 50% of the knowledge and gain for (probably) 25% of the cost.

Thinking about it, resistance to the water weight still looks really iffy. Consider concrete  forms and the need for whalers - fluid weight applies a _lot_ of horizontal force. I don't think a sloped face will change that force much - just maybe allow more down force. Which may not be enough to hold the thing in place w/out multiple _structural_, not hydraulic, keys. The more I think about the more it looks like "low" and "short term" may be the defining characteristics. And that would point to the question of how much benefit such a thing would provide you. Viable life span figures pretty important when trying to price out the ROI.

Another thought: If a small dam would cause enough sediment buildup quick enough, maybe it would effectively "grow" and maintain some effect even after it begins to fail.

Here are two links I dug out from another life. They are just random pages, examples - neither addresses the question here. But I think the sites themselves can be mined for good info on this type of project so I pass them along.

https://www.eng-tips.com/viewthread.cfm?qid=162089
https://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html

Rufus

Again, valid points.

My problem has been, if I am going to rip into the area behind the dam with a bulldozer, why not just spend and hour and form an earthen dam for the outlet? I could do what Darron is doing with his homemade beaver dams and not do any excavation, just haul in a few dump trailer loads of rock and form up a dam, but that would take considerable time to do. My trailer does not load all that fast unfortunately.

I did have a problem with erosion along a roadway that is on a horrific 9% grade. I added rock dams to ward off erosion, but really need to add more, or really add rock from the bottom of the hill to the top; 500 feet. That is a LOT of rock. I have it, but I have no quick way to load, haul, dump and return for another load way to do it. In that situation, maybe Timbered Geometry Dams might help with erosion. You can see the rock check dams on this heavy haul road here.

The above picture does not look bad, in fact it looks flat, but it is actually a 9% grade. That is the maximum grade a truck can climb if that tell you anything.

This makes me think about those plastic modular dams used for controlling emergency flooding, and since you brought up 4ft height I also thought that perhaps concrete barriers could be used with a skim coating or lathed coating of concrete to seal the pieces together.

I have a small pocket pond I've been wanting to build that would be fitting for this sort of solution I think... I'll have to look at this option