Multipurpose dam/pond project

Hi all! I thought I'd share the progress of one of our (way too many) side projects on our land. Yeah, I know, it's probably daft to start digging a pond when one doesn't have a house yet, but I can always blame my brother He wanted a project when visiting, and I went "well, we'd like a pond here at some point..." and so he started digging. And then we had a couple of very warm days when standing up to the ankles in water felt just right, so I continued.

When finished, the idea is that the pond/dam will be multipurpose. The most important function is probably to reduce erosion. Our small stream is extremely variable in size, and after heavy rains (not rare in our area) it's a monster. In the bends, it eats through its banks quite quickly. At the pond site, the stream passes below a steep slope (which was created by the stream in the first place). The slope, and the area just above it, are among our few areas with full sun, so we'll want to use it for growing stuff, but it would not be nice if our apple trees fell into the stream after a few years. Hence the need for erosion control.
Another function that's connected to growing things is heat storage. I figure a few cubic metres of water might delay first frost quite a bit, and since some of the things we have planted (and a bunch more that we want to plant) are on the edge of their hardiness, that could be a very good thing.
Also, we want to grow a lot of aquatic plants, so that's an obvious one. We hope to grow water pepper (Persicaria hydropiper) around the edges of the pond next year, and then go on with Sagittaria sagittifolia and Butomus umbellatus. Since the stream passes through the pond and then empties into a rather big lake, it's probably not advisable to grow non-native plants there, alas. Well, there will be more ponds.
While water storage is a non-issue in our area most years, it's still good to have, for the years that aren't like most years (and there will likely be more of those in the future...)
I have a dream/plan about introducing native crayfish at some point, we'll see if it happens. Also, bathing is a good function for a pond! Can you think of any others?

Some pictures:

great work, Eino!  Keep us posted with updates.

I don't know what your experience is with building dams so I'm going to throw out some questions for you to ponder/consider.  

How High is your dam wall in comparison to your raised outlet?  There should be about a meter or yard or so of 'freeboard' (height above high water level) for the safety of avoiding a breach.  If that is the only spillway, will it be adequate to take a large surge in the event of a hundred-year storm?  What if a really epic storm happens next year before you have your bank plants established?  What does that look like on your newly built earthen dam bank?  

Also, here are some further thoughts: all dams settle over time as the air will be driven out of them by water infiltration, and compaction from above.  Adjust your predictions with 5-15% settling depending on the dam's materials.  Armoring the upstream side of the dam with stones is probably in your interest.  Also, putting larger rocks down stream of your outlet spillway might be a good idea to bounce the energy of the waterfall when high water pours water over the outlet.  Experiment with rock placement so that the splashes bounce against one another, dissipating the force.  This will reduce the potential for erosion downstream of and at this site of concentrated force.      

Here's a civil engineering site's page about small dam projects:

https://www.aboutcivil.org/How-build-small-dams-Construction.html

Hi Roberto, thanks for the reply! Much appreciated, especially since my dam-building experience is very limited. To answer your questions: The freeboard (new word for me, thanks for that too!) is not one meter. I haven't really measured, but I'd guess about 40-50 cm above highest "normal" (non-rainstorm) water level. The plan was to keep adding to the dam wall as we dig the actual pond, dumping the excavated masses on the outside of the existing wall. Suppose we could make the new part taller, to try to attain 1m freeboard. The dam wall as it is now has been seeded with some sort of meadow-flower mix, and I also planted mint and a couple of other things to hold it together, so adding more material on top of what's there now would mean starting over with that.

Regarding settling and dam wall materials: the wall is made from alternating layers of rock and excavated turf (including grass roots) and I've tried to compact it as much as possible by stomping after adding each layer. The bottom layer is rocks driven into the ground as far as possible by jumping on them. It should manage a bit of pressure even before the plants have established. If there is a really huge storm next year it might end poorly, but on the other hand, last year we had a crazy rain event where we got 55 mm of rain in less than 24 hours. The stream did flood its banks in the place where the pond is now, but not by that much. I wonder if it would be a good idea to add an overflow spillway armored with rocks and lower than the rest of the dam wall (but higher than the main outlet) to prevent the wall breaking when we get massive amounts of rain? There is a natural place for it, between the spruce tree and the birch to the right of me in the second picture. That's where the overflow water went last year...

We'll definitely armor the inside of the wall with more rocks when we start the excavation for real. There are already a bunch more rocks on the upstream side of the outlet than when the pictures were taken, so the slope of the bottom up towards the outlet isn't as sharp. Good idea with placing rocks downstream to dissipate the force! They'd have to be big, though, and probably dug down into the streambed a bit.

As a Civil Engineer, I may be able to help.
Earthfill dams of the style you are talking about and also detailed in the link by Roberto, need a central core of impervious clay to eliminate 'piping ' through the wall.
Your soil looks very organic rich which one would expect adjacent to a stream. It has a high chance of being washed away with your big flood.

You will most certainly find the huge flow from upstream is caused by degradation of the area upstream catchment, and the best way to deal with that is look and try to create ' slowing ' habitat.
IE not have pasture.
How deep is a flood with respect to the left hand bank?

You speak of water supply, have you considered Rainfall collection and use?

One thing to watch for too is burrowing wildlife. It may just look like a cute home for an animal, but their burrowing will create a MAJOR piping source and blow out your dam.

Any plans for microhydro?

I just bought a house with a river out back. I am thinking up a microhydro design that can turn some of that tumbling water into power myself.

John C Daley wrote:You will most certainly find the huge flow from upstream is caused by degradation of the area upstream catchment, and the best way to deal with that is look and try to create ' slowing ' habitat.
IE not have pasture.
How deep is a flood with respect to the left hand bank?

Actually, there is no degradation whatsoever upstream, and no pasture. Just primary forest, some of which has been lightly logged (not clearcut by any means, I suspect they cut out most of the pine and left the spruce) a few decades ago. There is plenty of wetland around the stream further upstream. I think the massive flow is just a result of massive amounts of rainfall in a steep-sided valley.

The biggest flood we've seen in the two summers we've been on the land (the 55mm rain event mentioned above) did flood the right-hand bank and sent a part of the stream over the area that will be the pond, but it seems to have just barely risen above the bank. The effect on the left-hand bank is mainly that the stream eats away at its base little by little. The vast majority of the time, the water level in the pond will be lower than the original right-hand bank of the stream, even with the dam raising the water. So I'm actually not so worried about seepage through the dam wall, it's mainly there to contain/direct the water during rainstorms, and to prevent the stream eroding a new outlet for itself in a bad place.  Most of the storage capacity of the pond will be below original ground level, and although the topsoil is rich in organic material, below that is a layer of silty clay, and below that hard, old deposits of marine clay (with pieces of sea shells in it, even though the site is about 100 m above present-day sea level... That's cool.) So there shouldn't be too much seepage through the bottom of the pond at least.

Well, we'll see. Worst case, there is still nothing hugely important immediately downstream, so even if the dam wall breaks it's no disaster.

Steve Zoma wrote:Any plans for microhydro?

I just bought a house with a river out back. I am thinking up a microhydro design that can turn some of that tumbling water into power myself.

We did think about it, but since the flow is so variable we'd either have to build something extremely sturdy that can withstand the full fury of the stream at its biggest, or have something mobile and remove it every time the water gets high (and risk losing it if we forget...) Maybe one day.

Nice, I wish I had that on my property.

All my creeks a wet weather creeks as the creek only has water when it rains.

What multipurpose dam/pond project is in the future?

Do you have some beaver in your area?  I have read the beaver is very beneficial animal to have.

Anne Miller wrote:
Nice, I wish I had that on my property.

All my creeks a wet weather creeks as the creek only has water when it rains

What multipurpose dam/pond project is in the future?

Do you have some beaver in your area?  I have read the beaver is very beneficial animal to have.

Yeah, it's really cool to live in an area where water is mostly not an issue, although of course it does have its downsides... The plan is to dig loads of ponds in suitable places all over our land and grow a lot of edible and otherwise useful aquatic plants.

According to the species finding map I've looked at there are some beavers in the general area, but so far we haven't seen either the animals or any sign that they're around. If they are there they can't be so many. I wouldn't mind having some of them around, but maybe not in our stream. I have a feeling that they would find our apple, pear and mulberry trees a lot yummier than all the boring, everyday native trees...

Beavers are great.  I'm a huge fan of them.  But, would I want them on my land...  probably not. There is only so much flatish cleared, land with good soil on my property, and it is all in the plain of my little creek.  There is no doubt in my mind that my current garden space was a beaver pond many years ago, it is a relatively flat area, and is deep silty mix.  If I gave beavers free reign at my place, I would not have a safe garden or house site unless I cleared a large area of my forested land which is steeper and less likely to be flooded.  Beavers will definitely favor fruit trees.  You would have to encircle them with wire mesh or fence the whole area really well.  I do encourage the reintroduction of beavers into our waterways, but I would caveat that with the need for a live trapping and relocaton program so that people can choose to have them on their land or not.  We need Federal and State/Province run programs that fund both of those things in order to facilitate the re-beavering of the landscape.  It is one of the primary keys to reversing the damage that we have done to our hydrological systems.  There is no greater or cheaper way to facilitate the revegetation of the desert, to refill the watertables and aquifers,  to rebuild season streams into perennial flows, to ensure water security, than to have beavers back in our creek systems, lakes, and rivers.  

I guess this thread is long overdue for an update. Last year, when we arrived on the land, we found that while the dam wall had survived unharmed, the spillway had been utterly demolished, presumably by the snow melt. A bit surprising, given that we considered the spillway to be sturdier than the dam wall. But well, I guess it's all a question of water velocity, and as long as the spillway held sway, the water wasn't moving very fast anywhere except at the spillway itself.

Anyhow, with the help of some friends visiting, we rebuilt the spillway. This time from larger rocks, and with a wider rock-and-gravel "ramp" leading up to it, to direct the water over it. Then, step by step over the course of the summer, we continued digging out the pond (to a large degree, I confess, because we needed the excavated soil for raised beds etc. But hey, function stacking!) We also tried to restore some of the eroded parts of the left-hand bank.

When we left last autumn, the pond was completely open to the stream. It still needs to get deeper, but it is on its way. The dam wall and spillway survived some quite heavy rains (though not as heavy as the worst we've had) without issues. Hopefully the snow melt won't have been too much for it. Well, we'll see soon enough, we are going to be there in a week or two...

Thanks for the update!

That water flowing over the rocks is so beautiful! The addition of little ponds along the way sounds also beautiful! My permaculture earthworks guy said it wasn't worth doing more ponds on our property except maybe a little one for fun near the house. I would love ponds feeding into each other but I will be content with my rather large run-off pond and the berms/swales he put in.

The pond really reminds me of how an old river on a floodplain meanders, then as silk builds up, meanders turn into Oxbow lakes by being cut off. Often this happens when spring run-off cuts a new meander.

Eino, it looks like these recent pictures were taken during what would still be a high water period. Is that correct?

How straight is the river over your land and above and below it?

Water is powerful stuff - during bad flooding in my Province a couple of years ago, people reported hearing the crashing sounds of boulders being moved down-stream. This has been happening since rivers existed! I expect that even as you improve your dam and spillway, it will need ongoing watching and management.

Yep, the pictures were taken right after a decently heavy rain, so that's definitely above average water level. The dam wall and spillway survived spring unharmed, so things look roughly the same now as in the latter pictures (except some parts of the pond are a little bit deeper).

The character of the river changes a bit over its course. The highest part runs through a boggy valley bottom, so not very steep and making a million little turns, but doesn't change its overall direction much. The lower part is way steeper, and makes fewer but bigger turns. The pond is somewhere in between, the river starts getting steep just downstream.

And yeah, just a few days after we arrived on our land for the first time, we had this crazy rain storm. At the time we were camping way further down, so near the steep part of the river, and I remember that we were in our tent listening to boulders being pushed down. Equal parts cool and frightening...

Regarding the spill way, I would drop a few large rocks in a checkered pattern to slow the water flow a bit and limit the erosion of the down hill side of the spill way. other than that looks like a fun project.