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Milkwood Blog

The saga of the middle dam

March 16, 2011 | Permaculture Design, Water Harvesting + Reuse | 20 comments | Author:

First off, i would like to make an important point: we are yet to meet a challenge at Milkwood Farm that we could not fix with careful thought, good advice, relentless research, a strong dose of creativity and a stronger dose of humor. That said, the saga of the middle dam nearly had us stumped. But we got there in the end, with a strong brew of the above.

Secondly, I would like to point out that sharing our challenges so nakedly on this blog is not something I really enjoy doing. Sometimes I would rather paint a rosy picture of first-generation farmers awash in successfully implemented permaculture solutions and photogenic fields of nitrogen-fixing perennials. But hey – where’s the fun in that?

The middle dam being built, in December 2007

Our first dam on Milkwood was built in December 2007, as part of the very first course we ever ran at Milkwood Farm. 1.8 megalitres in capacity, it was built partly to form part of our immediate living environment – our cottage (still in progress at writing) is emerging from the earth on the dam’s edge.

I’ve talked to a lot of earthworks operators and permaculture designers about the moment when a hydrological earthworks system fills with water for the first time – and they all say the same thing – it’s a big relief when you finally see the system working as it should.

In our case, we waited nearly 3 years to see if the system really worked – it took that long to get decent rain!

The middle dam in August this year - fuller than full.

When our grand moment of watching the system do its thing came in August, we made some necessary adjustments in response to seeing the system at capacity – mostly re-setting the spillways.

Nick is no stranger to solving dam problems. Aside from his consultancy work, we’ve had a couple of on-farm challenges with dams at Milkwood already. As outlined in The saga of the top dam, we’d spent the year before this struggling to make our top dam hold water, due to multiple factors, one of them being dispersive clays.

Nick cracked that problem in the end, but it took a lot of thought, hand wringing and hard work to get it right.

In the months that followed with all this new water about, things started to happen for our middle dam. One day in December we noticed that the whole wall was slumping. We were about to loose the dam! First off, Nick leaped into action and drained the dam as much as he could, as fast as he could, with our trusty small diesel pump.

After some discussion with various earthworks expert friends, Nick did some thinking and some research, counted our pennies, made a plan, and called the earthworks company. One excavator to Milkwood, please – stat.

Dam repair method diagram: before and after

What it turned out had happened is this: when the wall was built, some topsoil was left behind on the ground at the base of the wall. With heavy rain, this topsoil liquefied and moved downhill, causing part of the wall above it to slump (marked in red on the diagram above).

When a dam ‘goes’, it’s a really big deal, because it means the end of that dam – that’s it, show’s over. The amount of material washed away by the force of the water when a dam wall fails is, in almost all cases, just too costly to replace. We really didn’t want to loose the dam right next to our house. So Nick began coordinating the emergency repair immediately.

Our house dam with its water all pumped out, awaiting repair

After waiting 3 years to see this dam full, it was hard to pump it out again and let the water go! But go it did, down into the creek. Now everyone could work properly, without so much fear of the dam bursting.

The dam wall with the topsoil all scraped off, and the gunk layer removed

We removed all the topsoil from the outside of the wall with an excavator, and dug into the wall to the source of the slippage – the liquefied topsoil at the wall’s outer base. Nick had the excavator remove that quicksmart.

He then needed to re-build the dam wall at a more shallow angle. Part of the reason for this dam having problems was that the wall was, in retrospect, too steep for the non-ideal materials used. The wall also needed to be ultra-compacted to ensure it stayed put from here on in.

To build more wall, we needed more material. To get more material, we needed to make a hole somewhere else to obtain that material from. Hmm. I thought we had enough holes on this farm.

Digging out a pond to create material for reparing the dam wall with

Turning another problem into a solution, Nick elected to dig a pond below the dam wall, where the slow leak comes out. Ah yes – did I tell you about the slow leak? This dam has a leak out it’s base, due to a fissure in the bedrock at the bottom of the dam when it was first built (did i also mention that we surprisingly hit bedrock when we built this dam? We did). Not much we can do about this leak. It is a slow one, and it pops out as a wet spot a little further down the hill.

So a new pond seemed like a good solution to two problems – we would get more material for the wall, and create an aquatic environment which was a good use of our wet spot, rather than letting that precious water create a mushy patch at the bottom of our system.

We then mixed the material from the new pond and the material from the slippage together and applied it in layers to the dam wall, compacting each layer as we laid it. Enter the sheep’s-foot roller, a nifty little device fitted to the end of the excavator’s arm.

sheeps foot roller in action

A sheep’s-foot roller exerts great force over the small area it rolls, and is great for compacting hard-to-compact material, like ours. Micah the excavator driver rolled and rolled and he rolled again, until that entire wall was well compacted. No more slipping happening to this damn wall on our watch!

The new pond and dam wall, all re-dressed in topsoil, and ready to grow stuff

Following this step we replaced the topsoil all over the dam wall and around and inside our new pond. Taking no chances, Nick and Ryan (our fabulous intern) also applied a generous amount of gypsum to the inner side of the dam wall, to help bond our dispersive clays and improve water retention.

using the sheeps-foot roller on the inside wall of the dam, squishing in the gypsum

Last of all, Nick and Ryan seeded and feather-mulched the new dam wall with cow pea as a cover crop, toasted the efforts all around, and then went and had a little lie down.

The dam wall a couple of weeks later, with it's cow pea cover crop coming up

Designing and implementing earthworks for water catchment and storage on your farm is a big deal. But there a plenty of books and DVD’s and people that can tell you how to do it, and who make it seem very easy and fool-proof if done well. Hurrah for that.

What we struggled to find in our hour of need, however, was resources on how to fix a failing dam, what to do when the wall slumps and other worst-case scenarios. Where are all these resources? Earthworks are not always a case of ‘set and forget’ – our landscape, especially those bits with non-ideal aspects, just don’t work that way.

So that’s why we wrote this post – to share a near-miss. Maybe it can be of use to someone else who has difficult terrain, a big rain, a slumped wall, and a lot of determination. It was great to work through this problem and find a solution that we’re confident will work well.

The total cost of repairing this dam wall was $3,000 which mostly went in excavator hire and excavator operator fees.

the new pond at the bottom of the wall, filling up nicely (with log for added underwater ecosystem)

Considering our future home’s immediate surroundings will be so positively influenced by having a large body of water right next to our front door, we consider the investment of re-building this dam wall very much worth it. We learned a lot, and we got an extra pond, which the geese have taken as their own. All’s well that ends well!

Thanks to: Cam Wilson + Darren Doherty for their solidarity and suggestions, and to Ryan Rutley  for the pictures and his efforts throughout this project. The excavator we used was a Komatsu pc120 from A1 Earthworks in Mudgee and the driver, Micah, was awesome.



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