Rosella Waters – Earthworks Phase 1, Part A

After many months of planning and waiting for the earthmovers to have their equipment available, the transformation of our humble 6 acres has begun.

As with any major event in one’s life, there was an air of excited anticipation and a slightly sick feeling in our stomachs. Just like the one you can get when you go travelling in far-flung places. You feel as ready as you’ll ever be but not 100% sure how things will pan out. Since doing our PDCs at the PRI over 2 years ago, these are our first major earthworks and time to put all that theory into practice!

During our April PDC with Darren Doherty, we dug 8 test holes with a 5 tonne excavator to see what kind of material we had to play with and if the quality of the clay was going to be there to hold water in our proposed dams. I would definitely recommend this practice when considering building a dam, as the expense of $300 to get the holes dug gave us piece of mind knowing that we had decent material before we began.

After that, we had a survey of the half of the property where the earthworks were to be done, by a friend, which made our little place look like the Swiss Alps. Marking out the swales connecting to the dams and the correct placement of the spillways became quite a challenge.

The thing to remember is that the height of the level sill spillway is the same height as the high water mark for the dam. What we were placing down on paper was essentially a concept plan as a few mm here or there doesn’t really concern a 20 tonne dozer…. as we were to find out.

The start of the process was to clear the site of close to 20 trees, mostly white gums and a couple of boxwoods. 90% of the trees were hollow after being eaten out by white ants so their core was filled with mud and not much cop for chain sawing through. The little bit of chain sawing we did to cut off the crowns and root systems, quickly blunted the blades. The dilemma we faced was what to do with close to 20 cubic meters of wood?

One suggestion we had was to use the 20 tonne excavator to crush it all into smallish pieces and spread it out over the landscape where the proposed swales were going. This way the whole lot could be cast with cover crop and in a couple of wet seasons it would all break down and feed the soil microbiology. On suggestion of this to the earthmovers, they somewhat politely responded in typical north Queensland colourful language.. No #$%!*&.way! The guys are married to their machines and the ones we have on site look like absolute dinosaurs. Both are relics from a bygone era having been built in the early 80s, leaking oil and with battle scars from years of “clearing scrub” and logging in Papua New Guinea. Still, the boys swear by them, hate the modern ones with computers and after 100 years of combined earthworks experience between the two brothers, they wouldn’t use anything else.

We’re saving a couple of the logs for fence posts, sawing some for nesting boxes for native animals, birds and bats, a little for fire wood and the rest will be sent through a “Tub Grinder”. As the name suggest, this massive washing machine gets brought in on a semi trailer and grinds whole logs and branches into mulch. It’s the machine councils often hire to mulch all the green waste at dumpsites. It’s actually quite reasonably priced at only $6 per sq metre of mulch produced. Needless to say we’ll have enough mulch to compost and spread out over the entire site.

Following clearing of the site, all the valuable topsoil was scraped off and set aside, to be placed back over the dam walls at the end. A cover crop can then be sown and lightly sprinkled with mulch to help stabilise things after the machines roll out. After that, the keyway of the first dam was dug. We decided to call this dam the Lap pool as it looks like one and is likely to be used often for such. Prior to the machines arriving we had marked out the approximate position of the middle of the dam wall and the proposed water line that we wanted to achieve.

The base of the dam wall was to be 15 meters wide at the base, tapering up to a 3-meter wide road at the top. That’s quite a lot of material required to reach our wall height and the 750mm freeboard that we were after. For those that don’t know, the freeboard is the safety margin between the highest level of water in the dam and the top of the dam wall. Together with correctly set spillways, it ensures you’ll never get water go over your dam wall, which would be an absolute disaster. More on that later.

Putting in the key required the excavator to dig down 2 feet wide enough to cater for the 6 tonne roller that would be used to compact the best clay we had on site to construct the key. It’s important to use the best clay and compact the key very well as this is one of reason why dams can fail if not done properly. Our earthwork brothers thought we we’re mad building dam in such a way for a body of water that was only going to hold approximately .3 of a mega litre. They’re used to suing this kind of technique on HUGE dams that cost $100,000 to build. Still, they did admit that it is the best way to do it. Ofcourse at $150 per hour for the machines it’s more expensive that simply pushing up a wall, but as the saying goes…poor man pays twice.

Something else we needed was a water truck on site. Fortunately the brothers had one of those, which meant we didn’t need to pay someone $100 per hour to sit there all day and get the odd load of water to help with compaction. We only got charged for the hours the truck worked and a neighbour at the top of the hill was happy for us to take some water from dam that he wants to drain and empty of “those invasive bulrushes”. I told him we’d be happy to take them too as long as he didn’t threaten them with roundup!

Once the key was dug, the dozer swung into action and started the repetitive process that lasted the rest of the day. Pushing material in the key, building the wall in layers, watering it down as required and track rolling it for compaction. One point on moisture is that you only want enough moisture so that when you squeeze the material it holds it shape. If you make the material too wet and compact it, it’ll crack as it dries and end up leaking water. The dozer had 3 rigid tyne rippers on the back and used them to rip the soil before pushing it up onto the wall with its front bucket, followed by water and track rolling and so on and so on…

All was going swimmingly until…a hiss of spurting oil and we we’re down for 1.5 days. One of the hydraulic hoses that control the rippers split in half. The hose was as old as the hills and the entire undercarriage of the dozer had to be taken off to access it. Over 20 years of oil mixed with dust was an extra addition to our soils as the machine was fixed on site by one of the brothers who’d been “doing it for 20 years”. The hose required wasn’t available in Mareeba (10 minutes away) so a 2.5 hour return trip to Cairns later we had a new one. By Thursday afternoon the machine fired up again and went back to ripping and pushing dirt up the face of the wall. A channel 1 metre or so deep was dug back up the gully to the high water level we had sprayed blue after checking things again with our laser level. At the point in the gully where the water will enter our property we have marked out a suitable location for a 3-metre wide rock wall otherwise known as a gabion. This will trap an enormous amount of silt each year and provide us with all the potting mix we need. The gully has an annual catchment of 5 mega litres and the small test gabion we made by hand earlier last year showed us we really needed to build a bigger version. The 3-metre wide rock wall will also provide access across the back of the dam to the orchard area.
In the process of digging out the channel at the back of the gully we were starting to struggle a little for material to reach our proposed dam wall height with a 750mm freeboard. The pressure of making decisions with a dozer putting away at $150 per hour led us to make an error of judgement and a small scar was created. We decided to cut a 500mm wide bench AT water level, which created quite a vertical drop from the original landscape. Luckily we realised our mistake and with the sun setting we had a good chat to the operator who next day admitted it was probably not the best and we should have benched BELOW the high water mark. However, with a little excess topsoil put aside at the end of the construction, we’ve decided to leave it as is, use the topsoil to back fill and what we’ve mistakenly ended up with is a wet/dry extended edge at the back of the dam which when its full will support many a beneficial aquatic species and habitat. In fact, we’re now looking at putting a slightly narrower bench around the entire inside edge of both dams to create a green edge, which will look really nice we think. We have also decided to bring the proposed gabion forward by 2 metres and make it much larger (3 metres wide) than we initially expected. This will heal the worst of the vertical drop and incision in the side of the gully.

The following day we begun work on the keyway of the second larger dam we have called the Mushroom because of its obvious shape. The excavator had dug it the day before so now the process was much the same as previously stated with the dozer ripping, pushing, followed by the water truck and then 6 tonne track roller. Once we had come up to original ground level, we then set about digging a channel to place our 27 metre long lock pipe fitted with 3 baffle plates. There were a couple of reasons for placing this in the wall, and once again the brothers shook their heads in disbelief as usually much larger dams holding much more water and being used for stock water or flood irrigation install such things. Still, being the bottom of our system we wanted the ability to be able to drain the dam if required, as this will be our primary aquaculture body of water on the property. The lock pipe also faces directly west and in the event of a fire we also now the option of opening up the taps to drain close to a mega litre of water in the likely direction of an incoming fire, making it nice and damp in between it and our structures which lie east of the dam. Living in the wet tropics we have designed our system to cope with the 1:100 year events that DO happen. If we were to get a 100 inch in a week event as we did in 1979 then we like having the ability to again open the pipes, leaving next to no chance of any water reaching the top of the dam wall. On that point, with the Mushroom we designed for a 1-metre freeboard and will place two level sill spillways on both swales that extend from the dam wall, 3 metres wide to deal with overflow.

Once the trench was dug, three of us carried the lock pipe and set it in place. The lock pipe consists of PN10 grade polypipe and is made from four 6 metre long sections. Each baffle plate (also poly) is 750mm x 750mm and has been electro welded to two .5 metre lengths of the same PN10 poly. All seven pieces were assembled onsite with couplings that fit over two sections of pipe and fuse together using an electrical charge. The couplings have a + and – node to which clamps are attached sending an electric charge through which melts the poly inside the coupling, fusing it forever to the poly. We have gone with a butterfly valve for the exit end as it allows us to regulate the flow from the lock pipe easily. At the intake end we have an elbow with a further 1 meter length of pipe which we drill ½” holes in to act as a filter. The cap on top of that section is screwed on with three stainless steel screws just in case we drain the dam and need to take it off. We hired a “wacker packer” from the local hire centre and with subsequent layers of material placed on top of the pipe, compacted the soil around it and the baffle plates. It is important not to over compact using such a machine that compacts soil much differently than a roller. It is possible to “over compact” the soil which when it inevitably soaks up moisture can lead that area to crack once again leading to potential leakage. Once we had a good 400mm of compacted soil over the lock pipe we then started to layer material again using the dozer followed by more track rolling.

During this whole process I had the lazer level out and was marking a clear high water mark line in blue spray paint for the operators as a guide. We had asked them to grade the sides of the dam walls at 1:1 starting from the high water mark line. This is nice and gentle and gives the dams a nice shape. It also means that any stock and humans can enter and exit the dam without shear drop offs into the void. With red paint I marked on a tree, at both ends of the dam wall, our target dam wall height, which in this case was 1 metre above the high water mark.

The rest of the process for the Mushroom went pretty much according to plan and what we had thought would be a 3.5 metre deep body of water ended up being 4.5 metres deep, which will greatly assist with evaporation. With that depth you get thermal cycling of cool and warm water, which is important in our scenario as we have 9 months of the year when the evaporation rate is over rainfall. We found a couple of patched of rock in the hole which I have subsequently picked out and set aside for future gabion plans in the small creek at the back of the property. There were also a couple of sand streams which are unlikely to be a problem but if it does end up holding water like a bucket full of holes there is enough clay left in the bottom of the dam to have the guys come back, dig them out and plug the holes with good clay. There wasn’t any point in doing this until we see what happens as everywhere else we dug through a bit of sand, we hit good clay behind it.

Another thing, which will be of interest, is the little spring we found when we initially had test holes dug. Sub-surface streams of water run through this place and in general are caught in a layer of sand between two layers of clay, top and bottom. These clay layers can be top/bottom, side/side in fact any which way, and we wont know until later exactly where the spring decided to surface. Once all of our water harvesting structures are in place, and there is another 5 days work to do, the landscape will be fully hydrated come the wet season. The little spring we found was at the back of the Mushroom and the intent was to dig through it and hopefully provide the dam with an offset of water to compensate for evaporation. We had placed a 90mm PVC pipe in the back filled hole when the tests were done and with a length of bamboo kept a regular check on the level of water moving through the landscape. What we notice was that the height of the water fluctuated with the moon and on full moon it was at it’s highest at close to 1 metre of water. Leading up to that time early in August will be interesting and we’re waiting to see if we get any water seeping through. At the moment there is nothing. Depending on the pressure of the mass of water that will sit on top of that point once the dam is full, the spring exit point could be in a totally different location. Water always finds the path of least resistance.

The following day, Sunday, was a half day spent cleaning up the site a little and spreading the last of the remaining top soil over the back of the dam walls. What we now have is two dams within the one gully that combined will hold close to 1.5 mega litres of water when fully charged. That still leaves a substantial 3.5-mega litres of water to flow through the system, charge the connecting swales, which are yet to be built, and fill the other 3 bodies of water we have planned. To do the remaining earthworks of Phase 1, we have enlisted another operator with a much smaller 5 tonne excavator with a tilt bucket. He will be putting in swales either side of the Mushroom and their spillways, a 50cm deep kids bathing area on the edge of the Mushroom under a large tree, a swale and diversion drain to another small duck pond on the Lap Pool, holes for a 3m x 3m jetty and 6m x 6m jetty on wither dam, another diversion drain and further duck pond with connecting swale with spillway, 150 sqm of terraced rice paddy, approximately 200 meters of other swales, another small water feature at the entrance of the property shaped like a dew drop, the Lap Pool 3-metre wide gabion and finally reshaping of the access roads and instillation of new irrigation lines. More on all of that later! So at this stage we have only cover cropped the back of the mushroom with cow pea as he’ll need full access to the site and will be roughing things up a little more yet.

All in all it’s been a pretty full on but very interesting and rewarding experience thus far. The bill so far has been just over $9000 plus the lock pipe, which was $1200. At first glance it seems like a bit of money but when you consider the value we have added to the property, the potential for growing systems with entrapped water, aquaculture and the eco-systemic services we are providing by keeping all that silt of the Barrier Reef, it’s OK.

A big thanks to our friends, Geoff & Nadia Lawton of the PRI, Darren Doherty of Australian Felix Permaculture, Paul Taylor of Trust Nature, Nick Ritar & Kirsten Bradley of Milkwood and all our past PDC students for their advice, support and ideas, many of which you’ll be happy to know have been put in place! Part B of Phase 1 coming soon….

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