|100 Lambert Street , Ararat, Victoria, AU|
(projects i'm involved in)
Project: Mudlark Permaculture
Posted by Carolyn Payne-Gemmell almost 8 years ago
Straw bale gabions-an experiment
When it comes to doing Permaculture related work, I often find it a good idea to call everything I do an experiment, it is a word that covers lots of bases, and when the results of activities are a little unpredictable, the word experiment is an easy cover for the words 'I have no idea, but its an interesting result". The word 'experiment' piques the interest of those who are curious, and gives me licence to explain my actions in detail.
For me personally, there are no black and white actions or answer to anything, the whole of life is shades of grey, and as everything is connected to everything else, we can never really predict anything with certainty, there are too many variables.
When searching to buy land for the home of Mudlark Permaculture, a site with excellent water harvesting demonstration potential was the highest priority. Certainly the most striking feature of this property is the 275 x 50 metre dam and adjacent swamp land.
The previous owner of the property and builder of the dam, John, is an interesting man and now my neighbor. John is a bit of a rebel, and he went over the heads of the local council and straight to the regional water authority with a plan to build a permanent wetland for bird life. He built it in a location that was damp and boggy, but not wet enough to be considered a swamp.
Luckily for me he had the vision to create this very large structure, as it is now the main feature of Mudlark Permaculture. It was a good effort, but with a few flaws in the design, however I am confident some well placed Permaculture design techniques will improve the overall capacity.
The dam was dug in a marshy area that was waterlogged in winter and the soil is fine silty clay. The wall height was built approx 3 metres with a free board of 1 metre above the high water mark. The depth of water when the dam is at spill point is somewhere between five and six metres deep.
Surprisingly, the dam was initially built without a spillway of any kind, clearly a very dangerous omission for a dam wall of this length. Ironically, very few dams built in this regon have spillways, maybe a metre wide at best and a few I have seen are about a shovel width, when the owner has run out in a shower of rain and cut a trench through one end of the bank, it is the sort of behaviour that gives me nightmares.
In the first winter, when the dam filled, the water began to over top and erode the bank, fortunately, this was only in a small section and there was enough spare material to fix it, and a 10 mt wide spillway was hastily pushed around the end.
---Spillway at the end of the dam bank-the water turns and runs behind the bank- safer re-design coming soon
The onset of drought saw the dam only holding a modest amount of water for the next six years, and, with the best of intentions but the worst of consequences, the previous owner John spent three years keeping the high water mark at spilling point by running very salty bore water into it night and day. This created a biological desert, with very little plant growth, in the water or around the bank. There were only a few insects and frogs and despite John having put fish in the dam, they were not breeding, but dying in the hot weather. The bird life was limited to various hardy duck species and if any other species called in, they moved on after a day or so.
Central Victoria had a predictably dry autumn in 2010, however good winter rainfall that year saw the dam fill and spill in rapid bursts on three occasions in July and August.
When the dam was built, a large flat drain was constructed across the paddock, bringing water from a culvert. This culvert passes under the road, bringing large amounts of water from the nearby hills onto the property. This run-off water previously bypassed the marshy area, but now it is captured and transported for 250 meters cross country to the swamp, which is made by the back flooding of the dam.
From a Permaculture earthworks perspective, this drain is far from ideal, it has a compacted quartz gravel base and mound, and on first appearances, looks like a swale but can not function as one, despite the fact that it is very flat, dropping only 1 mtr over its middle 200mt section, and is 2-3 mts wide.
With each rain event, the water rocketed through the culvert, flowed quickly along the drain; carrying clay, leaves, small sticks and quartz gravel into the swamp, creating a headwall cut at the end of the drain where it turned and dropped sharply.
However, thanks to a little experiment, the addition of bales of pea straw, I slowed the water flow down so it seeped, rather than flushed, this caused the water to continually flow into the swamp/dam complex and over the dam spillway for four months instead of a few days.
I wanted to try and slow down the flow of water and catch the sediments, and see if any water would infiltrate the soil on top of the compacted bank, with the hope more grass would grow there.
I began by placing 3-5 bales of pea straw across the drain at twenty five to thirty metre intervals. Water flowed under and around the bales initially, but as they became waterlogged and softened, with the help of a little jumping, they molded into the drain base and began trapping the sediment; they started silting up from behind, essentially gluing them into place. The water began to back up and was being held in the drain for several days after it rained, when previously it was gone in just a few hours. As the water level began to be held higher, I was inspired to add more pea straw bales and bale biscuits to the ends. I raised the level of water in the drain until it almost overtopped the lower sections of drain wall.
These bales were essentially acting as straw gabions, allowing a small but constant flow of water through the straw, slowing the water flow, trapping sediment and vegetation, and buffering the highs and lows associated with intermittent rain fall.
There were some great side effects apart from producing a constant water flow from an inconsistent source. The water clarity was greatly enhanced, with water that starts out various shades of brown from the gravel road runoff and tea coloured tannins from the hillside eucalypts , to being visibly clearer after passing through six sets of straw gabions.
---Pea straw bales sprouting peas- this was the highest and widest section of bale gabion
A great benefit of holding the water consistently in the drain over many months was the extra carrying capacity for the local wild life, dragonflies, frogs, lizards and the odd snake, and all the associated bird life which prey on all of these.
Up to this point, the story and photos had been six months in the making. I was hoping to show a 12 month diary, and I was predicting a green grassy drain which was trapping sediment, creating its own soil and any other interesting observations along the way. But whoops, we all know what happened in January 2011 : rain, rain and more rain…………..
So, in the spirit of one of Geoff Lawtons sayings, “show everything…… win, lose or draw”, here is what happened next.
On January 14th 2011, large and sudden flushes of water began coming through the culvert -and a lot not even making the culvert- but flowing directly across the road out of the bush land carrying gravel, silt, leaves and branches. It tore along the drain, ripping up the bales and shredding them.
On the first few gabions, the water flowed around the sides, disintegrating them from the edges, and eating away whole bales on the outside edges. This damage dropped the water level a bit but still held well.
---Centre bales unglued and moved
--- Further along the drain-Pea straw battering rams
Several other gabions came apart in the middle, this was where the drain narrowed a little, with the bales becoming ‘un-glued’ they started to move along the drain. This was a more serious matter as these sections lost all their water and I suspect it would have been sudden, setting up a tidal wave effect, and turning pea straw bales into battering rams. The effects became more pronounced the further the drain went , until reaching the end where the last few bales were thrown aside and all the water, dirt, sticks, leaves and pea straw made their way into the dam.
The water quality in the dam had been improving over the months of clean water infiltration, this set up the complex series of new plant growth, insect breeding, fish spawning and the arrival of fish predator birds on a permanent basis.
When the dam spilled in this sudden major event (once in one hundred years, I am told) the fish spawn washed down the paddock in their millions, becoming a short term easy bounty for the birds, I also suspect they became "fish fertilizer" as they rotted, making very lush grass growth.
---Dead Silver perch- bird food/fish fertilizer
--- Dam to the left- spillway water on right travels behind dam bank- new earthworks will soon amend this situation
In hindsight the only thing I would have done differently with this "Experiment" would be to anchor the bales with steel posts or large wooden stakes, they would have still disintegrated but it would have stoped the suddenness of the battering ram effect.
As all things are inter-related the story doesn't really end here - Mudlark Permaculture is an ongoing process, an Experiment, so this is just one chapter of many.
Stay tuned for the next one!
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