Article Macadamia Growers Assoc – Erosion solved in Macadamia Orchards with Concrete Mats

Concrete matting an option for erosion control

High rainfall in most growing areas this year has highlighted the importance of managing erosion, especially for those growers whose orchards are on undulating or steep topography and for areas that are subject to high traffic. A range of solutions is possible, including installing drains and diversions such as hay bales, redesigning roadworks and retaining vegetation in the inter row to slow the flow of water and decrease its destructive impact. Importantly, whatever solutions are decided on, they should be part of an integrated plan whose aim is to maintain healthy, productive soil in the orchard rather than allow it to be eroded or washed away from trees.

The Macadamia Integrated Orchard Management Drainage Guide, published in 2017 by NSW DPI, emphasised that an effective integrated plan to manage drainage in the orchard should be designed to ensure that:
• there is minimal soil movement during rain events
• concentrated water flows are managed away from macadamia trees
• blocks are protected from run-on water
• good conditions for macadamia feeder roots are maintained
• the orchard floor is trafficable and harvestable.
Recently, several growers in the Northern Rivers have incorporated a different solution into their drainage management plans – flexible concrete erosion matting. The matting has traditionally been used in remediation, construction and maintenance works in civil and infrastructure projects as a way to overcome erosion issues. Now the agricultural sector has discovered that they are a cost-effective option for controlling run off, surfacing roads and for crossing waterways.

About the concrete erosion mats

The mats are manufactured by Australian Concrete Mats at their factory near Ballina. According to the company, the matting has multiple uses on farm. For example, it has been used by farmers on roads to prevent washouts and to allow access to orchards sooner than would be normal during wet weather. It has also been used on dam spillways and overflows, and to minimise issues with orchard drains being filled and blocked by debris. The mats are made with concrete and geogrid. Textured concrete shapes create a hard, protective surface for the soil, while the space between the concrete shapes makes the mats flexible and permeable, which helps stop water from pooling on top of the ground and creating boggy conditions. The flexible mat protects the soils from washing away and prevents deep rills and gullies advancing into the orchards. Mats are delivered pre-rolled with an underlay geotextile. The underlay is biodegradable and helps to promote the growth of vegetation, which also helps to anchor the mat in the long term. The vegetation also has a secondary role in filtering sediment contained in water flowing over the mats during wet weather and flooding. This helps minimise valuable sediment and top soils from washing away into waterways.

Concrete matting comprises concrete shapes embedded into a high strength, polyester, woven geogrid with a geotextile underlay. Mats can be either flexible or firm. No special equipment is needed to install the mats once the area where they are to be laid has been prepared, e.g. grading or forming of drains. Rolls are unrolled into position and anchored on each side with steel U-bar anchors. After vegetation has established the matting can be mowed, and machinery can be driven on it as soon as it is installed.

Grower experience

At the beginning of this year, Kurt Braunstein installed concrete matting in his orchard at Alstonville in the
Northern Rivers. Kurt has 3,450 trees between 20 and 25 years old with a 50/50 cultivar mix of A16s and assorted
Hawaiians. As with most of the Alstonville Plateau, the soils are red ferrosols. While most of the orchard is flat, a gully runs through the middle. Usually this is not a problem but when it rains Kurt said that it can fill up with water and become a
fast flowing river. With all the wet weather in the last two years, water running down the gully has resulted in flash flooding
and erosion, especially towards the bottom of the orchard where the gully gets steeper and deeper
making crossing it hard. Two years ago, Kurt started looking at his options, dismissing his first thought, which was profiling.
“Unless the soil was compacted and grassed immediately, the effort would have been wasted in the first rain event, particularly as there has been no opportunity to do this with the continual wet weather.
As well, taking soil from the sides to fill the gully would have been like robbing Peter to pay Paul,” he said. Instead, Kurt decided to install a 250 mm pipe underground along with a sump at the steepest section at the bottom of the gully. This ended up being a
“waste of time” because the volume of water coming down the gully was too great to be channelled through the pipe. Taking a different tack, six months ago he decided concrete matting could be the solution. Initially, he prepared the base of the gully by getting in several loads of cheap blue metal road base. Based on his experience, Kurt said that this preparation was important as it provided a bed to roll the concrete mats onto.

To keep his costs down as much as possible, he bought reject matting, which came in two widths – 2.4 m and 1.2 m – and in lengths ranging from 6 to 12 m. He laid the narrower mats at the top of the orchard where the gully was not as wide or the sides so steep. The total length of the gully area laid was about 240 m. Once the mats were unloaded using a front-end loader,
installation was a breeze and took a day and a half. Kurt used the forks on his tractor to unroll the mats and said that a bobcat would work well too. He did note that some of the heavier rolls of matting were about 1.5 t so it is important to have a tractor or loader that can manage this weight when unloading and rolling them out. After rolling the mats out, he applied a layer of soil so the concrete blocks were just covered and “threw out” a seed mix to quickly establish some vegetation to hold the soil. He said that this mix established quickly, and he is now looking to put in something with runners such as kikuyu as a permanent cover.
He was able to drive over the matting soon after it was installed. While he can mow it, this is with the blades sethigher than normal, otherwise they will hit the concrete blocks and be damaged. Kurt believes that when the permanent cover has established itself, mowing will be easier. While he doesn’t need his mats to provide a harvestable surface because of its location in the orchard, he said that once they are bedded down and vegetation permanently established, nuts should be able to be harvested from them. A test of the mats was in the most recent heavy rains in October and Kurt couldn’t be happier, saying that they performed “very well”. “Some soil was washed out by the rains and water flowing down the gully, but it wasn’t a big issue and, importantly, there was no erosion,” he said. “I can’t think of anything else that would have worked in this situation, especially with the continual wet weather.” According to Kurt, it is obvious that you can’t stop water from going where it wants to go, as instanced when he installed the underground pipe, so the best solution is to work with the environment rather than trying to install barriers or redirect it.

Lesson learned

When asked what he would advise growers looking to install concrete mats, Kurt said that it was essential to prepare the area well by getting the profile shape right and providing a good bed to lay them on. He thought the blue metal he used worked very well.
One of the things he learnt from experience was that the mats should be pegged. “I didn’t do this when I installed them and after the first heavy rains, the mats at the top of the gully start to roll up so I had to go back and put the pegs in,” he said. And finally, he said that a key is to get grass cover as this helps keep the mats in place and is essential to water flow and to stop more soil being washed away in rain storms.

Article by Australian Macadamia Association. Summer 2022 News Bulletin

https://australianmacadamias.org/industry

https://australianmacadamias.org/industry/news/ams-news-bulletin-summer-2022-version1-version1