USING TURF FOR EROSION CONTROL

Research into turf has shown it to be the safest and most reliable method of achieving full greenlife coverage on erodible surfaces, while continuing to be one of the lowest cost methods. Turf achieves far more reliable coverage than hydro mulching, and when calculated as a percentage of coverage, turf has proved to be economically viable in comparison to other products. In a side-by-side study, Hydro mulch achieved a 23 percent coverage, while turf had 100 percent coverage. Turf also performed far better than silt fences, biodegradable mats, hydro mulching, and erosion coir socks.

Previously, barriers to the success of traditional varieties of turf products, such as Couch or Kikuyu, resulted from the fact that they required mowing maintenance at least 5-8 times per year on a traditional drainage swale. Enter the Zoysia revolution. In 2006, the roads department began experimenting with Empire™ Zoysia japonica ‘SS500’ PBR turf on roadsides and drainage swales, and since 2010 Empire™ Zoysia has been the go-to-turf for many roads departments for non-irrigated areas on roadsides. The switch to this particular product has resulted in maintenance savings and reduced energy expenditure, as Empire™ turf is mowed twice per year on average.

Empire™ Zoysia can be mowed when it reaches approximately 12-15cm, though growth will significantly slow beyond 20cm. This reduced maintenance on slopes and drainage channels means that the turf can be mowed or brush cut every couple of years or not at all if the area has a low volume of pedestrian traffic. Research demonstrates, having not been mown for 8 months, Zoysia reached 127mm in height, while Couch reached 250mm and Kikuyu reached 422mm in height. An added benefit to this reduced growth rate is that Zoysia does well in low nutrient situations, meaning the product works well without fertiliser.

A sister product to the Empire™ Zoysia, the native version of Zoysia, Nara™ Native Turf has been used by a number of clients since 2011. Nara™ Native has the same low-maintenance, low-cost benefits of Empire™ Zoysia, yet it differentiates itself with a better winter colour and higher salt tolerance. Zoysia macrantha only occurs in Australia, and as a native product it cannot be mixed up with exotic turf types. Like all Zoysia, Nara™ copes excellently with wet feet, so it is ideal for drainage swales. If for example, water sits on Zoysia for a week it will not be adversely affected like Couch, Kikuyu or Buffalo would be. For this reason, turf farmers often keep their wet areas available to grow Zoysia.

Research has shown Nara™ strengthens the soil 105kPa or 58 percent, which is far higher than Couch at 68.1kPa and Kikuyu at 72.4kPa. Both Nara™ and Empire™ have a natural mesh growth inside them and have extremely strong stolons and Rhizomes, which form a natural mesh that greatly increases the turf rolls tensile strength. This makes Nara™ the perfect solution for pegging with specialised biodegradable anchor pegs. Pegs such as E-staples have been designed with a type of arrowhead that makes them very hard to pull out and should water try to remove turf types such as Couch or Kikuyu, these pegs will stay in place and the turf will rip through the pegs. With Zoysia, the pegs will fail first as a testament to the products improved strength. When Zoysia is used for erosion control it is likened to having very strong jute mesh grown into the turf.

In the past, the cost of maintenance has prevented erosion control professionals and engineers from utilising turf in their projects. Now, with Zoysia two mows per year is all that is required to keep a drainage swale looking pristine. To maintain a pedestrian grass area, one mow or brush cut per year is satisfactory. On slopes or areas where pedestrian traffic is not a problem, a brush cut every 2-3 years will keep it looking clean and well persevered. If the presentation of the area is not a major factor in the project, no mowing is required and Zoysia will continue to provide adequate erosion protection.

Both Empire™ Zoysia and Nara™ Zoysia are superb at outcompeting weeds as the product grows thick enough that weeds find it difficult to survive. They have both proven to be extremely drought tolerant in research and will live on natural rainfall in the Eastern states of Australia once established.

Turf can withstand between 2-2.5m per second water flow without reinforcement. If the turf is laid on top of non-biodegradable long-lasting reinforcement netting, such as bird netting or similar, tests have shown it can cope with up to 5m per second flows for longer periods, and for short durations reinforced turf can withstand far higher flows.

Unlike concrete and other hard surfaces, a Zoysia lined channel or slope will help remove heavy metals and nutrients, catch sediment, and return water to the water table, whereas hard armour simply transfers pollutants to the final destination. Normal Zoysia turf or site reinforced Zoysia provide instant erosion protection, with 100 percent success if watered for establishment. In comparison, temporary mats or seeded hydro mulch both have a much lower success rate. The lower success rates of these products result from the long-term process of the vegetation’s growth from seed or plants and its spread into the groundcover. Uneven cover and performance are also a concern for such methods.

For projects that require use through the bottom of a constantly wet channel, Dryarna is the answer. Dryarna is a form of native turf commonly called Water Couch and it will survive in permanently wet conditions even as water flows along it. The product is not, however, overly drought tolerant, and is best used at the bottom of the swale where it is regularly wet. In dry situations, such as the sides of channels or if the bottom is only periodically wet and has periods of dry, research demonstrates that Nara™ native turf is a suitable alternative for these cases.

COST COMPARISONS
Average cost including installation according to Landscape Association and other research.

The Zoysia revolution is now upon us. It has been tested and proven to work for over 10 years in Australia. Over in the US, project managers have been using Zoysia for decades with excellent results. The greatest barrier for professionals previously attempting to use turf for erosion control, on roadsides, on slopes and in swales was the cost of maintenance. Zoysia has solved the problem of high maintenance and high-cost functions. And alongside its low-cost benefit, Zoysia’s functions as the strongest performing erosion control grass thanks to its higher shear strength and ability to better protect the ground from high-velocity water flow.

For more information visit www.ozbreed.com.au 

Words by Todd Layt

Using Turf for Erosion Control

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