What is Hydromulching?

Hydromulching is a hydraulic technique to efficiently apply a mixture of organic material (mulch), a binding chemical (tackifier), fertilisers, and seed as a slurry in water, onto open or disturbed ground to prevent erosion while supporting revegetation. It is commonly used to rehabilitate land disturbed by land development, earthworks, bushfires, mining and road construction amongst many other reasons. Disturbed land can result in accelerated erosion attributed to loss of vegetative cover and soil fertility, alteration of the natural topsoil and changes in topography. Hydromulch mixtures typically include a soil stabilizer in a liquid slurry that is sprayed with a specialized water truck that incorporates a tank, agitators, and spraying equipment such as hoses and cannons. In general, hydromulch effects do not last the entire growing season and require up to a day of drying to be most efficient although hydromulches solidify after application which decreases soil movement due to a binding effect. In addition, hydromulches promote seed growth by trapping moisture and increasing soil and seed stability. However, the limitations of hydromulching include bulkiness of mulching equipment and the requirement of a ready supply of water to properly mix components. The article seeks to evaluate hydromulching, compare aerially applied vs ground applied hydromulching techniques and sum up with the role Global Road Technology plays in hydromulching innovative solutions. 

What goes into a hydromulch mix?

Hydraulically applied mulches known as hydromulches have continuously evolved and improved over the past 50 years with inclination towards enhanced performance and greater productivity over the traditional methods of erosion control and revegetation.  Hydromulching is a recent variant of mulching which tends to bind strongly to the soil surface by the action of the soil-binding agent hence it is particularly useful for steep slopes and strongly modified areas such as quarries, construction sites and cut and fill slopes along roads. A range of organic materials is used in the industry from thermally or non-thermally refined wood fibre, paper, straw, cane thrash, and even non-organics such as plastic fibres are used to provide the mulch layer. Binding chemicals, often called tackifiers, help bind the mulch to itself as well as stick it to the ground surface. Tackifiers are polymers – and a wide range of chemistries have been utilised in the industry. When used in revegetation, seeds are included in the blend and are intended to increase the vegetative cover, especially when the mulch starts to decompose. Careful selection of seeds for the target environment is crucial as it guarantees both adequate cover from seeding and avoids introduction of species that become invasive. Hydromulching treatments can be of different combinations, depending on what the intended outcome of project is. For example, a land development for housing may only require short-term establishment of a grass species in order to stabilise blocks for sale, whereas a road development through a National Park may require stringent seed selection, native species, and permanently established vegetation. 

How is hydromulch applied?

Ground hydromulch technique involves spraying from a specialized water truck that incorporates a tank, agitators, and spraying equipment such as hoses and cannons. These vehicles apply the water-based slurry of the mulch, tackifiers, and seed at a certain litre per hectare with spraying limited at maximum spray distance from infrastructure such as roads. Aerial hydromulch treatment is applied using an aircraft and the hydromulch is applied as a water-based slurry at a certain liter per hectare usually with the same seed mixture as for ground hydromulch application and can also include soil-binding agents such as polyacrylamide. The benefit is that applications aren’t limited by road or rail access, although obviously the application skill set is far less common. Site research shows that aerial hydromulch treatment can have greater initial and a longer-term effect on ground cover than the ground hydromulch. Effectiveness of aerially applied hydromulch was noted in reducing erosion owing to significantly higher percentage of ground cover provided by the mulch. Progressively, aerially applied hydromulch has shown reduction in sediment yield over time which is attributed to mulch cover decomposition or redistribution or natural vegetative recovery. In terms of retention and germination of seeds aerial hydromulch slurry forms a relatively cohesive cover on the soil surface which boosts seed germination and initial mulch cover. The components of ground hydromulch determine its effectiveness hence in cases where the soil binding agent is not added the mulch is less cohesive and hence much more susceptible to removal by wind and water. 

Bushfire recovery example

In cases of reducing post-fire runoff and erosion hydromulching has been utilized. Studies show that its effectiveness was analyzed through substantial reduction in erosion rates although the reductions were restricted to the first year after hydromulching was found to be ineffective owing to the breakdown of the mulch layer. For erosion control hydroseeding is mostly effective although the sediment control yields can be constrained if the severity of the storm is of a high intensity. Hydromulching is normally effective for short slopes where interrill erosion is the dominant process and the hydromulch mat is less likely to be detached by rill incision. Studies second that aerial hydromulching to be highly effective on long hillslopes with elevated rill densities. The agronomic practice of placing mulch on the soil surface for soil and water conservation and to favor plant growth is known as mulching. Greater plant growth using mulch may be attributed to the mulch’s effect on microclimate conditions on the soil surface, with mulch’s ability to reduce soil surface temperature by up to 20 degrees Celsius through interception of incoming radiation. Mulch is also known to prolong the process of evaporation from the soil surface with the resultant higher soil water content decreasing soil surface temperatures through its effects on soil thermal properties.

Hydromulching conclusions

The purpose of hydromulching is to foster revegetation in high risk areas by immediately creating temporary cover for the soil in the process reducing raindrop impact and binding together loose soil and ash to protect from overland flows and wind whilst increasing soil water retention. Hydromulch is known to have positive effects on soil moisture conservation and the inhibition of evaporation. Hydromulching can increase soil’s saturated hydraulic conductivity, water use efficiency, microbial and enzyme activity and production and temperature. Runoff and soil erosion are reduced owing to hydromulch’s ability to increase interception storage and protect the soil surface. Different hydromulch materials can be used as either permanent or semi-permanent protective cover over the soil surface. For all revegetation needs Global Road Technology offers tackifier chemistries as well as seed germination and strike rate through their biocatalyst technology in GRT Nature Plus, which allows for mobility and increased uptake of nutrients by plants and is an ideal product to give excellent growth, root development and sustainable vegetation health.

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REFERENCES 

• Baharanyi, A. 2010. Techniques for Establishing Vegetation for Erosion Control on Disturbed Slopes in Alabama. Master’s thesis from the Graduate Faculty of Auburn University. 1-87.
• McLaughlin, R.A. and Lee. G. 2013. Field Evaluation of Hydromulches for Water Quality and Vegetation Establishment. North Carolina Department of Transportation. FHWA/NC/2010-15.
• Prats, S., Vieira, D.C.S., and MacDonald, L. 2013. Effectiveness of Hydromulching to Reduce Runoff and Erosion in a Recently Burnt Pine Plantation in Central Portugal. Land Degradation and Development. 
• Ricks et al. 2020. Evaluation of Hydromulches as an Erosion Control Measure Using Laboratory-Scale Experiments. Water. 12:515. 1-17. 
• Rough, D. 2007. Effectiveness of Rehabilitation Treatments in Reducing Post-Fire Erosion After the Hayman and Schoonover Fires, Colorado Front Range. Master’s thesis from Colorado State University. 
• Wilson, W.T. 2010. Evaluation of Hydromulches as an Erosion Control Measure Using Intermediate-Scale Experiments. Master’s thesis from the Graduate Faculty of Auburn University.