Erosion Control Series Pt 1: Erosion control blanket 

In the late 1980s, the Texas Transportation Institute steered efforts set to tackle storm water management issues faced by the Texas Department of Transportation. The research focused on methodologies used for the evaluation of field performance of various erosion control technologies which gave birth to the Hydraulics and Erosion Control Laboratory. Core to their works was erosion control blanket research with objectives to provide concrete evidence for effectiveness of erosion-control blankets on the growth of warm-season perennial grasses and their role in prevention of sediment loss in sloped conditions. Sediment retention and vegetation establishment performance for erosion control blankets on sandy soils yield better results regardless of slope conditions or material type as compared to clay soils. Critical to the performance of erosion control blankets in the field were: 

• Soil-fabric interactions, 
• Vegetation establishment, and 
• Installation methods. 

This article will seek to highlight what a soil erosion blanket is, followed by where it can be utilized and finally assessment whether it can be used with other technologies available under the Global Road Technology selection. 

Blanket requirements

The background to the necessity for erosion control blankets emanates from the practice of growing vegetative cover such as grass on flat land or slopes, channels and bare earth with the intention to inhibit erosion due to the effects of wind and rain. The successful establishment of ground vegetation seldom happens due to soil erosion eroding the soil before vegetation takes hold. The role of the erosion control blanket is to prevent that from happening. Patent work dating back to 1964 shows use of erosion control blankets with one good example of excelsior fiber blanket manufactured in America utilized to control soil loss and runoff into adjoining areas. The components of an erosion control blanket include:

• A top sheet of a netting material having a color which tends to blend in with the surrounding area
• A bottom sheet of a netting material
• A synthetic filler material being disposed between said top sheet and said bottom sheet 
• The synthetic filler material consists of crimped polymer fibers arranged to form a three-dimensional matrix

Definitions

An erosion control blanket is defined as articles which resemble a form of fibrous matting in which two outer layers of netting or other material are commonly used to form an envelope or covering about a fibrous interior filler layer. The netting and loose fiber filler permits the blankets to be fairly light in weight and also permits the ingrowth of grasses and other vegetation into and through the blanket. The netting primarily serves to hold the loose fiber filler together while providing a large number of openings for plant in growth. Preference is given to use of biodegradable materials for the inner fibrous layer of the blanket as it will become a fixture in the installment site. When the erosion control blanket is placed on top of the ground it serves as a shield to the earth area from wind and water erosion forces and as the ground vegetation grows it eventually replaces the blanket which decomposes and furnishes the ground vegetation with a nutritive mulch. There are two generic classifications of materials that can be used for making erosion-control blankets namely organic and synthetic classes. In the organic material classification the examples include excelsior, jute, recycled fiberized waste paper, coconut husks, strips of palm leaves, fibers of palm leaves, strips of bamboo, stalks of maize and straw of rice and blends of straw and coconut and gypsum. In the synthetic classification examples are polypropylene blends, PET from post-consumer polymer material from the recycling of soda bottles and PVC. 

Soil loss modelling

Modelling scenarios related to different aspect of soil erosion events have been performed using a variety of models. The mean annual soil loss model in the late 90s, the sediment yield and ephemeral gully erosion model in the early 2000s provided a foundation for understanding hydrological and erosion processes through predictive tool for land use management. The European Soil Erosion Model (EUROSEM) alternatively offered an event-based soil erosion model which was used to predict runoff and sediment discharge for different environmental conditions. Their use of erosion control blankets in applications of EUROSEM 1998 was complementary to its applications on the soil surface to offer surface seal formation and topsoil properties in the process controlling runoff and soil erosion rates during a rainfall event. Limitations owing to within storm conditions led to improvement in the physically-based erosion model of 1998 to cater for errors in runoff and soil erosion predictions which resulted in EUROSEM 2010. In addition EUROSEM 2010 simulates erosion, sediment transport and deposition over the land surface by rill and interrill processes in single storms for both individual fields and small catchments. Key to modelling in the 2010 version was incorporation of the effects of roots on soil cohesion, improvement in computations for flow velocity and modification of fluid viscosity based on sediment load. 

More sustainable options

Interestingly, erosion control blankets can also consist of just biodegradable material as such is the example in a study conducted 10 years ago. Site specific best management practice required for erosion and sediment control was effectively performed using fine compost, mulch and 50-50 mixture of compost and mulch. Preference was for mulch and the 50-50 mixture for better erosion control compared to using compost alone. In general covering bare soil with an appropriate cover increases water infiltration and surface storage by enhancing the soil structure and porosity. In addition, it also reduces the impact of precipitation and surface sealing decreases runoff velocity and helps to restore some of the lost soil nutrients. Phosphorus and nitrates that reach surface waters encourage eutrophication of local water resources with sediment increasing turbidity and resulting in undesirable siltation which drives for more stringent land use management practices regardless of the types of land use. In essence the biodegradable erosion control blankets help protect the soil long enough to establish the vegetation that would offer permanent erosion control although its applications might not be for large areas it can be best utilized for discrete areas such as military training grounds, small farms, horticultural plots and recreational parks. In Australia, biodegradable erosion control blankets have been used with waste ballast rock mulch to aid grass establishment on steep slopes as part Gregory Erosion Project in Central Queensland. 

Industry best practice

A soil erosion blanket allows for growth of vegetation and deter soil erosion and can either be a type of matting with a top and bottom netting material embedded with organic or synthetic material, or a biodegradable material. These biodegradable options such as mulch or fine compost provide appropriate erosion and sedimentation control long enough for vegetative cover to establish. Global Road Technology offers GRT: Enviro-Binder and GRT Nature Plus, specially formulated liquid polymer and highly effective source of phosphorus and nitrogen, which are key in revegetation as it increases germination and strike rate. These products provide the benefits that erosion control blankets can provide, however as there is 100% direct soil contact, you avoid undermining of blankets that often occurs. Because GRT: Enviro-Binder and Nature Plus are hydraulically applied, they are cost effective, much quicker and also much safer to apply on slopes and difficult terrain. 

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REFERENCES 

• Bhattarai, R., Kalita, P.K., Yatsu, S., Howard, H.R., and Svendsen, N.G. 2011. Evaluation of compost blankets for erosion control from disturbed lands. Journal of Environmental Management. 92. 803-812.
• Bohannon, G.D. 2000. Synthetic Fiber Filled Erosion Control Blanket. Patent No. US006855650B1. 
• Godfrey, S.H., and Curry, M.K. 1995. TTI hydraulics and erosion control laboratory research field performance of erosion-control blankets. Landscape and Urban Planning. 32. 161-167.
• Gyasi-Agyei, Y. 2004. Optimum Use of Erosion Control Blankets and Waste Ballast (Rock) Mulch to Aid Grass Establishment on Steep Slopes. Journal of Hydrologic Engineering. 9. 150-159. 
• Prunty, T., Johnson, W.E., and Johnson, J.W. 1998. Erosion Control Blanket and Method of Manufacture. Patent No. US005786281A
• Smets, T., Borselli, L., Poesen, J., and Torri, D. 2011. Evaluation of the EUROSEM model for predicting the effects of erosion-control blankets on runoff and interrill soil erosion by water. Geotextiles and Geomembranes. 29. 285-297.