Municipalities, counties, and states are increasingly discovering the benefits of erosion control and adoption of sedimentation ordinances. However, user knowledge of new products and applications have not kept pace. There is a lack of understanding between erosion control and sediment containment as well as temporary or vegetative control and permanent or mechanical control. There is also a need for clarification of product terminology. Beforing addressing these issues, some statistics on soil erosion are in order.

Before the colonization of North America, the average rate of soil erosion was equal to the natural geologic rate at which soil profiles were being formed from parent material, i.e., about 0.2 tons per acre per year. Today, as a result of man’s utilization of land, forest soils are eroding at a rate of 0.5 tons per acre per year, pasturelands at 1.5 tons per acre per year and cultivated fields at 20 tons per acre per year.

Lands undergoing urbanizing development are experiencing losses at an even greater rate. Normally, an unprotected construction site experiences soil losses exceeding 150 tons per acre per year.

New Vocabulary

In a relatively short period of time, a new vocabulary has emerged: geotextile, geogrid, and geomatrix are now becoming familiar terms in the treatment of disturbed lands or turf reinforcements. Obviously, each of the catagories cannot address all problems, therefore, and understanding of their uses is important to the specifiers and users.

Erosion occurs in three stages:

• Detachment–rainfall dislodges soil particles.
• Transportation–water transports soil particles.
• Deposition–sediment is deposited

Selecting the right product to address each of these stages is important. Temporary fabrics or blankets are applicable to protect soil from rainfall. Three dimensional geomatrix products address soil transportation. Geotextile fences contain the deposition of sediment.

Suppliers of manufactured products have an increasing responsibility to the engineering community to provide reliable information as to product application for specific projects.

A viable temporary erosion control fabric or mat should functions as follows:

• Protect the seed, soil, and fertilizers from impact of rainfall.
• Provide a mulch.
• Allow the moisture needed to promote seed germination, while planning for excessive water. The plaining off of excessive water is crucial on cut and fill slopes in preventing over absorption of water by the fill and resultant slope failure.

In mechanical control, permanent materials or products are used that will not degrade and will perform with or without vegetation. Some dissipation of energy created by velocity and room for deposition of sediment should be characteristic of products used in these applications. The geomatrix products perform very effectively in this environment. The rough surface of this product offers energy dissipation while its three dimensional quality allows sediment to build up in the mat, bonding it to the earths surface. A real plus of the geomatrix is the fact that vegetation may be established through the mat, giving the impact of a vegetively controlled area when in fact it is mechanically stabilized with the geomatrix. Another benefit is the more cover established, the better the mat’s control of soil movement. The same geomatrix is very effective when used on athletic fields as turf reinforcement.

Geogrids, usually heavy non-woven geotextiles formed in a honeycomb configuration with depths of two to tour inches, when placed on the surface of the slopes or roadways, allow filling of the grid with almost any type fill and hold the fill within the individual combs. When used on slopes, water percolates through the grid preventing the waterfall effect, which could work out the fill. Although any material may be used to fill the grid opening, small rock is most common. By holding materials in place on steep slopes, even though vegetation cannot be established, this system offers significant economic advantages.

Making distinctions as to material use for optimum results in varying situations is essential for the engineering community if erosion control decisions are to serve the public interest.

This article first appeared in the December 7987 issue of Public Works, reproduced with permission.