In simple terms, erosion is the removal and loss of soil by the action of water, ice, gravity or wind. In the UK and elsewhere, on construction sites of all kinds, erosion by water is the major problem. There are three steps involved in water erosion: detachment, movement and deposition. Of these three phases, detachment is the most critical, since this is where erosion starts.

In this first phase soil particles are detached from the soil mass by raindrop impact or by water running over the soil. Initially some moisture may be absorbed, depending upon soil condition, but eventually water impact causes splashing and brings the soil particles into suspension by breaking down soil aggregates. This process has two consequences. The first is the obvious: direct erosion. Since the soil particles are detached from the soil mass they are much more susceptible to being transported by surface run-off. The second, the splashing soil particles, creates a turbid fine suspension which, when it enters the soil, clogs the soil pores. Continued raindrop impact compacts and seals the soil surface and reduces the possibility of penetration by water. This then allows greater run-off and more erosion. This secondary factor can be important and it has been demonstrated that rain falling on to an exposed soil for less than 4 min can decrease water penetration to the point where 98% of water falling thereafter runs off.

The problems associated with erosion are both environmental and economic. Detachment and movement of soil means a

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