1.   Water.  Erosion from water typically occurs in the following ways.

a.   Raindrop Splash and Sheet Erosion. The first step in the erosion process begins as raindrops impact the soil surface. Raindrops typically fall with a velocity of 20-30 feet per second. The energy of these impacts are sufficient to displace soil particles as high as two feet vertically.  In addition, the impact of a rainfall on a bare soil can compact the upper layer of soil, creating a hard crust that inhibits plant establishment.

Sheet erosion occurs as runoff travels over the ground, picking up and transporting the particles dislodged by raindrop impacts.  The process of sheet erosion is uniform, gradual and difficult to detect until it develops into rill erosion.  If runoff is maintained as sheet flow, the velocity remains low and there

The method used to prevent erosion from raindrop splash and sheet erosion is stabilization. Stabilizing techniques such as temporary and permanent vegetation, sodding, mulching, compost blankets, and rolled erosion control products absorb the impact of raindrops and protect the ground surface.  By protecting the surface, soil particles are not dislodged and transported by sheet flow. Typically, sheet flow does not have sufficient volume or velocity to dislodge soil particles from a bare surface by itself.  It is dependant on raindrop impacts to disturb the surface. Therefore, stabilizing a surface, protects the ground from both raindrop and sheet erosion.

b.         Rill Erosion.  Rill erosion occurs as runoff begins to form small concentrated channels.  As rill erosion begins, erosion rates increase dramatically due to the resulting concentrated higher velocity flows.  Construction sites that show signs of rill erosion need to be re-evaluated and additional erosion control techniques employed.  Rilling can be repaired by tilling or discing and should be repaired as soon as possible in order to prevent gullies from forming.

c.         Sheet Erosion. Sheet erosion is the process by which transportation of soil particles begins. Sheet erosion occurs as runoff travels over disturbed ground, picking up and transporting particles dislodged by splash erosion. The process of sheet erosion is gradual, and difficult to detect until it develops into rill erosion. The potential for sheet erosion is dependent on the soil type, velocity, and quantity of flow over the surface. Long slopes, steep slopes, and slopes that carry higher volumes of runoff are more susceptible to sheet erosion

The same stabilizing methods used to prevent splash erosion are used to prevent sheet erosion. These methods slow the flow of water over the surface and trap dislodged particles, preventing them from being transported.  Diversion structures may be used to reduce the volume of flow over a bare slope, and surface roughening techniques can be used to reduce the effective slope length of the surface by breaking up sheet flows.

d.        Gully Erosion.  Gully erosion results from water moving in rills, which concentrate to form larger channels. When rill erosion can no longer be repaired by merely tilling or discing, it is defined as gully erosion.  Gullies must typically be repaired utilizing earthmoving equipment. Gully erosion can be prevented by quickly repairing rill erosion and addressing the cause of the rill erosion.

e.         Stream Channel Erosion. Stream channel erosion consists of both stream bed and stream bank erosion. Stream bed erosion occurs as flows cut into the bottom of the channel, making it deeper. This erosion process will continue until the channel reaches a stable slope.  The resulting slope is dependant on the channel materials, and flow properties. As the stream bed erodes, and the channel deepens, the sides of the channel become unstable and slough off; resulting in stream bank erosion. Stream bank erosion can also occur as soft materials are eroded from the stream bank or at bends in the channel.  This type of stream bank erosion results in meandering waterways. One significant cause of both steam bed and stream bank erosion is due to the increased frequency and duration of runoff events that are a result of urban development.

Figure 1: Types of soil erosion on an exposed slope.

                   Source: USDA NRCS, 2002