Certain of these processes, such as saltation, where grains bounce along the soil surface, depend on the depth and velocity of flow and therefore are more likely in deep fluid flows found during heavy rainstorms or large wind movements. In the case of rain impact, the dislodging of soil is caused by momentum in the raindrop transferring to soil particles in a splash. This is an important process but only moves soil relatively short distances. It does however provide loose material for sheet or rill flow. 5.2 Macro Scale - Erosion at Field Size Sheet wash occurs when rainfall exceeds infiltration.
Rills are continuous narrow channels a few centimetres deep.
They form mainly on recently cultivated soils where water is easily able to form a path between loose soil clumps. Gullies, the natural development of rills, can be between centimetres and over 30m deep, are distinguishable in that they cannot be ploughed out. When broad rills merge, they form sheet erosion. As rills and gullies are constricted at the sides and base they are more likely to carry more erosive turbulent flow. Northwest England As can be seen on Figure 5, Northwest England does not suffer from excessive wind erosion and for this reason, the first step must be to identify those areas of the field affected by runoff.