Once rills are large enough to restrict vehicular access they are referred to as gullies or gully erosion. This results in deeply incised gullies occurring along depressions and drainage lines. Material commonly moved is the size of fine to medium sand or may be derived from slaking, when large aggregates disintegrate upon wetting. In headward erosion the gully extends upstream as a result of waterfall undercutting and gravitational slumping of the gully head. In both cases gullies may widen through lateral erosion, where water undercutting causes subsequent slumping of the sides.1 Gully sides may also be subject to splash, sheet or rill erosion.
Large gullies disrupt normal farm operations, creating access problems for vehicles and stock.
Gully erosion often occurs on lower slopes, but can form quite high in the landscape in particularly susceptible areas. Alternatively, high rates of run-off may lead to a concentration of water in existing rills, especially in sloping paddocks or on hills. Remember that rill erosion is a preliminary step to gully erosion and under the right climatic and topographic conditions can extend into gully erosion, exposing subsoil and sometimes bedrock. Attempt to reduce the amount of water reaching the gully by increasing plant water use where the rain falls, so reducing run-off. Control depends on the size and scope of the problem, but may include diversion banks, gully battering, drop structures, groynes and detention dams.