Gully Erosion

Gully erosion is a phenomenon that can be observed worldwide and is often related to actual or former agricultural land use. Gullies are a form of linear (soil) erosion that emerge due to the erosive power of concentrated overland flow. Soil and unconsolidated rock are removed up to depths of from 0.5 to several meters, and in the worst case, badlands develop.

The process of gully erosion occurs mainly in landscapes with (climate- and/or human-induced) discontinuous vegetation cover and more or less periodic heavy rainfall events when gullies can develop excessive short-time drainage. These precipitous watercourses are also a sign of severe land degradation and are mainly triggered by land use change, inappropriate cultivation and irrigation systems, overgrazing, urbanization, deforestation, and global change.

Gully erosion can be distinguished into ephemeral and permanent gully erosion. The former can be easily erased with ordinary farming equipment, [Page 1393]but the latter means an irrecoverable loss of land resources.

Gully erosion in southeast Spain derived from aerial photography: (1) very active main headcut(s), (2) side headcuts, (3) densely vegetated main drainage line(s), (4) headcut retreat by piping, and (5) very sparsely vegetated almond grove.

Source: Dr. Irene Marzolff, University of Frankfurt, Germany. Note: Arrows = flow direction.

The process of gully erosion depends on numerous variables—for example, catchment size, vegetation density, land use, substrate characteristics, and slope inclination and length. Furthermore, the velocity and extent of gully erosion depends on the amount of overland flow reaching the top end of the gully (often by convergence of rills), the so-called headcut, which is regressively moving upslope. Thus, gully erosion is conditioned by the properties of the catchment of the gully.

The main processes responsible for headcut retreat are, as mentioned above, headward erosion by overland flow, piping, and undercutting of the headwall by plunge pool development (see images).

The consequences of gully erosion are commonly divided into “on-site” and “off-site” damages. On-site gully erosion destroys and dissects valuable farmland and paths. Off-site, it fills reservoirs or small channels and causes flooding and loss of water quality at the lower reaches. Both damages cause severe socioeconomic problems as loss of arable land or loss of reservoir capacity, which is especially problematic in semiarid to arid regions. Especially in loess areas (Belgium, China, Germany) and regions with a temporary or permanent water deficit (Mediterranean), gully erosion is widely observed and explicitly described. In Belgium and Germany, for example, relicts of former gullying can be found under forests and are believed to be a consequence of former land use changes that can be connected to social problems such as devastating diseases or to climatic change affecting the maintenance of agricultural land and the vegetation cover.

Several techniques have been applied to determine the velocity of gully erosion, such as measuring with benchmark pins, aerial photography (at different spatial scales), GPS (global positioning systems), or dendrochronology. For prevention and control of gully erosion, the variables controlling gully erosion have to be taken into account. Possible controls include restoration of vegetation, changes in land management practices, and technical solutions such as check dams.

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