Vegetation restoration in gully beds enhances sediment deposition
Shortcut URL: http://serc.carleton.edu/31995
Country: South America
UTM coordinates and datum: none
Climate Setting: Tropical
Tectonic setting: Continental Collision Margin
Type: Process, Computation
It is known that relatively small changes in land use or cover can have major implications on sediment production and delivery at the catchment scale, as vegetation cover exerts a non-linear control on the production and transfer of water and sediment (Figure 2, Vanacker et al., 2007). This means that a relatively small increase in vegetation cover (10-25%) can lead to a significant (60%) decrease in erosion. Not only total vegetation cover is important, but also its spatial distribution. Landscape structure controls the connection and disconnection of water and sediment fluxes in the landscape. Any spatial reorganization of land units that is modifying the spatial distribution of sediment sources and sinks within the catchment can have major effects on the transfer of water and sediment downslope. It is therefore not surprising that the establishment of vegetated buffer zones on hillslopes or in valley floors has been shown to be an effective means of erosion control for agricultural areas (e.g. Fiener and Auerswald, 2006)
The vegetation control on slope processes is critical in badlands, as their low vegetation cover and reduced soil development often result in rapid generation of overland flow on the gully slopes, which is transported efficiently downslope through a dense network of active gullies leading to a rapid and sharp hydrological response (Sole-Benet et al., 1997). Restoration projects in degraded environments often target on badlands as being important sources of runoff and sediment production.
Observations were carried out on 13 small ephemeral gullies, located in highly eroded sites that have been developed on poorly consolidated and deeply weathered argillites, argillaceous sandstone/siltstone and volcanic deposits. The time of formation of this small-scale badland topography is not known at present, and most of them pre-date the first aerial photographs (1962) of the area. Their length varies between ~40 m and ~100 m, and they drain an upstream area of 287 to 1009 m2. Gullies were selected based on the density and age of the gully bed vegetation so that a wide range of vegetated gully systems could be included in the analysis (Figure 3).
As a general rule, the sediment transport capacity within a gully will increase more rapidly with drainage area than the sediment supply to the gully, or else gully formation would not be possible. Yet, this assumption may no longer hold once gully beds are vegetated. Vegetation growth in active gully channels will decrease the sediment transport capacity of the flow firstly by reducing its average velocity and absorbing a portion of the boundary shear stress, and secondly by reducing runoff amounts through runoff transmission losses. A sudden drop in sediment transport capacity because of vegetation growth may then lead to deposition of the sediment entering the vegetated channel reach.
Field measurements from 138 steep gully segments with strong variations in vegetation cover show that gully bed vegetation is the most important factor in promoting short-term sediment deposition and gully stabilization. Local sediment deposition in steep vegetated gullies is observed even when the sediment transport capacity purely based on local topographic controls, such as drainage area and channel slope gradient (here assessed as A2.1S2.25 following Istanbulluoglu et al., 2003) is expected to increase. This observation holds for different densities of vegetation cover of the gully bed (Figure 4). Our data indicate that the establishment of herbaceous and shrubby vegetation in gully beds gives rise to the formation of vegetated buffer zones, which enhance sediment trapping in active gully systems in mountainous environments. Vegetated buffer zones modify the connectivity of sediment fluxes, as they reduce the transport efficiency of gully systems, which then evolve from sediment sources to sediment sinks.
These findings highlight the potential of relatively small, but well-focused revegetation programs to reduce the transfer of sediment generated in the upstream area to the river system
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