Vignettes > Vegetation restoration in gully beds enhances sediment deposition

Vegetation restoration in gully beds enhances sediment deposition

Veerle Vanacker
University of Louvain
Author Profile

Shortcut URL: http://serc.carleton.edu/31995

Location

Continent:
Country: South America
State/Province:Azuay
City/Town:
UTM coordinates and datum: none

Setting

Climate Setting: Tropical
Tectonic setting: Continental Collision Margin
Type: Process, Computation


This picture of the central part of the Inter-Andean valley around the city of Cuenca (Ecuador) shows the intense degradation of the environment. This area has favourable climatic and topographic conditions for the development of agriculture, and was historically densely populated and intensively cultivated. Rapid socio-economic and demographic growth resulted in a very dynamic land use system, which is now dominated by agricultural and residential land use. Natural forest was increasingly converted to arable land or rangeland to augment agricultural production. In a later stage, agricultural lands affected by intense soil erosion and gully development were abandoned. As a response to the growing demand for firewood and timber, reforestation with quick growing exotic species has taken place on highly degraded land since the early 1970s. Details


This figure illustrates the acceleration of erosion rates following removal of the vegetation. Based on the concentration of cosmogenic isotopes in river sediment, we estimated long-term, natural erosion rates, against which human impact can be measured. Present-day erosion rates were calculated from reservoir sedimentation measurements for 37 catchments; and the fractional vegetation cover of the catchments was estimated from vertical photographs of 1 × 1 m vegetation plots. Our data indicate that vegetation cover exerts first-order control over present-day erosion rates at the catchment scale. Areas with high vegetation density erode at rates that are characteristically similar to those of the natural benchmark, regardless of whether the type of vegetation is native or anthropogenic. Details


Three examples of surveyed ephemeral gullies, each one with different type and density of vegetation. (A) Restored gully with dense vegetation cover of the gully floor with native species, (B) Restored gully with intermediate cover of the gully floor with native and exotic species, and (C) Active gully system. Active gully systems developed on highly weathered or loose parent material are an important source of runoff and sediment production in degraded areas. However, a decrease of land pressure may lead to a return of a partial vegetation cover, whereby gully beds are preferred recolonization spots. Details


This figure shows the observed sediment deposition volumes as a function of downstream change in stream power (here assessed as a function of Area and Channel Slope). This concept is used as a surrogate for topographically controlled change in sediment transport capacity at surveyed gully segments. Our results clearly show that sediment deposition in gully beds occurs even when the topographically controlled sediment transport potential of the channel is increasing. This indicates that local gully topography alone cannot explain the observed deposition patterns. Our observations from the 138 gully segments indicate that the ground vegetation cover strongly controls sediment deposition in steep gully beds. (Note that sediment deposition values are plotted on a logarithmic scale) Details


Description

Mountain ecosystems in developing countries are suffering from rapid land-use/land-cover change, induced by demographic growth and socio-economic development (Figure 1, Vanacker et al., 2003). Given their steep topography and shallow soils, they are particularly vulnerable to accelerated runoff and soil erosion (Harden, 2001). To assess the effect of rapid land-use/land-cover change on the hydrological and geomorphological functioning of these ecosystems, it is important to understand the effect of vegetation on the transfer of water and sediment from slopes towards intermittent or permanent river systems.

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

Associated References

Fiener P, Auerswald K. 2006. Seasonal variation of grassed waterway effectiveness in reducing runoff and sediment delivery from agricultural watersheds in temperate Europe. Soil and Tillage Research 87: 48-58.

Harden C. 2001. Soil erosion and sustainable mountain development. Experiments, observations, and recommendations from the Ecuadorian Andes. Mountain Research and Development 21: 77-83.

Istanbulluoglu E, Tarboton DG, Pack RT. 2003. A sediment transport model for incision of gullies on steep topography. Water Resources Research 39(4): 1103. DOI: 10.1029/2002WR001467

Sole-Benet A, Calvo A, Cerda A, Lazaro R, Pini R, Barbero J. 1997. Influences of micro-relief patterns and plant cover on runoff related processes in badlands from Tabernas (SE Spain). Catena 31: 23-38.


Molina A, Govers G, Cisneros F and Vanacker V., 2009. Vegetation and topographic controls on sediment deposition and storage on gully beds in a degraded mountain area. Earth Surface Processes and Landforms 34: 755-767.

Vanacker V, Govers G, Barros S, Poesen J, Deckers J. 2003. The effect of short-term socio-economic and demographic changes on landuse dynamics and its corresponding geomorphic response with relation to water erosion in a tropical mountainous catchment, Ecuador. Landscape Ecology 18: 1-15.

Vanacker V, von Blanckenburg F, Govers G et al. 2007. Restoring dense vegetation can slow mountain erosion to near natural benchmark levels. Geology 35: 303-306.


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