The sediment delivery problem revisited

Abstract

Understanding the sediment delivery process at the drainage basin scale remains a challenge in erosion and sedimentation research. In the absence of reliable spatially distributed process-based models for the prediction of sediment transport at the drainage basin scale, area-specific sediment yield (SSY; t km—2 y—1) is often assumed to decrease with increasing drainage basin area (A). As the measurement of A is relatively simple, this assumption is frequently used for prediction of SSY in ungauged basins. However, over the last two decades several studies reported a positive or non-linear relation between A and SSY. Various authors have suggested diverse explanations for these opposing trends. This paper provides an overview of the different observed trends and summarizes the explanations for each trend. Furthermore, three typical trends are identified to conceptualize the main driving forces of the relation between A and SSY. First of all, it is emphasized that erosion and sediment deposition processes are scale dependent, and going from small (<m2) to larger areas (km2) more erosion processes become active leading to a rise in SSY with increasing A. However, for larger areas (>km2) erosion rates generally decrease and deposition in sediment sinks increases due to decreasing slope gradients, and so SSY decreases with increasing A. Next, land-cover conditions and human impact determine if hillslope erosion is dominant over channel erosion or vice versa. In the first case, SSY is expected to decrease with increasing A, while in the latter case SSY will show a continuous positive relation with A. Only for very large areas (A > ~104 km2) a decrease in SSY is observed when drainage density decreases or channel banks are stabilized. Finally, spatial patterns in lithology, land cover, climate or topography can cause SSY to increase or decrease at any basin area and can therefore result in non-linear relations with A. Altogether, with increasing A often first a rise and then a decrease in SSY is observed. The decrease can be absent or can be postponed within a region due to local factors of which lithology, land cover, climate and topography are the most important ones. The large regional, local and even temporal variability in the trend between A and SSY implies that prediction of SSY based on A alone is troublesome and preferably spatially distributed information on land use, climate, lithology, topography and dominant erosion processes is required.

Sediment yield as a desertification risk indicator

“…Robinson 1977). This approach has, however, received a lot of criticism, mainly due to its black box nature (Walling 1983;de Vente and Poesen 2005;de Vente et al 2007). Especially in (semi)-arid and Mediterranean environments (i.e.…”

“…environments with a potential desertification risk) SDRs are hard to apply, as they are very variable and difficult to assess. Studies of A-SY relationships for various (semi-)arid regions indicated both negative trends, with often much stronger decrease of SY with A than in humid regions, as well as insignificant or even positive relationships (Walling & Kleo 1979;Verstraeten et al 2006;de Vente et al 2007). In a study of SY and its scale dependence in Europe, Vanmaercke et al (2010a subm.…”

“…through splash, sheet and rill erosion). As spatial units increase, other sediment sources (such as gully erosion, bank erosion or mass movements) may become dominant (Walling 1983;de Vente et al 2007). Gully erosion, for example, only occurs if a specific slope and drainage area threshold is exceeded (Vandekerckhove et al 2000;Poesen et al 2003).…”

“…Walling 1983). However, analyses of large datasets of SY indicate that these decreases are generally limited or insignificant for various (semi-)arid regions (de Vente et al 2007;Vanmaercke et al subm.). Target levels can, eventually, also be defined for specific catchment areas, based on earlier calculated regional A-SY relationships.…”

Sediment dynamics and the role of flash floods in sediment export from medium-sized catchments: a case study from the semi-arid tropical highlands in northern Ethiopia

“…Descheemaeker et al 2006;Haregeweyn et al 2008;Nyssen et al 2009a), the integrated effect of these measures at the medium-sized catchment scale is not well understood. The response of catchments to soil erosion and human interventions is often complex due to scale effects which can buffer changes in sediment supply (Walling 1983;Trimble 1999;Verstraeten et al 2002;Walling 2006;de Vente et al 2007). Further, many rivers in the northern Ethiopian highlands have a hydrological regime characterized by flash floods.…”