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

Abstract

Purpose The Ethiopian highlands are a fragile environment, characterized by steep slopes, intense rainfall, a sparse vegetation cover and the occurrence of flash floods. Although important efforts have been made to mitigate the ongoing soil erosion and land degradation problems, the sediment dynamics at medium-sized catchment scale (100-10,000 km²)are not fully understood. Therefore, this study aims to provide a better of sediment export processes and the importance of flash flood events in semi-arid tropical catchments. 2Methods Measuring campaigns were conducted in 10 subcatchments of the Geba, a tributary of the Tekeze, representative of the northern Ethiopian highlands. During 2 to 4 rainy seasons, the rivers were sampled for their suspended sediment concentration (SSC) and runoff discharge.Results and discussion Variations in SSC and sediment grain-size distribution indicate changes in sediment supply during the rainy season, due to the depletion of readily available sediments and the development of a vegetation cover.Also during flood events, changes in sediment supply are observed. Sediment yields (i.e. 497-6,543 t km -2 yr -1 ) are higher than suggested by previous studies and correlate with rainfall depth. The majority of sediment export occurs during a few short but intense flash floods. No clear effect of implemented soil and water conservation measures could be detected in the sediment yields of the catchments.Conclusions Sediment export rates in the Ethiopian highlands are high, are characterized by important changes in sediment supply and are mainly controlled by the occurrence and magnitude of flash flood events. Mitigation measures to reduce sediment yield at the catchment scale should therefore not only focus on the reduction of hillslope erosion rates, but also on the magnitude of these floods.

Sediment yield as a desertification risk indicator

“…However, at the scale of medium-sized (i.e. 100-5000 km²) catchments no clear effect of soil and water conservation measures on SY was found so far (Vanmaercke et al 2010b). Although other factors (such as rainfall patterns) also play an important role, this is partly due to the reactivation of alluvial sediments.…”

“…The occurrence of floods can have a major impact on SY, especially in (semi-)arid environments as the episodic nature of floods and the lack of a good vegetation cover provide opportunities for very large volumes of sediments to be transported (e.g. Schick et al 1997;Vanmaercke et al 2010b). In the Israelian Negev highlands floods lead to the headcut migration of gullies in valley bottoms.…”

“…This type of sediment transport is rarely measured at gauging stations, although it can be very important, certainly in Mediterranean and mountainous regions (Verstraeten et al 2006). Owens et al 2010;Vanmaercke et al 2010b). As discussed in a previous section, the response of catchment SY to disturbances can differ significantly over time and in magnitude from the response at the hillslope scale.…”

Impact of Soil Depth and Topography on the Effectiveness of Conservation Practices on Discharge and Soil Loss in the Ethiopian Highlands

“…In most watersheds, sediment concentrations are elevated in the beginning of the rain phase after the soils are plowed and rills form [22,23,28,[44][45][46][47]. Once the rills have formed, sediment concentrations decrease.…”

Sediment research, management and policy

“…to mitigate high water flows in channels as well as hillslope erosion rates (Vanmaercke et al 2010); & in the Trinity River (Texas) modern fluvial sediment input to the delta is occurring at a rate approximately 20-30% of the mean Holocene accretion rate, which is insufficient to combat subsidence and wetland loss and keep pace with the future sea level rise (Slattery et al 2010); & in the Rhine Delta, the many proposed landscaping measures against the potential combined effects of climate and land use changes will considerably rehabilitate the natural role of the floodplains as traps for fluvial sediment conveyed from the continents to the coastal zone (Middelkoop et al 2010). …”