Dynamics of ephemeral streams at the foot of degraded catchments in northern Ethiopia

Abstract

The magnitudes of flash floods and their effect on channel width changes were investigated for Hara River in the Raya graben (northern Ethiopia). Precipitation was measured using rain gauges evenly distributed over the study catchment. Event peak discharges were measured with crest‐stage gauges at a reference cross‐section. Changes in channel width were measured at six monitoring sites along a 1.5 km long reach. Decadal channel width change was analyzed over the period 1965–2014. The average rainfall depth of the rainy days was 14.2 ± 9.4 mm whereas the highest was 41.9 mm and the lowest, capable to generate some flow in the river, was only 1.8 mm. The maximum calculated discharge peaked at 408 m3/s whereas the lowest flow was 19.5 m3/s. An average widening of 0.11 ± 0.19 m per flood (for a total of 3.74 m) was measured. The cumulative precipitation and cumulative channel widening suggest that larger and longer lasting rains have substantial influence on channel width change. A marked increase in channel width commonly accompanies 2–3 days of flood duration; it is less marked where there is riparian vegetation on the banks. The long‐term (1965, 1986, 2005, and 2010) measurements indicate that channel width increased rapidly over the last 50 years; nearly 25 m of widening. Generally, channel changes are mainly induced by the duration of floods rather than by their intensity and the data clearly show that ephemeral streams channel dynamics can be triggered also by floods much smaller than bankfull.

Flood hazard in a semi‐closed basin in northern Ethiopia: Impact and resilience

“…In one of the study rivers, a bankfull discharge of 5.4 and 589 m 3 /s were estimated, virtually corresponding to a discharge with a return time of 1.58 and 2.33 years. The flow with 1.58 return time corresponds to the modal flood in the Gumbel extreme values probability curve and it is considered most effective in shaping river channel morphology and probable channel overflow (Demissie et al, 2019). Hence, in the study area flood hazards happen every 2-3 years.…”

“…Channel width changes of the Hara River between 1965 and 2014 (Demissie et al, 2019); and (b) stream direction change (Gobu River) due to a diversion structure. Flow direction is towards the back of the photo (photo taken by Biadgilgn Demissie, 2014).…”

Spatiotemporal drought occurrences in the semi‐closed Raya graben along the northern Ethiopian Rift Valley

“…The area has two agro‐ecological zones with six districts, including Alamata, Kobo, Raya Azebo, Ofla, Endamehoni, and Alaje. The first three, in the eastern lowland side, are part of the Raya agro‐ecological zone, and the last three belong to the Ofla‐Alaje highlands agro‐ecological zone shouldering on the western escarpment (MoARD, 2007a; 2007b; Demissie et al ., 2020). It is the major sink for groundwater coming from the escarpment in the western escarpment and is hence known for its thick and extensive sediment fill and huge ground‐water potential (Kebede, 2013; Demissie et al ., 2017; Meaza et al ., 2019).…”

Geomorphic natural hazard on loess terrain: expansion on the gully sidewall

“…The main reason for this is that runoff velocity rises with increasing slope gradient and rainfall intensity (Wang et al 2013), which leads to the component of the tractive force of flowing water parallel to the slope surface to be greater than the soil resistance, eventually causing retreat of the gully shoulder line, leading to gully widening (Xiao and Tang 2007). Demissie et al (2020) also suggested that heavier and longer-lasting rainfall events had a substantial influence on channel width. As the initial slope gradient increases, the retreat rate of the gully shoulder line also increases.…”