Hydro-geomorphological features at gully heads in the humid northern Ethiopian Highlands, Birr Watershed

The study was conducted in the Birr watershed at twelve gully heads located close to each other. The survey includes measurements of morphological features, soil properties, water table elevations and catchment characteristics including erosion at each gully head. The analysis showed that gully head morphology could be explained by the role of different gully head controlling factors. The result suggested the maximum rate of head cut retreat reaches from 0 to 22.5m. There was no head retreat recorded from the arrested heads relative to unprotected heads. Compared to semiarid highlands of northern Ethiopia, the average short term head cut retreat was 12 fold greater. From the direct shear test, angle of internal friction by far greater than the slope of gully heads which are located at flat lands. The width depth ratio showed that the shallow depth heads were controlled by fluvial erosion whereas for the deep gully heads both fluvial and mass wasting due tension cracks are operating. In this study a significant power relationship established between the volume of the gully head and the length of retreat at the active gullies with V = 4.85 L1.05 (R2 = 0.91 and P= 0.042) which is different from the relation obtained from the entire gully system as a result of varies controlling factors.