E IAG REGIONAL CONFERENCE 2011 X GEOMORPHOLOGY FOR HUMAN ADAPTATION TO CHANGING TROPICAL ENVIRONMENTS C ADDIS ABABA, ETHIOPIA FEBRUARY 18-22, 2011 U ORGANIZED BY EAG - ETHIOPIAN ASSOCIATION OF GEOMORPHOLOGISTS R S I O N G U Post-conference excursion I Geomorphological hazards, land degradation and resilience in the northern Ethiopian highlands D 23 February – 2 March 2011 E Jan Nyssen, Asfawossen Asrat, Francesco 1 Dramis, Mohammed Umer (Eds.) Table of contents of the excursion guide 1. The western Rift Valley escarpment ....................................................................................... 4 2. Coseismic surface faulting in the Kara Kore area (Wollo) caused by the 1961 earthquake .. 5 3. Geomorphological evolution and present-day processes in the Dessie graben ...................... 8 4. Repeat photography for studies on environmental changes and geomorphological processes in northern Ethiopia ...................................................................................................................... 16 5. Land degradation and resilience in Wollo from the 1930s onwards, as derived from aerial and terrestrial photographs ............................................................................................................ 22 6. Hydrogeomorphology in a marginal graben of the Rift Valley ............................................ 30 7. Land use change and related changes in hydrogeomorphology of an ephemeral mountain stream at Gra Kahsu (Rift Valley escarpment) ............................................................................. 36 8. Long-term gully and river channel dynamics in Tigray ....................................................... 39 9. Upper tree limit and land use on the upper slopes of Ferrah Amba, the peak of Tigray (3939 m a.s.l.) .......................................................................................................................................... 45 10. Geological and geomorphological framework of the Tigray Highlands .............................. 52 11. Geomorphology of the Mt. Amba Aradam south-eastern slope ........................................... 61 12. Sediment transport in the Geba River system ....................................................................... 65 13. Soil-landscape relationships in the basalt-dominated uplands of Tigray – example in May Leiba ............................................................................................................................................. 72 14. Rainfall-triggered slope failures on shale hillslopes; the case of Amba Raesat ................... 77 15. Using the sediment of Mayleba reservoir for land reclamation ............................................ 83 16. The effect of soil and water conservation treatments on rainfall-runoff response and soil losses in the Northern Ethiopian Highlands: the case of May Leiba catchment .......................... 86 17. Apple tree introduction and adaptation in the Tigray highlands........................................... 93 18. Rural marketing in Tigray: the case of Hagere Selam market .............................................. 97 19. Beekeeping innovation by farmers in Tigray for generating income and restoring natural vegetation .................................................................................................................................... 102 1 20. Livestock fodder development as an essential component of catchment management: case study in Mai Zeg-Zeg.................................................................................................................. 104 21. Monitoring of the remobilisation of the May Ntebteb landslide near Hagere Selam ......... 109 22. The sediment budget of May Zeg-zeg catchment and its components ............................... 114 23. Hydro(geo)logy and impact of soil and water conservation measures on the hydrological response in May Zeg-zeg catchment........................................................................................... 121 24. Soil and water conservation through introduction of conservation agriculture in Dogua Tembien ...................................................................................................................................... 128 25. Monitoring gully headcut retreat rates in May Bati ............................................................ 134 26. The travertine dammed sedimentary sequence of Mai Makden ......................................... 136 27. The limestone caves of Tigray: their potential for speleothem palaeoclimate studies ....... 141 28. Planation surfaces on the road from Mekelle to Adigat ..................................................... 143 29. Geology of the rock-hewn churches of Tigray ................................................................... 145 30. Glacial geomorphology: the paleozoic sedimentary successions of Northern Ethiopia ..... 148 31. Rockfall shapes the amba landscape ................................................................................... 153 32. Geology of the Negash area ................................................................................................ 158 33. Land use and land cover changes in Sinkata village ........................................................... 160 34. Impact of surface water reservoirs on ground water reserves in the highlands of Northern Ethiopia, Tigray region ............................................................................................................... 165 35. Adwa phonolite plugs and the temple of Yeha ................................................................... 170 36. Geomorphology and geoarchaeology of the Axum archaeological area ............................ 172 MAPS Overview map of the excursion ............................................................... 3 2 MAP OF THE EXCURSION 1/3 28/2 Axum Mekelle 27/2 26/2 Alamata 25/2 24/2 Kombolcha 23/2 Addis Ababa 3 1. The western Rift Valley escarpment Location of the field presentations (1) ―Afar window‖ (locally known as 'Gemasa Gedel'): amazing view from the Rift shoulder extending to the Afar depression; the Afar window represents the major rift escarpment where the main fault plane (scarp) has a vertical throw of more than 700 m. (2) Chira Meda, in the middle of the escarpment, with a nice view to both the Rift to the East and the plateau/escarpment to the West. See the section on road geology between Addis Ababa and Maichew in: Asrat, A., Demissie, M., Mogessie, A. (2009) Geo-tourism in Ethiopia. SHAMA Books, Addis Ababa, 186 pp. 4 2. Coseismic surface faulting in the Kara Kore area (Wollo) caused by the 1961 earthquake (mostly taken from Gouin, 1979) G. Fubelli & F. Dramis Department of Geological Sciences, ―Roma Tre‖ University, Rome, Italy 1. The 1961 Kara Kore Earthquake In 1961, from the end of May to the end of September, over 3500 earthquake shocks were recorded from the Kara Kore area at the Geophysical Observatory in Addis Ababa. A first series of earthquake shocks, which began on 29 May, reached a maximum frequency of 150 per day; a second series began on 1 June with a peak frequency of 350 per day. Two shocks had a magnitude >6.4 and seven >5.0. The felt area was estimated as about 300,000 km2 and a relatively higher intensities were observed in the southeastern sector of the zone. The maximum intensity at the centre of the epicentral zone was estimated as VIII-IX on the Mercalli-Modified scale. Cracks, fissures, and subsidence of up to 1 meter deep developed on the Addis Ababa - Mekelle highway: many culverts and retaining walls along the road had to be rebuilt. Gravitational movements were observed on steep escarpments slopes and a 15-20 km long fissure, in places 6- 7 meters deep, formed in unconsolidated soil along the eastern scarp of the Borkena marginal graben (Fig. 1). There were no casualties. Figure 1. Panoramic view of the Borkena graben in the Kara Kore area. Coseismic deformation affected the road on the right of the image during the 1961 earthquake. The maximum damage was localized along the Addis Ababa – Mekelle highway between latitude N 10° and 11° and longitude E 39.7° and 37.9°. At these latitudes, the highway runs along the upper margin of the eastern escarpment of the Ethiopian Plateau, which is dissected longitudinally by the Borkena graben, and transversely by NE-SW faulting curving in from Afar. In the epicentral area, the village of Majete was completely destroyed while the village of Kara Kore was affected only in part, maybe because of the type of buildings. In Kara Kore only the masonry collapsed, while the tukuls (traditional houses) withstood the shocks very well. Along the highway, damage was spectacular. Large boulders from rockslides, some estimated to weigh 12 – 15 tons, blocked the road. Bridge pillars were fissured and parapets destroyed; cracks as 5 wide as 60 cm and as deep as 150 cm were opened in the road surface; heavy slumping and subsidences with a resulting difference of some 100 cm in the surface level rendered the road impassable. All the bridges and culverts between kilometre posts 240 and 255 from Addis Ababa had to be rebuilt. Heavy alterations in the landscape had been also recognized. In addition to the numerous landslides, a piedmont scarp in unconsolidated materials opened along the escarpment of the Borkena graben. This scarp could be followed over 12 – 15 km until it became obscured by rubble. In some places, the vertical differential displacement reached 2 m, the depth