Application of GIS on Environmental Degradation Due to the Offshoots of Highway Development Projects: Central Ethiopian Highlands

Abstract

Physical land degradation after road building has been observed in Central Ethiopian highlands through gully erosion. In this research work by using a number of methods such as Field observation, gully measurement, GIS technique, the 1997 and 2006 topomap and socio-economic questionnaires, the impacts of the road on land degradation has been studied. It investigates how highway construction in the Ethiopian Highlands affects the gully erosion risk by quantifying the catchment area before and after road construction, the number of gullies created, and its characteristics in two selected cases: Addis Ababa-Fiche and Addis Ababa-Ambo. Accordingly; since the building of the road, 17 new gullies were created immediately down slope of the studied road segments and 8 other gullies at a radical change in its dimensions. The average catchment area is now 58.28 hectares and 74.52 on the road segments of Fiche and Ambo respectively, which is significantly different (p<0.001) from the average pre-road catchment area of 8.45 and 14.52 hectares (paired average). The total surface area occupied by gullies in the side of Fiche road and in the side of Ambo road transects was about 63,892.6 m² and 59,214.25 m² respectively. The volume of soil loss was calculated between 12,530.38 m³ and 71,420 m³ from each road segments. The result of statistical analysis indicates that variation of the gully length contributed 95% of variation in the volume of soil loss. The Gully density (5.7m/ha to 14.06m/ha) implies that the sampled roadside areas were moderately to severely degraded. The damages and associated problems of the gullies, as explained by farmers, include loss of land, dissection of farms, and deposition of sediments on growing crops and in extreme cases putting agricultural fields out of production. Hence roads should be designed in a way that keeps runoff interception, concentration and deviation minimal. Techniques must be used to spread concentrated runoff in space and time and to increase its infiltration instead of directing it straight onto unprotected slopes.

Key words: Gully erosion Risk, catchment area, Road design, Ecological destructions