Hydraulic Performance and Effectiveness of Trees, Shrubs and Grasses as Riparian Vegetations in Reducing Flow Velocity near Riverbanks, Subsequent to Riparian Erosion and Sediment Generation Control in Humid Tropics

摘要

This paper described a methodology followed to quantify hydraulic performance and effectiveness of trees, shrubs and grasses in reducing flow velocity near riverbanks with the help of a field experiment conducted in Jimma zone (South-West Ethiopia) which fell in the humid tropics. Jimma zone is one of the regions with most eroded riverbanks, increasing population pressure, torrential rainfall, rugged topography and lack of proper land management. These problems impose two major impacts as in-situ soil loss and siltation of hydroelectric dams, consequent to reduction in efficiency of hydropower generation. In Ethiopia, several tons of sediments are transported annually from the highlands to downstream rivers and entail huge costs to Ethiopia such as dredging costs of clogged channels, desludging of reservoirs and hydroelectric dams. One of the primary sources of sediments for the dams is associated with riverbank erosion. The most sustainable and economical means of stabilizing riverbanks is the use of appropriate vegetation. This study was carried out on locally available, eco-friendly and economically motivating vegetation species that could be planted by all local people on banks of rivers that run along their lands. Six vegetation species were selected and contrasted with bare bank treatment: Salix purpurea and Sesbania sesban as shrubs; Pennisetum purpureum and Pennisetum macrourum as grasses and finally Syzigium guineenseand Saccharum officinarum as trees. This assessment was achieved with the help of a field artificial trapezoidal flume with water from a diverted river and data were collected with the help of a 10-MHz Acoustic Doppler Velocimeter and Horizon ADV software and were analyzed with Win ADV and Microsoft Excel. The results revealed that the vegetation characteristics and planting arrangements affected much their impact on water flow velocity. Almost all vegetations showed power in reducing lateral shear stresses responsible for riverbank erosion except Saccharum officinarum and Syzygium guineense which were less effective due to their big diameters compared to the other species. They also showed that Salix purpurea, Pennisetum macrourum and Sesbania sesban were the most effective species to reduce water velocities near the banks due to their small diameters, stem density and leaves’ density. However, less leafy species didn’t increase surface roughness, the major parameter, to reduce water flow. As the fluvial erosion control is an intensive project, it is recommended that the participatory involvement of local people should be encouraged to cover maximum possible area.