Effects of dam construction on the Ribb River bed topography

Dam construction affects the discharge regime and the sediment transport of rivers (e. g. Williams & Wolman, 1984). Dams reduce the annual peak discharges (Graf, 2006) and may store all sediment or a large part of it. The downstream river reach adjusts its morphology to the new conditions. This involves changes in planform, slope, width, depth and sediment characteristics through time. The Ribb River is located in the North Western part of Ethiopia where it drains to Lake Tana. A 73 m high dam and a diversion weir 30 km downstream are under construction to irrigate 15,000 ha of land (WWDSE & TAHAL, 2007). The river reach downstream of the dam is 77 km long. This part of the river is strongly affected by flooding, pump irrigation, sand mining and backwater effects from Lake Tana and will be even more impacted by Ribb Dam operations. Lake Tana level was regulated for hydropower production at a higher elevation between the years 1995 and 2001, at a lower elevation between the years 2001 and 2010 and then to a higher elevation again. Embankments have been constructed in the lower part of the river for flood prevention. The objective of this study is to analyse the effects of the dam on downstream river bed topography and channel alignment. This work presents the assessment of pre-dam morphological trends of the river.