Effects of Land Deals on Peak Discharge and Sediment Transport in the Catchments Around the Grand Ethiopian Renaissance Dam

Land degradation poses a threat to reservoirs because of siltation; hence, sustainable hydropower development necessitates the conservation of upstream catchments. Ethiopia is currently constructing Africa's largest hydropower dam, the Grand Ethiopian Renaissance Dam (GERD), on the Blue Nile. Given the commercialization of large tracts of land in the closer catchments, there is a need to explore the effects on peak discharge and suspended sediment concentration. A field survey was conducted on 20 ephemeral streams to compare annual maximum peak discharges between catchments (0·4 – 15·5 km2), eight of which drain commercial farms and twelve others, land under traditional management. We measured channel characteristics, observed flood marks and applied the empirical Manning equation to calculate peak discharges. SSC samples were taken during each runoff event from a stream in each of the two dominant land management categories. Results indicated that commercial farms contributed for the 51% increase in peak runoff compared to traditionally managed lands. Catchments dominated by commercial farms also have a significantly higher SSC (mean = 6·44 ± 2·23 g l−1) compared to catchments without commercial farms (mean = 2·77 ± 2·31 g l−1). Forests and woodlands have a strong buffering effect upon flood generation while higher peak discharges and sediment transport have generated from crop lands. However, leasing forests and woodland to agricultural companies has resulted in deforestation. Consequently, the increased runoff response may lead to downstream bank erosion and affect downstream communities; the increased sediment transport poses a threat to the multibillion dollar hydropower reservoir of the GERD. Copyright © 2016 John Wiley & Sons, Ltd.