Sediment transport of the Blue Nile River at Khartoum

The Blue Nile is a typical allogenic river since it is fed by the monsoon type rainfall on its headwater catchment in Ethiopian highlands and flows downstream trough an almost dry land in Sudan to join the White Nile at Khartoum, giving rise to the Main Nile. The Blue Nile supplies the largest proportion (about 60%) of the flood water of and about 75% of the sediment to the Main Nile. For this reason the relevance of the Blue Nile goes beyond the confluence with the White Nile since it remarkably imprints the hydrologic and sediment characteristics of the Main Nile. Notwithstanding the critical importance of the Blue Nile sediment load, few data of suspended sediment yield have been published and such data are non uniform and sometimes contrasting. Bedload yield is extrapolated mainly from reservoir siltation since field measurements of bedload transport during floods are not yet available (or unpublished). This led a few authors to consider bedload as negligible though the marked delta erosion after the Asswan dam construction and the worrying volume loss due to sedimentation would suggest the opposite. This presentation is based on a large, unpublished data set of flow, suspended load and sediment grain-size characteristic measured in the sixties on the Blue Nile at Khartoum. Suspended load data are analysed and compared with modern data available from the literature. Some recent data of the Blue Nile measured in Ethiopia are reported and analysed as well. Bedload is obtained by using the dune migration method and compared with the only data measured in the field on the lower reach of the Main Nile downstream of Aswan dam. To calculate bedload, several dune geometry models have been used and the resulting yields are compared with suspended load. The data demonstrate that bedload is not to negligible (this result is particularly relevant in view of the construction of the Great Dam of Merow in Sudan and the lack of bed sediment transport data) and that the dune methodology could be applied successfully as it was for the lower reaches of the Main Nile. Moreover, recent climatic variations and land use change in Ethiopia would suggest to intensify the field measurements of the Blue Nile sediment load as it is expected to have changed during the last decades making old data unsuitable for proper dam design and river mangement.