Dramatic variations in water discharge and sediment load from Nanliu River (China) to the Beibu Gulf during 1960s–2013

Abstract

River discharge and sediment variation is vital to material transport between river and sea, which is the focus of research on river–sea interaction. This study takes Nanliu River, a typical independent river into the sea in Beibu Gulf as an example, analyzing river regimen and river bed morphology variations during 1960s–2013 in response to climate change and human activities based on wavelet analysis and Man–Kendall test methods. The results indicate that river discharge and sediment in Nanliu River have significant seasonal characters with over 70% and 90% river discharge and sediment occur in summer half year. Compared with 1960s–1980s, the time of peaked monthly river water discharge and Suspended Sediment Concentration (SSC) during 1990s–2000s had shifted from June/August to July and from April to July, respectively. Meanwhile, both river flow and SSC present 4–6 years and 11 years fluctuations. In the recent 50 years, annual river discharge and SSC present downward trends with discharge decreased by 13.9% and 22.28% respectively in upstream and downstream while SSC decreased by 33.72% and 49.05% in upstream and downstream, respectively. Rating-curve between flow and SSC turns from clockwise rotation with enveloped area during 1965–1989 to a relatively narrow appearance during 1990–2012 in upstream, but indicates relatedly mild variation in downstream. Evolution of river bed morphology is characterized by “erosion in flood season and deposition in dry season”. Moreover, the river flow entering the sea is dominated by precipitation while the sediment entering to the sea is controlled by middle and lower reaches supply. Human activities, including soil erosion, forest conservation and hydraulic engineering along the river, are responsible for the decrease of river water and sediment discharge entering the sea.