Fluid dynamics, sediment transport and mixing about the confluence of Negro and Solimões rivers, Manaus, Brasil

Mark Trevethan, Marco Ianniruberto, Andre Martinelli, Carlo Gualtieri, Marco Oliveira

Friday 3 july 2015

9:15 - 9:30h at Mississippi (level 1)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 14B. Sediment - River

In the last decades a wide body of theoretical, experimental, and field research has emerged on the fluvial dynamics of river confluences. To date most experimental studies have focused either on laboratory confluences or on small natural confluences, whereas an extremely limited number of investigations conducted on large river confluences. Presently little is understood about how river confluence hydrodynamics may vary with river size, especially in the largest rivers. The Clim-Amazon Project aims to study temporal sedimentary records to understand the mechanisms involved in climate and geodynamic changes and the processes involved in dissolved and suspended load evolution of the Amazon River basin from the Miocene to the present. As part of the Clim-Amazon Project a study was conducted about the confluence of the Negro and Solimões Rivers to help understand the fluid dynamics, sediment transport and mixing about the large confluences of Amazon River. Since this confluence ranks among the largest confluences on Earth the outcomes of this study may also provide some general insights into large confluence dynamics. Within this study a field campaign was conducted inside and downstream the Confluence Hydrodynamic Zone (CHZ) of the Negro/Solimões Rivers. A detailed series of ADCP, water quality and seismic profile measurements were collected to investigate key hydrodynamic and morphodynamic features about this confluence. Presented here are the key hydrodynamic features observed about this large confluence and how these relate to findings in previous studies conducted in flumes and small confluences. The observed hydrodynamic features are then compared to some past ADCP data collected about the confluence by Geological Survey of Brasil (CPRM) under different flow conditions to illustrate how the location of these hydrodynamic features may vary. One famous feature about the confluence of the Negro and Solimões Rivers is the meeting of their black and white waters (called locally "Encontro das Aguas"), which may be visually observed not mixing for over 100 km downstream of the confluence. Therefore some insights into how the differences in water characteristics and the hydrodynamics of these two rivers may influence the rate of mixing downstream are presented.