Multi-threaded Congo River channel hydraulics : field-based characterisation and representation in hydrodynamic models

Hydrodynamic processes that occur along the Congo Middle Reach are a key determinant of risks pertaining to biogeochemical cycling, ecology, public health, transportation, and flood risk. Knowledge of channel hydraulics is paramount to understanding and modelling these hydrodynamic processes, yet such knowledge is severely lacking here. The aims of the research presented in this thesis were twofold. The first aim was to assess the water surface and in-channel hydraulic conditions along the Congo Middle Reach, and the capacity of satellite observations to determine these conditions. The second aim was to evaluate methods of channel geometric representation in hydrodynamic models of the multichannel Congo mainstem. Fieldwork was central to achieving these aims; the field data having been used to characterise hydraulics, assess satellite altimetry datasets, model bathymetry, and model fluvial hydraulics and hydrodynamics. A key finding of the hydraulic characterisation was a complete absence of river flow constrictions that cause backwater effects, which partly explains the relatively subtle nature of inundation here. Assessment of existing satellite profiling altimetry datasets showed their spatial coverage adequately captures the water surface profile along more than 1,200 kilometres of the middle reach. However, coverage was insufficient through the Chenal entrance, where a downstream increase in bed-slope generates a significant drawdown effect. Satellite altimetry deviated from field measurements by two metres here, which is half the annual flood wave amplitude. The findings show that these satellite profiling altimeters cannot be relied on to capture significant water surface slope variability resulting from gradually varied flow conditions, even on the world’s largest rivers. Modelling work showed that the Congo’s multi-threaded channel geometry can be simplified to an effective single channel in a hydrodynamic model, without introducing significant error. The resultant root mean square error in water surface elevation was estimated to be less than 0.25 metres, providing channel friction and shape parameters are calibrated to observations obtained across the entire flow range. This finding may apply to other large multi-threaded channel reaches, which are commonly found on the world’s largest rivers.