Estimation of Bathymetry for Modelling Multi-thread Channel Hydraulics: Application to the Congo River Middle Reach

Bathymetry is a key input into numerical models that simulate river hydrodynamics, but it is particularly challenging to obtain on multichannel rivers where mid-channel islands prevent survey boats from undertaking bank-to-bank navigation. This is problematic, as there is a need to explicitly represent individual channel threads in hydrodynamic models, to ensure sufficient in-channel process representation, and obtain hydraulic information in individual channel threads. Here, we evaluate an approach to estimating the bathymetry of low gradient multi-thread channel systems, which involves the use of Manning's equation to derive depth from observations of reach-averaged discharge, water surface slope, and river width. The approach is applied to a 70 km reach of the multichannel Congo main stem near Mbandaka, DR Congo, and is evaluated initially through geometric validation with sonar track data. The evaluation also involves 2D hydraulic modeling to assess the estimated bathymetry hydraulically, by comparing its ability to predict observed in-channel flow conditions with that of observed bathymetry. We find that mean channel depths and general flow conditions are well approximated by the estimated bathymetry in the wide multi-thread reaches, the observed bathymetry offering little or no performance improvement here. However, bathymetry at two single-channel width constrictions is poorly approximated. The results indicate the Manning's equation bathymetry estimation approach is widely applicable to reaches of large, low gradient multichannel rivers such as the Congo, providing there are no major irregularities in effective channel width within the reach.