Unstructured-mesh modeling of the Congo river-to-sea continuum

Abstract

With the second largest outflow in the world and one of the widest hydrological basins, the Congo River is of a major importance both locally and globally. However, relatively few studies have been conducted on its hydrology, as compared to other great rivers such as the Amazon, Nile, Yangtze, or Mississippi. The goal of this study is therefore to help fill this gap and provide the first high-resolution simulation of the Congo river-estuary-coastal sea continuum. To this end, we are using a discontinuous-Galerkin finite element marine model that solves the two-dimensional depth-averaged shallow water equations on an unstructured mesh. To ensure a smooth transition from river to coastal sea, we have considered a model that encompasses both hydrological and coastal ocean processes. An important difficulty in setting up this model was to find data to parameterize and validate it, as it is a rather remote and understudied area. Therefore, an important effort in this study has been to establish a methodology to take advantage of all the data sources available including nautical charts that had to be digitalized. The model surface elevation has then been validated with respect to an altimetric database. Model results suggest the existence of gyres in the vicinity of the river mouth that have never been documented before. The effect of those gyres on the Congo River dynamics has been further investigated by simulating the transport of Lagrangian particles and computing the water age.

Appendix A:: Data collection and treatment

In this section, we briefly describe the procedures we had to develop in order to extract and merge different sources of information in order to construct the bathymetry and coastline datasets. Although quite technical, this preliminary step was absolutely necessary due to the scarcity of data readily available for that region.

1.1 A.1 Bathymetry

Several global bathymetric datasets are available. As a basis for our digital bathymetry, we used gebco 2008 (Monahan 2008). This bathymetry dataset provides a 30^″ regular grid of sounding points. This is sufficient in basin areas but not close to the coasts and inside the river. To complement gebco 2008, we have therefore collected nautical charts. These charts were edited in 2003 and 2008 but cover only the Congo River estuary. The shallow water part of the river that spreads about 35 km downstream of Boma is very poorly charted. There are no certified bathymetry dataset for this region. This is obviously problematic since water depth strongly influences bottom friction, which plays an important role in shallow regions. In this area, a channel for commercial navigation is maintained. The maximum allowed draught is known along this channel, but neither its actual depth nor its width.

To collect data from paper charts, we have developed a piece of software, GeoDesk⁴, which is freely available. First, charts need to be digitalized, using wide size scanners. Then, GeoDesk can georeference the resulting images and collect data such as bathymetric sounding and isobathymetric lines.

Gebco 2008 reference is the mean water level, while in nautical charts we have used it is minimum water level. In our model, the reference is the mean water level, which is a standard choice. To make data from nautical charts fit this reference, we have added half the tidal range on them, which we have obtained from our model. This correction has been done until convergence, i.e., until no impact is noticeable on the tidal modeling. In the case of our Congo River model, only one iteration was necessary.

In the unobserved area, we have assumed the depth to be 5 m. The navigational channel is assumed to be 20 m deep at its maximum and 1 km wide, with a parabolic depth profile. These choices are consistent with the little information available.

All of these developments lead to the bathymetry displayed in Fig. 3. This digital bathymetry uses an unstructured approach: the resolution is a function of how many sounding points we had. Hence, in the areas where nautical charts are well defined, the resolution is of the order of 10 m. In the deep ocean, the resolution is the same as the one of gebco 2008. The bathymetry is then interpolated on the unstructured mesh of the hydrodynamic model (described in Section 2).

1.2 A.2 Coastlines

As a starting point, we have used the Global Self-consistent, Hierarchical, High-resolution Geography (GSHHG) database (Wessel and Smith 1996), which is a digital database of global coastlines. At the river mouth, we have completed this coastline using the previously digitalized charts. These images have been vectorized using Gimp⁵ and Inkscape⁶. We have then obtained a vectorial image in svg format, and we used GeoDesk to georeference it. A locally-developed tool finally converts the image into a format our model can use.