Innovative research on underwater dunes large rivers

Published: 24 February 2020

River dunes and sand ripples form a submerged yet dynamic landscape. In January 2020, an article was published in Nature Geoscience, describing the characteristics of these features in five large rivers around the world and in the Dutch river Waal. Co-author Thaiënne van Dijk is a marine geologist at Deltares. ‘They have a different appearance than expected.’

Field measurements

The subject of the Nature Geoscience article is the quantification of dunes in large rivers worldwide. Van Dijk: ‘Our knowledge of the formation of dunes, sand ripples and comparable structures on riverbeds mainly originates from experimental research in flumes, such as our Delta Flume for waves. The analysis described in this article, however, is based on field measurements in large rivers, involving data taken by thirteen international institutes involving high-resolution multibeam images. Our work provides insight into the morphology of bedforms in rivers like the Mekong, Amazon and Mississippi.’

Variable slope angles

Thaiënne van Dijk is affiliated to the Department of Geology at the University of Illinois (USA) as a Visiting Research Professor. Colleague Jim Best, a world-famous river morphologist, is the article’s second author. ‘Main author is Julia Cisneros, on whose PhD research the article is based’, said Van Dijk. ‘She has developed a method to analyse huge numbers of measurements – in some cases up to 125,000 analysed points per river segment – and to translate these into generic information.’ The huge datasets make it possible to determine exactly how the bedforms looks. ‘They appear to look different than we thought’, stated Van Dijk. The classical image is that of a structure with a gentle stoss side, i.e. the upstream side, and a steep lee side, the side that lies downstream, with a slope angle of around 30°. ‘In reality it appears that lee-slope angles in large rivers are on average merely 10°, and the steepest parts often occur on the lower half of the lee side.’

Multibeam echo soundings displaying river dunes in the Amazon, Brazil. Data from R.P. de Almeida, 2016 (Figure S1a and reference in the paper)

This shape has consequences for interpreting various hydro- and morphodynamic processes. ‘For example, water flows faster over a gentle slope and, depending on the slope angles, flow separation may either occur or be absent.’ The same applies to deposition as a consequence of dune formation. ‘This new information also leads to different geological interpretations of the deposits that we see in ancient river sediments.’ The paper further examines the relationship between the dune height and flow depth, which in these large rivers may be up to 60 metres. ‘These relative dune heights also appear to be lower than we thought, approximately one tenth of the water depth. This knowledge enables us to reconstruct the palaeo-hydraulics of ancient rivers.’

Advantages for river managers

‘These new insights are important in improving our understanding of river system processes’, stated Van Dijk. ‘Particularly where a river still has space to change its course.’ More realistic input of bed roughness in river system models will provide better calibrated outcomes. ‘River managers will benefit from this.’ Accurate knowledge of water depths is crucial for the navigability and maintenance of a primary waterway such as the Dutch Waal. ‘It may be possible to better estimate the critical under keel clearance (the distance between the bed and the ships’ keel) and therefore how heavily a vessel can be loaded.’ Another possible application of the new knowledge is the calculation of sand transport. ‘The downstream-migrating dunes and ripples are in fact a form of sediment transport. If you quantify their migration over time, you can determine how much sediment passes through the river.’ In a follow-up research project, Van Dijk hopes to be able to gain an even better understanding of the interaction between deposition and erosion of the sandy bed structures in relation to the storage of sediment in the bed itself. ‘For this, we will be examining the internal structures of the dunes!’

The article is published on Nature GeoScience (via Springer Nature SharedIt Initiative).