Storyline Judith Zomer

Natural dynamics and human interventions

Key message:

The properties of small dunes migrating on top of large dunes may be the key to quantifying the transport of sand and gravel.

Data on water and sediment flow, and morphological developments are needed to understand the processes in the river, to calibrate and validate prediction formulas and models, and to support policy and river management. The transport of sand and gravel in lowland rivers can be quantified from the properties of migrating bedforms. These properties can be quantified relatively easily from bed level surveys at appropriate time steps. A method to track these dunes – ‘dune tracking’ – therefore may prove a fast and cheap alternative to quantify the transport of sand and gravel in comparison to time-consuming methods of trapping the sediment transport in so-called bedload samplers.

The analysis of bedforms on the river Rhine shows large dunes – up to 180 m long and 1,6 m high – migrating downstream at about 15 cm per hour. On top of these dunes are small dunes migrating ten times faster. The sediment transport according to the migration of these small dunes agrees very well with the sediment transport according to the transport formula of Engelund & Hansen. Is this a sign that the migrating small bedforms are a key to quantifying the transport of sand and gravel?

Future developments

Key message:

Long-term monitoring of bedform properties from bed level surveys is needed to quantify future impacts of human interventions and climate change.

Sediment transport in the river will change because of human interventions and climate change. A gradual shift to – relatively – less sand and more gravel has already been observed. Future developments in the transport of sand and gravel can partly be monitored from regular bed level surveys at several selected locations in the river system. New insights urge to expand existing river discharge surveys to monitor sediment transport. These discharge surveys are conducted at a number of locations during the rise and fall of flood waves in winter. In combination with the discharge measurements, sediment transport monitoring improves our understanding of future morphodynamics of the river Rhine.

Sources

• Zomer, J.Y., Naqshband, S., Vermeulen, B. and A.J.F. Hoitink, 2021. Rapidly migrating secondary bedforms can persist on the lee of slowly migrating primary river dunes. Journal of Geophysical Research: Earth Surface, 126, e2020JF005918, https://doi.org/10.1029/2020JF005918.