Scaling Morphodynamics in Time with Lightweight Sediments in Hydraulic Models of Riverine, Estuarine, and Coastal Environments

Hydraulic scale models with movable beds have been and are still widely used to investigate morphodynamic and sediment transport processes in fluvial and coastal environments. Compared to fixed bed scale models, the scaling criteria and the design of movable bed models are much more sophisticated, as a prerequisite for such models is not only similarity in flow features but also similarity in sediment transport processes. In fact, for morphodynamic investigations, the similarities defined by the Froude number (hydraulic processes), the Shields number (initiation of sediment motion and transport regime), the particle Reynolds number (near bed flow regime), relative roughness, relative sediment density (particle weight) and the Rouse numbers (particle settling velocity) cannot be fulfilled simultaneously.

Moreover, an open question for movable bed models is the accurate determination of the morphodynamic time scales. Hydrodynamic time-scales can be estimated based on the Froude-similarity but the morphological time-scale is more difficult to determine as it depends on the transport mode (bed load or suspended load), morphodynamic processes (single grain movement or morphodynamic evolution of the bed), and sediment characteristics (sediment density). The time-scale also depends on which scaling criteria are intentionally relaxed to facilitate the scale model study which in turns generates higher uncertainties in the processes to be quantified.

On the other hand, movable bed models can be a powerful tool to study the effect of Climate Change on the morphological evolution of water bodies, which is characterised by long time scales. Particularly the use of lightweight material is a promising modelling approach as it allows for an additional acceleration and modification of the time scales to reproduce a miniature facsimile of real-world fluvial and coastal systems, or to create conceptual models aiming at the formation of small-scale and large-scale morphological features. In fact, using different materials with different densities and size combinations, lightweight sediments provide more possibilities to achieve smaller spatial scaling ratios and to cover larger temporal scales.

The objective of the present paper is to summarize the knowledge gained from many different studies with lightweight materials that have been reported in the grey and published literature and which have reviewed within the Hydralab+ EU-project. Current scaling methodologies and experimental advances are presented and discussed and an outlook is provided towards suitable physical scale modelling protocols to improve the representation of sediment transport processes over large time scales, such as time scales relevant to the management of climate change adaptation.