Non-Fickian dispersion in open-channel flow over a porous bed
Authors: Andrea Bottacin-Busolin
Journal: Water Resources Research
Publication Date: 28 August, 2017
School of: Mechanical, Aerospace and Civil Engineering
New insights into the physics of pollutant transport in natural water courses
The development of cost-effective measures of pollution control relies on modelling tools for predicting the fate of pollutants in rivers and their catchments. The interaction between streaming waters and the surrounding aquifer produces flows into the sediment, where contaminants can be retained and undergo biogeochemical transformations. Quantifying these fluxes is important for assessing environmental impacts of point and diffuse sources of pollution, and supports the development of strategies for water quality remediation and pollution control in river basins.
Now, a researcher at the University of Manchester has addressed a fundamental question about the physical mechanisms that drive the dispersion of solutes in natural channels. Simulations show for the first time that, at sufficiently large times, the temporal evolution of solute concentration in the surface water is controlled by the transverse (i.e. vertical and lateral) variation of the transverse mixing rate in the subsurface. By appropriately considering this variation, excellent agreement is found with experimental observations which cannot be explained by classical transport models that are widely used for water quality studies.
In collaboration with international partners, researchers are now working to incorporate these findings into decision support systems for water quality control, and to develop new techniques for assessing the vulnerability of water bodies.
- In the context of one-dimensional transport models for channel flows, the dispersion of a solute is said to be Fickian if the change of concentration over time about a point moving at the mean speed of the flow can be described by Fick’s second law of diffusion. Accordingly, if a solute is injected at a point in a channel flow, the solute concentration along the channel follows, after some time, a symmetric bell-shaped distribution, and the variance of the concentration (a measure of the extension of the solute plume) increases linearly with time.
- Experimental evidence from several field-scale tracer studies has shown that evolution of solute concentration in streams and rivers is inconsistent with conventional transport models that are widely used in water quality studies
- The results open new possibilities for predictive and inverse modeling of contaminant transport processes in rivers and their catchments