In Chapter 4, the transport of a dissolved substance was discussed without taking diffusion into account. This is not very realistic as there are a number of causes by which a substance (such as salt or a waste material but also temperature) is spread out in addition to being transported with the mean flow: molecular diffusion, turbulent mixing, and (very importantly) variations of flow velocity over the river cross-section. For a detailed discussion see, e.g., Fischer et al. (1979). You will find it intuitively plausible that a substance tends to be transported by these processes from locations where the concentration is high to locations where it is small, such as shown in Fig. 11.1. This is called diffusion here, whatever the cause. For the spreading by velocity variations, the term dispersion is often used, which has, however, other meanings as well. The transport with the mean velocity is called convection and the combined phenomenon convection—diffusion.
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