Transport and Mixing in Coastal Waters: Introduction

The transport and mixing of pollution in coastal waters is a complicated process, one which at times seems inherently unpredictable. Chaos, or dynamical systems theory, offers a way of seeing order in processes that were formerly thought random or unpredictable. Consider a small parcel of effluent which is being discharged into a bay. Rather than spreading evenly and smoothly throughout the bay, it quickly becomes stretched, thinned, and folded, such that some points within the parcel that were originally close become widely separated, while others are drawn close together. As the parcel stretches and folds, it seems to divide the bay into patches which seem impervious to mixing (see picture below). The stretched parcel seems to act as a barrier which inhibits transport and mixing. In short, an underlying structure in the bay becomes evident, a structure which gives rise to barriers and patches. This type of structure is not a rare case, but one that is common. The creation of this structure is often called chaotic stirring. J. Zimmerman in the Netherlands was the first to note such stirring in coastal waters during a study of the Wadden Sea. Since then, many others have studied or noted the existence of chaotic stirring.