This chapter describes semienclosed basins that receive freshwater input in relatively low volumes or with highly seasonal periodicity. In temperate basins, density gradients are dominated by salinity. In contrast, in basins with low or no freshwater discharge (described in this chapter), density gradients may also or exclusively be dominated by thermal gradients. Therefore, research on these basins, which has been scantily reported in the literature, should consider cooling and evaporation. In general, studies in these basins should include assessment of heat and water exchanges with the atmosphere, in addition to land-derived freshwater input. We know less about these systems than those in temperate latitudes because they are most likely found in tropical or subtropical latitudes or near the poles, where access to study sites is less frequent.

This chapter describes tidal residual flows expected from dynamic considerations in semienclosed basins without the influence of density gradients. The chapter first qualitatively connects the concepts of Chapter 4, shallow-water tides, to the generation of residual flows. It then introduces a qualitative and quantitative description of the mechanisms that may generate these tidal residual flows in basins with a main channel flanked by shoals. It follows with an explanation of the flow structure in this type of basin, considering ‘long’ and ‘short’ basins, relative to the tidal wavelength. It then depicts examples that compare observations to theoretical expectations. The chapter concludes with descriptions of flows around headlands or sites with coastline curvature.

Any given semienclosed basin may change from vertically homogeneous to strongly stratified in one tidal cycle. These stratification changes can occur at various temporal scales and that is why it may sometimes be irrelevant to classify a basin with these guidelines unless we specify the gamut of variability and its temporal scales. A basin typically changes geomorphology over hundreds to thousands of years and its classification will likely remain the same in our lifetime. However, a basin may change water balances in monthly to seasonal scales and vertical stratification in even shorter periods. Because of this variability, we should be cautious when trying to classify any semienclosed basin. Other robust approaches to classifying these basins are presented in Chapter 12, which includes a dynamical approach. So, before understanding such a dynamical approach, we need to familiarize ourselves with the fundamentals of the dynamics of semienclosed basins. This acquaintance is formulated in Chapters 2 to 11.

This chapter describes residual flows driven exclusively by density gradients. Chapter 6, in contrast, presented residual flows forced only by winds, and Chapter 5 treated residual flows from tides. This chapter begins with a qualitative description of the flow arising from density gradients established by freshwater input to a semienclosed basin, that is, the density-driven exchange flow or gravitational circulation. It then goes into a dynamical description, grounded in fundamental physics. The dynamical description first considers lateral homogeneity and then allows for lateral variations caused by Earth’s rotation. It follows by considering lateral changes in bathymetry and of the relative contribution of frictional effects versus Earth’s rotation effects. The chapter concludes by exploring the influence of basins' width on the gravitational circulation. Along-basin variations in residual flow are considered in Chapter 8.

This chapter continues studying flows in homogeneous semienclosed basins. The main flow driver in this case is the wind. Density gradients and their dynamic impact continue to be neglected. Flows driven by density gradients will be treated in the Chapter 7. The present chapter begins with a presentation of fundamental concepts related to wind-driven flows in homogenous fluids. Then it goes into a quantitative presentation of the fundamental dynamics related to these flows in semienclosed basins. It describes the effects on wind-driven flows of lateral variations in bathymetry. The chapter concludes with a comparison of theoretical results to observations of wind-driven flows.

This chapter covers basic concepts on the forces that drive tides. It presents a description of the main harmonics or constituents related to the driving forces, followed by a presentation of the concept of spring and neap tides, as well as their analogous tropic and equatorial tides. It then introduces the fundamental physics for a frictionless tidal wave, while depicting progressive and standing waves. It continues with the conditions for tidal resonance. The chapter then includes the effects of bottom friction on tidal currents and covers the effects of convergent coastlines by presenting the concepts of hypersynchronic, hyposynchronic, and synchronic basins. It follows with the effects of Earth’s rotation on frictionless tides and the generation of amphidromic points. The chapter goes further with an exploration of the effects of lateral bathymetry on tidal flows.

This chapter describes fronts in, or originating from, semienclosed basins. It begins proposing a general definition of a front and then presents possible spatial and temporal scales attributable to these features. It proposes arguments that support possible values for vertical velocities at fronts. This chapter discusses different types of fronts arising from the conservation equation for property C, which can be salinity, or density, or any dissolved or suspended material. Such different types of fronts are plume and tidal intrusion front, mixing (mostly tidal) fronts, and shear fronts. Concepts in the chapter refer to fronts in semienclosed basins and in coastal oceans, although, in some instances, the concepts may be applicable to oceanic regions.

The basic quantitative tools to study the hydrodynamics, and any suspended and dissolved matter in water, of semi-enclosed basins are the conservation equations. Conservation of momentum and conservation of mass are used for water motion, while conservation of salt and conservation of heat address their distribution in space and time. An equation of state, the Thermodynamic Equation of Seawater, relates temperature (heat content in the water column), salinity, and pressure to water density, which plays a dynamic role in the conservation of momentum. The conservation of suspended or dissolved matter has an advective contribution that depends on water motion, a diffusive contribution, and a source/sink contribution. The source/sink contribution represents the greatest uncertainty in water-related studies.

Studies of any semienclosed basin frequently ask the question, How long does it take for water renewal in this basin? The question is rather simple, but the answer is far from it. The response is difficult because one single value (e.g., NT number of hours or days or years) falls short in representing the possible variability of the agents that cause such water renewal. This chapter presents different ways to represent the renewal and the potential variability in those representations. The chapter is anchored in previous syntheses that attempt to address persistent inconsistencies in the use of the various terms that refer to such water renewal. The chapter proposes distinctions among flushing time, residence time, age, transit time, and exposure time.

This chapter considers interactions between different forcings. It first describes the interaction between tidal currents and density gradients at intratidal (within one tidal cycle) time scales. One outcome of this interaction is the phenomenon known as tidal straining. The chapter continues with the treatment of intratidal variations of density that can also result from the interaction of density fields with tides and bathymetry. Subsequently, the chapter presents a description of the interaction between tides and density gradients at subtidal time scales, that is, at periods greater than one tidal cycle. The chapter then describes how advective accelerations from tidal currents can interact with density gradients to modify residual flows. It follows with a description of the competition between tidal stresses and density gradients in driving residual flows. It then deals with the competition between density gradients and wind stresses, to later add tidal forcing. The chapter then includes the influence of river discharge on estuarine circulation. The last two subsections present salt (or solute) budgets and their linkage to hydrodynamics and approaches to study saltwater intrusion

This chapter describes the mechanisms by which tides may be deformed or distorted or rectified (all terms referring to the same phenomenon) when they enter semienclosed basins. The chapter first presents qualitative and quantitative arguments for the presence of distortions and the generation of high-frequency harmonics, that is, of periodicities in the tide that are shorter than the fundamental (semidiurnal and diurnal) tidal periods. The chapter continues by exploring the possibilities of the generation of high-frequency harmonics in what are referred to as shallow-water tides. These shallow-water tides can be overtides or compound tides, depending on what harmonics generate them. The chapter goes further with an explanation of the physical meaning of processes causing distortions. It concludes by providing a pair of examples of overtides and compound tides.

A classification of semienclosed basins is proposed in this chapter by following a tidally averaged momentum balance that compares drivers and modifiers of residual flow. Residual flow drivers are characterized by the nondimensional tidal Froude number, while balancing forces are typified by the Ekman number. Thus, the classification is contained in a parameteric space that considers estuaries, tidal rivers, vertically homogeneous lagoons, and frictionless tidal basins. The scheme can be regarded as the baroclinic tendencies shown by any basin, presented in the abscissa, versus the mixing tendencies, as represented in the ordinate. Also considered are situations in which wind stress competes with (a) density gradients and (b) tidal stress. This additional approach allows a dynamic description of basins where the dynamics are purely frictional, non-frictional (geostrophic), and Ekman-type. This classification includes semienclosed basins beyond estuaries.