Seawater intrusion and mixing in estuaries
- 1 Introduction
- 2 Seawater intrusion mechanisms
- 3 Random walk
- 4 Analytical expressions for the longitudinal dispersion coefficient
- 5 Dispersion by residual circulation
- 6 Dispersion by tidal straining
- 7 Dispersion by “dead zones”
- 8 Time scales for vertical and lateral mixing
- 9 Dispersion by deterministic chaos
- 10 Dispersion by tidal pumping
- 11 Residence time scale
- 12 Experimental determination of the longitudinal dispersion coefficient
- 13 Related articles
- 14 References
Estuaries are generally defined as semi-enclosed transition zones between river and sea. The intrusion of seawater in estuaries is mainly due to tides and buoyancy (related to the density difference between seawater and river water, see Estuarine circulation). Seawater intrusion in estuaries is an important phenomenon to man and nature: it limits fresh water availability for human and agricultural use and it determines the type of habitats and species that can develop in an estuarine environment. Besides, density driven currents and salinity play a role in estuarine turbidity and sedimentation processes.
We describe in this article the physical processes involved in seawater intrusion and mixing in estuaries and explain some simple methods for deriving quantitative estimates. Several of the dispersion mechanisms discussed in this article are illustrated by dye experiments shown in Estuarine dispersion: dye experiments in the Eastern Scheldt scale model.
Seawater intrusion mechanisms
The fresh water discharge and the salt flux in an estuary are given by
where is the estuarine cross-section, the instantaneous local water depth and the estuarine width, the longitudinal current velocity and the salinity. The brackets stand for averaging over the tidal period (assuming a cyclic tide) and the overbars stand for averaging over the depth and the width. The coordinate follows the upstream positive longitudinal direction (along the thalweg), the coordinate the lateral direction and the coordinate the upward positive vertical direction.
We call the salinity averaged over the estuarine cross-section and the tidal period. We may then decompose
This decomposition singles out the fresh water discharge (the term ) as a mechanism for flushing seawater out of the estuary, while the term represents the sum of all processes contributing to seawater intrusion. These processes are:
- Horizontal circulations in the estuary (mainly the net relative displacement of water masses circulating between ebb and flood channels and the net relative displacements due to geometry-induced eddies, followed by lateral mixing of these water masses);
- Horizontal tidal straining (lateral mixing between water masses which are advected at different speeds, due to lateral gradients in the longitudinal velocity);
- Vertical circulation in the estuary, also called estuarine circulation (seawater intrusion induced by the density-driven net displacement of near-surface water relative to near-bottom water, followed by mixing over the vertical);
- Vertical tidal straining (vertical mixing between water masses advected at different speeds due to vertical gradients in the longitudinal velocity);
- Lateral mixing of water masses captured in "dead zones" with the main flow;
- Chaotic dispersion, related to the chaotic character of particle trajectories when travelling through a complex field of tide-generated eddies;
- Tidal pumping at the inlet (partial replacement of the ebb tidal prism with ‘new’ seawater flowing in from the nearshore zone during flood).
Water parcels move some time forth and back in an estuary before they are evacuated offshore. We call the flushing time of water parcels in the estuary (the average residence time fluid parcels entering the estuary at the upstream boundary). During this time, water parcels also move in lateral and vertical directions, due to flow circulations and turbulent eddies. The time scale for vertical mixing, , and the time scale for lateral mixing, , are related to the vertical and lateral turbulent diffusion coefficients, and , by the relationships and , respectively. If the vertical and lateral mixing time scales are both much smaller than the flushing time of water parcels in the estuary, the longitudinal path of a water parcel follows a random walk. The longitudinal displacements in successive time intervals are uncorrelated, by choosing equal to an integer number of cyclic tidal periods such that . The cross-sectional mixing time , assuming that lateral mixing takes more time than vertical mixing, as is the case for most estuaries. In estuaries satisfying the condition , salt transport by seawater intrusion processes, , can be represented by a gradient-type transport formula,
where is the longitudinal dispersion coefficient and . The dispersion coefficient has the important property that it does not depend explicitly on the salinity distribution in the estuary , but only on the flow characteristics during the tidal cycle (which may be influenced by the salinity distribution, by the way). According to random walk theory ,
where is the average of the squared successive random displacements,
The magnitude of the random displacements depends on the location in the estuary; the longitudinal dispersion coefficient is thus a function of . This is illustrated in figure 1 for the Eastern Scheldt and Ems-Dollard estuaries.
Analytical expressions for the longitudinal dispersion coefficient
Under certain simplifying conditions it is possible to derive analytical expressions for the longitudinal dispersion coefficient. These assumptions are:
- the estuarine geometry does not vary strongly in -direction over distances comparable to or larger than the tidal excursion;
- the cross-section of the estuarine main channel has approximately a rectangular shape.
We also have the condition . In the following we consider different seawater intrusion processes under these conditions and present an approximate analytical expression for the dispersive transport produced by each process.
Dispersion by residual circulation
Fist we consider seawater intrusion caused by estuarine circulation: the up-estuary near-bottom flow caused by the higher density of seawater relative to estuarine water. The estuarine circulation is represented by the velocity component
where the brackets stand for averaging over the tidal period (in fact, the averaging is done in a frame moving with the cross-sectional mean velocity), and for averaging over the vertical. The longitudinal dispersive transport can be estimated by a procedure outlined by G.I.Taylor . The result is
For the dispersion coefficient related to lateral horizontal residual circulation a similar formula can be derived, replacing in the expression for everywhere by .
Estuarine circulation is an important seawater intrusion mechanism in estuaries with a single deep (dredged) channel and small to moderate tide. Lateral circulations are important in wide natural estuaries with a complex geometry (meandering main channel , secondary channels, channel bars and tidal flats) and strong tides. The dominance of lateral circulations for seawater intrusion relative to vertical circulations appears in the analytical expression of through the much larger lateral mixing time compared to the vertical mixing time . The presence of distinct ebb and flood channels is a major cause of lateral circulation in wide estuaries, see for example figure 2. However, density gradients related to seawater intrusion also produce lateral circulations (see Estuarine circulation), which contribute often even more to longitudinal dispersion than the vertical density-induced circulation .
Dispersion by tidal straining
If residual circulations are weak, dispersion is mainly caused by tidal straining, the relative displacement of water masses due to vertical and horizontal gradients in the tidal current velocity (In river flow, the usual term is shear dispersion). Seawater intrusion is primarily caused by vertical tidal velocity gradients in narrow deep estuaries, whereas lateral tidal velocity gradients are important in wide estuaries. We present formulas for vertical tidal straining; the expressions for lateral tidal straining are similar. The process of longitudinal dispersion through tidal straining is explained in figure 3.
The velocity component responsible for vertical tidal straining is
where is the depth-average current velocity. By a procedure outlined by Holley, Harleman and Fischer , the following first-order estimate of the longitudinal dispersion coefficient is obtained:
The coefficients depend on the velocity profile; order-of-magnitude estimates are and .
Longitudinal dispersion produced by lateral tidal straining can be expressed by a formula similar as for vertical tidal straining. Dispersion by tidal straining is largest if the time scale for vertical or lateral mixing is on the order of . The time scale for vertical mixing is generally smaller than the tidal period and the time scale for lateral mixing is generally larger. Dye experiments illustrating dispersion by lateral tidal straining are shown in Estuarine dispersion: dye experiments in the Eastern Scheldt scale model.
It should be realised that longitudinal dispersion is not simply the sum of transport processes related to circulation and straining. Circulation causes not only a net relative displacements of water masses in the estuary, but it also influences tidal straining.
Dispersion by “dead zones”
The formula for lateral tidal straining includes the influence of "dead zones", if they are considered part of the estuarine cross-section and if they are distributed regularly along the estuary. Dead zones are areas along the main estuarine channel where water is not transported in longitudinal direction, for instance, tidal flats or lateral creeks. The longitudinal dispersion coefficient is given by an expression of the type 
where is the maximum tidal velociy and is the HW storage cross-section of the dead zones relative to the channel cross-section. The coefficient depends on the mixing rate within the dead zone; in case of complete mixing during the tidal cycle , assuming that filling of the dead zones starts at low water (LW) .
Even without any mixing, storage areas along the channel contribute to longitudinal dispersion, because of a non-zero phase shift that generally exists between horizontal and vertical tidal motion (between and , respectively, where is the tidal level). The process is illustrated in figure 4. We assume dead zones with bed level at low water (or below), which are distributed evenly along the estuarine main channel. Filling and emptying of the dead zones during the tidal period then produces a net transport through a plane at given by
where is the dead zone volume at time per unit estuarine length, and is the net distance travelled by fluid parcels from the time of low water (LW). Other symbols are shown in figure 4. The integral evaluates mass transport due to water parcels passing through the plane x=0 during ebb but not during flood (because of a net seaward displacement related to storage in the dead zone, represented by the first term in square brackets), taking into account an equivalent landward water flow in the channel (second term in square brackets). By evaluating the integral we find for the coefficient the expression .
A usual order of magnitude for the phase shift is 30-60 minutes, yielding . In estuaries with large tidal flats dead zones can significantly enhance longitudinal dispersion.
Dye experiments illustrating longitudinal dispersion by dead zones are shown in Estuarine dispersion: dye experiments in the Eastern Scheldt scale model.
Time scales for vertical and lateral mixing
A difficulty for practical use of the previous expressions for longitudinal dispersion, results from the uncertainty related to estimating the vertical and (especially) lateral mixing times, and . In case of a logarithmic velocity profile, the vertical diffusion coefficient is given by , where is the friction velocity and the flow velocity. This yields a longitudinal dispersion coefficient , for steady flow . However, in case of buoyant flows, vertical diffusion can be much slower (smaller ), leading to stronger longitudinal dispersion. Lateral diffusion depends strongly on the geometry of the estuary. The lateral diffusion coefficient is generally expressed as . An empirical estimate for moderately meandering channels is  and a model estimate is , where is a characteristic channel bend radius.
Dispersion by deterministic chaos
Dye experiments show that dispersion in wide estuaries with complex geometry generally proceeds in an irregular way, by advection through a field of geometry-induced tidal eddies. The result is very different from diffusion by a cascade of turbulent eddies of progressively decreasing size. Parts of the dye can be trapped within gyres with almost no diffusion, while other dye patches can be highly stretched in the flow direction. Strong stretching occurs in particular in the interface zones between tidal eddies. Zimmerman described the dispersion process in such systems as the result of Lagrangian chaos produced by the tidal whirlpool . Fluid parcels can be dispersed over the entire length of the estuary before lateral mixing has taken place. In this case, the random walk description of tidal dispersion is no longer valid. Zimmerman showed that longitudinal dispersion can still be described as a random process, even if turbulent mixing is completely absent. He called this random process “deterministic chaos” . In his model, fluid parcels are dispersed by moving along chaotic orbits through a lattice of tidal eddies. Most dispersion is generated by eddies with a size comparable to the tidal excursion length . This suggests that the longitudinal dispersion coefficient for chaotic mixing should be proportional to
The size of the eddies also depends on the basin with . A field study in Willapa Bay (US Pacific coast) suggests that chaotic dispersion could be described by a dispersion coefficient . If the lateral mixing time is comparable to or larger than the flushing time , the representation of the dispersive transport by the product of a dispersion coefficient and the local salinity gradient is no longer valid.
Dispersion by tidal pumping
Salt intrusion by tidal pumping is related to the higher average salinity of seawater entering the estuary during flood compared to the average salinity of estuarine water flowing out into the sea during ebb. It depends on the rate of renewal of estuarine water in the outflow region and therefore on tidal inflow-outflow hydrodynamics. Outflow of estuarine water often has a jet-like character, whereas flood water enters the estuary more distributed from different directions, depending on the tidal phase, see figure 5. The inflowing flood water therefore contains ‘new’ seawater from the sides of the ebb channel. Replacement of outflowing estuarine water by "new" seawater is enhanced when river discharge is high, because the ebb jet is concentrated in a surface layer (salinity stratification), whereas seawater inflow is more evenly distributed over the vertical.
We call the replacement rate of outflowing estuarine water by seawater during a tidal cycle. For small fresh water discharge, the corresponding dispersion coefficient at the estuarine mouth is given by
where is the tidal excursion at the estuarine mouth. Savenije  derived the following empirical expression for :
where is the depth and the convergence length of the estuary in the outflow zone (assuming exponential convergence of the cross-section in the outflow zone, );
is the Richardson estuary number ;
is the relative density difference seawater-fresh water and the estuarine width at the mouth. The value of should not exceed 1. Tidal pumping is a major mechanism of seawater intrusion in estuaries during periods of high river flow.
Residence time scale
The residence time is defined as the average time a water parcel located at a distance from the sea boundary, will take to leave the estuary. If the fresh water discharge is zero or very small, and if the dispersion coefficient is assumed constant along the estuary, the residence time is given by random walk theory :
The flushing time is the average time it takes for a fresh water parcel to move through the estuarine zone to the sea. According to the random walk model, for small discharge ,
where is the estuarine length. This is equivalent to , where is the fresh water volume in the estuary.
Experimental determination of the longitudinal dispersion coefficient
The dispersion coefficient can be determined experimentally in situations where the freshwater discharge is constant over a period longer than . In that case the salinity distribution is close to equilibrium (). The total residual salt flux approximately equals zero. We thus have (with the sign convention for )
Values of the dispersion coefficient can be derived from measurement of the residual discharge and the salinity distribution . Examples of longitudinal dispersion coefficients determined in this way are shown in table 1. It should be noticed that the dispersion coefficient is a function of and . The dependence of on has two causes. It is related in the first place to the influence of the salinity distribution on the velocity flow field ; such an influence is due to salinity-induced density gradients (see Estuarine circulation). In the second place, it is related to the location of the freshwater-seawater transition zone. If this zone is situated near the estuarine mouth, the dispersion coefficient is strongly influenced by tidal pumping. This explains the high longitudinal dispersion coefficients for Rotterdam Waterway, Seine and Loire in table 1, which are determined for situations with important river flow . The same holds, to a somewhat lesser degree, for the Elbe, Weser, Mekong and Sinnamary.
In estuaries with a complex geometry and river flow several orders of magnitude smaller than the tidal velocity, the influence of salinity-induced density gradients on the longitudinal dispersion coefficient is generally small. This is often the case for tidal lagoons with small river inflow.
If a non-buoyant dissolved substances is introduced in the estuary, it will be mixed over the cross-section after a time . From that time, the longitudinal dispersion of the substance is similar to salinity dispersion (same dispersion coefficient). For a permanent discharge of a non-buoyant dissolved substance, the same dispersion coefficient applies in the estuarine zones where the substance is mixed over the estuarine cross-section.
|estuary||tidal range [m]||depth [m]||width [km]||discharge [m3/s]||dispersion coefficient [m2/s]|
|Bay of Fundy (Canada)||10||20||20||150||300|
|Bristol Channel (UK)||8||30||20||480||60|
|Chao Phraya (Thailand)||2.5||7.2||0.6||30||330|
|Eastern Scheldt (Netherlands)||3||12.5||2||60||200|
|Gambia (The Gambia)||1.2||8.7||4||2||200|
|Mae Klong (Thailand)||2||5.2||0.2||30||200|
|Mekong-Co Chien+Cung Hau (Vietnam)||2.1||7||4||2125||570|
|Mekong-Tran De+Dinh Anh (Vietnam)||2.8||8||3||2250||530|
|Rotterdam Waterway (Netherlands)||0.9||15||0.6||1000||1000|
|St. Lawrence (Canada)||3||74||48||8500||200|
|Tha Chin (Thailand)||2.9||5.3||0.2||10||270|
|Western Scheldt (Netherlands)||3.8||16||3.5||100||200|
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