Encyclopedia of Environmental Science and Engineering, Volume I and II

MODELING OF ESTUARINE WATER QUALITY 717

salt water extends as a wedge into the river and the inter-

face between the fresh and salt water slopes slightly down-

ward in the upstream direction. The steep density gradient

at the interface, amounting to a discontinuity, reduces the

turbulence and mixing to a very low level. The effect of

the Coriolis force causes the interface to slope downward

to the right in the northern hemisphere looking toward

the sea. In the moderately stratified estuary, the dominant

mixing agent is turbulence caused by tidal action, rather

than velocity shear at the interface between the salt water

and overlying fresh water layer as in the previous case.

With a tide of moderate amplitude, random water move-

ments at all depths occur and turbulent eddies transport

fresh water downward and carry salt water upward, in con-

trast to the dominantly upward advection of salt across the

interface which constitutes the vertical flux of salt in the

river dominated estuary. The result of this two way mixing

is that the salt content of both the upper and lower layers

increases toward the sea. At any given point the bottom

layer is always higher in salt content than the lower layer.

The boundary between the seaward flowing upward layer

and the counter flowing lower layer occurs with a mid-

depth region of relatively rapid increase in salt content

with depth, compared to the vertical gradient in either the

upper or lower layers. This type of mixing contributes a

greater volume of salt water to the upper, seaward flowing

layer than in the salt wedge estuary. The rate of flow in the

upper layer of the moderately stratified estuary is therefore

much greater in volume than in the highly stratified estu-

ary, necessitating a correspondingly larger compensating

up estuary flow in the lower layer.

When tidal mixing is sufficiently vigorous, the vertical

salinity stratification breaks down, and the estuary approaches

true vertical homogeneity. The type of circulation which

exists in a vertically homogeneous system depends upon the

amount of lateral homogeneity. Owing to the Coriolis force in

the northern hemisphere, the water on the right of an observer

looking seaward may be lower in salinity than the water to his

left. A cyclonic circulation pattern is developed, with fresher,

seaward flowing water concentrated to the right of center and

a compensating up estuary flow of higher salinity water to the

left of center. Although a vertical salinity gradient is absent

in a vertically homogeneous estuary, vertical transfer of salt

is not lacking. There is also a strong lateral transfer of salt

which represents the dominant circulation pattern in this type

of estuary.

Certain vertically homogeneous estuaries, particularly

those which are relatively deep and narrow, do not exhibit

these cyclonic circulation patterns. The direction of water

movement is symmetrical about the longitudinal axis, and

fluctuations in velocity are related to the tides and the net

flow averaged over several tidal cycles is directed seaward at

all depths. There is a tendency for salt to be driven out of the

estuary by the action of the advective process. There must

be a compensating non-advective longitudinal flux of salt

directed toward the head of the estuary (Pritchard, 1967).

It is very important to note that the quality or character

of the water at any point in the stratified, partially stratified,

or vertically homogeneous estuary will be strongly corre-

lated with the salinity content of the water. For example, the

high salinity, bottom water in a stratified estuary will have a

quality much like that of the offshore ocean water. The water

at the geographical midpoint of a vertically homogeneous

estuary will be a mixture of river and ocean waters. Also,

materials introduced into an estuary will be influenced at

any point in time or space by the circulation patterns in the

estuary. Estuaries which have not felt man’s influence either

in the estuarine zone or the fresh waters which flow into them

have biological systems adapted to whatever water quality

patterns exist. Since these water quality patterns are strongly

influenced by the circulation patterns and/or introduction

or removal of materials, they will have a beneficial or del-

eterious effect on the biota of the estuary depending on the

extent of the change or the nature of the material introduc-

tion or removal. Thus, it is important to examine the circula-

tion patterns of estuaries as well as the material introduced

or removed to understand the water quality and biota of the

estuary and the uses which may be made of the estuary.

Estuarine Circulation Models

Numerous attempts have been made to model the hydraulic

processes which occur in estuaries. Originally, these models

were developed to determine circulation modifications

which might occur because of physical modifications to the

estuary. These models have been extended in recent years

to include constituents of water and the prediction of their

transport and fate in estuaries.

One of the first type of models developed for estuaries

was the hydraulic model. This type of model is a physical

representation of an estuary on a small scale. Such models

are usually distorted in the vertical direction so that water

depth may be represented on a larger scale than a lateral

dimension. For example, if an estuary were modeled on a

scale of 1:100, the width of the estuary, if it were 10 miles,

would be 0.1 miles in the model, but the depth of the water,

if it were 10 feet, would be 0.01 feet which would be not

much more than a film of water in the model. To avoid this

situation which would make the model unusable, the ver-

tical scale is reduced to a lesser extent than the horizontal

scale such that the 10 foot depth of water mentioned above

would be about 1 foot. While the hydraulic models are capa-

ble of representing tidal currents, momentum entrainment,

and gravitational circulation, they are not able to represent

local currents and turbulent eddies. For this reason, there is

considerable distortion of diffusive processes in the physi-

cal model that makes its utility in quantitative concentra-

tion distribution studies dubious (Ward and Especy, 1971).

From a qualitative standpoint, the physical model possesses

an excellent demonstration capability for the visualization

of flow patterns in resultant concentration distributions, and

this capability should not be under-rated.

The other types of models developed for estuaries are

mathematical models which may be intended to model tidal

currents, net advective movement, or tidal stage in an estu-

ary, or they may be intended to model the transport of salt

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