Title Page
 Table of Contents
 List of Figures
 Part 1 - Flow and mass transport...
 Nearshore hydrodynamics in the...
 Mass transport in the vicinity...
 The starting jet
 Measurement of jet width from aerial...
 Part II - Dissipation and wave...
 Theoretical model for wave...
 Approximate solutions for scattering...
 Experimental parameters
 Incident, transmitted, and reflected...
 Bibliographical sketch

Title: Flow separation and related phenomena at tidal inlets
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00084174/00001
 Material Information
Title: Flow separation and related phenomena at tidal inlets
Physical Description: xvi, 328 leaves : ill. ; 28 cm.
Language: English
Creator: Özsoy, Emin, 1950-
Publication Date: 1977
Subject: Tidal currents   ( lcsh )
Inlets   ( lcsh )
Engineering Sciences thesis Ph. D
Dissertations, Academic -- Engineering Sciences -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis--University of Florida.
Bibliography: Bibliography: leaves 325-327.
Statement of Responsibility: by Emin Özsoy.
General Note: Typescript.
General Note: Vita.
 Record Information
Bibliographic ID: UF00084174
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000084927
oclc - 05301838
notis - AAK0273

Table of Contents
    Title Page
        Page i
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
        Page vi
    List of Figures
        Page vii
        Page viii
        Page ix
        Page x
        Page xi
        Page xii
        Page xiii
        Page xiv
        Page xv
        Page xvi
    Part 1 - Flow and mass transport in the vicinity of tidal inlets
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Nearshore hydrodynamics in the vicinity of tidal inlets
        Page 10
        Page 11
        Page 12
        Page 13
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        Page 15
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    Mass transport in the vicinity of tidal inlets
        Page 83
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    The starting jet
        Page 172
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        Page 187
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    Measurement of jet width from aerial photographs
        Page 189
        Page 190
        Page 191
        Page 192
        Page 193
        Page 194
        Page 195
        Page 196
    Part II - Dissipation and wave scattering by narrow openings
        Page 197
        Page 198
        Page 199
        Page 200
        Page 201
        Page 202
        Page 203
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    Theoretical model for wave transmission
        Page 207
        Page 208
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        Page 210
        Page 211
        Page 212
        Page 213
        Page 214
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        Page 216
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    Approximate solutions for scattering and dissipation
        Page 218
        Page 219
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    Experimental parameters
        Page 289
        Page 290
        Page 291
        Page 292
        Page 293
    Incident, transmitted, and reflected wave amplitudes for experiments 1-10
        Page 294
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    Bibliographical sketch
        Page 329
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Full Text








The author would like to thank Dr. Umit A. Unluata,

who supervised the research. His guidance, encouragement,

and enthusiasm will not be forgotten, in spite of the fact

that he could not be present at the time of submission of

this dissertation. Suggestions and enthusiastic support

received from Dr. Ulrich H. Kurzweg, Dr. Joseph L. Hammack,

and Dean Morton Smutz are deeply appreciated.

Mrs. Nancy V. McDavid worked many arduous hours to

type a highly professional final draft of the manuscript.

The talent of Mrs. Lillean Pieter,who did the drafting,

and Mr. Ron Franklin's photographical work were also of

great value. Special thanks are given to Mrs. Evelyn Hill

who helped the author in every respect. Support received

from Dr. Ashish J. Mehta and the efforts made by Mr. Howard

Groger made possible the experiments reported in Part I.

During his graduate studies the author was supported

through a NATO Scholarship awarded by the Scientific and

Technical Research Council of Turkey. Part II of this

research was supported by the National Science Foundation

through a research initiation grant to the Coastal and

Oceanographic Engineering Laboratory, University of Florida.

The use of COEL facilities and the funds made available

for the publication of this report are greatly appreciated.






. . . xii



1.1 Background . . .
1.2 Objective and Scope of the Present
Study. . . . .

TIDAL INLETS . . .. ..
2.1 Shallow Water Equations . .
2.2 Turbulent Jets in Shallow Water. .
2.2.1 Simplified Equations for Jets
2.2.2 General Solution for Bottom
Frictional Jets Over Variable
Topography . . .
2.2.3 Jets on a Frictionless Bottom
of Constant Depth . .
2.2.4 Bottom Frictional Jets with
Constant Depth . .
2.2.5 Bottom Frictional Jets on a
Linearly Varying Topography
2.2.6 Jets on Arbitrary Bottom
Topography. . . .
2.3 Curved Jets in the Presence of Cross-
Currents . . . .
2.3.1 Equations for Deflected Jets.
2.3.2 Flows in the Presence of Cross-
Currents . . .


S 2

. 8



INLETS . . . ..
3.1 Shallow Water Diffusion Equation
3.2 Diffusion in Turbulent Jets .
3.2.1 Simplified Equations for Jets
3.2.2 General Solution. . .
3.2.3 Transport of Pollutants .
3.2.4 Transport of Suspended Sedi-
ments . . .
3.2.5 Effects of Ambient Concentra-
tion . . .
3.2.6 Effects of Cross-currents
3.3 Tidal Exchange and Mixing in Bays. .




REFERENCES . . . . . .






1.1 Background . . . .
1.2 Objective and Scope of the Present
Study . . . .


3.1 Approximate Analysis for Monochromatic
Wave Incidence . . .
3.2 Approximate Analysis for Multiple
Harmonic Wave Incidence. . .
















4.1 Experimental Set-Up and Procedures
4.2 Ranges of the Parameters a and B
4.3 Shallow Water Nonlinearity of the
Incident and Scattered Waves ..
4.4 Transmission, Reflection and Dissipa-
tion Characteristics of Narrow
Openings . . . .
4.4.1 Sharp Edged (Type I) Openings
4.4.2 Long (Type II) Openings .
4.5 Determination of the Drag Coefficient
from Experiments . . .
4.6 Parametric Dependence of Wave Scatter-
ing Observed in Other Experimental
Studies . . .......






. 328













Figure Page


2.1 Definition sketch. . . .... .. 13

2.2 Definition sketch for shallow water jet flows. 25

2.3 Variation of centeriine velocity for jets on a
frictionless, constant bottom. . .. 38

2.4 Variation of half-width for jets on a friction-
less, constant bottom. . . ... 39

2.5 Volume flux and energy flux variations for
jets on a frictionless, constant bottom. 41

2.6 Bottom frictional jet over constant bottom;
jet half-width, centerline velocity. . 44

2.7 Ebbing flow at Redfish Pass, Florida .. 46

2.8 Jet characteristics over a linearly varying
bottom; jet half-width, centerline velocity. 52

2.9 Laboratory layout of the bay-inlet-ocean
model. . . ... . . 56

2.10 Centerline velocity for a jet on linearly
varying bottom topography. . . ... 57

2.11 An example of arbitrary depth variations;
depth variations, jet half-width, centerline
velocity . . . . 59

2.12 Jet development at Jupiter Inlet, Florida. 61

2.13 Bathymetric map of Jupiter Inlet and vicinity. 62

2.14 Three dimensional plot of the bottom topog-
raphy near Jupiter Inlet . . 63


LIST OF FIGURES (continued)

Figure Page

2.15 Comparison with the jet characteristics at
Jupiter Inlet; depth variations, jet half-
width, centerline velocity . ... .64

2.16 Polar coordinates. . . . ... 66

2.17 Definition sketch for jets in cross-currents 70

2.18 Cross-sectional pattern of flow in the y,-z
plane. ...... . . . .. 73

2.19 Jet characteristics for jets in cross-currents;
jet half-width, centerline trajectory. . 79

2.20 Curved jet at Bakers Haulover, Florida 81

2.21 Strongly curved jet at South Lake Worth Inlet,
Florida. . . . . ... 82

3.1 Definition sketch for diffusion in shallow
water jets . . . . 92

3.2 Jet centerline concentration for constant
depth. . . . . 107

3.3 Jet centerline concentration for linearly
varying depth. . . . 107

3.4 Lateral variation of the bottom deposition rate
K(E,S) in the ZOEF, in the ZOFE. . .. 112

3.5 Diffusion and settling characteristics; center-
line concentration, centerline deposition rate,
integrated deposition rate ... 118

3.6 Diffusion and settling characteristics; center-
line concentration, centerline deposition rate,
integrated deposition rate . ... 120

3.7 Diffusion and settling characteristics; center-
line concentration, centerline deposition rate,
integrated deposition rate . ... 121

3.8 Diffusion and settling characteristics; center-
line concentration, centerline deposition rate,
integrated deposition rate . ... 123


LIST OF FIGURES (continued)

Figure Page

3.9 Contours of bottom deposition rate K(E,C)
for 1 = 0.05, v = 0,y = 0.1, = 1.0 . 125

3.10 Contours of bottom deposition rate K(E,c)
for V = 0.05, v = O,y = 0.1, p = 1.4 . 126

3.11 Sand depositions near four inlets in the vi-
cinity of Big Marco Pass, Marco Island,
Florida. . . . . .. 128

3.12 Sand depositions and jet interaction at
Little Hickory Pass and Big Hickory Pass,
Florida. . . . . ... 129

3.13 Sand depositions at East Pass, Destin,
Florida. . . . . 130

3.14 Terminology for tidal inlet sedimentation
patterns . . . .. ... 133

3.15 River mouth sedimentation patterns ... .. 135

3.16 Land area of distributaries at the Mississippi
River delta. . . . ... 138

3.17 Thermal infrared imagery of Mississippi River
effluents . . . . 139

3.18 Schematic diagram showing sand pockets and
distributaries of the Mississippi River delta. 140

3.19 Contours of bottom deposition rate K(E,C) for
= 0.05, v = 0, y = 0.1, p = 1.0 and
CA = (E/2)exp(-2/2). . . .. 142

3.20 Centerline concentration for jets in cross-
currents, y = 0.02, v = 0. . . ... 145

3.21 Contours of bottom deposition rate K(,S) for
a jet in a cross-current p = 0.02, v = 0,
6 = 0.1, p = 1.0, UA = 0.4 . ... 146

3.22 Curved shoals at Captiva Pass, Florida 147

3.23 Schematic representations of a bay-inlet-ocean
system, jet flow during ebb tide, sink flow
during flood tide. . . . ... 149

LIST OF FIGURES (continued)

Figure Page

3.24 Jets issuing from three bays on the Texas and
Louisiana Coast. . . . . 151

3.25 Definition sketch for ocean mixing . .. .159

3.26 The ocean mixing coefficient, T, as functions
of Tf and p. . . . ... 162

3.27 Schematic representations of water surface
displacement in bay, velocity at inlet, evolu-
tion during flood tide, evolution during ebb
tide . . . . . 164

3.28 Total amount of rhodamine dye in Card Sound
during continuous injection test, T = 0.3. 171

4.1 Unsteady development of a starting jet . 173

4.2 Starting flow at the beginning of ebb tide at
Cape Hatteras, North Carolina. . .. 175

4.3 Starting jet at Fort Pierce Inlet, Florida 176

4.4 Starting jet at Ponce de Leon Inlet, Florida 177

4.5 Jet development at Little Pass, Clearwater,
Florida. . . . . 178

4.6 Frames from motion picture showing jet
development. .. . . . 180

4.7 Distance to jet front QF versu' time T . 183

A.1 Guide for the measurement of jet width from
photographs. . . . ... 191


1.1 Tsunami induced vortex shedding at the harbor
entrance of SA-ME District, Hachinohe Port,
Japan. . . . . . 203

2.1 Definition sketch, sharp edged (type I)
opening with L/d << 1, long (type II) opening
with R/d >> 1. . . . ... 209

LIST OF FIGURES (continued)

Figure Page

2.2 Definition sketch illustrating the concept of
effective mass; plan view, side view ..... 214

2.3 Inertial reactance for sharp edged (type I)
openings, long (type II) openings. . ... 216

3.1 Equivalent block diagram for equation (3.1),
block diagram for first approximation quasi-
linear system. . . . ... 219

3.2 T1 as a function of a and B. . ... 228

3.3 R1 as a function of a and B. . ... 229

3.4 e as a function of a and a. .. . 230

3.5 T1 as a function of p2, p3, and B. . ... 238

3.6 T2 as a function of P2 and B . ... 239

3.7 T3 as a function of p3 and B . ... 240

4.1 Laboratory layout of the wave tank .. 244

4.2 Narrow opening placed in the wave tank .. 246

4.3 Array of capacitance type wave gauges. ... 248

4.4 Strip chart recorder and controls for the
wave-maker . . . . 249

4.5 Data acquisition system and magnetic tape
recorders. . . . . ... 249

4.6 Visualization of the wave-induced flow through
a narrow opening . . . ... 258

4.7 T as a function of 8 . . ... 262

4.8 R as a function of 8 . . .... .263

4.9 e as a function of 8 . . .. ... .264

4.10 Incident, transmitted and reflected wave spec-
tra for experiment 1 . . ... .266

LIST OF FIGURES (continued)

Figure Page

4.11 T as a function of a for different ranges of
the parameter . . . . 268

4.12 T1, R1, and E as functions of a1l/h. .. 271

4.13 T1, R1, and e as functions of all/h. ... 272

4.14 TI, R1, and e as functions of all/h. ... 273

4.15 T and R1 as functions of Kb for a long (type
I ) opening. . . . .. 275

4.16 The drag coefficient f as a function of d/b. 280

4.17 T, and R1 versus B for experiments due to
Hayashi et al . . . 282


Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment
of the Requirements for the Degree of Doctor of Philosophy



Emin Ozsoy

August, 1977

Chairman: Dr. Ulrich H. Kurzweg
Major Department: Engineering Sciences

Various phenomena associated with flow separation

at tidal inlets and other narrow entrances are analyzed.

The solutions are then used in discussing relevant appli-


In Part I, flow and mass transport are analyzed for

turbulent jets issuing from tidal inlets during the ebbing

phase. Frictional resistance and topographical variations

of the bottom and ambient cross-currents are taken into

account. In studying mass transport, settling of materials

to the bottom and ambient concentrations are also allowed.

The flow is analogous to that of a classical two-

dimensional turbulent jet when bottom friction and topo-

graphical variations are neglected; and the solutions com-

pare favorably with available theories. When bottom friction

is present, the jet expands rapidly and gets retarded.

Effects due to an offshore-sloping bottom counteract the

rapid expansion due to friction. For arbitrary depth

xii i

variations, the jet may go through a series of expansions

and contractions with distance. Comparing these solutions

with experimental and field evidence gives satisfactory

results. In the case of pollutants, bottom friction re-

duces dilution within the jet due to a decrease in convec-

tive transport. An offshore sloping bottom opposes this

effect. The transport of suspended sediments depends on

the fall velocity of sediments and the velocity at the

inlet, in addition to bottom friction and depth variations.

Finer sediments are transported to larger distances off-

shore as compared to coarser sediments. The largest depo-

sition occurs at the location of two marginal shoals

flanking the jet centerline. When the velocity at the

inlet exceeds a critical value, deep scour holes are formed

near the inlet mouth. These results and their implications

on the geomorphology near tidal inlets and river mouths are

discussed qualitatively.

Exchange and mixing in a bay-inlet-ocean system is

studied, based on the results of the previous analysis.

Materials within the bay are jetted to great distances off-

shore during the ebbing phase and this provides an efficient

flushing mechanism.

Finally, the unsteady development of a tidal jet is

described qualitatively and a simple method for calculating

the speed of the jet front is proposed. Preliminary experi-

ments are also provided.


In Part II, an experimental approach is used in

determining the wave scattering by narrow openings such

as tidal inlets, tsunami breakwaters, and harbor entrances.

Flow separation and generation of jets and vortices in

the near field of the opening determines the subsequent

loss of energy and the scattering of waves in the far field.

Approximate methods are used in analyzing the non-

linear behavior of a narrow opening in the presence of flow

separation. Experimental results are interpreted by making

use of these approximate solutions.

Experimental procedures are complicated because of

the shallow water nonlinearity, which causes the incident

waves to be almost periodic in space and to have harmonic

distortion. Therefore, the measured waveforms are Fourier

analyzed and the spatial amplitude variations of the inci-

dent and reflected harmonics are obtained.

The transmission of waves is reduced by two main

factors; one being due to the inertial reactance resulting

from the disturbance of the flow by the opening, and the

other being due to the drag resistance caused by flow

separation. When inertial reactance is dominant, the re-

sponse is selective of frequency; whereas, when separation

resistance is the dominating factor, wave transmission

becomes dependent on amplitude only. The dissipation of

energy is maximal in the second case.

It is also shown that the scattering of the funda-

mental component is not affected significantly by the

presence of higher harmonics in the incident wave. How-

ever, the same result is not true for the scattering of

higher harmonics.






1.1 Background

Tidal inlets play important roles in a broad range

of coastal activities such as commerce, nagivation, small

craft operations, and achievement of adequate water quality

standards. Since they act as interfaces between estuarine

or lagoonal waters and the coastal ocean, they deserve,

and to some extent have received, utmost attention. It

is on these grounds that the prediction and understanding

of tidal currents in the vicinity of inlets gains particu-

lar significance.

Tidal currents at inlets often transport pollutants

or suspended sediments. Mass transport of these materials

determines the ultimate distribution of pollutants and

the bathymetric changes near tidal inlets.

Tide-induced currents in coastal waters become es-

pecially significant near tidal inlets and estuary mouths.

Patterns of flow change with time during a tidal period.

More specifically, the flow on the ocean side separates from

boundaries during ebb, as opposed to the non-separated flow


during flood. Therefore a turbulent jet is often formed

during ebb-flows. Depending on the strength of inlet

currents, the jet may extend to distances of considerable

magnitude, hence, contributing significantly to the small-

scale mixing and circulation on the continental shelf.

During flood, the currents in the semi-infinite ocean

region may be modeled as a sink flow, since water is drawn

towards the inlet.

There has been one serious attempt by French (1960)

in modeling the ocean flow patterns during both stages of

the tide, but the model is inadequate in that the effects

of bottom friction and variable bottom topography are

neglected. In reality, these factors may become important

especially during ebb-tide, when a jet flow is formed.

In the presence of bottom friction, the jet loses

its momentum against the resistance of the bottom and

therefore expands more rapidly in width as compared to

a frictionless jet. Bottom topographical variations

seaward of tidal inlets also contribute to the jet develop-

ment. For example, if the bottom slopes downward in the

offshore direction, the expansion rate of the jet is

reduced due to the increased vertical extent of the

flow area.

Bottom frictional effects have been hinted at in a

number of other studies. Taylor and Dean (1974) and

Borichansky and Mikhailov (1966) have included bottom

friction in their analyses, but have been led to results

that are of limited value since they have utilized various

inaccurate assumptions. The latter have also accounted

for depth variations. Experimental studies by Savage

and Sobey (1975) and Gadgil's (1971) analysis of the effect

of Ekman friction on laminar jets also indicate the fast

jet growth.

Tidal flows near inlets may have a complex nature

due to a number of other factors. For example, waves

incident on the coast and local winds often set up circu-

lation patterns in addition to tidal currents. These cur-

rents in the ambient waters often change the behavior of

tidal jets. Furthermore, secondary circulations within the

jet may also occur.

Density stratification is also a commonly observed

phenomenon at tidal inlets, especially when fresh water

outflows are present in the adjacent bay areas. In the

event of stratification, the jet exhibits buoyant spread-

ing and thus a salt water wedge may be generated at the

inlet. An example of a tidal inlet where this phenomenon

occurs is reported by Wright and Sonu (1974).

On a speculative basis, the earth's rotation may

be expected to influence tidal jets, since they often reach

to large distances offshore. However, Savage and Sobey

(1975) have shown both analytically and experimentally

that shallow water jets (extending through the full depth

of the fluid) can support pressure forces arising due to

rotation and therefore are not deflected. This result

appears to be a confirmation of the Taylor-Proudman theorem

(see Savage and Sobey, 1975). On the contrary, for deep

water jets, i.e., for jets which do not cover the whole

depth, they have shown that the jet encounters deflection

due to Coriolis forces. Therefore, for jets issuing from

tidal inlets, deflection may occur when the flow separates

from the bottom either due to a steep bottom slope or due

to buoyancy of effluents.

A tidal inlet serves as an interface between the

interior waters and the ocean and thus plays an important

role in the circulation and flushing characteristics of

the interior waters. To date much effort has been directed

toward the analysis of tide-induced circulation in bays

and estuaries (see Wang and Connor, 1975). However, while

the exchange and flushing characteristics of inlets and

entrance regions have been generally recognized as having

a significant effect on the quality of interior waters,

there has been very little effort made to understand

related phenomena. An approximate analysis of inlet ex-

change characteristics has been done by Taylor and Dean


With regard to sedimentation, a basic knowledge of

the hydrodynamic phenomena in the vicinity of an inlet,

a river, or an estuary mouth is crucial to the understanding

of the evolution, persistence, and maintenance of the

geomorphology of the neighboring regions, including inner

and outer shoals, swash channels, navigation channels and

the configuration of the adjacent shorelines. Indeed, a

significant amount of sand in a littoral system can be

trapped by an inlet. Briefly, the flow issuing from an

inlet in the ebb-cycle is in the form of a turbulent jet,

laterally entraining the adjacent waters and, therefore,

carrying littoral material into the inlet zone. During

flood cycle the flow radially converges toward the inlet

carrying sand with it. The sand in the adjacent littoral

system is thus transported toward the inlet in all phases

of the tidal cycle and is either jetted off-shore or

carried into the interior waters to form extensive shoals.

It has been found in some areas of Florida that the ma-

terial extracted in this manner from a littoral system has

caused 40 feet retrogressions of the shoreline each year

for a period of 50 years (Dean and Walton, 1974).

Deposition on the seaward side of an inlet often

creates bar structures that encircle the inlet mouth.

Sedimentation patterns mainly depend on tidal currents,

but other effects due to waves, stratification, etc. may

also have an influence. For instance, Dean and Walton

(1974) have discussed the effect of wave energy on limit-

ing shoal volumes. They have shown that in areas of low

wave energy, the bar structure consists of two massive

shoals flanking a central channel; whereas, on high

energy coasts, the shoals take a crescent shape due to

the shoreward transport of sediments. This balance be-

tween tidal currents in the offshore direction and wave

induced onshore currents has also been discussed by Oertel

(1974); who also incorporated the effects of longshore


The above cited features of the flow and mass transport

are by no means reserved to tidal inlets and in fact ex-

tend to the case of a river that terminates at a coast.

Fresh water discharging from a river into coastal waters

also forms a turbulent jet. However, a buoyant spreading is

associated with the flow in this case due to the lower

density effluents. Essential features of river effluents

have been thoroughly outlined and documented by Wright and

Coleman (1974) and Garvine (1977). The buoyant spreading

of river plumes has been analyzed by Bondar (1970) (see

Wright and Coleman, 1974) who neglected lateral entrainment

and lateral shear stresses. An analysis including buoyancy,

lateral shear stresses, and earth's rotation has been done

earlier by Takano (1954) (see Officer, 1976) but his

analysis applies only in the far field of the river mouth;

since,the nonlinear convective terms in the equations of

motion have been neglected.

River mouth sediments also exhibit deposition pat-

terns that are similar to those at tidal inlets, even

though buoyant spreading of the jet influences the deposi-

tion of sediments to some degree. Sedimentary forms

gain particular significance at river deltas since they

contribute to delta building. Computer simulations of

the deposition of suspended sediments at river mouths have

been made by Bonham-Carter and Sutherland (1967); however,

their model is limited since they have assumed the effluent

to be initially separated from the bottom. Experimental

studies by Butakov (1971) and Mikhailov (1972) have shown

that bars similar to the marginal shoals of a tidal inlet

are formed in the vicinity of a river mouth. Wright

and Coleman (1974) also report similar features for

Mississippi River mouths.

1.2 Objective and Scope of the Present Study

While inlet hydrodynamics and the mass transport

of pollutants and sediments has been explained on a quali-

tative basis in the available literature, the quantitative

aspects of such phenomena are virtually nonexistent or

simply inadequate.

The present work is an attempt in analyzing the

phenomena associated with flow separation at tidal inlets.

The approach herein is based on the theory of turbulent

jets; however, the effects of bottom friction and topo-

graphical changes have been included. Diffusion of pollu-

tants and the diffusion-settling of suspended sediments are

also analyzed. Several additional factors such as cross-

currents and distributions of pollutants or sediments in

the ambient waters are accounted for. The results of the

analysis are then used in studying the exchange mechanisms

of a bay-inlet-ocean system.

Some of the conclusions drawn from the analysis

also have a bearing on river mouth processes.

Hydrodynamics of the flow is studied in Chapter 2.

Mass transport of pollutants and suspended sediments is

analyzed in Chapter 3. Chapter 4 addresses itself to the

unsteady development of the flow during the early stages of

the ebb tide. Conclusions reached by the present study

are outlined in Chapter 5.



In this chapter,the hydrodynamics of the flow issu-

ing from an inlet is studied in the ebbing phase of the

tide. During this phase, the flow is similar to a two-

dimensional turbulent jet. Tidal flows in the neighbor-

hood of inlets are ultimately determined by the ocean tides

and hence have an unsteady character. However, it will be

shown through an order of magnitude analysis that the

unsteadiness is of little importance in the near-field

of the inlet. Therefore, a steady jet is assumed.

The analysis also applies to the near-field flow

caused by a river terminating at a coast, although river

outflows may have a slightly different structure. Fresh

water discharging from a river mouth gradually rises above

the sea water due to its buoyancy. Nevertheless, river

effluents also form jet flows which may in some cases

"touch" the bottom in the near field of the mouth and

therefore the present theory may still be applicable.

The analysis in this chapter covers the hydro-

dynamic aspects of the jet flows in shallow water. The

depth averaged equations of motion are derived and later

solved analytically in some cases. The effects of

bottom friction, variable depth and cross-currents are

included in the theory.

2.1 Shallow Water Equations

Consider the long period oscillatory motions in

a fluid medium of limited vertical extent. In such cases,

vertical motions are usually much less significant than

the horizontal motions and thus the flow can be con-

sidered to be vertically uniform. This assumption implies

a horizontal, two-dimensional flow and a hydrostatic

pressure distribution. In this section, the shallow

water equations are derived, based on this approximation.

The continuity equation states the conservation

of mass within the flow domain and for an incompressible

flow (Schlichting, 1968, p. 44) it is

+ + 0 (2.1)
3x ay az

where u, v, and w are components of the velocity vector

in the cartesian coordinate system (x, y, z).


The conservation of momentum is stated by the
Navier-Stokes equations of motion (Schlichting, 1968,

p. 61). Rearranging the convective terms by making use

of equation (2.1), the Navier-Stokes equations can alterna-
tively be written as

u + +u2 + vu + w+1 X I + + ] (2.2a)
at ax @y z p ax p x 8y [z

+v .uv v2 8wv 1 [ + + yz] (2.2b)
-t ax ay 2t p ay p x ay 2z

+w + auw + avw + w2 -g- 1p + 1 zx z + ] (2.2c)
at x y az p a-z p ax +y az

where t denotes the time, p the pressure, p the density,
g the gravitational acceleration pointed in the negative
z-direction. Here TaB is the deviatoric stress tensor
(Schlichting, 1968, p. 55) with symmetrical properties

where the subscript B denotes the direction of a stress
component and a the direction of the normal to its plane.
Depth and time averaging of equations (2.1) and (2.2)

yields the corresponding shallow water equations.


Vertical averaging. Consider the fluid domain

sketched in figure 2.1, where the free surface and the

bottom topography are described by the equations

Z=n(x, y, t) and z = h(x, y) respectively. The velocity

field is described by the components u, v, and w in the

coordinate system (x, y, z) shown in the figure.

x T(x,y,t)

[ {x,y)

h(x, y) H(x, y,t)
7= +h

Figure 2.1 Definition sketch

Since the volume flux through the bottom and

surface boundaries has to be zero, the kinematic free

surface and bottom boundary conditions

w(z=n) = + u(z=n) + v(z=n) (2.3)
at ax ay

w(z=-h) = -u(z=-h) h -v(z=-h) A (2.4)
ax ay

have to be satisfied in addition to the dynamic equations

(2.1) and (2.2).

In the forthcoming derivations, the shorthand

X will be used to denote the vertical average

X = Xdz (2.5)
X = -h

of a quantity X, such that HX is the integrated quantity.

Similarly, the deviations of X from the vertically aver-

aged quantity X will be denoted by

X = X X. (2.6)

Vertically integrating equation (2.1) and making

use of Leibniz's rule together with equations (2.3) and

(2.4) yields

+ (Hu) + (Hv) = 0. (2.7)
at ax u y

Before integrating the momentum equations (2.2)

some a priori assumptions have to be made in regard to

the shallow water approximations. Assuming that the

water depth is small compared to the horizontal length

scale (say, the tidal wavelength) but large compared to

the vertical amplitude of the motion, it follows that

<< g (2.8)


and therefore the vertical particle accelerations are

negligibly small. Also neglecting the vertical components

of the viscous stresses, the z-momentum equation (2.2c)

reduces to

0 = -g (2.9)
p az

When integrated, the above equation yields the hydrostatic

pressure distribution

p = pg (n-z) + Po (2.10)

where po is the atmospheric pressure of the water surface.

Integrating equations (2.2a,b) vertically by

making use of equations (2.10), (2.3), (2.4) and of

Leibniz's rule one arrives at

aHu a. H2 + aHuv ar 1 aH02 aH09 H 3Txx xy (2
+ + -gH + + (2+11a)
at ax ay ax p bx ax -y P ax +y

aHv a3Huv + Hv2 Ta 1 3Hv9 9aH2 H yx y
W+ .+ = ~ y yx x+y (2.11b)
at x y -gHy p -by ax ay p ax -y


Tx Txz (z=-h) (2.12a)

by yz ( h) (212
T T (z=-h) (2.12b)
Tby yz

are the bottom shear stresses. It has been assumed

here that no shear stresses or atmospheric pressure

variations are acting on the free surface.

Time averaging. Since turbulent flows are con-

sidered here, equations (2.7) and (2.11) have to be

averaged over a representative turbulence time scale T in

order to obtain equations describing time mean properties.

For this purpose, the time average of a quantity X will

be denoted by

= x t X dt (2.13)

and deviations from this time mean property by

X' = X X (2.14)

Some preliminary assumptions will be made before

carrying out the time averaging. In free turbulent flows

(Schlichting, 1968, p. 531), the molecular diffusion terms

are usually much smaller than their turbulent counterparts.

Based on this fact, the laminar shear stress terms for

momentum (last two terms in equations 2.11a,b) can be

omitted. It will also be assumed that the water surface

displacement is small compared to the depth, that is,

n/h << 1. Therefore the total depth H will be replaced

by the still water depth h.


After the time averaging, the shallow water
equations (2.7) and (2.11) take the following form:

S+ (h-) + (hV) = 0 (2.15a)
at ax ay

h U x- (hu2) + (huv) = -gh (h 2)_
at ax ay ax p bx ax ay

(hu,2) -- (h -'') (2.15b)
ax ay

h + (hv) + (h2) = -gh 1 p by (h) (hz)
at ax ay ay p byax ay

(hu'V )- ) (hV ) (2.15c)
ax ay

It may be noted here that similar forms of the shallow
water equations (2.15) have also been derived elsewhere
(for example see Wang and Connor, 1975).
The bottom shear stresses in equations (2.15) are
usually expressed in terms of empirical formulae. Follow-
ing Dronkers (1964), the average bottom stresses in shallow
water are given by

1 f (2.16)
p bx u q

1 f
P by v q (2.17)
p Tby -


q = (u' + V2) (2.18)

represents the magnitude of the horizontal velocity and

f is the Darcy-Weisbach friction coefficient. The friction

factor f is in general a function of the Reynolds number

and the ratio of the bottom roughness to the hydraulic radius

except in fully rough regimes, where the dependence on

Reynolds number vanishes. The values of f for free

surface flows can be calculated through a procedure

analogous to that in Daily and Harleman (1966) used for

pipe flows.

2.2 Turbulent Jets in Shallow Water

2.2.1 Simplified Equations for Jets

For turbulent jet flows in shallow water (sketched

in Figure 2.2), the equations derived in the last section

can be simplified considerably. Jets, in being confined

to coherent flow regions, are similar to boundary layers

in which the lateral velocity and length scales are much

smaller than the longitudinal ones. In addition, equa-

tions (2.15) can be simplified further by considering

shallow water motions in the near field of the inlet,


following an order of magnitude analysis. For this pur-

pose the scales of the motion are selected as in Table 2.1.

Table 2.1

Variable Scale Variable Scale

t T1 u uo

x L v Su

y 6Lo 0 pu0
h ho v 61uo

n ao u' euO
a0 eu 0

v' EU0

The parameters T L h a u correspond re-

spectively to the time, length, surface displacement, and

velocity scales that characterize the flow. The coordinates

are selected such that the x-axis coincides with the jet

axis. The scale UL is a measure of the boundary layer

thickness which corresponds in this case to the width of

the jet. It is therefore obvious from equation (2.15a)

that the small number 6 has to be the ratio of the veloci-

ties v and u. In later sections it will become appar-

ent that the lateral velocity v = O(au ), where the


entrainment coefficient a is of order a = 0(10-2).

(Here, as well as in later usage, the notation x = 0(10")

will imply 5 x 10n-1 < x < 5 x 10n.) Therefore, the

order of magnitude for 6 is estimated as

6 = = 0(a) = 0(10 2). (2.19)

The number p characterizes the deviations of the flow

properties from their vertical means and e characterizes

the ratio of turbulent fluctuations to the time mean

properties. These are small numbers roughly of the orders

S = 0(10-1) (2.20)

S = 0(10-1). (2.21)

After scaling equations (2.15) based on the

parameters in Table 2.1, the following dimensionless

parameters appear:

a L gho fL0
K X = Too A = 2 r 2 8h (2.22)

Since the application here addresses itself to jets issu-

ing from tidal inlets, magnitudes of these parameters

can be estimated at the outset. For tidal periods,

T 0 105 sec., typically. Based on field observations (and

the discussions in Chapter 4) it may be argued that these

tidal jets may extend to offshore distances of around

L 0 10 Km. The tidal velocities usually observed at

inlets are on the order of u 0 Im/sec. Assuming also

h 10 m, a 1 m, f = 10-2 and taking g = 10 m/sec

it is obvious that 6, C, e, K, and X constitute a set of

small numbers < 0(10-1), while the numbers A and r may

be 0(1) or larger.

An additional small parameter arises due to the

boundary layer structure of turbulence since the apparent

shear stresses may be one order larger than the apparent

normal stresses, that is,

O(u) (2.23a)

= 0( ) (2.24b)

where p is a small number.

Finally, the relative magnitudes of the terms in

the governing equations (2.15) and the auxiliary expres-

sions (2.16) and (2.17) are shown below each term in the


+ (hu) + (hv) : 0 (2.25a)
ut ax xy
KX 1 1

h u + (hu'i) + (huv) = -gh u q (h) 2 (h0^)
t ax ay 3x 8 ax ay

x 1 1 KA r 2 p2

(hu'2) (h V') (2.25b)
ax ay
E2 i2

h + T (huv)+ ( v (h (ay

6X 6 6 KA Fr 2 6p2

S(hu'v') (h '2) (2.25c)
ax ay
E2 2

Terms which are < 0(10- ) are in general negligible with
respect to the 0(1) or larger terms. However, note that
the ratio E2/6 of two small numbers is not necessarily
small. In fact, by virtue of equations (2.19) and (2.21)

S = 0(1) (2.26)

and therefore the corresponding terms cannot be


Since each individual term in equation (2.25c)

is of small order, say 0(6), the pressure gradient term

as a reactional force has to be the same order:

S= 0(6) (2.27)

and by virtue of this result, the pressure gradient term

in equation (2.25b) is even smaller, with an order of


KA = 0(62). (2.28)

Again, since fv/u = 0(6), the local magnitude of

the velocity is approximately equal to the longitudinal


= 1 + 0(62) 1 + 0( ) (2.29)

and therefore q can be replaced by uij in the governing

equations. Thus, neglecting all small terms in equations

(2.25), the simplified equations for shallow water

turbulent jets read as

(hu) +-- (hI ) = 0 (2.30a)
ax 3y

S(h2) + (hu) = -f j (hu'v'). (2.30b)

2.2.2 General Solution for Bottom Frictional
Jets Over Variable Topography

Consider the turbulent jet flow produced by water

ebbing from a tidal inlet into a semi-infinite ocean as

shown in figure 2.2. The water issuing from the inlet

entrains and is mixed with the ambient waters due to the

turbulent exchange of eddies. The flow is retarded by

the frictional resistance of the bottom and the depth

variations also affect the expansion of the jet. The
equations of motion (2.30) for this flow were derived in

the last section. Omitting the notations used before for

the vertical and time averaging these can be written as

S(hu2) + (huv) u2 1 F (2.31a)
ax y( 8 (p y xy

' (hu) + (hv) = 0 (2.31b)
ax 3y


.. -- -s



F"igr2 D n th (xyt)


Figure 2.2 Definition sketch for shallow water jet flows

where F = ph u'v' (using the old notation) is the
depth integrated turbulent shear stress acting laterally

on the flow. Since the flow is in the ebbing phase,

ujul has been set equal to u2. These equations which

account for the effects of bottom friction and variable

depth can be reduced to the classical two-dimensional

turbulent jet equations (Schlichting, 1968, p. 682) for

a constant, frictionless bottom by simply taking f = 0 and

h = constant.

In obtaining solutions to equations (2.31),

the flow variables of interest are the velocity distribu-

tion u(x,y) and the characteristic width b(x) in which

the motion is confined (see figure 2.2). Solutions to

these equations are then possible by assuming the velocity

profiles to be self-similar. The similarity hypothesis

has a firm basis in the case of classical turbulent jets

and wakes as demonstrated by both theory and experiments

(for general information on the subject see, for example,

Abramovich, 1963, and Schlichting, 1968). The self-

similarity of velocity profiles will also be assumed in

the present case although it may be argued that this may

not hold in the case of a frictional bottom and variable

depth. Nevertheless, this assumption may be justified

intuitively if one reconsiders the shallow water approxi-

mations. On this basis it is expected that any possible

vertical gradients in velocity are not as strong to affect

the horizontal velocity profiles. Therefore it may be

assumed that the velocity profiles u(x,y) are self-similar.

This facilitates the integration of the equations of

motion (2.31) across the jet. The velocity u and the

shear force Fy vanish as y b, but there is a lateral
velocity ve due to the entrainment of the surrounding fluid

into the jet. Therefore, allowing only one dimensional

variations in depth: h = h(x) and integrating equations

(2.31) across the jet yields

b b
d (h udy) u2dy (2.32a)
dx -b -b

d (h J u2dy) = 2h v (2.32b)

Various different similarity functions have been

developed based on different sets of hypotheses. Tollmien

(1945) has used Prandtl's mixing length hypothesis whereas

Reichardt (1951) and Goertler (1942) have found a differ-

ent solution based on Prandtl's second hypothesis. Abramo-

vich (1963) has shown that a simpler similarity function

can be used to represent the velocity distribution and has

compared it with experimental data. The functional form

he used was adopted from Schlichting (1930) who initially

derived it for turbulent wakes (see Schlichting, 1968,

p. 691). Later, Stolzenbach and Harleman (1971) have

generalized the same functional form to the core region

of the jet and seem to get reasonable results from their


The profiles used here will be the ones adopted

by Stolzenbach and Harleman (1971). Taking b(x) and r(x)

to be the jet width and the core width respectively, and

xs to be the length of the core region as shown in

figure 2.2, the velocity profiles are

u = F(y) (2.33)

where the function F(y) is defined as follows: in the

Zone of Flow Establishment (ZOFE), x < xs:

0 ; -y > b

F(y) = (1- -5)2 ; r < |yl < b = J (2.34a)
( o o b r
1 ; 0 < lyl < r

while in the Zone of Established Flow (ZOEF), x > xs:

F(y) = 0 2 C = b (2.34b)
S ; 0 < ly < b. b

The entrainment velocity v can be related to the center-

line velocity uc by

e = auc (2.35)

where a is defined as the entrainment coefficient. The

above statement, which implies a proportionality between

the mean inflow velocity across the edge of the jet and
the local jet centerline velocity was initially introduced

by G. I. Taylor and was used later by Morton et al.
(1956). It has consequently been shown to be a useful

approximation in a variety of applications. The numerical
values of a will be obtained later by comparing the results

with experiments.
Now, using the self-similar velocity profiles

(equations 2.34) and the entrainment assumption (equation

2.35) in equations (2.32) yields the two ordinary differ-

ential equations

d Uf 22 T2

(h b u2 TI) uc T (2.36a)

d (h b uc I) = h uc (2.36b)
df xc1c


T = f (-u-) d
0 c

12 1



(U) d .

The inlet is modeled by a rectangular slot of width 2bo

and depth ho as in figure 2.2. The initial velocity of the

jet at the inlet is u Using these as the appropriate

scales, equations (2.36) can be written in the normalized


d (2 H B U2) = T2 B U2 (2.38a)
d E 2

(I1 H B U)

= a H U

where E= x/bo and



B(e) = -

U() uc






The core width r(x) will also be normalized as

R() r

Noting by virtue of equations (2.33) and (2.34) that

= -R/B R R
o 1-R/B o- 0 o +

equations (2.37) can be written as

R R+
Il 1(1-

12 + 12(1- )


I1 = I

12 = f


(1C1.5) dC

are numerical constants. Therefore it is seen from equa-

tions (2.41) that the quantities 1T and T2 may vary with

distance only within the ZOFE since, as E Es = xs/b ,

R 0. For E > s, the core width R = 0, and therefore





= 0.450


= 0.316


Il and 12 take on the constant values I, and 12 by virtue

of equations (2.41). The integration of equations (2.38)

for the unknowns R and B in the ZOFE and U and B in the

ZOEF is quite straightforward using the initial conditions

H(O) = 1, R(O) = 1, B(O) = 1, U(O) = 1. (2.43)

Thus, equation (2.38a) can be integrated at once to


12 H B U2 = exp[- p J H1T d '] J(() 2.44)

in which the initial conditions (2.43) have been invoked

and it has also been noted that 12B 1 as -* 0.

Equation (2.38b) has to be integrated separately

in the two zones of the jet. In the ZOFE, the centerline

velocity u = u by virtue of equation (2.33) and the

initial conditions (2.43). Therefore, by setting U = 1

in the ZOFE, equation (2.38b) is integrated to yield

I1 H B = a H(E') dE' + 1 = G() (2.45)

where it has been noted that I1B + 0 as E + 0. Therefore

the solutions for the two unknowns R and B in the ZOFE fol-

low from equations (2.44) and (2.45) by setting U = 1 in

the former equation and utilizing equations (2.41):

llJ( ) 12G(5)
R = G ) (2.46a)

B = -(2.46b)
(11-12) H(E)

To obtain solutions for the unknowns U and B in

the ZOEF, equation (2.44) is first utilized to put equa-
tion (2.38b) into the form

al HJ
d 2J J (2.47)
d-C Il T U

where it has been noted that I = I, and 12 = 12 by
virtue of equations (2.41) since R( > Es) = 0. Then,
equation (2.47) can readily be integrated from x = xs/b0

to E > E,, yielding

1 2 ( 2 I H(') J(S') d' + 2 (s) (2.48)
2 s

The constant of integration y (Ts) is determined by
requiring continuity of the centerline velocity U at = (s.
Thus by evaluating equation (2.48) at E = Es it is seen

1 1 J2(5s) 1
T (2 ( = J '), (2.49)
s 2 U2( s)

since U(Es) = U( < s) = 1. Substituting equation (2.49)
into (2.48) there follows

U = J(- ) (2.50a)
[J2( s) + S()]r2

and substituting this result back into equation (2.44) and
taking 12 = 12 gives

[J2(s) + S(0)]
B = s(2.50b)
12 H(E) J(S)


2ctI2 /f
S( ) I= 2 H((') J(E') d '. (2.51)
1 s

This completes the general similarity solutions to
equations (2.31), with equations (2.46) and (2.50)
constituting the respective solutions in the ZOFE and the
ZOEF. The point Es = xs/bo where the ZOFE terminates is
found from the roots of the equation R(Es) = 0 with R given
by equation (2.46a).

2.2.3 Jets on a Frictionless Bottom
of Constant Depth

It has been remarked earlier that the governing

equations (2.31) can be reduced to the classical jet equa-

tions in the case of a frictionless bottom with constant

depth. Similarly the solutions in equations (2.46) and

(2.50) can be reduced to those describing classical two-

dimensional jets by substituting h = const and f = 0. Thus

in the ZOFE, the simplified solutions are

R = 1 2 (2.52a)
I 1-I2

B = 1 + 1a 2 (2.52b)
11 2

whereas in the ZOEF they take the following form:

2al -2
U = [1 + 2-2 (-s)] (2.53a)

1 2al?
B = [1 + (2-~ )] (2.53b)
12 I1

Thus, the dependence on E of the jet properties observed

from the above solutions are identical to those obtained

from classical turbulent jet solutions; i.e., the core

and the jet widths are shown to obey linear variations,

whereas the centerline velocity decays as (" (see for

example Daily and Harleman, 1966, p. 415).

The numerical value of the entrainment coefficient

c, which has been left unknown earlier, can now be obtained

by comparing the simplified solutions in equations (2.52)

and (2.53) with the available experimental data and/or

analytical solutions for classical jets. For example, if

one had empirical knowledge of the growth rate e of the

shear layer thickness B-R (in the ZOFE) and the growth

rate 2 of the jet width B (in the ZOEF); by virtue of

equations (2.52) and (2.53b), the relations

d(B-R) (2.54a)

in the ZOFE and

dB 2a
dB = 2 = 2 (2.54b)

in the ZOEF could be used for obtaining the numerical

values of a. Stolzenbach and Harleman (1971) have also

used this approach in obtaining their entrainment coeffi-

cient and have arrived at the same results as in equations

(2.54a) and (2.54b). For the growth rates they have

assumed l = E2 = 0.22. Abramovich (1963) has observed

different growth rates in the two regions of the jet and

reports them as el = 0.27 and E2 = 0.22. By using results

due to Abramovich, it follows from equations (2.54) that

a = a1 = 0.036 (2.55a)

in the ZOFE and

a = a2 = 0.050 (2.55b)

in the ZOEF.

Next, the simplified solutions in equations (2.52)

and (2.53) are compared with the available information on

two-dimensional turbulent jets. The predictions of the

centerline velocity and the jet width shown in figures

2.3 and 2.4 indicate a fair agreement with the previous

theories and experimental results. Thus, it can be said

that the present theory predicts jet properties accurately

when the bottom is frictionless and has constant depth.

Other possible comparisons include jet properties such

as volume and energy flux variations with offshore distance.

The normalized forms of the volume flux Q/Q0 and the energy

flux E/Eo as given by Albertson et al. (1950) are calculated

0.8- a\

U() 0.6




Figure 2.3


Variation of centerline velocity for jets on a frictionless, constant
bottom (f = 0, h = constant)


0 Dota by F6rthmonn (after Abramovich)
Present Solution

8() 10-

0 10 20 30 40 50

Figure 2.4 Variation of half-width for jets on a frictionless, constant bottom
(f = 0, h = constant)

from the present solutions as

u dy
Q o
Q u= BUIl
Qo u b
o o

f (u2 + v2) u dy
E o
0 u3b
0 0

S(1 + 2) B U3 T6.

Here 11 is the previously calculated function given by

equation (2.41a) in the ZOFE and 16 is a similar function

which can be expressed as

6 = B+ (1 )
6 B 6 B



16 =

(1- 1"s) de = 0.251


is a numerical constant corresponding also to the value of

the function T6 in the ZOEF. The fluxes of volume and

energy calculated from equations (2.56a) and (2.56b) are

compare: in figures 2.5(a) and 2.5(b) with the experimental

and theretical results due to Albertson et al. (1950).




& Experiments, Albertson, et. al.
--- Theory, Albertson, et. al.
5.- Present Solution

Q .01

2 5 10 20 50 100 300

1.0 --


E 0 Experiments, Albertson,et.oal.
---- Theory, Albertson, et. al.
2 -Present Solution
0.2 -

0.1 I
2 5 10 20 50 100 300

Figure 2.5 (a) Volume flux and (b) energy flux variations
for jets on a frictionless, constant bottom
(f = 0, h = constant)

The accuracy of the present solution is also demonstrated

2.2.4 Bottom Frictional Jets
with Constant Depth

In this section bottom frictional effects are
examined for a constant depth. Therefore, taking H = 1
in equations (2.46) and (2.50), these solutions simplify
to more specific forms. Thus, in the ZOFE, E < (s:

I1 e 12 (1 + cE)
R = -
I1 12

(-12 )(1+ac ) (1-11) e
B =
I 2

and in the ZOEF, 5 > (s:

U = e
S-2S + 2al2-
e + e ee

B = [-2e5 + 2
12L I1





- e


The above solutions reveal the exponential

dependence of the centerline velocity and the jet width on

distance. In fact, the jet width grows and the centerline

velocity decays exponentially for sufficiently large

distances away from the inlet as opposed to the linear

growth of the classical two-dimensional turbulent jet

in which the centerline velocity would decay as E -,

hence much slower. In the presence of bottom friction,

a linear behavior is found only near the inlet and in the

ZOFE since, .s E + 0, B 1+[(1-I2) + (1-1 )] /( 2)

Nevertheless, even in this situation the rate of growth

of the jet width is altered from its frictionless value

(1-I2)a/(Ii-12) because i = 0(a), typically.
The features of the bottom-frictional jet are

displayed in figure 2.6 where p = fb /8ho is kept as a

parameter. The parameter p combines a number of effects,

such that when the friction is large or the inlet width

is large compared to depth, the jet merely explodes as

it faces the bottom resistance and loses its momentum.

Since a value of i = 1/20 may be considered as typical, it

may be inferred from figure 2.6 that the bo,:tom friction

plays an important role in the jet dynamics provided that

the variations in depth are sufficiently small. One also

notes in figure 2.6 that the important features of the jet

8(() 20

.= 0.05


0 10 20 30 40 50



0.2 -=0.10

0 10 20 30 40 50

Figure 2.6 Bottom frictional jet over constant bottom
(a) jet half-width, (b) centerline velocity

characteristics can be inferred without considering the

ZOFE whose extent is suppressed by the frictional re-

sistance. This will be done in the later analysis.

A possible field evidence for the explosive growth

of the bottom-frictional jet may be seen in an aerial

photograph of Redfish Pass, Florida, shown in figure 2.7.

The ebb currents which are superposed onto the photograph

indicate a rapid widening of the jet. This is perhaps

further strengthened by the sedimentary features.

The fast growth rate of the bottom-frictional jet

has been demonstrated in a number of other studies. In

investigating the structure of a jet in a rotating system,

Gadgil (1971) has examined the effect of laminar Ekman

boundary layers which develop at the top and bottom solid

boundaries of a two-dimensional laminar jet. It has been

found that when the rotation effects are sufficiently

large, the jet spreads out much faster than the classical

jet in the same manner as in the present solution. It

has also been shown that the jet, having lost all of its

momentum against friction, begins to eject fluid into the

surrounding. In experiments carried out for jets in

rotating basins, Savage and Sobey (1975) have found that

jets in shallow waters exhibit faster spreading rates than

jets in deep waters. In a study concerning exchange

Figure 2.7 Ebbing flow at Redfish Pass, Florida
(University of Florida, Coastal Engineering

characteristics of inlets, Taylor and Dean (1974) have

also found an exponentially growing jet which expands as

e p/2 whereas the growth in the present theory is as e"

hence, faster. This is because they neglect the lateral

entrainment and assume the velocity to be uniform across

the jet. Borichansky and Mikhailov (1966) have analyzed

flows near river mouths in a tideless sea. Their theory

includes both the bottom friction and the variations in

depth, however, in accounting for the lateral friction on

the jet they have used a friction factor analogous to the

bottom friction coefficient, and have also neglected the

lateral entrainment into the jet. It is interesting to

note that due to the bottom friction Borichansky and Mikhai-

lov also find an exponentially decaying mean velocity and

an exponential increase of the jet width, but, they are

led to erroneous results since they have assumed the

integral momentum of the jet to be conserved as opposed to

the present theory.

2.2.5 Bottom Frictional Jets on a
Linearly Varying Topography

The effects of depth variations are investigated

by taking a linearly varying depth:

h(x) = h + mx (2.61)

where m is the slope. In normalized form this is

H(E) = 1 + v~ (2,62)

where v = mb /h is the slope parameter. Using equation
(2.62), the general solutions in equations (2.46) and
(2.50) can be expressed as

I H v I2(1+ca+ 0" 2)
R =1 12) H (2.63a)

1 1

(1-I2)(1+at + 2) (1-11) H v
B = (I1 12) H (2.63b)

in the ZOFE (5 < ) and

U = 2 l (2.64a)
[H-2p/+ 2 (H -1 H )]
s Il(2v-') 2(Hp H2

S2a2 a2-P/ 2-U/V
H -2 + 2 (H H2s )
s I (2v--) s
B = -(2.64b)
12 H1Iv

in the ZOEF ( > Es), where Hs is the depth when E = Cs"

In the solutions (2.64), the effect of variations

in depth, the bottom friction and the lateral entrainment

onthe dynamics of the jet are expressed, respectively,

by v, p, and a. The roles of parameters p and a have al-

ready been considered in the last section by setting

v = 0. It is therefore appropriate to first inquire in

the effect of the variable bathymetry by neglecting fric-

tion and the lateral entrainment. Taking p = 0, a = 0 it

follows from equations (2.64) that

B = 1/12H, U = 1 (2.65)

that is, with increasing depth, the jet contracts and the

centerline velocity remains a constant. This agrees with

Arthur's (1962) results. The contraction of the jet with

increasing depth is due to mass conservation because the

velocity U does not vary with distance.

Next, taking a f 0 but keeping 0 = 0, that is,

allowing for lateral entrainment, the jet width in equa-

tion (2.64b) simplifies to

B = 1 1 2 (H2 H ) (2.66)
12H 1Is

It can be inferred from equation (2.66) that near S = s

(i.e., H = Hs) the effect due to the depth change will

dominate while, as E + -, B (aC/I1 + a/vll) so that

the jet growth is linear as in the case of the classical

jet; however, it is worth remarking that the rate of growth

is only half that given by equation (2.53). Consequently,

the jet will tend to contract first and then expand. Note

that this result will hold for v > 0. For v < 0 (decreas-

ing depth), the bottom will intersect the surface at a

distance E = 1/IJv and near this region the jet will grow

as B u 1/H. Thus, for negative slopes the jet expands both

due to decrease in depth and the lateral entrainment.

On the other hand, if the entrainment can be

neglected (a = 0) but not the bottom friction, then it

follows from equation (2.64b) that

B = Hs2/v / IH v (2.67)

It is now seen that for positive slopes (v > 0) the jet

will contract if v > p and expand if v < p. In the former

situation the depth effect, which contracts the jet, is

suppressing the jet growth resulting from the bottom fric-

tion while in the latter case the bottom friction effect

dominates. If p = v the two effects are in balance so

the width of the jet remains the same with distance off-

shore. Finally, for v < 0 the jet expands since the bottom

friction effect is enhanced by the decrease in depth.

It is clear from the preceding discussions of

the various limiting cases that, under general circum-

stances, the effect of increasing depth is to counteract

the effects of the bottom friction and the lateral entrain-

ment. For decreasing depth with distance offshore, the

three effects will be acting the same way to lead to the

expansion of the jet. These points are further demon-

strated in figure 2.8 keeping v as a parameter and fixing

the values of i and a. The values of a are used as given

by equations (2.55). It is seen that the effect of bottom

topographical variations on jet expansion are overwhelming

because, even for slopes as small as that corresponding

to v = 0.01, the exponentially growing extent of the

bottom-frictional jet is altered significantly. The

velocity, however, does not exhibit any significant vari-

ations with v.



0.4- V = 0.01
S= 0.05= A
L :=-0o03
O 10 20 30 40 50

Figure 2.8 Jet characteristics over a linearly varying
bottom, p = 0.05 (a) jet half-width
(b) centerline velocity (Dotted line is an
assymptote for v = -0.03)

It is worth noting that for p =vequations
(2.64) lead to

U = H -2+ (H Hs) (2.68a)

B H 2 + 2 (H Hs) (2.68b)
12 s I I

that is, the bottom-friction effect is in balance with
the effect of the depth increase so that the jet expands

linearly with distance as a result of the lateral entrain-
ment alone as in the case of the classical jet. In fact,
the jet width given by equation (2.68b) is identical to

that given by equation (2.53b) apart from a slight shift
in the coordinate E. The velocity variations, however,
differ from the classical jet because the decay here is
as H"3/2 hence, faster.
Finally, it can be seen from equation (2.64b) that
there is an apparent singularity in jet expansion for
2v = p. Upon carrying out the limit as p + 2v it can
be shown that

4 4 2al2 H
B H + 2c i n (2.69)

that is, the jet grows linearly near the inlet (H HS)

but the growth is logarithmic as (H 9n H) at far distances

away from the inlet.

In conclusion, it can be stated that depending on

the relative orders of magnitude of the parameters i and v

and a, the behavior of the jet is changed dramatically

from an exponential to a linear to logarithmic behavior.

Experiments. Although experimental data on jet

properties are highly desirable for comparing with the

present solutions, undertaking such detailed measurements

are not attempted within the context of the present work.

Therefore measurements of only a preliminary nature were

conducted to complement the an lytical results. The model

used in the experiments consisted of an inlet constructed

on a sloping bottom. The general layout of the model is

shown in figure 2.9. The inc ined bottom pre-existed in

the modal basin and had a slcje of m ~ 0.025. The inlet

half-width and depth were b = 10 cm and h = 5 cm, re-

spectively. Thus, assuming a friction factor of f = 0.02

yields p ~ 0.05 and v ~ 0.05 for the values of the friction

and the slope parameters. This assumption for the friction

coefficient is not unreasonable when considering a rough-

ness height of Ks = 10-2 cm (for concrete bottom) and

assuming that the flow is in the fully rough regime. The

value of f = 0.02 can then be calculated from Moody dia-

grams that are available, for example, in Daily and

Harleman (1966).

The measurements of the jet centerline velocity

were done as follows: A jet flow is formed on the ocean

side of the inlet model by discharging water on one side

of the basin and withdrawing the same amount from the other

end. A steady flow was assured by adjusting the discharge

rates and letting the water levels reach constant values

at the points Hi and H2 (see figure 2.9) where they were

measured by point gauges. Then the velocity was measured

along the centerline A-A' (figure 2.9). The velocities

were obtained with a Type 265 Kent Miniflometer kit that

had previously been used and documented by Jones (1975).

Measurements of the centerline velocity obtained

in five individual experiments demonstrate a fair agree-

ment with the analytical solution as shown in figure 2.10.

The importance of the bottom friction is also displayed

in figure 2.10 since the analytical solution for a fric-

tionless bottom (p = 0) differs significantly from the

observed variation of the centerline velocity.


Figure 2.9 Laboratory layout of the bay-inlet-ocean model

1.0.8 ----
0.8 o 8,^ o "- 1

U(C) 0.6 -

04 -0
Analytical Solutions
02 -- v = 0.05, p= 0.05 8
.Z -- 0.05, .= 0

0 10 20

Figure 2.10 Centerline velocity for a jet on linearly
varying bottom topography

2.2.6 Jets on Arbitrary
Bottom Topography

The general solutions obtained in section 2.2.2

are valid for arbitrary one-dimensional depth variations

H = H(). Therefore, only a minimal effort has to be

made, i.e., the solutions have to be evaluated by numeri-

cal quadrature, when the depth varies in an arbitrary manner.

Such example calculations are performed in this section.

An example: Now, consider an example where the

depth changes are modeled by

H = 0.826 {1 + 0.02 + 4.0 [ 1 tan-1 0.2 (&-30) + 0.5]} (2.70)

as shown in figure 2.11a. The variation in equation

(2.70) is such that the bottom slope changes smoothly from

a mild to a steeper one and then back to the same mild

slope; with increasing distance E. The solutions obtained

from equations (2.46) and (2.50) are shown in figures 2.11b

and 2.11c. The jet expands initially, stays of constant

width over the steep section and starts expanding when

the slope becomes mild again. Over the steep slope the

jet expansion due to the bottom friction is counter-

balanced by the contraction due to the increase in depth.

The velocity variation does not demonstrate significant


0 10 20 30 40 50 60 70

S-------- (a)


B(C) 10








Figure 2.11

An example of arbitrary depth variations,
i = 0.05;(a) depth variations, (b) jet half-
width, (c) centerline velocity

Calculations for a prototype inlet: In this

section the present results are compared with what can be

obtained from the aerial photographs of an actual inlet.

A series of aerial photographs of Jupiter Inlet,

Florida, taken on a relatively calm day in June 1973 are

unique in showing the jet boundary clearly (figures

2.12ab). The bathymetric map of the inlet's vicinity is

shown in figure 2.13. A three-dimensional view of the bottom

topography is provided in figure 2.14. Using figures 2.13

or 2.14, it can be inferred from the aerial photographs

that the jet expands in passing over the shoals and con-

tracts afterwards as the depth increases (unfortunately,

the view of the section of the jet beyond the region where

its width is varying slowly was not photographed).

A sample calculation is made by finding an average

one-dimensional bottom variation shown on the top of

figure 2.15. In the calculation, the inlet half width

and the depth are taken to be bo = 50 m and h = 3 m,

respectively. The friction coefficient f was set equal

to 0.02 implying p = 0.04. The variation of the jet width

calculated from the theory is compared in figure 2.15 with

its actual values estimated from the aerial photographs

(see Appendix A). In view of the uncertainties, the

result seems to be satisfactory.

Figure 2.12(a)(b) Jet development at Jupiter Inlet,
Florida (1973 photographs, Ziya


kc~r. c




Figure 2.13 Bathymetric map of Jupiter Inlet and vicinity
(depth in meters based on 1967 data)

----- -C-, -__ ___-. -

c ~--- _2m. 200m.

.< --- _____..- _

Figure 2.14 Three dimensional plot of the bottom topography near Jupiter Inlet
(1967) C
-- -





o ----

Figure 2.15

Comparison with the jet characteristics at
Jupiter Inlet; (a) depth variations,
(b) jet half-width, (c) centerline velocity

-----1975 Data
-.-1967 Data
Used in the Calculations

2.3 Curved Jets in the Presence
of Cross-Currents

2.3.1 Equations for Deflected Jets

Jets in cross-currents will be deflected sideways

in reaction to the forces exerted on them. These cross-

currents may have originated in the receiving water body

due to a multiplicity of reasons such as wind stresses

acting on the water surface, waves breaking at an angle

with the shoreline or alongshore components of tidal cur-

rents. In the last section equations (2.30) were derived

for shallow water jets; however, these did not account for

the jet curvature due to cross-currents. Since gently

curving jets are most suitably described in locally cylin-

drical coordinates, a change of coordinates is performed

in this section.

In the last section, equations (2.25) were simpli-

fied by omitting terms pertaining to the unsteadiness,

pressure gradients, dispersion and normal components of

the turbulent diffusion. The lateral component of the

momentum equation was also dropped. However, in deflected

jets not all of these terms can be neglected. Pressure

gradient terms must be retained since the deflection of

the jet materializes in response to pressure forces

exerted by the cross-current. In addition, since the

Jet is curving, both components of the momentum equation

and the turbulent diffusion terms will be kept. However,

the unsteady terms and dispersion terms may be omitted at

the outset, since they contribute nothing. Furthermore,

since the dispersion terms are neglected, the overbars

denoting vertical averages may also be dropped. Thus,

equations (2.25) can be written as

V (ho) = 0 (2.71a)

(. V)h = ghVi | ('.V)h (2.71b)

where u = (u,v) and u' = u',v') are vectors, representing

respectively the mean and the fluctuating components of

velocity and V =i + j denotes the two-dimensional
ax dy
gradient operator.




e r

Figure 2.16 Polar coordinates

Now consider the system of polar coordinates

shown in figure 2.16. Equation (2.71a) can be expressed


_htr hQr aha-
r+ + 1 (2.72)
ar r r 38

where r and u0 are the velocity components parallel to

the unit vectors er and e respectively (see figure 2.16)

and r and e the new coordinates.

For writing equation (2.71b) in polar coordinates,

first note that for any arbitrary vector a = (ar,a ) it

can be shown that

aha ha a a )ha
(I.v)ht = (a, + er e aI ,
r r r r 0 ae

aha, a, aha ha
+ (ar ar +- r r ) er (2.73)

Thus by virtue of equation (2.73) the components of the

momentum equation in polar coordinates are


ahU hU ahU B ahu' hu'u' u;' hu'
S + + fu q -u (2.74a)
r ar r r ae 8 r ar r r e

ah r hr 6 fhu' r 6
3hu U 3hiU hUh2 hu' u; hu'
"r + r -u + (2.74b)
r 3r r ae r Urq r ar r 36 r

The above equations may then be put into a more compact

form by making use of the continuity equations for the

mean and fluctuating velocity components. The continuity

equation for fluctuations may be obtained by subtracting

equation (2.15a) from (2.7). Thus, by neglecting the

unsteady terms and converting into polar coordinates,

ahu' hu' ahu
rr r + r H =0. (2.75)

Next, equations (2.72) and (2.75) are multiplied by bu and

u' respectively, time averaged, and then added to equation

(2.74a). The same is done for the radial component of

the momentum equation but this time equations (2.72) and

(2.75) are multiplied by Ur and ur respectively, time

averaged, and added to equation (2.74b). Thus,


ahWu 3h uD 2hU U ahu ;2
1 eOr Or _gh f~~ 1 h
+ + uf- 1q
r -9 3r r r 0 8 uq r g9

ahu'ur' 2hu;u'
Sr (2.76a)
ar r

DhU U 3hu2 hU2 hU2 hu'
1 rO r + r-gh fT 1 rU
r 8 ar r r ar 8 Ur r Be

Shu 2 h'2 hu,2
-+ r (2.76b)
ar r r

are the final forms of the momentum equations (2.74).

2.3.2 Flows in the Presence
of Cross-Currents

The equations derived in section 2.3.1 can now be

applied to jets bending in response to cross-currents.

The cross-currents are assumed to be constant in the

alongshore direction but variations in the offshore direc-

tion are allowed. The velocity profiles are modified

locally as shown in figure 2.17 due to the co-flowing

-u -(x)

Figure 2.17 Definition sketch for jets in cross-currents


E:: -,b~, *T


component of the cross-current velocity. The following

additional assumptions are made:

1. The jet bending is assumed to be gradual,

such that

u, cos 0
a << 1 (2.77)

2. It is assumed that the velocity profiles and

other jet properties are symmetrical with respect to the

jet centerline.

3. The entrainment by the jet is assumed suf-

ficiently small with

v sin e
e<< 1 (2.78)

such that the entrained flow does not modify the cross-

current in a significant manner. These assumptions are

tantamount to the assertion that the interactions between

the jet and the cross-flow are weak. In other words, the

approach used herein is not applicable to strongly bending

or attached jets. For a gradually bending jet (see

Stolzenbach and Harleman, 1971) one can define a local

cylindrical coordinate system

dx, = r dO

dy. = dr



(see figure 2.17), where the coordinates x,, y, are

attached to the centerline of the jet. After setting

= uu ur u' = v' (2.81)
r 6 r

making use of equations (2.79) and (2.80) and neglecting

small terms, equations (2.72) and (2.77) can be written


ahO + ah ( d
S h .-L hV = 0
ax, vy, dx


9h U 2 MO 2 9 do ) U2 hu'v' d
+2 + 2 (d+) h2 = d- hu'v +2 ( ) hu'v' (2.82b)
ax*, @y dxy 8 ay, dx,

hN +ah02 deO dO aU f ahu'v'
+ + ( h) h ( ) h2 -gh uv -
ax* ay Tx, d, ay ax

do de
hu'2 + ( ) hv'2 (2.82c)


These equations can now be integrated across the jet

cross-section assuming that the velocity profiles are

self-similar. Furthermore, a potential flow is assumed

external to the jet.

The entrainment velocity Ve is pr-

portional to the centerline velocity Uc as before:

Se uc


The pressure force which deflects the jet can be calculated




+b +b

dy = -gh b = (-h )b
\-b = _-b


where p = pgn is the dynamic pressure. Taking a cross

section as shown in figure 2.18 and writing the Bernoulli

v -(** +Co)
Ua,sine -^Ve

v(y,---- -4
:-Z. sine +oe

7/ 7 7-,,

Figure 2.18 Cross-sectional pattern of flow in the y,-z plane

equation between the jet boundaries y* = +b and points
away from the jet at y. + + -one obtains:

+) + 2 +2) ( v2) (2.85a)
S=y+b 2 '3+ b y,4+

( +(2 u2) = v2) (2.85b)
= =-b y= r- b y*+- 0

Noting that

V(y* = +b) = -
V(y* = -b) = + Ve
u(y* = +b) = (y* = -b) = Ua cose (2.86)
v(y.++~) = a sine e
v(y.-_) = Ua sine + e

and substituting equations (2.85) into equation (2.84)

f+b -gh dy = 2 Ve Ua sine (2.87)
-be a*

Thus, with the substitution of the pressure force
term in equation (2.87), the integrated equations (2.82)

d- (h f U dy*) = a h

d (h f
_R* 0


b b
U2dy*) = f 2dy, + ah Uc Ua cose

h I L2dy*) = ah UcUa sine




Now, making use of the notations for I1 and 12 in equa-

tions (2.37) and dropping the unnecessary time averaging

notation these can be written as

d (h b u, T ) = h u
dx, c 1 c

d f 2 2
-dxd (h b Uc T2) = u hu uc cose b uc
dr, c 2 ac8 c2





(hbuc T2) = h ua u sine
S2u a c

or in the normalized form as

d H B U) = HU (2.90a)

d (12 H B U2) = H UA U cose pl2 B U2 (2.90b)

I2 BU = U sin (2.90c)
2 d, "A

where UA = Ua/Uo and x. = x,/b has been defined in addi-
tion to the normalized variables in equations (2.39). Two

extra relations arise by relating the cartesian coordinates

S= x/bo and X = y/bo to the curvilinear coordinate E,:

S= sine (2.90d)

dX = cose .(2.90e)

Equations (2.90) constitute five simultaneous ordinary

differential equations to be solved for the sets of five

unknown variables R, B, e, E, X in the ZOFE and U, B, 0,

E, x in the ZOEF. Solutions to equations (2.90) will
be obtained by numerical integration.
The functions T1 and 12 can be obtained by carrying

out the integration in equations (2.37), but this procedure

is slightly different than before since the self-similar

velocity profiles are altered by the co-flowing component

of the cross-current as seen in figure 2.16. Following

Abramovich (1963) and Stolzenbach and Harleman (1971),

the similarity profiles in a co-flowing stream can be

expressed as

u u, cose
u = F(y ) (2.91)
uc ua cos
c a

where the function F(y,) is defined by equations (2.34)

when y is replaced by y*. Thus, it follows from equation

(2.91) that

u u
u- a cose [1-F(yj)] + F(y*)
c c

= cose [I-F(C)] + F() (2.92)

where r = y,/b is the normalized coordinate. Using equation

(2.93) in equations (2.37) yields expressions for T1 and

12. In the ZOFE, making a change of variables as in equa-

tion (2.40) yields

S= B+ [I+(1-II) cos 6] (1-R) (2.93a)

2 B + [12+2(1-12) coso + (1-21 +2) cs28](l-) (2.93b)

where I1 and 12 are constants defined in equations (2.42).

It is worth noting that in the absence of cross-currents

(UA = 0), equations (2.93) reduce to equations (2.41).

The centerline velocity is constant in the ZOFE, therefore

it follows that U = 1 in equations (2.93). The correspond-
ing expressions in the ZOEF can be obtained carrying out

the integration in equations (2.37) using (2.92) and

(2.34b), or by simply setting R = 0 in equations (2.93).

Thus in the ZOEF

T1 = + (1-1I) cos6 (2.94a)

U U2
T +2(11-12) A1 2
2 + 2(1-1^ U cos9 + (1-21+12) cos e (2.94b)

A set of example calculations obtained by numeri-

cally integrating equations (2.90) is displayed in figure

2.19. The equations were solved on IBM 370 using the IBM
Scientific Subroutine Package routine DHPCG. The


B() 10 -


X -10

Figure 2.19

Jet characteristics for jets in cross-currents,
p = 0.02; (a) jet half-width, (b) centerline

routine solves sets of first order differential equations

with given initial values using Hammings Modified Predictor-

Correc:or Method. The cross-current is assumed to be

constant with offshore distance. With increasing magnitude

of the cross-current, the jet bends more and the jet expan-

sion is greatly reduced. The suppression of the jet expan-

sion is due to the addition of momentum by the co-flowing

component of the ambient current. The jet centerline

velocity however does not seem to be influenced to a great

extent and therefore it is not shown.

Finally, photographical evidence for jet bending

is presented in figures 2.20 and 2.21. Wind driven cur-

rents are probably responsible for the gentle curvature

observed in the first picture. In the second photograph,

a shoreline attached jet is observed. The inclination

with respect to the shoreline of the breaking waves indi-

cates the presence of a strong longshore current which

bends over the jet and attaches it to the beach. The

curved jetty on the north side of the inlet may have also

helped the jet bending. The present theory obviously

does not apply to such recirculating flows.

Figure 2.20 Curved jet at Bakers Haulover, Florida (University of Florida,
Coastal Engineering Archives)

Figure 2.21 Strongly curved jet at South Lake Worth Inlet, Florida (University
of Florida, Coastal Engineering Archives)



Effluents from tidal inlets and fresh water outflows

from river mouths usually carry materials in solution or sus-

pension. The diffusion of these materials and the mixing

with the ambient water determine their ultimate distribu-

tion. The material may consist of pollutants or suspended

sediments which may originate in the bay or lagoon areas or

may be entrained into the effluent from the adjacent littoral

system. Therefore a generalized formulation will be used

in this chapter in analyzing the turbulent jet diffusion

processes in shallow water based on the hydrodynamic aspects

studied in Chapter 2. The effects of bottom friction, vari-

able depth, cross-currents, as well as ambient concentra-

tions of material in the receiving water are included.

The theoretical analysis is applied to the diffusion of

pollutants and suspended sediments. In the case of sedi-

ments or other pollutants that may settle, the bottom

accumulation or depletion rates are also predicted. Re-

sults of the theory are then used in studying the mixing
and exchange mechanisms of a bay-inlet-ocean system.

3.1 Shallow Water Diffusion Equation

The conservation of mass of the materials in solu-

tion or suspension is expressed by the diffusion equation.

For a homogenous fluid carrying passive constituents which

experience gravitational settling, the diffusion equation

(Nihoul and Adam, 1975; Csanady, 1973, p. 7 and p. 43) is

ac + uc +uc + wc 2 c +2c +2c Ws (
+ -x + D (z +-+ ) + z (3.1)
Tt ax ay @z -5 { 3 W 3 z

where c is the concentration (in arbitrary units) of the

substance, D is the molecular diffusivity, and ws the

settling velocity. The last term in equation (3.1) ex-

presses the rate of change of the vertical flux due to

settling. When the material in suspension consists of

particles with a density slightly greater than that of

the fluid medium, they settle to the bottom with a terminal

velocity ws. One of the main assumptions used in deriving

equation (3.1) is that the concentration (in units of mass/

volume) of the material in solution is small with respect

to the density of the original fluid; hence, the density

of the mixture is nearly uniform throughout the fluid domain

and the small density variations do not contribute to the

momentum balance in equation (2.2).

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