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LSU Historical Dissertations and Theses
Graduate School
1970
Circulation, Effluent Diffusion and Sediment
Transport: Mouth of South Pass, Mississippi River
Delta.
Lynn Donelson Wright
Louisiana State University and Agricultural & Mechanical College
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Recommended Citation
Wright, Lynn Donelson, "Circulation, Effluent Diffusion and Sediment Transport: Mouth of South Pass, Mississippi River Delta."
(1970). LSU Historical Dissertations and Theses. 1814.
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71-3446
WRIGHT, Lynn Donelson, 1940CIRCULATION, EFFLUENT DIFFUSION AND
SEDIMENT TRANSPORT:
MOUTH OF SOUTH
PASS, MISSISSIPPI RIVER DELTA.
The Louisiana State University and Agricultural
and Mechanical College, Ph.D., 1970
Geography
University Microfilms, Inc., Ann Arbor, Michigan
THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED
CIRCULATION, EFFLUENT DIFFUSION AND SEDIMENT TRANSPORT*
MOUTH OF SOUTH PASS, MISSISSIPPI RIVER DELTA
A Dissertation
Submitted to the Graduate Faculty of the
Louisiana State University and
Agricultural and Mechanical College
in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
in
The Department of Geography
and Anthropology
Lynn Donelson Wright
B.A«, University of Miami, 19^5
M.A., University of Sydney, 1967
May, 1970
ACKNOWLEDGMENTS
This study was supported by the Coastal Studies Institute of
Louisiana State University under contract with the Geography
Programs of the Office of Naval Research, Project No* NONR 1575(03)»
Task Order No* NR 388 002, and by a Gulf Oil Corporation fellowship*
The writer is indebted to Drs. William G* Mclntire and J.M* Coleman
for invaluable advice and encouragement throughout the research pro­
gram and during the preparation of the manuscript*
The writer
wishes especially to thank Dr* J*M* Coleman for field assistance and
for innumerable hours of consultation during which many of the con­
clusions presented herein were conceived.
Pah American Oil Company
made available the crew boat MANTARAY, aboard which the data of May
23-25 were collected.
The writer is grateful to the U.S. Coast Guard
for making available the U.S. Coast Guard Cutter POINT SPENCER during
the period November 11-12, 1969? to the U.S. Army Corps of Engineers,
New Orleans District, for essential data, and to the Louisiana Wild­
life and Fisheries Commission for providing lodging for field parties.
Field assistance, without which the study could not have been com­
pleted, was rendered by Drs. J.M. Coleman, S.P. Murray, C.J. Sonu,
and D.S* McArthur and by F.J• Swaye, D.J. Arndorfer, E. Weisblatt,
J. Peake, and Wm. Hart.
For advice and criticisms in the final prepa­
rations the writer is grateful to Professor R.J. Russell and Dr s. D*
S. McArthur and S.P. Murray.
Dr. M. Young wrote the Fortran IV pro­
gram which was used to calculate mixing and stratification parameters.
ii
TABLE OP CONTENTS
JPage
ACKNOWLEDGMENTS..............................................
.11
TABUS OP CONTENTS....................... ...............
ill
TABLES...
.........
1?
FIGURES..........................................
▼
^
ABSTRACT.........
INTRODUCTION......
1
The Dynamic Environment of the Study Region....... ••••••••
Parameter Definitions and Determinations....... •••••••••••
Data Collecting and Sampling Procedures.•••••••••••••••.•••
6
10
14-
CIRCULATION AND MIXING IN THE LOWER SOUTH PASS CHANNEL.........
17
Channel Geometry....................*..«•••...... •••••••••
Seawater Intrusion..........................................
Hydraulics and Mixing in the Lower Channel.•••••......
Salt-Wedge Intrusion at High River Stage....••••••......
18
18
22
28
VERTICAL STRATIFICATION AND MIXING SEAWARD OP THE MOUTH........
29
Statistical Analyses and Identification of
Salient Control Variables.
;••••••............ •••••••
Bottom Topography, Mixing and Stratification
••••••«••
Two-Factor Interactions............ ••••••••••••......
29
4-3
4-5
HORIZONTAL EFFLUENT, CURRENT AND DISCHARGE PATTERNS............
4-5
General Surface Patterns...................................
Foam Lines and Fronts.
....
••••••••••••••
Implications of Overall Surface Patterns
•••••••••••
The Effects of Tide, Wind and Waves on Surface Patterns.,..
Current and Discharge Patterns.....
.•••••«••••••••••..
45
48
SEDIMENT TRANSPORT PATTERNS
.... ..........
Bed Load......,•••••••••.. . . . . . a . . . . . . ........ ••••••••••••
Suspended Load........
Surface Turbidity Patterns
Flocculation and "Sludge" Formation.•••••••••••••••••••••••
50
52
52
6l
6l
62
69
74
SUMMARY AND CONCLUSIONS...................................
75
REFERENCES, ............. ................
78
ill
TABLES
Table
1»
Page
Summary Statistics and F-Ratios for Mixing,
Stratification and Flow Parameters between
Tidal Ebb and Flood for Stations In South
Pass Channel Near the M
o
u
t
h
o
25
2.
Summary Statistics for Mixing and Stratification
Parameters Seaward of the Mouth.*..*.********•••••••••• 31
3*
Results of Analysis of Variance for Mixing
and Stratification Seaward of the Mouth*•••••••■•....
32
4A* Regression and Correlation Coefficients
with No Lag Effect for Station SP
November 11-12, 1969**••••••••.••.•••••••••*••••*••••. 3^
4B* Regression and Correlation Coefficients
with Three-Hour Lag Effect for Tide for
Station SP 3-4, November 11-12, 1969*..................35
5.
6.
Summary Statistics and Simple Correlation
Coefficients for Discharge Parameters as
Functions of Tide for Station SP 3-^»
November 11-12, 1969........
••••••••.•••.
60
Average Suspended Load Concentrations,
River Stages and Water Temperatures
for Six Observation Periods*!...•*.*•,•••••••••••••••••• 63
4v
FIGURES
Figure
1*
Page
Air photo of the mouth of South Pass showing
South Pass effluent, ambient freshwater band,
and Gulf w
a
t
e
r
*
*
.
.
*
.
.....
3
5
2*
Location map of the study area.
3*
River stage at New Orleans (Carrollton gage)•
A* Average distribution for 19^9-1967*
B. Distribution for the study period....... *.......
7
Average wind and wave regimes of coastal Louisiana.
A* Wind direction and velocity (after Scruton, 1956).
B. Average annual distribution of wave height and
period (after Bretschneider and Gaul, 1956)
C. Average monthly distribution of onshore wave
energy in Mississippi Delta......... ••••••*••••
9
5*
Location of survey lines and sampling stations.... ••••
10
6*
Current and cf+ cross-sectional profile in the
South Pass channel on October 1, 1969* during
flooding tide showing typical salt-wedge intrusion
pattern and associated circulation*.••*•*.••••*•••••••
21
Water surface elevations at Head of Passes and
Port Eads showing variation with tide of hydro­
static gradient*
A*
Low-stage - January 25-27* 1969*
B.
Normal-stage - January 31 - February 2, 1969*
C*
Hlgh-stage - April 19-21, 1969*•••••••••••....
23
Current and density profiles at the mouth of
South Pass (Station SPCG) on October 1, 1969*
A.
Flooding tide.
B*
Ebbing tide......
26
0+ cross sections for line 3 at low river
stage on October 2, 1969*
A*
Ebbing tide.
B* Flooding tide*........... ••••*•••••.......
37
7*
8*
9*
10. a. cross sections for line 2.
A.
Low river stage (flooding tide)
on October 2, 1969*
B.
High stage (ebbing tide) on June 30,1968........
v
38
"Figure
Page
.
11
a. cross sections transverse to the South Pass
effluent (approximately on line 5) at low
river stage on October 3# 19&9*
A. Ebbing tide.
B. Flooding tide..... ..........................
12. at variations with depth and time during
a complete tidal cycle, November 11-12, 1969..... .
13.
Generalized surface effluent patterns
and location of boundaries.
A. Flooding tide.
B. Ebbing tide........... ............ •••••••••••
14.
Two modes of effluent expansion.
A. Turbulent jet diffusion (after
Albertson et ad., 1950)•
B. Lateral spreading as a discrete layer
(Modified by Walsh, 19&9# after Defant, 1961)••••••
15.
Current vectors transverse
and along the distributary
A. Ebbing tide just prior
October 3# 1969*
B. Flooding tide, October
C. Flooding tide, October
to the effluent
mouth bar.
to low water,
3# 19&9*
1, 1969............... .
. Current vectors along line 3*
16
A.
B.
C.
D.
Ebbing tide, October 2, 19^9.
Flooding tide, October 2, 19&9*
Ebbing tide, February 12, 1970.
Flooding tide, February 12, 1970......... •••••
17.
Freshwater and saltwater discharge
vectors along line 3# October 2, 1969.
A. Ebbing tide.
B. Flooding tide............
18.
Freshwater and saltwater discharge vectors
at 1-hour intervals for a complete tidal
cycle at station SP 3-4# November 11-12, 1969......
19.
Vertical distribution of suspended-sediment
concentration at the mouth of South Pass
(station SPCG).
A. High stage, flooding tide, May 30, 1969*
B. Low stage, ebbing tide, June 30, 19&9*
C. Low stage, flooding tide, June 30, 19&9*
D. Low stage, ebbing .tide, February 12, 1969.
E. Low stage, flooding tide, February 12, 1969....
'Vi
Figure
20.
21.
22.
Page
Vertical distribution of suspended-sediment
concentration for line 3» July 1> 19^91
low river stage, ebbing tide, calm seas.• • • • • . • • . 6 6
Vertical distribution of suspended-sediment
concentration for line 3» October 2, 19&9S
low stage, calm seas...............................
67
Vertical distribution of suspended-sediment
concentration for lines 1 and 2, May 23 and 24,
1969} high river stage, rough seas...............
68
23.
Surface distribution of suspended-sediment
concentration at high river stage, April 12, 1969.......70
24.
Surface distribution of suspended-sediment
concentration at low river stage.
A. Ebbing tide, September 4, 1969*
B. Flooding tide, September 3» 19^9* ......
25*
Suspended-sediment concentrations at
the surface along several different
survey lines at high and low s t a
vii
g
e
72
.
.
.
73
ABSTRACT
A study was conducted at the mouth of South Pass, Mississippi
River, to ascertain the influence exerted by interaction between
effluent and ambient fluids; tides; waves; winds; bottom topography
and channel mouth geometry; regional coastal currents; horizontal and
vertical density gradients; and hydrologic regime of the Mississippi
River.
The time required for the tide to propagate up the passes caused
a significant phase lag between the mouth of South Pass and Head of
Passes.
This resulted in downstream hydrostatic gradient being
steepest at ebbing tide and reversed or reduced at flooding tide.
Consequently, downstream flow within the pass, as
well as efflux sea­
ward of the mouth, were swiftest during ebbing tide.
Salt-wedge circu­
lation in the lower reaches of the pass was characterized by upstream
flow at flooding tide and downstream flow at ebbing tide.
Position of
the salt-wedge interface and variations in the degree of interfacial
mixing were explicable in terms of a densimetric Proude number
F
■ u /ygD.
Interfacial mixing took place when F^ > 1.
Increased velocity during
ebbing tide was accompanied by poorer stratification and greater ver­
tical mixing, presumably because of an increase in F^.
Statistical analyses indicated that river stage, tide, sea state,
and position relative to the distributary mouth bar exert a signifi­
cant influence on vertical stratification and mixing seaward of the
mouth.
Stratification was most pronounced during high river stage,
flooding tide, calm seas, and along the effluent axis, and weakened
progressively from the mouth to the bar crest.
Freshwater concentra­
tion was significantly greater during high river stage and calm seas.
viii
An abrupt decrease in the depth of the salt-wedge interface between
the mouth and the bar crest was attributed to lateral spreading of
the upper layer of fresh water.
Ascent of the interface with
proximity to the bar resulted in increased
values, which were
deemed responsible for decreasing stratification.
Seaward of the
bar, stratification was sharp in association with a slower moving
stream of fresh water from outlets to the northeast.
In the fluvial-marine interaction zone immediately seaward of
the mouth, the freshwater effluent spread laterally as a relatively
discrete and homogeneous mass, with lateral turbulent diffusion
between effluent and ambient fluids inhibited by frontal boundaries.
The boundaries appeared to result from convergence of water masses of
different density; were most pronounced during flooding tide and with
light onshore winds; and were dispersed by high winds and rough seas.
Efflux from South Pass, coastal drift of ambient fresh water,
and tides dominated flow tendencies seaward of the mouth.
Maximum
flow velocities occurred near the surface within the main effluent.
The velocity gradient near the eastern boundary of the plume was ex­
tremely steep, with swift flow on the plume side and tranquil flow on
the ambient side, where flow normally converged toward the boundary.
During flooding .tide ambient and subsurface flow had a strong south­
westerly set which augmented deflection of the effluent.
Suspended-sediment concentration varied directly with river
stage and inversely with temperature.
Seaward of the mouth, the
highest sediment concentrations were normally in the upper layer of
fresh water; however, when vertical mixing was augmented by winds or
waves, suspended-load concentrations in the saltwater layer increased.
ix
Surface concentration patterns corresponded closely to the distribu­
tion of effluent fresh water*
Deposition during high stage was indi­
cated by a rapid seaward decrease in suspended-load concentration
along the effluent axis*
At low stage there was very little longi­
tudinal variation in concentration within the plume, where surface
concentrations were typically lower than those of the ambient fresh­
water band, the primary low stage contributor of suspended sediment*
It is concluded that fluid and sediment dissemination at the
mouth of South Pass are associated with an intricately interacting
multiprocess system.
Patterns cannot be adequately explained in
terms of any single process such as turbulent.jet diffusion.
X.
INTRODUCTION
Of the processes which contribute to the formation and develop­
ment of deltas, those which arise from the interaction of river and
sea at the river mouth are the most fundamental.
These processes are
responsible for the formation of the subaqueous delta, a prerequisite
to further deltaic development.
Gilbert (188*1-) stated that "the process of delta formation
depends almost wholly on the following law; The capacity and compe­
tence of a stream for the transportation of detritus are increased
and diminished by the increase and diminution of the velocity."
Though the implication of this statement is grossly oversimplified,
its validity becomes apparent if turbulence is also considered.
Generally, if chemical and biological processes are excluded, the pat­
tern of sediment deposition approximates that by which the transport­
ing forces diminish at the river mouth.
An understanding of river
mouth and prodeltaic morphology hinges, then, on a recognition of the
variations in transporting capacities and the factors which influence
them.
Gilbert (188*0 proposed the simplest model of flow deceleration
at river mouths.
He reasoned that loss of capacity and competence
and consequent deposition at the mouth were the immediate response to
the opposing inertia of the basin water.
This reasoning was based on
the assumption that all of the transporting energy is possessed by the
stream and none by the receiving basin.
Because of its straightfor­
ward a priori rationale, Gilbert's supposition became the classic
theory of delta formation.
In a treatise on the processes active at the mouths of the Missis-
sippi River, Bates (1953a, 1953b) advanced the "jet flow*' hypothesis
of delta formation.
This theory regards effluent momentum as a con­
servative property which is progressively deconcentrated seaward as a
result of turbulent jet diffusion into the ambient (basin) water.
Bates predicted the diffusion pattern to approximate the Tollmien
(1926) free jet.
Since Bates* original work, the jet hypothesis in
various forms has become widely accepted.
Borichansky and Mikhailov
(1966) have offered equations substantiated by laboratory experiments
describing the lateral velocity field and form of a river jet under
various conditions of channel mouth geometry.
Takano (l95^a, 195^b,
1955) has treated the case of fresh river water spreading above denser
seawater.
Jopling (1963, 1965) has carried out experiments on verti­
cal diffusion and flow separation, and Axelson (1967) has reviewed
river mouth jets in general.
Jet theories, like Gilbert's model, assume the basin fluid to be
"still" and such agents as tides, waves, and marine currents, to con­
tribute only to residual trends.
The nonfluvial agencies have usually
been considered only in terms of their ability to rework the deposits
already laid down, and not with regard to their role in controlling
the patterns of initial deposition (i.e., Bates, 1953a, 1953b; Santema,
1966; NEDECO, 1961; Zenkovitch, 1967).
At the mouths of the Mississippi River, circulation associated
with a pronounced vertical density stratification or "salt wedge" acts
in conjunction with the other factors.
Though there has been con­
siderable theoretical and experimental work on salinity-wedge intru­
sion in bounded estuaries, including the passes of the Mississippi
(Bowden, 1967; Ketchum, 1953; Pritchard, 1952, 1955* Schultz and Tif­
fany, 1965), very little has been done on the influence of strong
Figure 1. Air photograph of the mouth of South Pass showing
South Pass effluent, ambient freshwater band, and Gulf water.
stratification on diffusion and circulation seaward of the estuary
mouths.
The framework of the present investigation is the implicit
hypothesis that the transport and deposition of sediment (and hence,
depositional topography) in the vicinity of the mouth of South Pass,
Mississippi River, are direct products of circulation and diffusion,
which, in turn, depend on the following primary factors: (l.) inter­
action between effluent and ambient fluids;
(^.) wind;
(2.) tides;
(3.) waves;
(5 .) bottom topography and channel mouth geometry;
regional coastal currents;
(6.)
(7.) horizontal and vertical density
gradients caused by thermohaline variations; and (8.) hydrologic regime
of the Mississippi River.
This study was aimed at identifying the
influence of the factors listed above, evaluating the relative con­
tribution of each, and elucidating, as far as possible, the mechanisms
by which each plays its role.
The ultimate goal is to permit the
development of a realistic theoretical model explaining spatiotemporal
variations in river mouth and prodeltaic morphology,
The results are based primarily on the writer's field observa­
tions and analyses made over the period February 1969 - March 1970.
D.J. Ouellette's (19^9) field data from the period June 1968 - October
1968 and various published reports provided valuable supplement.
Measurement of sediment transport was limited to the amount of
material in suspension because of the difficulties of studying bed
load.
Also, logistics precluded observations under the gamut of
natural conditions.
No offshore data were collected when seas were
more than moderately rough, and no current measurements (the collec­
tion of which requires anchoring the research vessel) were obtained
MISS.
VENICE
WEST
BAY
EAST BAY
GARDEN
ISLAND
BAY
29° 00'
STUDY,
AR EA
5 Miles
Figure 2*
Location map of the study area*
during high river stage.
It is felt, however, that the data are
adequate to reveal many trends of general significance.
South Pass was selected as the locus of the investigation in
preference to other outlets because of its central situation, large
number of landmarks for positioning, and easier navigability.
The Dynamic Environment of the Study Region
An appreciation of the significance of the river mouth pro­
cesses requires familiarity with the environmental setting.
The
physiography, geology, and geologic history of the Mississippi Delta
have been amply treated (Russell, 193&? Russell and Russell, 1939?
Fisk, 1944; Fisk et al,, 1954? Mclntire, 1954? Kolb and Van Lopik,
1958? Welder, 1959)*
The hydrologic, sedimentological and oceano­
graphic regimes critical in the present study are summarily described
in this section.
The distributions of average river stage at New Orleans (Carroll­
ton gage) for the years 19^9-1967 and for the period of this study are
graphed in Figure 3A and B.
According to Holle (1952), a stage of 3
feet (low stage) on the New Orleans gage corresponds to a discharge
of about 300,000 cubic feet per second.
At normal (9 feet) and flood
(17 feet) stages discharges of about 600,000 cubic feet per second and
1,000,000 cubic feet per second prevail, respectively.
The sediments of the deltaic mass derive from a drainage basin
which covers 41 percent of the continental United States (1,245,000
square miles).
Holle (1952) estimates that on the average the Missis­
sippi River transports roughly 5°0 million tons of sediment to the
Gulf annually.
195*0
Bed load accounts for between 10 percent (Fisk et al.,
20 percent (Holle, 1952) of the total sediment load.
Henry
12
2
-
gl
1
I
I
I
t
,1.,
I ...
I
..I
I
Jon
Fab
Mor
Apr
Moy Jun* July A u g Sepl
Oct
Nov
Doc
Jon
Feb
I
.
Oct
I
-I
Nov
Oac
14
12
Slogs
ot
N««
O ri so ns
|fs st)
10
O Field excursions
b y D. J. O u e l l e t t e
a Field excursions
by the w r i t e r
Apr
Moy Juna July
Aug Sopt
Mor
Apr
Moy iu n « July
Aug Stpt
O d
Nov
Dtc
ion
Fs b
-----------1968---------- ►•--------------- 1969--------------- - *— 1970-*
Figure 3« River stage at New Orleans (Carrollton gage). A. Average
distribution for 1949-1967. B. Distribution during the study period.
(1961) reports that the suspended load typically consists of 40 per­
cent silt, 50 percent clay, and 5 to 10 percent very fine sand,
whereas fine sand predominates in bed load.
At Head of Passes the Mississippi River branches into three
major distributaries: Pass a Lotrtre, South Pass, and Southwest Pass
(Pig, 2).
Thirty-seven percent of the total discharge reaches the sea
by way of Pass a Loutre, 29 percent via Southwest Pass, 19 percent via
various secondary outlets, and 15 percent via South Pass, the locus of
the present study.
Proportionately, South Pass carries an average
annual sediment load of 75 million tons (Holle, 1952).
Much diversity in deltaic and river mouth morphology can be
attributed to geographic variations in the coastal energy regime, par­
ticularly of tide, winds, and waves.
The astronomical tides of the
Mississippi Delta region are diurnalj there is one high and low tide
per day (Marmer, 1954)•
Mean tidal ranges are 11 inches at Head of
Passes and 13 inches at Port Eads (ESSA, 1969* P» 239)•
Continuous
records for Port Eads reveal that tidal range varies from about 6
inches at equatorial tide to about 2 feet at tropic tide.
Tidal
curves are relatively symmetrical, rise and fall having approximately
equal duration.
Inshore along the delta coast tidal currents with
velocities as high as 2 feet per second accompany tropic tides (Drennan, 1968).
For further details regarding the tidal regime refer to
Scruton (1956).
Wind records are available only for Southwest Pass (Burrwood,
Louisiana) and Grand Isle, Louisiana.
Wind patterns at Grand Isle,
Burrwood, and Pensacola (Florida) differ only slightly; hence, the
records for any of these stations should approximate the situation at
9
6
GRAND ISLE, LOUISIANA
W-
PENSACOLA. FLORIDA
BURRWOOD. LOUISIANA
1944, *45, *47. '4 0
I9 Z 4 --4 S
I9 4 2 ~ *4 9
W!
C
s
s
ARROWS INOlCATE WIND VELOCITY
NUMBERS INOlCATE MONTHS
0
s
1
1
---- «0
1
Vpn
j a 111 , J w
MtlCHt
»C*00
Mt'&xr
’dies
t'T
<*•
.03
o
€
HV'.-I
*NVV
•W
BURRWOOD , LOUISIANA
■sw
— i oo —n -----------
1
Ik. r Ht»ica
r--‘\u
Jan
F ib
M ar A pr
M o y J u n i J u ly A g g S t p l
O el
Nov 0 • c
B
Figure
Average wind and wave regimes of coastal Louisiana. A. Wind
direction and velocity (after Scruton, 1956). B. Average annual dis­
tribution of wave height and period (after Bretschneider and Gaul, 1956).
C. Average monthly distribution of onshore wave energy in Mississippi
Delta.
South Pass. Monthly mean wind direction and velocity are indicated in
Figure ^A, which shows that winds from the easterly sectors dominate
throughout the year.
Average wave characteristics for each octant (Fig. 4b ) are based
on hindcasting computations for Southwest Pass (Burrwood, Louisiana)
for 1950, 1952, and 195^ by Bretschneider and Gaul (1956)•
Figure 4C
shows the average monthly distribution of onshore (northeast to south­
west) wave energy computed from the same set of data.
Aside from
observations of wave direction, height, and period by the U.S. Army
Corps of Engineers (1959) during the period January - October 1957*
no long-term observations of wave characteristics for the Mississippi
Delta are available.
Parameter Definitions and Determinations
The analyses of stratification, mixing, discharge, and sediment
transport presented in this study require quantitative indices to per­
mit objective comparisons.
Indices of stratification, mixing and dif­
fusion, and discharge are defined and their relevance is discussed in
this section.
Stratification of a fluid is expressed in terms of the concen­
tration gradient and range of any scalar property.
Vertical stratifi­
cation in areas of freshwater and saltwater interaction primarily
results from gravitational differentiation according to density (Far­
mer and Morgan, 1953? Ketchum, 1953? Williams, 19&2, pp» 150-158).
For this reason, density, the most functional and causally meaningful
parameter for describing stratification, is used conventionally (e.g.,
Sverdrup et al., 19^2; Defant, 1961).
Density varies directly with
salinity and inversely with temperature and may be readily computed
from data for the two.
In oceanographic studies, density is conven­
tionally expressed as cr^ (sigma-t) to eliminate unnecessary digits,
Slgma-t is related to the water density by
<j ® (density - 1 ) x 1000
(Defant, 1961, p. 4l).
values are determined from tables (U.S.
Naval Oceanographic Office, 1966) or from a set of formulae found in
any standard textbook on oceanography (e.g., Defant, 1961, P»
Sverdrup et al., 19^2, p. 56).
The stability of an interface between layers in a stratified flow
is often indexed by a densimetric Froude number F^ given by
A
Fj * u /ygD
(Bowden, 1967 ? Farmer and Morgan, 1953? Stommel and Farmer, 1952)
where u is the mean velocity of the upper layer, g the acceleration
of gravity, D the depth of the interface, and y the density ratio
Y " (P3~pf)/ps
where
P - density of the salt (lower) layer and p~ = density of the fresh
S
X
(upper) layer.
When F^ exceeds 1 (i.e., when u is greater than the
celerity of an internal gravity wave), interfacial waves form and
break and mixing ensues (Bowden, 1967* Farmer and Morgan, 1953? Stom­
mel and Farmer, 1952).
Stability is indicated by low values of F^,
which for given values of u and D varies inversely with y » the density
ratio.
In cases where current measurements are lacking, the value of
Y serves to gage the relative degree of stratification.
All values of y in this study are computed from the weighted mean
densities above and below the interface, the interface taken as the
depth of maximum a
gradient (Acf^/A2) ma3C or*
Y - (Ps - Pf)/P3
12
j -i
and
j »n
bottom
j denotes the interval, i and i+1 the interval immediately above and
immediately below the interface respectively, and z the depth.
In addition to indicating the depth of the interface, (AcX^/ A z)
max also serves as a measure of pycnoclinal sharpness.
Most theoretical and experimental studies of diffusion employ
the coefficients of eddy diffusivity and eddy viscosity.
Determina­
tion of these parameters involves complex procedures, numerous
theoretical assumptions, and quantities not obtainable by the methods
of the present study.
For these reasons the writer has devised more
easily determined indices of the degree of mixing.
These indices are
intended to facilitate comparison only, not theoretical analyses.
For practical purposes the salinity of pure river water is 0.00
o/oo, while pure seawater has a salinity in the vicinity of 36.0 o/oo.
In any zone of mixing between freshwater and saltwater masses, the
freshwater concentration G may be readily determined from
C - 1 - (s /s q)
(Bowden, 19&7) where S is the observed salinity at the point in ques­
tion and S
o
is the salinity of pure seawater (in practice, the maxi-
mum salinity observed in deep water during the particular period of
observations).
At any instant the total volume of fresh water Q present in a
column of unit horizontal dimensions and thickness h is equivalent to
z *=h
z -0
The total freshwater volume is approximated by
where j refers to the interval.
The freshwater volume Q . contained in any depth interval is estimated
'3
by
The fraction Q „ . of the total freshwater volume contained within an
interval is
Qf 3 “ V
Qf
The total weighted concentration of fresh water in the entire column
is then given by
C - 2 C Q .
j 3 *3
The maximum value of unity for this quantity indicates complete
integrity for the fresh water present in the column and hence no mix­
ing between freshwater and saltwater masses.
Extremely low values
suggest a high degree of freshwater deconcentration.
For unidirectional flow the discharge through a given section of
channel or ocean is the product of the area of that section and the
velocity.
All discharge values herein are expressed in cubic feet per
second (cusecs.).
For a vertical section of unit width the total dis­
charge M. is
t
,
z =n
where u is the velocity in feet per second.
estimated from
the discharge of fresh water from
In practice M may be
x>
and the discharge of salt water from
M - S u [ ( A z) - Q J •
8
i 3 '
J
0
Seaward of the mouth, directions of flow generally exhibit con­
siderable variation throughout the vertical.
In this case it is
expedient to compute discharge values for each direction and then sum
the vectors so obtained.
All vector sums are resolved graphically by
vector resultant diagrams.
Data Collection and Sampling Procedures
Water salinity and temperature were determined in the field with
a Beckman portable induction salinometer.
Prior to field trips this
instrument was calibrated in the laboratory against salt solutions of
known concentration, and all values were corrected to correspond to
calibration curves.
As an added check, occasional water samples were
titrated with silver nitrate (AgNo^).
percent.
The maximum error was about 5
Other parameters were computed on an IBM J60 computer from
salinity and temperature data.
A Marine Advisors Roberts-type Model B-3c current meter was used
to obtain current velocity and direction.
On one field excursion this
meter was checked against a Marine Advisors Q-15 meter, and the meters
showed close agreement.
Water samples collected in the field for suspended-load analysis
(by means of a 1-liter Van Dorn-type water sampler) were transferred
to 1-quart waxed cardboard containers, sealed, labeled, and returned
to the laboratory for analysis.
In the laboratory a Millipore 100-ml pressure filtering apparatus
and Millipore 47-mm filters with 0,45 4 pores were used to extract
suspended sediments from water samples.
Filters were oven dried at
90*?J and weighed to the nearest 0.0001 grams on a Mettler analytic
balance.
In most cases 400 ml of sample was passed through the fil­
ters; but only 200 ml (and on a few occasions only 100 ml) of highly
turbid samples was used.
liter (mg/l).
All values were expressed in milligrams per
Numerous samples were run in duplicate to determine the
error of the method: 5 percent for highly turbid samples and less than
10 percent for the least turbid.
Wind velocities were measured with
a portable hand-held anemometer.
Sea states were estimated by visual
observations and by a Raytheon fathometer equipped with a buoy-mounted
transducer.
Tidal elevations and rates of fall or rise are from con­
tinuously recorded tide gage records for the mouth of South Pass fur­
nished by the U.S. Army Corps of Engineers, New Orleans District.
Offshore stations were positioned by adjacent horizontal sextant
angles between fixed shore objects and buoys.
To facilitate navi­
gation and reoccupation of sampling stations (Fig. 5)» each survey
line was selected to be coincident with a range line on a pair of
fixed objects.
are:
Respectively, the range pairs for lines 1 through 5
(l.) East Jetty Light - Outer Buoy;
Inner Buoy;
(2.) East Jetty Light -
(3») West Jetty Front Marker - Pilot House;
Guard Radio Mast - Pilot House;
(4.) Coast
(5.) Inner Buoy - East Oil Platform.
Line 4 was discontinued following Hurricane Camille (August 17)
because the radio mast was destroyed.
The width of the South Pass channel at the mouth (700 feet) was
chosen as the distance unit to provide a dimensionless measure for
future comparison with other deltas.
Stations 1 through 8 on each
S o uth P a u
Reor lig h t
Sa mpl i ng Stations
1 00 0
1300
3000
Ft.
.,3
Figure 5*
*17
Location of survey lines and sampling stations
survey line were situated at 1, 2, 4, 7, 11, 16 , 22, and 29 channel
widths from the mouth, respectively.
This expanding grid provided
maximum detail in the zone near the mouth and over the bar, where
horizontal variations are most critical.
Other stations were occupied
on the basis of the ephemeral position of water mass boundaries.
In most cases, salinity and temperature were measured at the sur­
face and at depth intervals of 1 foot for the first 10 feet, at 5foot intervals between 10 and 30 feet, and at 10-foot intervals below
30 feet.
Current velocity and direction were measured at the surface
and at 5“foot intervals for the first 30 feet.
measurements were at 10-foot intervals.
Below 30 feet current
Occasionally, when critical
changes were anticipated within a small depth range, currents were
measured at 1-foot intervals.
Water samples were collected at the sur­
face and at 5-foot intervals for the first 30 feet.
Below 30 feet
sampling was at the observer's discretion.
Least-squares maximum likelihood general purpose and multiple
regression canned programs from the L.S.U. program library were used
in making statistical analyses on the L.S.U. IBM 360 computer.
Com­
putational details are presented in Harvey (1966) and Krumbein and
Graybill (1965, pp. 277-299 and pp. 391-399).
CIRCULATION AND MIXING IN THE LOWER SOUTH PASS CHANNEL
Fluid and sediment dissemination patterns seaward of the mouth
are partially predetermined by events within the confines of the pass
itself.
Mixing and interaction of river water and seawater begin up­
stream from the mouth, particularly at low stage.
As a result of
estuarine processes operative in the lower reaches of the channel,
vertical distributions of current velocity and fluid density at the
jettied mouth differ substantially from those at Head of Passes, 13
18
miles upstream*
Channel Geometry
South Pass has a total length of 13*5 miles from Head of Passes
to the mouth.
Channel width varies only slightly throughout most of
the length, averaging 785 feet (U.S. Army Corps of Engineers, 19^1).
The maximum width is 1,000 feet at Head of Passes; the minimum is 700
feet at the jettied mouth.
The mean channel depth is 35*^ feet with
a maximum of *K) feet and a minimum of 35 feet just below Head of Passes
and at the mouth, respectively.
The channel cross-sectional area
ranges from 35*000 square feet at Head of Passes to 25*000 square feet
at the mouth with a mean of 26,880 square feet (U.S. Army Corps of
Engineers, 19^1)*
The decrease in cross-sectional area at the mouth
is the product of artificial control (jettie$> coupled with a tendency
for the channel to shoal slightly downstream.
Immediately seaward of the mouth the bottom shoals abruptly in
the form of a distributary mouth bar, and within a distance of about
one-half mile water depths on a seaward extension of the channel axis
are as shallow as 15 feet or less.
For navigation the U.S. Army
Corps of Engineers maintains a dredged channel 35 feet deep across
the northeastern portion of the bar.
Seairater Intrusion
On the basis of the degree to which the discharging fresh water
mixes with the undiluted seawater, three main types of estuarine cir­
culation are generally recognized; the we11-mixed estuary in which
salinity exhibits little or no vertical variation; the partially mixed
estuary lacking steep gradients but having a large vertical range of
salinity around the mean; and the highly stratified estuary where
freshwater and saltwater layers are separated in the vertical by a
well-defined interface (Bowden, 1967? Ippen and Keulegan, 1965?
Pritchard, 1955? Schultz and Tiffany, 1965)*
The type (and degree) of
Circulation pattern depends on the ratio of the freshwater volume dis­
charged during one tidal cycle to the tidal prism (the total volume of
water exchanged owing to tidal flow between high and low tide) (Bowden,
1967> Ketchum, 1953? Simmons, 1952).
When this exceeds unity, the
highly stratified or salt-wedge type estuary will usually prevail
(ippen and Keulegan, 1965? Pritchard, 1952, 1955)•
The dynamics of the salt-wedge estuary have received considerable
attention (Farmer and Morgan, 1953? Bowden, 1967? Farmer, 1951? Stommel and Farmer, 1952? Keulegan, 19^9* 1955> 1965? Ippen and Keulegan,
1965? Bondar, 1963> 1967» 1968? Graya, 1951? Hamada, 1959? Harleman,
1961, 1965? Harleman et, al,, 1962? Ketchum, 1953? Macagno and Rouse,
1961, 1962? Otsubo and Fukushima, 1959? Pritchard, 1952, 1955? Schultz
and Simmons, 1957)«
These studies show that when the average velocity
of the discharging stream is below a critical value the density dif­
ferential (about 2 percent) between river water and sea water enables
the latter to underflow the former.
Intrusion proceeds upstream until
the pressure created at the salt-wedge interface by seaward flowing
river water suffices to balance the hydrostatic pressure arising from
the density contrast.
Two-layered flow in which salt water entrained
by the downstream flow of the surface layer is replaced by upstream
flow within the lower layer typically distinguishes salt-wedge estu­
aries.
Mass exchange is almost exclusively upward.
In Southwest
Pass, another distributary of the Mississippi, upstream flow prevails
within the wedge regardless of tidal phase (Henry, 1961? U.S. Army
Corps of Engineers, 1959).
The depth of the saltwater-freshwater interface and the average
velocity of the upper layer mutually adjust to yield a densimetric
/
Froude number F^ at the mouth equal to or slightly less than unity
(Bowden, 196?j Stommel and Farmer, 1952? Farmer and Morgan, 1953?
Bondar, 1967)•
Internal waves form at the interface when F^ > 1.
With further velocity increase these waves break, causing interfacial
mixing and erosion of the upper part of the salt wedge.
This erosion
diminishes the instantaneous salt-wedge volume until stability is
attained by reestablishment of the interface at a depth where F^ ^ 1 .
Upstream from the mouth, F^ progressively decreases to a minimum at
the toe of the wedge.
The requirements for the highly stratified situation are met with­
in the distributaries of the Mississippi River, whose mouths consti­
tute
classic salt-wedge estuaries (Bates, 1953a, 1953b? Holle, 1952?
Scruton, 1956? Simmons, 1952).
At mean low stage (3 feet on the New
Orleans gage) the toe of the wedge is typically situated at Head of
Passes (Holle, 1952).
Gulf water with salinities as high as 2? 0/00
has been observed at New Orleans at abnormally low river stages (Free­
port Sulphur Company, unpublished data).
Salt-wedge intrusion is
favored by maintenance of dredged channels in South and Southwest
Passes,
ages
Excessive intrusion is by way of Southwest Pass, which aver­
about 10 feet deeper than South Pass.
Figure 6 shows density and velocity distribution patterns and
densimetric Froude numbers in the South Pass channel for October 1,
1969* during flooding tide ( 3-k hours after low water).
was at 4 feet on the New Orleans gage (low stage).
River stage
An unavoidable
Coast Guard
S t a t i o n (Mouth)
c
C o n to u rs
in
at
25
30
Froude
number
35
•n
-n
40
o
In
o2.0
3. 0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
CD
1 2. 0
o
>o
13. 0
M i le s b e l o w H e a d o f Passes
Figure 6. Current and <j^ cross-sectional profile in the South Pass channel on October 1* 19&9*
during flooding tide showing typical salt-wedge intrusion pattern and associated circulation.
(Circled dot indicates null flow.)
time lag (nearly 4 hours) between observations at Head of Passes and
at the mouth precluded quantitative description of longitudinal vari­
ations in velocity and discharge; but the overall patterns are evident.
*Flow is downstream at all depths upstream from the toe of the wedge.
Upon encountering the wedge, seaward flow is concentrated above the
Interface.
Downstream from the toe, the thickness of the wedge and up­
stream velocity within the wedge increase.
Saltwater entrainment by
the freshwater layer is evidenced by the downstream currents in the
upper portion of the wedge.
This general pattern, which is consistent
with the ideal salt-wedge case, typifies South Pass during low and nor­
mal stage and flooding tide.
Hydraulics and Mixing in Lower Channel
Water-surface elevations (relative to mean Gulf level) at Head of
Passes and Port Eads (near jetties) for three ^8-hour periods represen­
tative of low, normal, and high river stage are shown in Figure 7*
In
all three of the examples Head of Passes experiences a tidal phase lag
of 1-2 hours behind Port Eads; this lag is attributable to the time
required for the tidal wave to propagate up the passes.
Consequently,
hydrostatic gradient within the channel is highly influenced by tidal
phase.
At low river stage the direction of the gradient reverses with
the tide (Fig. 7)' during flooding tide water-surface elevations at
Port Eads exceed those at Head of Passes by an average of about 0,3
foot; at ebb, the surface is higher at Head of Passes by approximately
O.Jj- foot.
With increasing stage height the upstream declivity attend­
ing flooding tide decreases; the downstream gradient is augmented dur­
ing ebb.
High stage results in downstream gradients throughout the
tidal cycle, and elevation differences vary from a minimum of 0.5 foot
during flood to a maximum of 1.7 feet during ebb.
Janu ary 2 5 -2 7 , 1969
S l a g * a l N e w O r l e a n s = 3 . 8 ft. (Low s t a g e )
O
* l
H e a d o f Passes
o
.a
a
-Port Eads
Nown
J a n u a r y 31- F e b r u a r y 2, 1 9 6 9
S t a g e a t N e w O r l e a n s = 7 . 5 ft. ( N o r m a l s t a g e )
o
*
>
o
-O
O
«•
Ik
jB
H o u r*
A p r i l 19-21, 1969
S t a g e a t N e w O r l e a n s = 11.7 ft. ( H i g h s t a g e )
A p r . 19
A p r. 20
A p r . 21
2
*OO
2
1
Hown
Figure 7. Water surface elevations at Head of Passes and Port Eads
showing variation with tide of hydrostatic gradient. A. Low stage
January 25-27, 1969. B. Normal stage - January 31 - February 2,
1969* C. High stage - April 19-21, 1969*
Because of variations in hydrostatic gradient, river stage and
tide exert a high degree of control on current and discharge within
the channel.
In turn, flow affects mixing and the degree of stratifi­
cation between freshwater and saltwater layers.
Variations in mixing,
stratification, and flow parameters are evidenced by observations made
just upstream from the mouth of South Pass during the periods May 2325, June 28 - July 1, and September 30 - October 3»
Table 1 lists
summary statistics and P-ratios for variation with tidal phase of
weighted concentration of fresh water ( c ) , density ratio (^ ~(j)/
depth of interface, densimetric Froude number, freshwater discharge
(per foot of surface), and the ratio of freshwater to saltwater dis­
charge.
This table indicates the significant role played by tide in
determining freshwater discharge, the ratio of freshwater to saltwater
discharge, the depth of the salt-fresh interface, and the density ratio.
Figure 8 shows typical low-stage density (0^) stratification
situations and current velocity profiles for flooding and ebbing tides
at station SPGG (near Coast Guard Station) 200 yards upstream from the
mouth.
The close spacing of the isopycnals during the flooding phase
signifies a high degree of stratification, and a sharp interface is
evident at a depth of 13 feet.
Downstream velocities in the upper
layer decrease progressively with depth from a maximum of 3*0 feet per
second at the surface to a null point at the interface, below which
flow is upstream.
stream only.
During ebb, flow is considerably swifter and is down­
The halocline is much more diffuse, and the maximum den­
sity gradient is at 18 feet.
Weighted means disclose that freshwater
discharge during ebb is approximately twice that of flood.
This is a
consequence of the consistently observed tendency for downstream
Table 1.
Summary Statistics and F-Ratios for Mixing, Stratification and Flow Parameters
between Tidal Ebb and Flood for Stations in South Pass Channel near the Mouth
Flooding
Tide
Ebbing
Tide
Degrees
of
Freedom
n
Weighted
Mean
in
17
13
.666
.0180
15.65
.0486
.0020
4. 50
oy
9
17
9
•
Total n
Overall
Mean
Standard
Deviation
-0
CD
00
Depth of
Freshwater
Fresh/salt
Density
Interface
Discharge
Discharge
C__________ Ratio____ _____(feet)_______ ^i,_____ (cusecs)
.,
____ Ratio
.181
6
.686
.0189
8
8
.644
.0169
Tides
1
1
1
1
Error
15
15
15
11
n
Weighted
Mean
F
3.753 NS
5.366
12.89
8
18.75
12.234**
.720
7
.846
1.64 NS
13
.
29.15
11.72
6
19.17
7
37.71
13
2.35
.716
6
2.88
7
1.90
1
1
11
11
22.713**
...
11.347**
**Significant at .01 level (highly significant)? ^Significant at .05 level (significant); NS Not significant
n = number of observations for the indicated class, i.e., n = 9 for flooding tide signifies that the
weighted means for flooding tide are based on 9 observations. Weighted means of dependent variables are
presented for each tidal phase (subclass); i.e., the weighted mean of .686 for flooding tide is the mean of
all observations of C made during flooding tide with correction for unequal subclass n.
Upstream
Downstream
-rB'
c
>20
a.
a
20
20
24
20*
■
24
28
Current velocity
i.o
0
1.0
2.0
C u rren t velocity
3 .0
4 .0
i.o
1.0
To
4.0
Ft/sec
Figure 8. Current and density profiles at the mouth of South Pass (station SPCG) on October 1, 1969*
A. Flooding tide. B. Ebbing tide. (Based on current measurements at 5-foot depth intervals and
salinity-temperature measurements at 1-foot depth Intervals.)
27
velocities as well as the volume of the upper layer to increase sub­
stantially during ebbing tide.
The higher velocity accompanying ebb tide accrues from the
'increase in downstream hydrostatic gradient#
With an increase in
velocity following high water, densimetric Froude number values should
increase initially since the upper boundary of the salt wedge will at
first be situated at a shallow depth inherited from the previous flood
phase,
Densimetric Froude numbers in excess of unity were recorded
during ebbing tide on two separate occasions.
With the development of
interfacial turbulence, exchange between the saltwater and freshwater
layers reduces the sharpness of the interface and probably accounts
for the significant decrease in density ratio (Table l).
The insigni­
ficant F-ratio (Table l) for the densimetric Froude number suggests a
tendency for conditions to adjust mutually so as to keep this parameter
approximately constant.
With an increase in velocity this constancy
is maintained by an increase in the depth of the interface (thus, the
highly significant F-ratio for depth of interface).
The highly significant tendency for the ratio 6f freshwater dis­
charge to saltwater discharge to be lower during ebb tide (Table l)
reflects greater seaward transport of salt water.
Figure 8 implies
that this increase is due both to augmented saltwater entrainment by
the upper layer and to downstream flow within the salt wedge.
The
writer has observed downstream velocities as high as 3*5 feet per second
in the lower portion of the salt wedge during ebbing tide (June 30,
1969)«
(Such flow would eventually remove the wedge entirely from the
pass; however, it appears that at low river stage restabilization of
the interface or flooding tide precedes complete flushing,)
28
Salt Wedge Intrusion at High River Stage
Under natural conditions (i.e., in the absence of dredged chan­
nels) the strong high-stage flow should hold the salt wedge outside
the mouth throughout the tidal cycle; Scruton (1956) reports that this
is, in fact, the case for Pass a Loutre, where the outlet is unaltered
by man.
According to Henry (1961, p. 25), however, in Southwest Pass
at high stage the toe of the wedge is situated just seaward of the jet­
ties at ebb tide but, permitted by artificial deepening, advances up­
stream about 1 mile during flood tide.
Because of the logistic diffi­
culties of operating in the channel under high discharge conditions,
the writer was able to obtain only two salinity profiles and no
velocity measurements upstream from the jetties at high river stage.
Salinity-temperature measurements at station SPCG during flooding tide
on May 23» 19&9 (river stage 10.5 feet at New Orleans), disclosed the
presence of the salt-wedge Interface at a depth of 12 feet.
Stratifi­
cation was strong (density ratio = 0 .0229) and the discharging fresh
a
water was highly discrete (C = 0.793)*
May 2b, again during flooding tide.
A similar situation existed on
Measurements made at the same
station by Ouellette (19&9) on June 20, 1968 (river stage 12.3 feet at
New Orleans), during ebbing tide indicated no saltwater intrusion at
any depth.
In South Pass two-layer circulation of the classic salt-wedge type
seems to be primarily a flooding tide phenomenon.
The significant vari­
ations with tidal phase of the degree of stratification and the volume
and direction of flow indicates that, contrary to the conclusions of
previous investigators (Bates, 1953a, 1953b; Simmons, 1952; Walsh, 1969) >
tides appreciably control circulation and mixing in South Pass.
Among
the distributaries of the Mississippi River, this degree of tidal in­
fluence is unique to South Pass? the greater depth of Southwest Pass
permits upstream flow within the salt wedge during all phases of the
tide (U.S. Army Corps of Engineers, 1959? Henry, 1961), whereas in
Pass a Loutre the time lag between the occurrence of a particular tidal
phase at Head of Passes and at the mouth is practically negligible
(ESSA, 1969, P. 239).
VERTICAL STRATIFICATION AND MIXING SEAWARD OF THE MOUTH
Seaward of the lateral restriction of the lower South Pass chan­
nel, river water continues to flow seaward, overriding the wedge of
denser, saline water.
Initially, the degree of efflux stratification
is a product of processes active within the channel.
Beyond the shel­
ter of the jetties, however, innumerable marine and riverine processes
interact to further influence vertical stratification and mixing.
The
rates at which stratification decreases in sharpness and freshwater and
saltwater volumes are exchanged across the vertical zone of contact
fundamentally determine the dispersion of effluent momentum and the
rapidity with which marine processes gain dominance of fluid and sedi­
ment transport.
Statistical Analyses and Identification of Salient Control Variables
Analyses of covariance were performed separately on weighted fresh­
water. concentration (C), density ratio ( P - P ) / P » depth of the inters
f
s
face, and maximum density (<j ) gradient as dependent variables with
w
distance (in channel widths) seaward of the mouth serving as a con­
tinuous independent variable and river stage, tidal phase, sea state,
and position relative to the distributary mouth bar as treatments.
The analysis of variance design was completely random with factorial
arrangement of treatments.
Weighted means and number of observations
of the response variables for each primary class are presented in
Table 2; F-ratios and significance levels from the analysis of variance
are given in Table 3*
The analyses are on data collected by Ouellette
(1969) during the periods June 20, 1968, July 24-25# 1963, September
4-5, 1968, and October 5-7# 1968? and by the writer during the periods
May 23-26, 1969> June 30 - July 2, 1969* September 30 - October 3* 1969#
and February 12-13, 1970.
The data may be regarded as representative
of a wide range of environmental conditions.
In addition, station SP 3-4, located near the crest of the dis­
tributary mouth bar (Fig. 5)» was occupied for 26 hours (one complete
tidal cycle) on November 11-12, 1969# and complete sets of observations
were taken at hourly intervals.
River stage was low (2.5 feet at New
Orleans) and tides were tropic.
Winds were light and seas were calm
at the start of the survey, but wind velocity and sea state increased
progressively throughout the period.
A least-squares multiple regres­
sion analysis related the dependent variables
weighted freshwater con­
centration (c), density ratio, maximum a. gradient, and densimetric
Froude number to height of tide (feet), rate of rise or fall of tide
(feet/hour) (where rate of rise is assigned a plus value and rate of
fall a minus value), wind velocity (m/sec), and wave height (feet)
(Table 4A).
The results of the same analysis with the introduction of
a 3-hour lag for height and rate of rise or fall of tide are shown in
Table 4B.
(A lag effect was anticipated from the fact that minimum
current velocities of 0.0 feet per second were not observed until 3
hours after high water on November 11.)
Several important relationships emerged from these analyses.
Table 2
Summary Statistics for Mixing and Stratification
Parameters Seaward of the Mouth
„
Density
Depth of
(C)_____ Ratio Interface (ft.)
Ag /Az
■...... V"
Total n
Overall mean
Stand, dev.
138
0.54-8
0.1918
138
0.0138
O.OO55
138
8.88
5.794
138
4.095
2.886
High
River
Stage
Low
River
Stage
n
Weighted mean
41
0.6?4
41 ‘
0.0157
41
11.096
4
3.205
n
Weighted mean
97
0.422
97
0.0119
97
6.673
97
4.984
Flooding
Tide
n
Weighted mean
58
0.539
58
0.0168
58
8.730
58
4.875
Ebbing
Tide
n
Weighted mean
80
0.55.6 ....
80
0.0109
80
9.039
80
3.315
Seas
Calm
n
Weighted mean
66
0.606
66
0.0146
66
7.874
66
5.247
Seas
Rough
n
Weighted mean
72
0.490
72
0.0131
72
9.895
72
2.942
Line
1
n
Weighted mean
16
0.514
16
0.010
16
11.431
16 •
3.296
Line
2
n
Weighted mean
36
0.562
36
0.0163
36
8.296
36
4.709
Lines
3 & 4
n
Weighted mean
57
0.557
57
0.0167
57
8.520
57 '
4.415
Line
5 .
Shore­
ward of
bar crest
Seaward
of
bar crest
n
Weighted mean
29
0.559
29
0.0123
29
7.291
29
3.960
n
Weighted mean
78
0.541
78
0.011
78
7.358
78
2.940
n
Weighted mean
60
0.525
60
0.0166
60
10.411
60
5.250
Weighted means are corrected for unequal subclass n and for the
effect of the continuous independent variable (distance seaward)•
. Table 3
Results of Analysis of Variance for Mixing
and Stratification Seaward of the Mouth
Source of
Variation
River Stage
g Tide (flood
5
or ebb)
8
Seas (rough
£?
or calm)
a
....
e
1
^ Line Number
u,
e
o
£
S
n
<D
d.f.
1
(5)
41.208**
F - Ratios
Depth
of
Density
Interface
Ratio
7 .31*4**
Aot/Az
7*794**
5.222*
0.031 NS
3.253 NS
1
0.138 NS
1
6.509*
0.940 NS
1.204 NS
6.482*
14.671**
3
0.387 NS
5.422**
1.449 NS
0.694 NS
Shoreward or
Seaward of Bar
Crest (bar pos»)
1
0.122 NS
14.561**
3.433 NS
8.140*
River Stage Tide
1
3.726 NS
31.547**
5.399*
6.743*
River Stage Sea State
1
0.011 NS
5.039*
1.232 NS
3.606 NS
River Stage Line Number
3
0.979 NS
0.872 NS
1.405 NS
0.162 NS
River Stage Bar Position
1
0.939 NS
5.367*
0.202 NS
Tide Sea State
.
1
0.668 NS
0.959 NS
0.859 NS
0.611 NS
3
0.066 NS
0.716 NS
0.888 NS
2.787*
1
0.983 NS
3.459 NS
0.008 NS
0.050 NS
Sea State Line Number
3
1.982 NS
5.139*
1.533 NS
3.830*
Sea State Bar Position
1
0.675 NS
1.383 NS
3.014 NS
3.580 NS
21.568**
t Tide M Line Number
8
t Tide rS Bar Position
CM
Table 3
(continued)
Source of
Variation
d.f.
Line Number Bar Position
3
# Distance Seaward
of Mouth (linear) 1
Error
111
Total
(uncorreoted)
138
( 5 ) ...
0.317 MS
25.71^*
** Significant at .01 level
*
Significant at .05 level
NS Not significant
#
Continuous variable
. F - Ratios
Deoth
of
Density
Interface
Ratio
Acrt/Az
0.217 MS
0.139 NS
0.561 NS
2.859 NS
7.963**
0.398 NS
Table 4a
Regression and Correlation Coefficients with No Lag
Effect for Station SP 3“^» November 11-12, 19^9 •
n » 26
O
C
0.0136
6.327
1.377
Stand, dev.
0.103
0.0033
3.368 '
1.353
*0
0.414
0.0077
Wind
velocity
(m/sec)
Wave height
(feet)
b
2
NS
r
0.33^ NS
b
0.422**
r
0.850**
0.0028**
0.691**
NS
12.048
3.783
NS
-1.128**
0.613**
NS
0.329 NS
-0.177 NS
-0.683**
NS
0.087 NS
b
NS
NS
-1.213**
NS
r
O.O96
-0.454*
-0.649**
0.385*
b
NS
NS
r
R
*i
0.416
Rate of tidal
rise or fall
(feet/hour)
2
Act/A z
Mean
Height of tide
(feet)
R
Density
Ratio
0.021 NS
NS
-0.39***
NS
-0.632**
0 .512**
before deletions
0.766**
0.579**
0.523**
0 .521**
after deletions
0.721**
0.475**
0.421**
0.*j66**
** Significant at .01
level
b Partial regression coefficient
*
level
r Simple correlation coefficient
Significant at .05
NS Not significant
o
R Coefficient of determination
All b values shown were computed after deletion of insignificant effects.
Table 4B
Regression and Correlation Coefficients with Three-Hour Lag
Effect for Tide for Station SP 3-4* November 11-12, 1969
n - 26
c
Density
Ratio
0.522
0.017
12.048
3.435.
NS
NS
-0.970**
a
Height
of tide
(feet)
(-3 hovers)
Rate of
tidal rise
or fall
(feet/hour)
(-3 hours)
Wind velo­
city
(m/sec)
b
-0 .005**
r
-0.297 NS
0.367 NS
b
0.399**
0.0099**
r
0.738**
0.666**
b
r
Wave height b
(feet)
r
2
R before
deletions
R after
deletions
NS
0.096 NS
NS
0,021 NS
Aff+./ A z
*i
0.629**
-2.306*
NS
0
-0.621**
.
NS
-0.413*
-0.00064*
-1.213**
NS
-0.454*
-0.649**
0.385*
NS
-0.394*
NS
NS
-0.632**
0.512**
0.687**
0 .586**
0.541**
0.514**
0.682**
0.564**
0.421**
0 .500**
36
Explicitly, spatial location, river stage, tidal phase, and sea state
were found to affect profoundly vertical mixing and stratification.
A
close relationship between stratification and situation relative to
the distributary mouth bar was also observed.
Weighted freshwater concentration exhibited a highly significant
decrease (Table 3) with increasing distance seaward of the mouth; a
linear regression coefficient of -0.011 relates the value of the con­
centration index to channel widths seaward.
This indicates the ex­
pected proclivity for river water (and indirectly for effluent momentum)
to be progressively diluted by seawater away from the mouth.
Overall
seaward decrease (b = 0.246) in depth of the interface, also highly
significant (Table 3), reflects thinning of the surface layer.
Both
of these tendencies are evident from Figures 9A and B, and 10A, from
which a general seaward ascent of isopycnals can be seen.
Contrary to intuitive expectations, the stratification indices
(density ratio and maximum
gradient) did not vary significantly
with linear distance from the mouth, because of a wide zone of high
stratification seaward of the bar.
This zone was associated with a
plume of relatively fresh to brackish ambient coastal water flowing
roughly parallel to the bottom contours.
It is conspicuous on Figure,.
9A and B as an indentation in the isopycnal
pattern.-
Traverse position (line number) exerted a highly significant
influence on density ratio (Table 3)» the. highest degree of stratifi­
cation was along the main axis of the South Pass effluent (lines 3 and
4) (Table 2).
The localization of effluent fresh water is apparent
from Figure 11 (as indicated by vertical arrows).
The constancy of other parameters between lines implies that con-
37
SP 3 - 4
SP 3 - 3
SP 3 - 6
SP 3-8
it'
[11= 1.J«
IlnJ.Ii
e
t
S outh P o s t Bor
z
C««t««f8 in «1
FI = D a n s i m a t r i c F r o u d a N u m b a r
40.
30
34
36
38
30
D is ta n c a in c h a n n a l w id t h s
SP 3 - 3
SP 3 - 3
SP 3 - 4
SP 3 - 3
SP 3 - 6
SP 3 - 7
Dopth
In laat
C .G .
S o u th P a s t B ar
35
30
Cm
im ii
S a m p lin g
I* <rt
S ta tio n
35
40.
14
D is ta n c a
16
in
18
30
33
34
36
c h a n n a l w id th s
Figure 9. at cross sections for line 3 at low river stage on
October 2, 1969 . A. Ebbing tide. B. Flooding tide.
30
38
SP 2 - 5
SP 2 .7
in fast
10'
Dapth
20
25
S outh P a s t B ar
Ceafearo la
#1
V S a m p lin g S ta tio n
35
20
22
24
30
D i s t o n c o in c h a n n o l w i d t h s
Mouth
.30'
C
a
CaalatMB la f t
a
▼ S a m p lin g S ta tio n
25
S o u th P a n B o r
30
35 ■
0
2
4
6
8
10
12
14
Figure 10. at cross sections for line 2. A. Low river stage
(flooding tide) on October 2, 1969. B. High stage (ebbing tide)
on June 30, 1968 (based on data by Ouellette, 1969)*
39
J
e
t"
>5»*•»
5
. V S a m p lin g
S ta tio n
30
\j
35
y
■/
j1 A p p r o x i m a t e
■ o u n di a
. ry of
B
Pl u m e
a
e
a
30
S a m p lin g
S ta tio n
35
A p p ro x im a te
Boundary o l
Plum e
’
Fi = D e n s i m e t r i c
Froude Num ber
40
Fi = D e n s i m e t r i c
Froude N um ber
C n n U v r*
in
It.
40
D is ta n c e in c h a n n e l w id t h s
D is ta n c e in c h a n n e l w id th s
Figure 11.
cross sections transverse to the South Pass effluent
(approximately on line 5) at low river stage on October 3* 19&9* A.
Ebbing tide. B. Flooding tide.
0
ditions on either side of the South Pass effluent are very similar to
those within and that freshwater alimentation is not solely by way of
South Pass,
The lack of significant lateral contrast in weighted
freshwater concentration may be attributed partially to the longshore
effluent of fresh and brackish water supplied by outlets to the east,
and partially to the lateral homogeneity of the South Pass effluent
(evident from Figure 11),
River stage is a major determinant of stratification and mixing,
as Table 3 reveals, with weighted freshwater concentration, density
ratio, and depth of interface experiencing highly significant varia­
tion between high and low stage.
Mean values for weighted concentra­
tion of fresh water, density gradient, and depth of interface, were
appreciably higher during high stage,
Bowden and Sharaf el Din (1966)
report similar associations between river stage and mixing and strati­
fication for the Mersey Estuary (England),
The greater interfacial depth at high stage results from the
larger volume of fresh water present at any instant.
The stronger
density stratification and greater discretion for the freshwater layer
are probably due to an increase in the river discharge/tidal prism
ratio; it was pointed out earlier that the degree of vertical strati­
fication depends to a very large extent on this ratio.
Increased
velocities at high stage do not generate mixing because of the greater
depth of the interface and proportionately reduced densimetric Froude
numbers.
The increase in interfacial depth may also reduce mixing and
promote sharper stratification by placing the interface below wind-and
wave-generated turbulence.
Tidal phase proved highly significant in influencing density ratio
(Table 3), with means of 0.0168 and 0.0109 characterizing flooding
and ebbing tide, respectively (Table 2).
The important role of tide
is even more apparent for the tidal cycle observations at station
SP 3-4 (Table
and B).
Weighted freshwater concentration, maximum
at gradient, densimetric Froude number, and density ratio displayed
highly significant associations with tide.
Weighted freshwater con­
centration, density ratio, and maximum <
3^ gradient varied directly
and densimetric Froude number inversely with rate of flood or ebb and
height of tide.
The stratification increase and densimetric Froude number decrease
with flooding tide at any given station is apparent from a comparison
of Figures 9A and B.
Figure 12 shows cr^ variations with time and in
relation to tide at station SP 3-^ Tor the period 1400 November 111500 November 12, 1969. High stratification is indicated by closely
spaced isopycnals.
These trends parallel those evidenced within the lower reaches of
South Pass and can probably be attributed to the same set of causes*
during ebb the densimetric Froude number increases because outflow
accelerates more rapidly than interfacial depth can readjust.
by interfacial waves results.
Mixing
Interfacial waves off the mouth of
South Pass were, in fact, reported by Walsh (1969) on the basis of
remote-sensing images showing alternating bands of •’slick" water.
Rough seas and high winds are also important mechanisms of verti­
cal mixing and stratification breakdown.
tration and maximum a
Weighted freshwater concen­
gradient varied significantly between conditions
of calm and rough seas (Table 3)> with maxima during calm seas (Table
2).
For the period of continuous observations at station SP 3“^»
12
in fe e t
20
Nov. 12
Depth
Nov. 11
Tide
24
Contours in O’t
Time In
Hours
20
23
25
Hours
Figure 12.
variations with depth and time during a tidal cycle, November 11-12, 1969*
&
43
maximum <t
gradient decreased with increasing wind velocity and wave
height (Tables 4a and B).
These observations agree with theoretical and experimental con­
clusions that wind-and wave-generated turbulence and fluid mass trans­
port cause mixing (ichiye, 1953? Johnson, i960; Johnson and Hwang,
1961; Masch, 1961, 1962; Weigel, 1964, pp. 437-439).
These studies
disclose that wind-induced currents and turbulent motion of wind waves
both contribute to vertical mixing.
Swell waves were found to affect
stratification negligibly (Weigel, 1964, p. 438).
Bottom Topography, Mixing and Stratification
A strong tendency for the isopycnals to rise toward the surface
and become more widely spaced in the upstream proximity to and above
the distributary mouth bar is prominent on a profiles constructed for
t
lines within the lateral limits of the South Pass plume (Pigs. 9» 10,
ll).
Tendencies for density ratio and maximum a. gradient to be lower
x>
over and shoreward of the bar were highly significant (Tables 2 and 3).
Upwelling takes place during both flooding and ebbing tide and at high
(Fig. 10B) and low (Figs. 9> 10A and ll) river stage.
Scruton (1956) witnessed a similar situation at the mouth of Pass
a Loutre during high river stage and attributed it to partial impound­
ment of discharging water upstream from the bar crest.
This seems
somewhat incompatible with theories which regard the efflux proclivities
as cause and the bar as effect (i.e., Bates, 1953a-j Borichansky and Mik­
hailov, 1966 ; Axelson, 1967).
On the other hand, some degree of feed­
back between process and response is inevitable in any natural system,
where it is possible for each event to be at once both cause and effect
of every associated event.
A more tenable explanation for the isopycrials* ascent and strati­
fication decrease may lie in the hydraulic repsonse to the cessation
of lateral boundaries at the mouth.
Stommel and Parmer (1952) have
shown experimentally that whenever two-layer (i.e., highly stratified)
; open-channel flow undergoes an abrupt increase in width, continuity of
energy is maintained by a decrease in the depth of the interface, with
the velocity of the upper layer remaining more or less constant.
Takano (195^) and Default (I96I, pp. 5bO-5k$) have formulated mathe­
matical theories predicting the decrease in thickness of a layer of
light river water expanding laterally over denser seawater.
Bondar
(1968, 1969) has observed stratification tendencies comparable to
those just described at the Sulina mouth of the Danube River, where a
highly stratified or salt-wedge situation similar to that of South
Pass prevails.
He regards this as a result of horizontal widening and
vertical thinning of the freshwater effluent.
The densimetric Froude number at the mouth is usually near unity
(previous section), so that a decrease in the depth of the interface
(D in the equation
Fl « u2/gVD)»
without a corresponding decrease in velocity of the upper layer, will
produce values exceeding the critical limit.
Densimetric Froude num­
bers shown in Figure 9A and B suggest that this is the case at the
mouth of South Pass.
F
values as high as those indicated should be
more than adequate to permit exchange across the interface, explaining
the lesser stratification and greater mixing in the offing (the region
between the mouth and the seaward limit of the bar where fluvialmarine interactions are most intense).
By extrapolation, overall
45
deconcentration of effluent energy by mixing processes should result
in loss of transporting capacity (and competence), contributing to
deposition and perhaps to the building of the distributary mouth bar.
Nevertheless, bar topography undoubtedly exerts some reciprocal
effect on stratification.
Topographic control is intimated by the
observed tendency for stratification indices to be somewhat lower
longitudinally along the bar and to the southwest of the offing zone
(line 5)»
Increased steepness, distortion, and fluid mass transport
(Raudkivi, 19^7» PP» 253-263) of waves shoaling over the bar may aug­
ment vertical mixing.
Two-Factor Interactions
Table 3 indicates a considerable degree of control on mixing
and stratification by various two-factor combinations of the indi­
vidual independent variables.
Variations in density ratio with tidal
phase were more pronounced during high river stage, with the inter­
action having a high significance.
Specifically, weighted means indi­
cate maximum stratification during times of high stage and ebbing tide.
Similarly, position relative to the bar most strongly influenced density
ratio during high stage with minima over and shoreward of the bar and
maxima seaward of the bar.
The interaction effect was highly signifi­
cant.
HORIZONTAL EFFLUENT, CURRENT, AND DISCHARGE PATTERNS
General Surface Patterns
Large contrasts in salinity, temperature, and turbidity between
outflowing Mississippi River water and ambient Gulf water outline the
plume surface.
Boundaries are easily recognized by remote-sensing
imagery and ground comparisons.
Surface-water patterns in the environs of the Mississippi Delta
have been described by Scruton (1956)» Henry (1961), Bates (1953a,
1953*0, Drennon (1968), Scruton and Moore (1953)» and most recently
by Walsh (1969).
These investigations have consistently disclosed
that a surface layer of fresh to brackish water surrounds the delta.
This band drifts westerly in response to the dominant winds and a
slight geostrophic component induced by the geopotential head of the
freshwater layer (Scruton, 1956? the mechanism is discussed by Sver­
drup et al., 1942, pp. 444-451).
Surface efflux from South Pass cor­
respondingly veers to the southwest under the combined influence of
the coastal stream just described, tidal currents, and Coriolis deflec
tion (to the right) of effluent momentum.
Seaward of the mouth three distinct water masses are identified
(Figs. 1, 13A and B):
(l.) the South Pass plume of fresh, sediment­
laden water emanating from the mouth and flowing seaward along a
course which, in plan, arches progressively to the southwest;
(2.) a
slow moving, highly stratified band of fresh to brackish turbid water,
supplied by outlets to the northeast; and
(salinity = 25
0/00
- 30
0/ 00)
Gulf water.
ly lies 10 or more miles offshore.
(3.) slightly diluted
Pelagic Gulf water general
The widths of each zone are, on
the average, greatest at high river stage but vary with tide and wind.
Fresh water within the intermediate coastal band is highly dis­
crete and comparatively well stratified (Fig. 9A and B).
At the sur­
face, water from South Pass discharges directly into this band, as
indicated by Figures 1 and 13.
Seaward of the distributary mouth bar
southwesterly transport of fresh water is primarily in the form of the
ambient band.
On every field excursion the intermediate water mass
Fresh C o a s t a l
Fresh C o a s t a l
Water
Water
-N-
-N -
G u lf
South
Pass
Effluent
•O
t/
South
*>/
•>V
Pass
0 ./
o ./
:&&■/
E ffluent
F l o o d i n g Ti de
3000
Ebbing Tide
3000
F t.
Figure 13. Generalized surface effluent patterns and location of boundaries*
B. Ebbing tide.
A.
Flooding tide.
flanked the South Pass plume; never did the writer witness South Pass
efflux directly into Gulf water.
Foam Lines and Fronts
Adjacent water masses are often sharply demarcated by thermohaline fronts and foam lines similar to those evident from Figure 1,
Walsh (1969) attributes the surficial water boundaries of the Missis­
sippi Delta region to surface convergence of water masses with con­
trasting density.
He also suggests that boundary sharpness attenuates
with decreasing convergence rates, and that transition zones replace
the boundaries when convergence is very weak.
The writer observed strong lateral density contrasts across the
boundary separating the intermediate band from saline Gulf water.
Though Bates (1953^, p. 62) reports a salinity contrast of only 1.8
0/00 (on the basis of a single set of two water samples), the present
investigation disclosed surface density differences as great as 10.5
at (or abour 14 0/00 salinity) between stations situated less than 100
yards apart.
Overall, the freshwater band thins progressively with
increasing proximity to the boundary (Figs. 9A and B, and 10B).
How­
ever, the vertical steepness of the front increases when convergence is
strong.
The freshwater layer was nearly 5 feet thick only 50 feet from
the front on the;afternoon of October 3» 1969, when a combination of
flooding tide and onshore breeze augmented convergence.
Boundaries delimiting the South Pass effluent are more complex.
The sharpest contrast between South Pass outflow and ambient (coastal
band) water occurs along the eastern plume boundary between the end of
the east jetty and the inner buoy (Fig. 13A and 3).
Here the front
generally extends vertically to a depth of several feet, as suggested
49
by the a
and B).
cross sections transverse to the South Pass plume (Fig. 11A
Surface densities are slightly higher (2 to k a.) east of the
x»
boundary.
Along the seaward slope of the distributary mouth bar where the
South Pass plume deflects to the southwest, the boundary is less well
defined, presumably as a result of the vertical mixing described pre­
viously,
Density, in this case, is usually lower on the seaward side
of the boundary within the intermediate band (Fig. 9A and B).
Despite comparatively low horizontal density contrasts the eastern
boundary of the South Pass plume in the zone immediately seaward of
the mouth appears to possess very real frontal qualities which pro­
hibit lateral mixing and diffusion.
Experiments with Rhodamine-W dye
were conducted at the mouth of South Pass on two separate occasions to
estimate qualitatively the degree of diffusion between the South Pass
plume and ambient water in the offing zone.
The first experiment, on
October 3# 19&9» was performed under calm sea conditions and slightly
prior- to slack low water.
Winds were light and from the east.
The
dye, injected at the mouth near the end of the east jetty, was photo­
graphed at rapid intervals as it spread seaward.
Visual observations
of the dye and inspection of the photographs indicated no fluid trans­
fer across the boundary.
The second experiment was carried out on November 2?» 19^9»
shortly after low water under conditions of moderately rough seas.
Dye was injected about 1 mile upstream in the South Pass channel, and
dye concentration (fluorescent intensity), was measured with a G.K.
Turner Model III Fluorometer.
Traverses were run repeatedly across
the dye patch as it ieft the jetties and spread seaward.
As in the
previous experiment, the dye elongated within the plume and along the
boundary but did not penetrate into the ambient water.
The fluoro-
meter indicated peak intensities immediately west of the boundary and
base-level intensities (zero conentrations) just east of the boundary.
These experiments demonstrate that very little lateral exchange takes
place.
Implications of Overall Surface Patterns
Conventional jet theories such as those of Bates (1953a, 1953b)
and Borichansky and Mikhailov (1966) explaining river mouth patterns
in terms of progressive lateral turbulent diffusion between effluent
and ambient waters (Pig. 14A) predict a gradual change in concen­
tration in the direction transverse to the axis of flow.
The abrupt
and frontal nature of the plume boundary and high degree of lateral
homogeneity (in the y direction) across the plume strongly suggest
that the turbulent plane-jet model is not applicable to surface water
debouching from South Pass under normal conditions.
The writer*s
findings are more compatible with Takano*s (1954a, 1954b, 1955? see
also Defant, 196l) generalized model of a homogeneous surface layer of
light fresh water which spreads laterally with variable lateral ex­
change above the denser seawater (Fig. 14b).
Continuity is maintained
by a corresponding decrease in the vertical thickness of the fresh
layer.
At South Pass a seaward increase in vertical mixing between the
mouth and the bar crest complicates the situation; however, for purely
lateral surface tendencies at the offing Takano's approach is the most
relevant.
It is conceivable that effluent patterns may more nearly
approximate a turbulent jet at times of river flood and ebbing tide
when
turbulence
and
lateral mixing are increased,
Bondar (1969)
Zone of
e s t a b l i s h e d flow
Zone of
fl ow e s t a b l i s h m e n t
Centre-line \
- - Q
"(j C - ^ u ma x =^o
0
-y o
^ 3
■
0
/'''u m a x < u o O
O
^ 0
O O O
O C O O O C
0^0° °cooo
of dif f usi on region
W
r
Figure 1^. Two modes of effluent expansion. A. .Turbulent jet
diffusion (after Albertson et al., 1950)• B. Lateral spreading
as a discrete layer (modified by Walsh, 1969» after Defant, 1961).
explains outflow trends at the Sulina mouth of the Danube River in
i
terms of a model similar to Takano’s.
The Effects of Tide, Wind and Waves on Surface Patterns
Tide exerts a considerable influence on surface effluent patterns
(Fig. 13)•
The South Pass plume is deflected more acutely to the south­
west during flooding tide, apparently in response to the lower dis­
charge and westerly tidal currents.
The writer has observed that
flood tide is also the time of both maximum frontal definition for any
given sea state and minimum width of the freshwater bands.
Southwest-
ward transport by tidal currents and consequent convergence are
probably responsible for these tendencies.
Winds have a comparable if not greater effect on flow direction
and surface layer widths.
Onshore winds cause abrupt plume deflection
and convergence by shoreward transport in much the same fashion as a
flooding tide; offshore winds spread the surface layer seaward, result­
ing in the replacement of boundaries by progressive gradations.
Irrespective of the overall direction of transport by either tide
or wind, when winds are strong and seas are rough, surface water bands
are more widely diffused and boundaries are poorer defined.
Accord­
ingly, theory and experiments show that wind-and wave-induced surface
turbulence increases lateral diffusion (Larsen, 1965? Wilson and
Masch, 1967? Weigel, 196^, pp. ^37-^39? Johnson, i960).
Current and Discharge Patterns
Figure 15 A-C shows current vectors at 5“f°°t depth intervals at
stations transverse to the channel axis along the crest of the distribu­
tary mouth bar for ebbing and flooding tide.
on data of October 1 and 3# 19&9*
These' diagrams are based
Surface velocities at stations
53
!>y\
Cu rr a n t Vac tor t
h.
S u ria c a
n»
10 Ft.
15 FI.
20 Ft.
1300
3000
30 0 0
C urra n t V a c to rt
” slurloca
10 Ft .
2 0 Ft.,
2 5 Ft.
3 0 Ft.
3000
Figure 15a Current vectors transverse to the effluent and along the
distributary mouth "bara A. Ebbing tide Just prior to low water,
October 3, 1969. B. Flooding tide, October 3> 1969. C. Flooding
tide, October 1, 1969 (seas rough).
r
aligned with the channel axis are seen to be greatest at ebbing tide,
a fact which is consistent with the trends described for the channel
mouth.
Contiguous with the southwesterly deflection of the South Pass
plume, surface vectors at both flooding and ebbing tide progressively
turn clockwise with increasing distance southwestward along the bar.
One of the most salient characteristics of the surface current
patterns is a sudden change in direction and increase in seaward
velocity in the general vicinity of the inner buoy near the northeast­
ern end of the South Pass bar.
This position coincides with the
northeastern boundary of the South Pass plume, and the rapid changes
in vector direction and magnitude are typical of the boundary zone.
Southwesterly currents immediately east of the boundary (Pig. 15 A and
B) confirm the earlier proposal of surface convergence at the boundary.
The boundary’s frontal nature and role in inhibiting lateral dif­
fusion are established by the sharp velocity differential across the
boundary.
This is strikingly evident from Figure 15A.
The current
data from the boundary zone were collected one-half hour after the
first of the dye experiments referred to earlier.
With the boat
anchored at the boundary, currents were measured within a few feet of
the boundary on either side.
Surface velocities averaged 2.5 feet per
second on the ambient side and Jj-.l feet per second on the plume side.
These values represent a velocity difference of 1.6 feet per second
within a lateral distance of approximately 5 feet.
Such a current
profile is inconsistent with that predicted by the turbulent plane-jet
theory (Fig. l^A).
During flooding tide, subsurface currents in the saltwater layer
exhibited a strong southwesterly set.
Combined with the decrease in
discharge and seaward momentum, this flow is the probable cause of
the greater southwesterly deflection of the South Pass plume and
increased convergence (sharper boundaries) at flooding tide.
The
currents also undoubtedly contribute to the southwesterly elongation
of the distributary mouth bar.
Current vectors for stations on line 3 at flooding and ebbing
tide on October 2, 1969* and February 12, 1970> are shown in Figure
16 A-D.
These figures are relatively self-explanatory, but it should
be pointed out that a moderate northeast wind was blowing on February
12 and was probably responsible for the somewhat extreme southwesterly
surface deflection as well as for the strong northeasterly subsurface
reverse flow observed on that date,
A maximum surface velocity of 6.7
feet per second (based on drift speed) was observed over the bar on
line 3 on May 2k, 1969, during high stage (10 feet at New Orleans) and
ebbing tide.
The South Pass plume was directed seaward with a minimum
of deflection, and velocities exceeded ^-,0 feet per second as far as
12 channel widths seaward of the bar.
Figure 17 shows freshwater and saltwater discharge vectors along
line 3*
A decrease in freshwater discharge (through the assumed 1-
foot section) is evident between the mouth and the bar crest.
This is
an expected response to the lateral spreading of the surface effluent.
The accompanying increase in the discharge of salt water relative to
that of fresh water is commensurate with the increase in vertical
mixing with proximity to the bar crest.
Seaward of the bar crest,
between the' 20-and 30 “foot contours, a consequential increase in fresh­
water discharge reflects the province of the intermediate coastal
stream.
C u rra n t V a c to rt
.— •
C urra n t V a c to rt
S u rfa c a
—
3 Ft .
— — 10 F t .
S u rfa c a
10 F t.
15 Ft .
2 0 Ft
1500
5000
2 0 Ft
3000
C urra n t V a c to rt
—
%
C urrant V a c to rt
Sur f oca
10 Ft .
—
S u rfa c a
— — 5 Ft.
* 10 Ft.
-
2 0 Ft .
-
2 5 Ft.
v a lx itr
1300
Figure 16a Current vectors along line 3« A, Ebbing tide, October 2,
1969a Ba Flooding tide, October 2, 1969* Ca Ebbing tide, February
12, 1970a Da Flooding tide, February 12, 1970a
D is c h a rg e V e cto rs
—
D is c h a rg e V e c to r!
Fresh w a te r
"S a lt w a te r
——
F reeh w a te r
■■■■"— S a l t w a t e r
$ ( a » t J e t ty L ig h t
30
30
•3 0 '
13 0 0
3000
3000
C « *U « « i U
Figure l?e Freshwater and saltwater discharge vectors along line 3#
October 2, 1969* Ae Ebbing tide, B, Flooding tide.
ll.
The 180-degree reversal in direction of saltwater discharge sea­
ward of the bar between flooding and ebbing tide further demonstrates
the significant influence of tide on subsurface marine transport.
Figure 18 illustrates freshwater and saltwater discharge result­
ants and corresponding positions in the tidal cycle at 1-hour inter­
vals for a complete tidal cycle on November 11-12, 1969* at station
SP 3-4,
Westward flow of both fresh and salt water accompanied
flooding tide for the first 6 hours of observations.
During this time,
the South Pass plume was so strongly deflected that the station lay
seaward of its regime, in the more tranquil intermediate band.
This
must have been primarily the result of flooding tide, because winds
and seas at the beginning of the observation period were almost
totally calm.
Flow ceased completely at hour 7 (approximately 3 hours
after high water), then changed direction as flow within the stillpresent intermediate band reversed with the ebbing tide and steadily
increasing southwesterly wind.
With continuing ebb the South Pass
plume swung more southward, and by hour 12 the station was within the
main effluent, as indicated by the increase in discharge and south­
easterly flow.
Surface velocities attained a maximum of 4.65 feet per
second at hour 17, about 1 hour after low water, again indicating a
lag effect.
An increase in the strength of southwesterly winds pro­
hibited recurrence of the westerly plume deflection during the second
flood phase, though velocity and discharge generally decreased with
the rising of the tide.
Table 5 indicates high statistical significance for an increase
in freshwater discharge with decreasing tidal level, maximum correla­
tion occurring when a 3“hour lag effect is taken into account.
59
H I I 1400
D f ic h o r g o t
o
H I 3 1600
H I 2 150 0
H I 4 1700
HI 5
11 0 0
EmtUef»
2M
i 9.96
•30
HI 6
1900
HI 7
3000
H I 11 3 4 0 0
H I 13
HI 8
3100
HI 9
3300
H I 10
0100
H I 13 0 2 0 0
H I 14
0300
HR 15 0 4 0 0
H I 17 0 6 0 0
HR 18 0 7 0 0
HR 19 0 8 0 0
HR 3 0 0 9 0 0
1200Llf
HR 2 4 1 3 0 0
H I 25
3300
1.08
>08
H I 16
0500
HR 21 1 0 0 0
HR 2 6
150 0
^ H R 22 11001 \ [
R « » u lta n ts fo r
26 h o u r p e r io d
V?->
^H R
23
1400
N o v . 12
j
\
Figure 18# Freshwater and saltwater discharge vectors at 1-hour intervals
for a complete tidal cycle at station SP 3“^» November 11-12, 19&9*
Presumably this lag represents the time required for tidal influence
to he transmitted to the effluent and is possibly unique to the cir­
cumstances prevailing during the observation period.
The highly significant tendency for the ratio of freshwater
discharge to saltwater discharge to be inversely related to tidal
height (with a 3-hour lag) suggests the presence of the South Pass
effluent at the station during ebbing tide.
Table 5
Summary Statistics and Simple Correlation Coefficients for Discharge
Parameters as Functions of Tide for Station SP 3-^» November 11-12, 19&9
Freshwater
Freshwater/saltwater
discharge________ discharge ratio
Mean
5.033
0.565
Standard deviation
3.^91
0.3^1
Height of
tide-(-no lasc)
r
-0.678**
-0.315 NS
Height of
tide (with 3
hour lag)
r
-O.923**
-0 .756**
r = Simple correlation coefficient
n = 26
** Significant at .01 level
NS Not significant
61
SEDIMENT TRANSPORT PATTERNS
If prodelta and river-mouth morphology can be regarded as ultimate
products of the circulation and mixing processes just described, then
the penultimate product must be the pattern by which the sediments are
transported prior to deposition.
Bed Load
Sediments transported both as bed load and suspended load con­
tribute to the development of subaqueous depositional topography.
Unfortunately, measurements of bed load transport, were logistically
precluded.
However, the available information permits some general
inferences.
It has been demonstrated that the coarsest fractions transported
to the mouth of a river as bed load are deposited within a short
distance seaward of the mouth and are the primary constituents of the
distributary mouth bar (Scruton, i960; Mikhailov, 1966 ; Fisk et al.,
195^5 Coleman and Gagliano, 1965)*
Indeed, the high percentage of sand
and silt in the bar sediments at the mouths of the Mississippi (Fisk et
al., 195^-j Henry, l<?6l; Scruton, I960; Shepard, i960) and the abundance
of cross laminations among the sedimentary structures of the bars (Cole­
man and Gagliano, I965J Coleman et al., 1965) suggest predominantly bed
load origin.
Previous investigators have concluded that seaward bed
load transport in the lower channel and beyond the mouth occurs only at
flood stage when the salt wedge is presumably held just seaward of the
bar crest (Scruton, 19^5? Bates, 1953^, 1953B? Moore, 1970).
For the
ideal case this conclusion has strong a priori foundations; however, in
South Pass appreciable downstream velocities occur in the lower layer
during ebb tide, even.at times of maximum salt wedge intrusion.
62
Evidence also shows that the salt wedge intrudes into the channel at
high stage when the tide is flooding.
It follows, then, that tide is
as much (or more) a determinant of bed load transport at the mouth of
South Pass as river discharge.
Purely on the basis of the observed current patterns it is inferred
that bed load reaches the bar primarily during ebbing tide, and has a
maximum rate of transport at flood stage.
Transport to the southwest
along the bar should take place at flooding tide, when strong south­
westerly components are present within the lower layer.
The accumulation of the bed load material as a distributary mouth
bar may at least partially be a function of lateral flow separation and
plane-jet turbulent diffusion of the lower layer.
This postulation is
strengthened by Jopling's (1963, 19&5) findings that when flow separa­
tion is restricted to the vertical, the bed remains horizontal to the
point at which seaward declivity begins.
Furthermore, the situation of
the bar at approximately four channel widths seaward of the mouth (Figs.
9A and B, and 10A) is predicted by the plane-jet theory (Bates, 1953a*
1953b; Albertson et al., 1950)•
Turbulent diffusion of salt water
issuing from the pass at ebbing tide would be favored by the low or
negligible density differential within the lower layer.
During flood
stage, however, and at times when fresh water occupies the entire depth
of the channel, bed load deposition probably results from saltwater
intrusion beneath the laterally spreading fresh water.
Suspended Load
Deposition beyond the offing and distributary mouth bar zone is
primarily from sediment transported in suspension.
Samples for sus-
pended-load analysis were collected on seven different field trips
during 1969 and 1970*
Surface concentrations of suspended load at the
mouth of South Pass varied considerably throughout the year.
The samp­
ling periods were too few to permit valid statistical analysis, hut
suspended-load concentrations were obviously controlled by river stage
and water temperature (Table 6).
The increase in suspended-load con­
centration with increasing discharge is accordant with thoroughly
established expectations (Hjulstrom, 1939)*
The inverse correlation
between concentration and temperature is predicted as the result of an
increase in fluid viscosity (Lane et al., 1949? Burke, 1966; Leliavsky,
195^» PP. 187-191).
Table 6
Average Suspended Load Concentrations, River Stages
and Water Temperatures for Six Observation Periods
Observation period
River stage
at N.O.
Water-surface
temperature
Average suspended-load
concentration at
surface for observation neriod
April 12, 1969
10 feet
12.8°C
305.0 mg/1
May 23-25, 19^9
10 feet
22ol°C
90.0 mg/l
June 30-July 2, 1969
4,3 feet
28»5°C
35.0 mg/l
September 3“^> 1969
3.9 feet
29.0°C
4.0 mg/l
September 30October 3, 1969
4.0 feet
26 • o°c
15 .0 mg/l
February 12-13, 1970
4.0 feet
7.30G
46.0 mg/l
Suspended-load variations with depth at the mouth of South Pass
for flooding and ebbing tide for three separate occasions are shown in
Figure 19A-E.
Overall differences in absolute suspended-sediment con­
centration between the three periods are explicable in terms of stage
and temperature; however, there are other apparent trends.
Most con­
spicuous are. turbidity maxima at the salt-wedge interface and the
tendency for these maxima to be most pronounced during flooding tide.
These proclivities were not unique to the examples shown.
The maxima
may arise from a combination of four different mechanisms:
(l.) a sud­
den decrease in the terminal fall velocity (of the sediment) in the
denser salt layer;
(2.) interfacial turbulence;
(3 .) a recycling up­
ward of sediment which falls into the salt wedge from above;
(4.)
flocculation of colloids in the zone of freshwater-saltwater mixing.
Postma (1967) reports that interfacial turbidity maxima are typical of
salt-wedge estuaries.
According to him, sediment which sinks into the
salt wedge returns upstream with the reverse flow.
The preferential
upward exchange caused by entrainment of saltwater from the wedge then
results in sediment concentration at the interface.
Such reasoning also
explains the greater prominence of the maxima during flooding tide,
inasmuch as upstream flow within the salt wedge is best defined at flood
tide.
Below the interface, in the mid-depth regions of the salt wedge,
suspended-load concentrations are minimal concurrent with the influx of
relatively limpid marine water.
The abrupt increase in sediment con­
centration near the bottom results from agitation of bottom sediments.
This was substantiated by microscopic inspection of the samples near the
bottom, which were found to contain a high fraction of coarse material.
Vertical patterns of suspended-load concentration seaward of the
mouth are highly complicated, as evidenced from Figures 20-22.
However,
65
1M
IS*
H
0
II
mJ
>0
JO
40
JO
•0
J»L
10
■I
Figure 19. Vertical distribution of
suspended-sediment concentration at the
mouth of South Pass (station SPCG). A.
High stage, flooding tide, May 30, 1969.
B. Low stage, ebbing tide, June 30, 1969.
C. Low stage, flooding tide, June 30,
1969. D. Low stage, ebbing tide,
February 12, 1970. E. Low stage, flood­
ing tide, February 12, 1970.
66
SP
3-3
SP
3-4
SP
3-5
Interface
■
o
13
13 T-
20
20
0
20
S u ip ttidcd
40
load
20
60
20
concentration
Suipandad
40
load
60
80
c o n c a n t r a t i o n (m g / I)
l»g/D
SP
SP
3-6
3-7
1SP 3 J 3 ^ /
Interface
Interfact
- a
20
40
60
80
10
20
40
60
20
40
|m g/l)
Figure 20, Vertical distribution of suspended-sediment concentration
for line 3t July 1, 1969J low river stage, ebbing tide, calm seas#
Upper layer turbidity maxima characteristic of low energy conditions
are shown.
60
67
1
feet
I I
1
II
|
Interface
in
\
si»
—
3 -:
Oeptk
Ebbing tide
::::::::::::
0
|
10
10
20
ll
iiiiiiiiiii'
-
Interface
3-6
Depth
Ebbing tide
/
1
-
10
I
I
1
SP
3-5
...... Ebbing
1
13 h * .............
e
1
.....
L
30
Suspended load
co ncentration |mg/l)
1
Interface
_
20
Interface
I
feet
SP
3-4
i
f
[blng tide\\\\\\
10
load
|m g/l)
0
5
t i : • j ;;! J»P
■Hi:!
20
S uipended
concentration
0
y
Interface
10
10
Seepended lead
concentration (m g /ll
1
I
bbing titie\\
10
in
1 1
Interface
SP
3-7.
20
■■■■■■Ebbing tide........
i-
f
10
0
Seiponded
vl
10
10
20
load
30
--- 1
10
cancentratian
Surpendcd
1
20
load
concentration
ii______
23
30
40
0
(m g/ll'
Suapended
("0 /1)
feet
SP
/
S
B
SP
..1...
.............
20
Depth
;I S P
3-2;;;
SP
i:flooding ;
I
10
Suapended load
concentration
( * /l)
0
20
3-3
Flooding
tide '
■
0
10
S u ip e n d e d lo a d
concentration
(mg/l)
N o defined
interface
10
20
Suapended load
concentration
1
I
10
V .
0
3-5
Flooding
tide
N o defined
interface
1 ..
10
30
load concentration
(m g/l|
in
interface
3-4
Flooding
tide
10
20
S u ip e n d e d load
concentration
Figure 21. Vertical distribution of suspended-sediment concentration
for line 3, Oetpber 2, 1969j low stage, calm seas. Note the increase
in concentration with depth at stations SP 3-2, SP 3~3> and. SP 3-5
during ebbing tide.
68
0
TT“
SP 1-3
SP 1-4
Interface
SP 1-5
Flooding tide
SP 1-6
Flooding
tide
5
Flooding tide
Flooding tide
• N o defined
n
interface
10
N o defined
interlace
N o defined
interface
a
is -
20
20
60
40
Suspended load
concentration (m g/l)
20
0
15
!
20
40
Suspended lood
concentration, (mg/l)
20
40
Suspended load
concentration (mg/l)
20
40
Suspended load
concentration (mg/l)
0
SP 2 - 3
SP 2 - 2
5
Ebbing tide
Interface
Interface
1
I
Ebbing tide
10
0
20
Suspended
IS
60
40
lood
100
80
concentration
120
20
40
Suspended load
concentration (mg/l)
(mg/l)
SP 2 - 4
SP 2 - 6
Ebbing tide
Ebbing tide
SP 2 - 8
Ebbing tide
N o defined
interface
a
a
e
N o defined
interface
N o defined
interface
1
20
Suspended
40
load concentration
|mg/l|
20
20
20
Suspended
25
20
Sutpondod
40
40
60
load
60
to ad concentration
|mg/l)
Figure 22. Vertical distribution of suspended-sediment concentration
for lines 1 and 2, May 23 and 2k, 1969? high river stage, rough seas.
Lack of significant turbidity contrast between fresh and salt layers
typical of rough seas and pronounced vertical mixing is shown.
60
close Inspection of the figures as well as additional data records from
the study discloses some broad trends.
The highest concentrations occur primarily in the upper layer of
fresh river water, and, on calm days, vessels of moderate draft often
leave a conspicuous wake of clear Gulf water in an otherwise turbid
surface.
Surface maxima are particularly apparent from Figure 20,
Deviations from this tendency occur when rough seas promote vertical
mixing (Fig. 22); when turbid water from the South Pass channel is dis­
charged seaward within the saltwater layer during ebbing tide (Fig. 21)
and over the distributary mouth bar, where the shallow depths favor
entrainment of bottom material by waves and currents.
The correlation
between sea state and vertical distribution of suspended sediments
testifies to the ability of rough seas to transfer sediment from the
domain of the river effluent to that of the marine forces.
The concen­
tration increase in the salt layer during ebbing tide (Fig. 21) implies
that ebb currents do indeed transport material from the channel to the
bar at low river stage.
Surface Turbidity Patterns
The transport of suspended material by surface effluents is
indicated by surface variations in suspended-load concentration.
Figure 23 shows a ‘typical high-stage surface distribution of suspendedsediment concentration based on data collected near high tide slack
water on May 24, 19&9*
These data illustrate the plume deflection as
well as the sharp concentration contrast near the mouth between the
South Pass plume and ambient water.
A concentration decrease with
increasing distance seaward evidences the progressive settling out of
sediment from the upper layer.
Suspended
sediment
concentration
-N-
(m g/l)
9 2 \ E a st J etty
light
5
\
■ %
\
U
V
7
Inner
\w••§
• 53
Buoy
14
18
m ~ y * 2o mg/l
73
66
• 50
50
Ft.
C o n to u r s in ft.
Figure 23* Surface distribution of suspended-sediment concentration
at high river stage, April 12, 1969* The plume boundary lies approxi­
mately between the 60 mg/l and 40 mg/l isolines*
iLow-stage surface concentrations at ebbing and flooding tide for
September 3 and 4, 1969* and the approximate positions of the South
Pass plume boundary are shorn in Figure 24A and B.
Abrupt south­
westerly deflection of the sediment outflow at flooding tide is evi­
dent.
In contrast to the high-stage situation, suspended-load concen­
trations of the ambient water exceed those of the South Pass plume.
In
addition, the seaward turbidity decrease which characterizes the highstage plume is lacking at low stage, signifying a low deposition rate.
These tendencies were typical of low-stage turbidity patterns.
Figure 25 shows variations with distance seaward in suspendedload concentration for several different survey lines and observation
periods.
On almost all of the lines a secondary (or primary, on the
low-stage profiles) turbidity peak is present in the zone between five
and ten channel widths seaward of the mouth.
The graph also illustrates
the more rapid seaward decrease in concentration which accompanies high
stage.
These trends lead to two fundamental conclusions regarding the
transport and deposition of suspended material:
(1.) For the most part,
deposition of sediment carried in suspension by the South Pass surface
effluent takes place in the prodeltaic region during high river stage;
(2.) during low river stage, particularly during the late summer months
when water temperatures are high, the intermediate band of fresh water
from the northeast is the main contributor of suspended sediment.
It
appears that much of the sediment carried by the ambient stream derives
from Garden Island Bay, immediately east of South Pass where waves and
tidal currents agitate the shallow bottom.
Suapaadad aadiaaaat
concaalrollea |aig/l)
Su apandad aadiraant
concanlrolion (aig/l)
^Innar Buoy
3000
Conloura
ia
il.
Figure Zk, Surface distribution of suspended-sediment concentration at low river stage*
tide, September 4, 1969* B* Flooding tide, September 3# 1969*
Contour* in It.
A* Ebbing
73
90 *.......
80
(mg/l)
concentration
50
40
Suspended
60
l oad
70
•.-i
30
20
10
ct
10
Distance
15
seaward
20
25
in c h a n n e l
widths
30
Figure 25. Suspended-sediment concentrations at the surface along
several different survey lines at high and low stage.
Flocculation and "Sludge" Formation
When fresh water charged with colloidal clays comes into contact
with seawater, an increase in the electrolytic content of the medium
and corresponding decrease in electrolytic potential of colloidal
particles result in flocculation and increased settling rates (Whitehouse and Jeffreys, 195**; Postma, 1967).
This mechanism is responsible
for "sludge", a dense, semisuspended, gelatinous ooze regarded by
numerous investigators as a significant contributor to deltaic sedimen­
tation (Bates, 1953a, 1953b; Cobb, 1952; Holle, 1952; Henry, 1961).
Most of the arguments advanced in favor of "sludge" as a major depositional mode have been based on commonly observed acoustic fathometric
reflections from 10 or more feet above the bottom.
Systematic observations of these reflections both in the pass and
over the bar during this study revealed them to be solely associated
with the salt-fresh interface.
A tendency for ships to lose way upon
approaching the passes of the Mississippi River, also cited as evidence
of "sludge", is explicable in terms of the drain on the ships* power
by the generation of internal waves when the ships* keel and the
density interface coincide (Neumann and Pierson, 1966, pp. 381-391;
Sverdrup et al., 19**2, p. 587)•
Undeniably, flocculation and coagulation near river mouths are
important geochemical processes and may even be partially responsible
for the turbidity maxima observed at the salt-wedge interface in the
lower reaches of South Pass.
Furthermore, a thin layer of "sludge"
is probably present at low river stage near the bottom in areas where
tidal currents are incapable of removing it (Bates, 1953b).
However,
suggestions of layers. 10 or more feet in thickness appear to be
overestimates of the abundance of this material.
This study reveals
that the relative contribution of "sludge" formation to prodeltaic
development merits reevaluation.
SUMMARY AND CONCLUSIONS
The roles of fluvial, marine, and fluvial-marine interaction
processes in controlling the dissemination of sediment-laden outflow
at the mouth of South Pass were investigated and the assumptions upon
which most existing models of river mouth and prodeltaic sedimentation
are based were reevaluated.
The salient findings are summarized.
In the lower South Pass channel a combination of saltwater intru­
sion and tide dominate circulation and mixing.
Seaward discharge and
vertical mixing in the channel are greatest during ebbing tide, when
the hydrostatic gradient is at a maximum.
Direction of flow within the
saltwater wedge is largely a function of tidal phase, upstream currents
prevailing .during flooding tide and downstream currents characterizing
ebbing tide.
Bed load transport in the lower channel, particularly at
low and normal river stage, is therefore considered to be tidedependent.
Immediately seaward of the jettied mouth the freshwater effluent
typically spreads laterally above the denser salt water as a relatively
homogeneous layer rather than progressively diffusing laterally in
accordance with the turbulent jet model.
Slight density contrasts
between effluent and ambient waters permit the formation of a frontal
boundary, which inhibits lateral mixing and facilitates the retention
of effluent discretion.
This boundary is sharpest at flooding tide,
when westerly transport of ambient water favors convergence and
decreased mixing within the channel results in a higher density
differential.
Mixing and deconcentration of the freshwater effluent take place
primarily in the vertical hy means of four major processes:
(l.) an
abrupt decrease in the depth of the salt-wedge interface in response to
lateral spreading of the upper layer increases the value of the densimetric Froude number, leading to mixing by interfacial waves;
wind-and wave-generated turbulence;
(3.) tidal currents;
(2.)
(U») inter­
action between the effluent and the distributary mouth bar.
Outflow in the lower (salt) layer is responsible for bed load
transport and probably for the formation and development of the dis­
tributary mouth bar.
With the possible exception of extreme flood
stage, efflux of this layer is highly tidal.
It is possible that flow
in the lower layer decelerates by lateral turbulent jet-type diffusion.
Flanking the South Pass effluent and seaward of the bar crest,
fresh water from outlets to the northeast drifts to the southwest as a
distinct band and at low river stage is
sediment.
the major source of suspended
This band is highly stratified during calm seas but mixes
with ambient and underlying salt water when winds are strong and seas
are rough.
Flow and net sediment transport are predominantly deflected southwestward by a combination of oceanographic factors, including south­
westerly coastal drift owing to the dominant winds and possibly a
Coriolis effect.
At any given time, the rate of southwesterly deflec­
tion depends on the tidal phase, being greatest during flooding tide.
The direction and magnitude of subsurface flow over and seaward of the
bar are chiefly tidal; strong southwesterly currents occur during
flooding tide and weak and occasionally reversed currents are observed
during ebbing tide.
It is concluded that the patterns of fluid and sediment dissemina­
tion and consequent sediment deposition at the mouth of South Pass can
be satisfactorily explained only in terms of complex fluvial-marine
interactions.
Numerous fluvial, oceanographic, and topographic
factors act as highly significant controls.
These factors interact
with one another and with their morphological "product", yielding a
unified system with a unique process signature.
Any significant over­
all change in tide, winds, hydrologic regime, or channel mouth
geometry would undoubtedly produce an appreciably altered end result.
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VITA
Lynn Donelson Wright was born December 13* 19^0* in Nashville,
Tennessee*
He graduated from St. Johns Country Day School, Orange
Park, Florida, June
i960, and entered the University of Miami
(Florida) In September of the same year.
He received a Bachelor of
Arts degree from that university in June 1965# majoring In geography
and geology.
During the same month he enrolled as a candidate for
the degree of Master of Arts in geomorphology in the Department of
Geography, University of Sydney (Australia) • He submitted his thesis
The Coastal Barrier System of the Shoalhaven Delta Region in June I967.
The degree of Master of Arts with honors was conferred in December
1967. He enrolled at L.S.U. and began working on the degree of Doctor
of Philosophy in the Department of Geography and Anthropology in
September 1967*
He is a member of the Association of American
Geographers and the American Geophysical Union.
EXAMINATION AND THESIS REPORT
Candidate:
Lynn Donelson Wright
Major Field:
Geography
Title of Thesis:
Circulation, Effluent Diffusion, and Sediment Transport:
Mouth of South Pass, Mississippi River Delta
Approved:
M a jo r Professor and Chairman
~
ean: Of
ofDthe Graduate School
EXAMINING COMMITTEE:
y
a/
/
^iJ
(X
Date of Examination:
April 13, 1970