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OCR for page 115
TIDAL HYDRAULICS*
F. A. Herrmann, Jr.
Introduction
The branch of knowledge applicable to studies of the physical
aspects of tidal waterways has become known as Tidal hydraulics. n
There is reason to regret the adoption of the term to include all tidal
waterway engineering, as many are prone to con side r its scope to be
limited to the rise and fall of the water surface in consonance with
the movements of heavenly bodies that generate the forces, and to the
currents that are caused by the alternately rising and falling tide.
It is emphasized that the term Tidal hydraulics" has come to be
understood as including, in addition to the purely hydrodynamic
considerations of such tidal waterways as inlets, estuaries, maritime
straits, and canals, the following: channel dimensions and alignment;
shoaling, including consideration of sources of the sediment, manner of
transport, and cause of deposition; training works and dredging
procedure (but not dredge design); jetty and breakwater layout; the
salinity of the water, including associated phenomena; and the
dispersal and flushing of pollutants. This paper, however, will
concentrate primarily on salinity conditions.
Tidal phenomena occuring in any waterway seldom result from a
single cause, but are more or less complex interactions of a number of
factors. Tbus, if a change in the regimen of a waterway is desired in
order to effect an improvement, the change in each contributing factor
and in the resulting interaction must be determined. The principal
factors to be taken into consideration are: tides, tidal currents,
freshwater disabarge, salinity intrusion, volume of sediment,
characteristics of beds and banks, wave action, littoral processes, and
dispersal and flushing of pollutants.
*Much of the information presented in this paper was developed under
the Civil Works Program of the U.S. Army Corps of Engineers.
Permission granted by the Chief of Engineers to publish this
information is appreciated.
115
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116
Tides
Tides are usually categorized as semidiurnal, mixed, or diurnal.
Semidiurnal tides {Figure 1) are typical on the East Coast of the
United States, and they exhibit two nearly equal tides (i.e., two
nearly equal high waters and two nearly equal low waters) per lunar day
(24 hr 50 min.). Mixed tides (Figure 2) common to the West Coast
exhibit two markedly different tides per lunar day. Along the Gulf
Coast, one of the tides per day often vanishes, resulting in a diurnal
tide {Figure 3~.
These tides generate an undulation in tidal waterways, and once
started, it propagates upstream to some point where further progress is
terminated by a barrier, or where the accumulating attrition causes the
undulation to disappear. If the length of the estuary exceeds the
length of the tide wave, as in the case of the Amazon River in South
America, the system may contain two or more tides at the same time.
Thus, the tide may be rising or falling in two or more reaches of the
estuary at the same time. In some situations, the geometry of the
waterway causes a stationary wave, but these cases are not so
frequently encountered as the so-called progressive waves. Progressive
waves travel upstream with a celerity related to water depth. Thus,
during the propagation of a progressive wave, the high-water portions
travel faster than the low-water portions of the wave, and this helps
to distort the shape of the wave. As the wave progresses up the
estuary, the duration of the rice decreases and the rate of rise
increase=; conversely, the duration of fall increases and the rate of
fall decreases. The shape of a curve representing tidal heights
plotted against time shifts from that approximating a sine or cosine
curve to that exhibiting a quick rise of relatively abort duration
followed by a slow fall of relatively long duration.
Tidal conditions within a waterway depend basically on the exciting
tide, the shape of the waterway, and tbe bottom friction. In a
converging waterway, where friction is a secondary factor, tidal
amplitude increases as the tidal wave progresses upstream. In
diverging waterways, or those in which friction is more important than
shape, tidal amplitude decreases as the wave progresses upstream. In
addition, reflections can either increase or decrease the amplitude.
Figure 4 ~how. the relative tidal amplitude along the Delaware
estuary. Rapid convergence in the bay causes an increase in amplitude
in the downstream area. In the next reach, the amplitude decreases,
apparently from reflections from two large island=. In the upper
reaches, convergence again causes the tidal amplitude to increase.
Tidal range varies greatly throughout the U.S. The tide range in Gulf
Coast estuaries is generally less than 2.0 feet, while tide ranges
greater than 30 feet are common in Cook Inlet, Alaska.
Tidal Currents
As the tide wave progresses through a waterway, tidal currents are
generated. Although a flood current basically occurs with a rising
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117
0~
2 _ ~
. ~O . 1 1 1 _
D 2 4 6
12r
10
8
. 6
C: 4
S 2
-2
CHARLESTON HARBOR
i1 1\ ~
8 10 17 14 36 18 20 ~24 0
TIME, HR
TYPICAL SEMIDIURNAL TIDE
(1)
GRAYS HARBOR, WA
~\ /:
_ 1 1 1 1 1 1 1 1 ~I I I
0 2 4 6 8 10 12 14 16 t8 20 22 240
TIME, HR
TYPICAL MIXED TIDE
(2)
Figures 1 and 2. Typical semidiurnal tide of East
Coast (1), and typical mixed tide of West Coast (2~.
ATCHAFALAYA BAY, LA
2.0
O~ 1.5
=~ 1.0
~ - 0.5
~ o
Sr
c, 3 1.0
Ul ~
~ C
C~ 0.5
O
AUG 1976
TIME, DAYS
1 1 1 1 1 1 1 1 1 1- 1 1 ~1 1 1 1 1 1
1 8 24 6 12 18
AUG 18, 1976 AUG 19,1976
TIME. HR
TYPICAL DIURNAL TIDE
Figure 3. Typical diurnal tide of Gulf Coast.
_
_
_
f ! t! ll t! ~ ~ I ~ ~ ~ ~ ~ ~ ~ ~ ~ I I I I I I I I I I I I 1 1 ~ I
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 1 2
| SEPTEMBER t976
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118
TRENTON
0.8
0.6
04r 1 1
O 2
- OCEAN
1.0 1 i ' - I. ll
'~ ~ ~ ~q
i _
4 6 8
D I STANCE,, FT X I O
DELAWARE ESTUARY
Figure 4. Tidal amplitude increases with con-
vergence in Delaware Bay; decreases with reflection.
3 8
J
~ 6
z
o
,- 4
J 2
C]
F o
14
CURR~/T~' _ _ _
· . · , · ~ · · · -
_ . . , . . . ~ . , · ,
, ~ ~/DE
1 2 3 4 5 6 7 8 9 10 1 1 12
TIME, HOURS
DELAWARE BAY
Fig~re 5. Typical tide-velocity relation.
20,000
~o,~
B.
~ 1,~
z 500
z
~r
.o
100
50
0
o 50
C}
2
J ~
~'
O ~
o
to J
2lD ~
I,,,, >
4
- /~?
5
.
· jz ~
S S -
1 1
100 lISO 200 260 300 350
1,000~fT CHANNEL STATIONS FROI~ f~llLADELPHIA
EFFECTS Of FRESH~WATER DISCHARGE
figure 6. ON eOTTOM SALINI" AT HIG^WATER S~K
~E" ~DE
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119
tide, there is generally a significant phase difference between the
current and tide. The current flow will generally be in the ~wrong.
direction for an hour or two after the tide has changed. A typical
tide-velocity relation is shown in Figure 5. Although low tide occurs
at he 4.0 in this example, the current continues to ebb until hr 5.5.
Similarly, high water occurs at hr 10.0, but tbe corresponding slack
current Is delayed until hr 11.5. In general, the tidal currents in
the lower portions of a tidal waterway will greatly exceed the
magnitude of freshwater currents resulting from upland discharges.
Freshwater Discharge
The freshwater discharges into the tidal waterway have profound
effects on the regimen. They affect the basic tide independently of
the effects of geometry, greatly modify the resultant current by
lengthening the ebb and shortening the flood, transport upland sediment
to the tidal waterway, and interact with salinity intrusion forces to
produce density currents. Additionally, the inflow of fresh water is
the means by which a tidal waterway purges itself of pollutants
introduced by man, and variations in the freshwater discharge rate
alter the extent of salinity intrusion, as shown for the Delaware
estuary in Figure 6.
Salinity
Engineers are interested in salinity conditions in an estuary for
several reasons. As I will explain later, salinity intrusion can have
a profound effect on the direction, magnitude, and duration of
currents. Salinity also plays an important role in determining the
sedimentation and circulation characteristics of an estuary.
Government agencies and private concerns use water from estuaries for
drinking water supplies, irrigation, and industrial purposes, and they
are thus concerned with the possibility of saltwater contamination of
their water sources. Salinity is also vital to the ecology of an
estuary. The various fish and wildlife are tolerant to salinity in
varying degrees. Thus, if salinity conditions were drastically
altered, certain species might be driven out of the area. For these
and other reasons, it is necessary to understand the significance of
existing salinity conditions and be able to predict the changes in
salinity conditions that might be brought about by some man-made change
in the estuary.
Tidal action and freshwater discharge normally provide the primary
mechanisms for mixing salt and fresh waters as a result of tidal
currents. Salinity characteristics in real estuaries can be classified
into three broad categories: highly stratified, partly mixed, and well
mixed. In a highly stratified estuary, a distinct saltwater wedge will
be present. In a well-mixed estuary, the salinity from surface to
bottom will be essentially uniform. The partly mixed case falls in
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fRESW WATE~
~. ~
~- ~.SALT WATER
. ~
1,,,~:,;,;;,.,,,,,,,,,;,.,;,;;;,,,,,.
CONDITIONS TYPICAL OF
I GH LY "STRATI Fl ED ESTUARY
Figure 7.
FRESH WATER
I SALT ~ VATER ~O
=~000~2 44
FLOOD EBB FLOOD EBB
TYPIGAL DISTRIBUTION OF VELOCITY
IN HIGHLY STRATIFIED ESTUARY
Figure 8.
+'o ~ 1 ~ ~ 1 1 ~
_ ~ Z3 ~ ~5 , ~O./ _
- o72 ~ r,-~/ ~76
, ~oZ/ ~ - ~/ , - 7. 6
6.B ~6.9, -74
- - 0-65 ~06.2 ~ o7.d
_ 06.2 . 4,5.3, oZ4 ~
- 05.0 ~0,fO ., .6.0 .¢
~5` ~`~55 · o68 a
~/~ Of ~EF TIES ~ '
J 10
~ 20
z
o
i_ 30
J 40
Figure 9.
_ ~/ ~0 750 , _6.5 ~ 0` /-
_ o6.d ~- 6.9 , - 5.9 ~3.t
_o6~3- ~5.5. ,~.5- as
_ ~5.8, 3.4 35 ~8ULE/JT
- oS. ~/.9 // ~;___
- d#. ,G ~0/,# o,`
~.0 ~ ~ APPROX L/WE OF REKE~SE QOW
- o~
_ ,~ ~ _~ /.6
, ,9 /4,
SO _
~ 1 1 1 1 1 1 1 1 1 1 1 ~-~
~0
19.S . 20.0 20.5 21.0
MILES eELOW HEAD OF PASSES
SOUTHWEST PASS
VELOC ITY Dl ST R IB U T I ON
FRESH-WATER DISCHARGE 300,000 CFS
OCR for page 121
121
between the other two, having a distinct vertical salinity gradient,
but not a wedge.
In an ideal case of a highly stratified estuary, the freshwater
velocity and the tidal currents are not great enough to create
appreciable mixing of the salt and fresh waters, but the shear stress
of the fresh water on the face of the saltwater wedge will cause a
reversal of current direction within the saltwater wedge (Figure 7~.
The salt water moves upstream at the bottom of the wedge but downstream
at the top of the wedge. In addition, the wedge will move upstream and
downstream in phase with the tide. The extent to which the wedge
intrudes upstream depends on the freshwater discharge, the channel
depth, and the density difference between the flab and sea waters.
Typical velocity distributions in a highly stratified estuary are
shown in Figure 8. Upstream from the limit of saltwater intrusion, the
direction of the current is the same at all depth", and since there is
normally no reversal of flow by tidal action in a highly stratified
estuary, the current direction is downstream at all times. In the
region of saline intrusion, the direction of the current from the
surface to somewhat below the salt-fresb water interface i" downstream.
However, that near the bottom is upstream to compensate for the salt
water being lost from the interface by mixing and for the salt water
flowing downstream within the wedge. Because there is virtually no
mixing at the interface, the water above the interface has essentially
zero salinity, while that below the interface is essentially seawater.
Southwest Pass of the Mississippi River is the best example of a
highly stratified estuary. Figure 9 shows the velocity distribution
(averaged over a tidal cycle) at the mouth of the Pass for a river
discharge of about 750,000 cubic feet per second (cfs). Flow in the
Southwest Pass was 300,000 cfs. The line of zero velocity is slightly
below the interface. For this discharge, the tip of the saltwater
wedge is located adjacent to the outer ends of the jetties. The
upstream extent of saltwater penetration is dependent on the magnitude
of the freshwater discharge, as shown in Figure 10. For an extremely
low river discharge, the wedge intrudes upstream about 140 miles, which
is upstream from New Orleans.
Whereas the highly stratified type of estuary was characterized by
a two-layered system with zero salinity in the surface layer and sea
salinity in the bottom layer, the well-mixed case is characterized by
essentially uniform salinity from surface to bottom (Figure 11~. The
salinity at the entrance to the estuary is that of seawater; and it
decreases with distance upstream from the entrance. Density currents
in a well-mixed system are not completely eliminated, but they are much
weaker than in a stratified system or than the tidal currents. There
is a complete reversal of flow direction at all depths with the
changing tide.
Velocity distributions typical of well-mixed estuaries are shown in
Figure 12. The currents reverse with tidal phase throughout the
estuary. In the fresh and brackish water regions, ebb currents at all
depths predominate slightly over flood currents because of the
freshwater disabarge. In the intermediate and highly saline regions,
however, the bottom flood currents usually predominate slightly over
OCR for page 122
n_
0 ~7
I 200,000 CFS _'
300,000 CfS _p
doo,ooo CfS =4
/~J ,
. .
1~ THALWEG
l
10 t2 14 18 10 ~r
. , __
,__.
i/'
~..!
__ 22
~ILES BELOW HEAD OF PASSES
PROFILES OF SALT-WATER INTERFACE
SOUT~ES' r~se. - ~m RIVER
Figure 10. Freshwater discharge determines penetration
of saltwater upstream.
/////,~H/GA/ T/DE
~LOW T/DE
I i!ll
,, ,~, , , ,,, ~,iY
r ~ / I ~ ~ / ~ ~ r r r I / ~ '.1 ~ r r ~ r [' ~ / ~ ~ ~ I / /~7;~/ /~/////////
Figure 11. Conditions typical of well-
mixed estuary.
FRESH WATER
~ 4 2 O 2
___ fLOOD EBB
1~(PICAL DISTRIBUTION OF VELOGITY
IN WELL MIXED ESTUARY
Figure 12.
rl
lool- I ~- -
fLOOD EBB
-
SALT WATER
I I I , ~ .c
l
20-
~ 3C
J
40 i_ 23
O ~
60 - ~x
Z 4,l
~ <
8^v ~ , 1!
~ O
__ 14
4.vv
~1
LO~ D/~
~ ~SC~~G~
° so 70 so 4~_D/S~RG~
~ILE5 ~e~ ~H
SALINITY F!ROFILES ~ DELAWARE RIVER
Figure 13.
OCR for page 123
123
the bottom ebb currents. Salinities decrease more or leas
progressively from seawater at the entrance to fresh water in the upper
reaches, and bottom salinities normally exceed those at the surface by
15 to 25 percent.
The Delaware River estuary is a typical example of a well-mixed
system. Surface and bottom salinity profiles are shown in Figure 13,
and it can be seen that there is essentially no vertical salinity
gradient anywhere along the estuary. As in the highly stratified case,
the upstream extent of saltwater intrusion is dependent on the
freshwater inflow, as can be seen in Figure 13.
The partly mixed type is obviously an intermediate step between
highly stratified and well mixed (Figure 14~. The tidal mixing forces
are great enough to break up the well-defined wedge, but not strong
enough to effect complete mixing. The "interface" at high tide Is
considerably steeper than at low tide, and the Interface moves over a
considerable distance in the estuary with each tide.
As In the well-mixed case, there is a reversal of flow direction at
all depths with the changing tide. When the current changes from flood
to ebb, the reversal at all depths occurs almost simultaneously.
However, when the current changes from ebb to flood in the lower
portions of a partly mixed estuary, reversal at the bottom occurs as
much as two hours before reversal at the surface. Thus, at the bottom,
the duration of flood flow is usually greater than the duration of ebb
flow.
In the region of the estuary just upstream from saltwater
intrusion, the current at all depths reverses with tidal phase, and the
vertical distribution of the current in either direction is similar to
that in an upland river; the downstream current at all depths
predominates over the upstream current because of the freshwater
discharge (Figure 151. In the region of saltwater intrusion, the
direction of the current both above and below the interface reverses
with tidal phase. Above the interface, the net flow is downstream;
below the interface, the net flow is upstream. The interface between
the fresher water in the surface strata and the saltier water
underneath is not so well defined as in the highly stratified type;
however, the presence of the Interface is often indicated by a
discontinuity of either the vertical salinity profile or the vertical
velocity profile.
Savannah Harbor is an example of a partly mixed estuary. Typical
salinity gradients at the entrance and about ten miles upstream are
shown in Figure 16.
It was previously pointed out that the length of saltwater
intrusion varies with the freshwater discharge. For any of the three
mixing types, it has been found that an increase in freshwater
discharge reduces the length of saltwater intrusion. However, the
turbulent mixing generated by the tidal currents is more important than
is the freshwater discharge. The Lower Mississippi River (highly
stratified) and the Delaware River (well mixed) both have controlling
depths of 40 ft. In the Mississippi River, a net downstream freshwater
velocity of 0.83 feet per second (fps) results In an intrusion length
of 125 miles for a river discharge of 128,000 Ifs. On the other hand,
OCR for page 124
h//Gh/ T/OE
LOW t/DE~ //~/
17,,,7
Figure 14.
~20
¢40
o
- 60
z
~0
CL
CONDITIONS TYPICAL OF
PARTLY MIXED ESTUARY
O FRESH WATER
l
_ I
I
l
l
I
l
I
l
100 11 _
4 2 O 2 4 4
fLOOD EBB
o:
SO
~,
< 100
o
z
v
SO
Figure 15.
\ l
\ ~ HIoH
~TIDt
\ ~
20
- ENTRANCE
~o
\\~/GH T/DE
LOW _,>< \
1 ~ \ I I
/
1_ 1/
t
_ SAL7 WAT ER O
t /
1, 1.
zo ~w
0
J
40
60z
80 ~
FLOOD E 68 100
TYPICAL DISTRIBUTION OF VELOCITY
IN PARTLY MIXED ESTUARY
c, 6
z
o
~ 2
l~ n
I,~ V
~ 2
: ~BB /
- - 1 1 1 1 1
TOTAL AREA SUBTENDED BY BOTH EBB AND fLOOD
CURVES =TOTAL fLOW
2
0m
Z
2-
o 4 AREA SUSTENDED BY EBB CURVE DIVIDED BY TOTAL 43
I3J AREA-PERCENT OF TOTAL FLOW DOWNSTREAM
6 1 1 1 1 1 1 6>
0 2 4 6 8 10 12
TIME IN HOURS
~FL OOD
~oo
0 20 40
SALI N ITY, P PT
10 M I L ES UPSTREAM COMPUTAT I ON OF FLOW Dl STR I BUTI O N
SAVANNAH HARBOR
Figure 16.
Figure 1 7.
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125
in the Delaware River, a net downstream freshwater velocity of only
0.13 fps holds the intrusion length to 70 miles for a river discharge
of 12,000 cfs. The Mississippi River is a very narrow estuary and has
a tide range of only about 1-2 feet. It thus teas a relatively small
tidal prism. The Delaware, on the other hand, has a very broad bay and
a tide range of 5-6 feet. It thus has a relatively large tidal prism.
Mixing
It has been found that the degree of mixing is generally a function
of the ratio of the freshwater discharge over a tidal cycle to the
tidal prism, where tidal prism is defined as the total volume of water
entering the estuary from the sea during the flood (rising) tide. When
the freshwater inflow is high compared to the tidal prism (ratio
greater than about 0.8), the stratified condition results. When the
opposite is true (ratio less than about 0.1), the well-mixed condition
results. It should be noted, however, that the degree of mixing can be
affected by several other factors such as wind, waves, ships, and
turbulence at the mouths of tributaries and channel constrictions.
The freshwater discharge/tidal prism ratio is by no means an exact
measure of the degree of mixing. It is not possible, for example, to
define accurately the relative degree of mixing among estuaries having
reasonably similar stratification conditions. Perhaps a more reliable
parameter for defining the degree of stratification in the Estuary
number n developed by Harleman and Ippen. The estuary number is defined
as
P F 2
estuary number = t o
QfT
where
Pt = tidal prism (the volume of seawater entering the estuary
on the flood tide)
u
Fo = Froude number =v~;~; uO is the maximum flood tide velocity
at the ocean entrance, and h is the mean depth of the estuary
Of = freshwater discharge
T = tidal period
The degree of stratification increases with decreasing value of the
estuary number.
An estuary may be changed from bighly stratified to partly mixed or
well mixed by reduction of the freshwater disabarge; conversely, one
may be changed from well mixed or partly mixed to partly mixed or
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126
highly stratified by increasing the freshwater discharge. Such a
change can be effected by a long-term change in freshwater discharge
resulting from upstream flow modification or by seasonal changes in
freshwater discharge.
Minor changes in mixing types are being constantly effected by
deepening, widening, lengthening, or other improvements to estuary
channels for navigation. As channels are dredged deeper and deeper,
the salt water penetrates farther into the estuary and the degree of
vertical stratification of the fresh and salt water is increased
because of increased tidal prism and reduced tidal current velocities.
Flow Predominance
I now want to introduce the concept of flow predominance. This is
a very useful concept for analyzing velocities, especially with respect
to density currents. For this method, velocity observations are made
at several depths at a given location, and the data are reduced to an
expression which tells whether the predominant flow at each depth is
upstream or downstream and in what percentage of the total flow at that
point. A conventional plot of velocity vs. time is made for the
observations at each point (Figure 17~. The area subtended by the ebb
curve is then divided by the sum of the ebb and flood curve areas. The
result defines what percentage of the total flow per tidal cycle at
that point is directed downstream, and is referred to as the ebb
predominance.
At the bottom of a saltwater wedge, the flow predominance can be
100 percent upstream; while in the freshwater layer at the surface, it
can be 100 percent downstream, as shown in Figure 18. Near the
entrance of a well-mixed estuary, the flow predominance will be
slightly upstream at the bottom, but more strongly downstream at the
surface, as shown in Figure 18. Farther upstream, the flow
predominance will be downstream throughout the entire depth. In a
partly mixed estuary, upstream bottom predominance will be fairly
strong at the entrance {Figure 18), and will extend a considerable
distance upstream. Surface flow predominance will be strongly
downstream througbout the estuary, except in areas under the influence
of large-scale eddies.
To obtain a broader impression of flow conditions in the estuary,
it is possible to plot a profile of flow predominance along the estuary
at various depths. That location along the channel at which the net
flow i" balanced (50 percent ebb) is called the null point. That is,
there is no net flow in either direction. The surface and bottom flow
predominance for Savannah Harbor are shown in Figure 19. The null
point on the bottom for this freshwater discharge is located where the
dashed line crosses the 50 percent downstream line.
An alternate means of determining time-average flow conditions at
any point is referred to as velocity predominance. In this case, it is
only necessary to determine the velocity at any point averaged over a
complete tidal cycle. The result is then ~nondimensionalized~ by
dividing by the freshwater velocity. An advantage of the velocity
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127
C)
~ 50
o
z
P~RrLY "/~a
,/1/ELL /~/J(ED-
~:
H/GA/LY /
sr~Ar/~/~>
1004 50 100
PERCENT OF FLOW DOWNSTREAM
T YP I CAL FLOW PRE DOMINANCES
Figure 18.
~ 75r ~1 1 1 1 1 1 17~
1r
z
3
o
o
~50
o
z
=
0
- MEANTIDE ~ ~~ z
_ RIVER O~5670 CfS ~_ ~_
~ _ ~ I I ~ I I ~ ~ 25
__
93 1 13 133 153 173 ~3
1000 Ft CHANNEL STATIONS
RELATION BETWEEN NORMaL SURFAGE
AND BOTTOM FLOW IN SaVANNAH HARBOR
,'A/~EL ~O~L/NG VFRY
\ /~/F~VY/W TH/S~F~CH
~ _ ~,
, `,
_
50
Figure 19.
4 ~ ~' ~ ~ ~ ~ r~ ~----_ _ ~
r 1 C af - 7,vCO CFS I I
3 `` n Q Qf- ,C30 CFS
~o Qf - ~8,230 CFS ~I _~
2 ~__\\~_~ ~ ~;'
O ~ DELAl2AR ~
. 1
0.01 0.05 0.1 0.5 1
l
SHOALING FOR TWO-~JEEK PERIOD
SOUTHWEST PASS, MISS. RIVER
Figure 2 1.
_1 L 1_ I I I ~ I I I I I I I I ~ I
, Vf
% 4(AP)
BOTTOM VELOCITY CORRELATION
Figure 20.
\~ ~ ( ArtANr/C
~'\ ~'
NF~
1 1 ' 1 ~ ~ I
SAVANNAH
· · I 1 1 · · .
I I I L f~LL'
l
,_
LOCATION MAP
SAVANNAH HAR BOR
ll
~/
Figure 22.
OCR for page 128
128
predominance method is that it appears to have a unique correlation
with the local densimetric Froude number for a given estuary, as shown
in Figure 20. It appears that this correlation is not affected by
changes in freshwater discharge, tidal amplitude, or channel depth.
Thus, it should be possible to predict the position of the bottom null
point for various disabarges or depths.
Salinity Effects on Shoaling
Saltwater intrusion plays an important role in estuarine
sedimentation. First, the salt water probably causes flocculation of
suspended clays, which prevents them from being carried to sea in the
upper flow layer. Second, density currents can move sediments upstream
along the bottom to the vicinity of the flow predominance null point.
For highly stratified estuaries, rapid shoaling will be experienced
at the tip of the saltwater wedge. As mentioned before, the tip of the
saltwater wedge at Southwest Pass is located at the outer end of the
jetties for a discharge in the pass of about 300,000 cfs. The shoaling
pattern developed in a two-week period with a freshwater discharge
varying from 248,000 to 294,000 cfs is shown in Figure 21. The
shoaling does indeed bracket the tip of the saltwater wedge. Note that
the maximum change in depth was 28 feet.
In partly mixed estuaries, the flow predominance null point usually
is an area of heavy shoaling. In well-mixed estuaries, however, that
is not usually the case, since density currents are quite weak.
Savannah Harbor
Savannah Harbor (Figure 22) is an excellent example of a partly
mixed estuary. More than 75 percent of the flow is carried through
Front River and North Channel. The relation between the bottom flow
predominance and the shoaling rate in the navigation channel is shown
quite dramatically in Figure 23. Of an annual shoaling of roughly
seven million cubic yards, more than two-thirds occurs in the six-mile
reach which brackets the bottom flow predominance null point.
Savannah Harbor also gives a striking example of the effects of
increasing channel depth on shoaling. The navigation channel has been
progressively dredged from 26 feet in 1889 to its present depth of 36
and 34 feet. Profiles of the various channels are shown in Figure 24.
Reliable dredging records are available for the periods indicated in
the figure. For purposes of analysis, the navigation channel was
divided into thirds, and the average annual shoaling rate was
determined for each of the four time periods for each channel section
(Figure 25~.
The shoaling rate in the downstream third has decreased steadily,
so that almost no dredging in now required. In the central third, the
shoaling rate increased rapidly until the present channel was
constructed, then it decreased significantly. The upstream third ebows
a very rapid increase in shoaling. By examining these data, we can
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follow the upstream migration of the zone of primary shoaling and, by
implication, the null point of the bottom flow predominance. The upper
dashed line in Figure 25 shows that the harbor-wide shoaling rate
increased essentially linearly until the final and most drastic
deepening was effected. In this final period, harbor-wide shoaling
exhibited a much smaller increase than for the previous channel
deepenings. This indicates that by the time of the final deepening,
density effects in the harbor had developed to suab a degree that
almost all potential shoaling material was trapped within the harbor.
Thus, there was an upstream shift in the region of heaviest shoaling
but relatively little increase in the total volume of annual shoaling.
The early upstream migration of the major shoal area was not a
significant problem because disposal areas for dredged material from
earlier dredging operations were readily available. In the last time
period, however, the region of major shoaling was in the port area,
where disposal areas are not readily accessible. Those that are
available are rapidly being filled. Thus, in addition to greatly
increasing the volume of material that must be dredged, the enlarged
channel shifted the location of the shoaling to an area where disposal
of the dredged material is increasingly difficult and expensive. Such
a change in maintenance operations and costs should be identified in
the design process to enable determination of overall project costs and
potential environmental problems.
DISCUSSION
WEBSTER: Have you investigated what forces act on the ships,
or how ships maneuver when the current is going one way on the surface
and another way below?
HERRMANN: No, we haven't. In the reports of some
investigations, we have noted areas where these types of currents seem
particularly severe. We communicate the results to the field offices
doing the design work. I am not sure whether they take that sort of
thing into account, but it certainly could be a problem.
SEARLE: Based upon a lot of experience finding and raising
ship" on the bottom of the ocean, T believe current curves that are
plotted over a tidal cycle are useless. You have to plot the tidal
current in the vertical column across the full lunar cycle. I can cite
you several examples. A salvage operation we are now engaged in, for
example, in Newfoundland: at one end of the tidal cycle, you get a
reversal of the current. To me, supervising divers on the bottom,
reversal of the current means that I get zero current for perhaps 5
minutes or 10 minutes or 30 minutes: I get a low-current window.
At other parts of the tidal cycle, the lunar cycle, there is no
reversal in some places in the world. If you need to know the forces
exerted by currents on a ship, you must know the tidal stage and the
lunar cycle stage.
131
BERRHANN: This has other implications. We have a model of
the Cbesapeake Bay and during this past year, I guess it was, we were
asked if we could help to locate some barges that had sunk. The Coast
Guard was concentrating its efforts downstream from the location where
they sank. With our knowledge of what the currents were in that area,
we said, the predominance of flow in the bottom is upstream. Go
upstream and look for them. They found them upstream.
SEARLE: Yes, T can cite you cases of looking for a sunken
barge. If you look for it at a particular time in the tidal cycle, you
will find it. At another part of the lunar cycle, you won't find it.
