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A PHYSICAL DESCRIPTION OF
THE SALMON RIVER ESTUARY
by
David Askren
Robert Hansen
Bruce Higgins
Scott Noble
Robert Pratt
CE 572
Marine/Estuarine Water Quality Dynamics
Spring 1976
Ocean Engineering Programs
Oregon State University
Corvallis, Oregon 97331
.•,
•
TABLE OF CONTENTS
Introduction.
Field
P~ocedure
............
And Equipment . . . . . . . . . .
1
1
...
5
Geography and Geology . . . . . . . . . . . . . . 6
Hydrography • . . . . . .
9
Tides •• . . . . . . . . . . . . . . . . . . . . 11
Currents. . .
. . . . . . . . . . . . 15
• • •
Water Quality . . . . . . . . . . . . . . . .
19
Flushing Prediction
. . . . . . . . . . . . . 26
Estimation Of The Dispersion Coefficient. . . . . 33
Pollutant Prediction. . . . . . . . . . . . . . • 35
Inlet Stability . . .
. . . . . . . . . . . . 37
Classification. . . . . . . . . . . . . . . . . . 38
Modeling Of The Salmon River. . . . . . . . . . . 39
Appendix A. . . . . . . . . . . . . . . . . . . . 41
Appendix B. • • • • •
. . . . . . . . . . . . 68
Water Quality Data Procedure.
..
•
LIST OF FIGURES
FIGURE
PAGE
1
Tidal and Water Quality Station Locations
2
2
Bubbler Tide Recording System. Station 0.7.
4
3
Average Cross-Section Depth by Segments
8
4
Observed Tides, Stations 0.5, 1.92, 4.86
12
5
Surface and Bottom Currents. Stations 0.5 & 0.7
17
6
Temperature and Salinity. Station 0.5
20
7
Temperature vs. Salinity at Station 0.5
21
8
Temperature vs. Dissolved Oxygen. Station 0.5
22
9
Average Salinity vs. Distance from Mouth
27
BOD vs. Distance from Mouth for Hypothetical
Pollutant Discharges Located at Various Points
Along the Estuary
36
10
,.
I
LIST OF TABLES
TABLE
PAGE
I
Reported Surface Areas of Salmon River Estuary
6
II
Average Estimated Discharge of the Salmon River
11
III Predicted Tides, and Observed Lags and Ranges
_, I
13
IV
Observed Tidal and Current Characteristics
18
v
Modified Tidal Prism Method Calculations
30
VI
Fraction of Fresh Water
31
VII Dyer-Taylor Segmented Tidal Flushing Method
32
VIII Estimated Dispersion Coefficients
34
IX
Taylor Dispersion Coefficients
34
X
Fresh Water Fraction and Pollutant Concentration
for the Fraction of Fresh Water Method
37
,_'
PLATES
PLATE
1
i
PAGE
Salmon River viewed from Riverhouse, Cascade
Ranch
· 10
'"'
INTRODUCTION
The Salmon River estuary lies roughly 137 km (85 mi) south of the
mouth of the Columbia River and 8 km (5 mi) north of Lincoln City.
The
estuary and some of its surrounding area are scenic natural areas and are
at present being considered for inclusion in the Siuslaw National Forest.
The Salmon River estuary is in fact one of the few Oregon estuaries which
has not suffered significant modification by man--no dredging or jetty
building has occurred--and thus offers one the opportunity to observe the
asthetic and scientific characteristics of a natural river estuary.
A modest amount of data is presently available concerning the
geology and hydrology of th.e estuary.
biology~
Detailed studies of tides, currents
and salinity distributions, however, had not been made prior to the field
--.,.,
study reported herein.
During a three-day period (14-16 May 1976), con-
tinuous tidal-elevation and current data were collected at three stations,and
conductivity, pH, temperature and dissolved oxygen were monitored along
the length of the estuary.
These data are reported herein and have provided
the basis for the characterization of the estuary as regards flushing,
friction effects, etc.
FIELD PROCEDURE AND EQUIPMENT
Station locations are shown in Figure 1.
the distance in kilometers from the mouth.
"
Station numbers indicate
Apart from the three tide sta-
tions and Station 0.5, which were located in advance so as to provide
input to various models, the water quality stations locations reflect infield sampling experience.
An
attempt was made to locate the salinity
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1. 33 st-ation Number And Distance
In Kilometers From Mouth
~ Water Quality Station
~
Tide Station
Figure 1.
/
. , ..,,
Tidal and water quality-station locations.
feet
+ --N
3
wedge and to sample at a sufficient number of stations to permit the
characterization of the estuary from the water quality viewpoint.
Tide stations 1.92 and 4.86 were equipped with the mechanical Stevens
tide recorders whereas station 0.7 had the "bubbler" type shown in Figure 2.
The bubbler system was of experimental design and was constructed from
available parts at Oregon State University.
The system consists of a
pressure transducer connected to an open-ended tube continuously supplied
with air from a scuba tank.
The open end of the system is mounted below
the lower low level and changes in the depth of the overlying water column
are registered by a corresponding linear increase in air pressure within
the system.
The air pressure is monitored via the transducer and recorded
on Rustrak strip-chart recorders.
gas supply to it.
The system is kept full of
ai~
by
a slow
Changes in atmospheric pressure have no effect as the
sensor is a differential type with one side open to the atmosphere.
The
system has the additional advantage of low interference with public use of
the area.
In this
study~
the open end of the system was fixed at lower low
water (-1.3 ft, approximately).
Station
o:s
consisted of an anchored skiff equipped to continuously
monitor current velocity and night-time water quality.
The skiff was
double-anchored mid-channel 0.5 km from the mouth where the channel was
most constricted.
A Hydro products Savonius rotor and a Gurley cup-type
current meter were suspended from the boat to depths of 0.3 and 0.5 m,
respectively.
Such shallow depths were required to prevent the equipment's
striking the cobbled bottom at low tide.
During night-time hours a Hy-
droproducts Hydrolab Surveyor S system monitored water quality parameters
0.3 m below the surface.
A six-channel Elnik NSC recorder was coupled
_,
Figure 2.
Bubbler Tide Recording
System. Station 0.7
5
to a!_l systems.
In addition to the tide sensor at Station 0.7, a Marsh-McBirney
electromagnetic current meter was installed on the bottom opposite the
River House of Cascade Head Ranch.
The data from this station and
Station 0.5 are tabulated in Appendix B and presented graphically in
Appendix A or within this text.
~-
WATER QUALITY DATA PROCEDURE
The water quality and current data for all other stations in the estuary
was collected from a boat which moved from station to station.
The water
quality data was obtained by lowering the Hydrolab sensor over the side of
the boat to the desired depth, as indicated by taped 1-foot intervals on
the sensor cable.
Readings of conductivity, temperature, dissolved oxygen,
and pH were taken at each depth and recorded manually.
Nearly all water
quality data was taken at 2-foot intervals from the even-foot of depth
nearest to the bottom, to one foot below the water surface.
If a large
conductivity gradient was evident between two depths, another reading was
taken at the intermediate-foot depth, to provide more accurate profiles.
Total water depth and time were also recorded at each station.
The Hydro-
lab sensor was kept in a bucket of estuarine water when the boat was in
transit, to speed the system response when placed in
the DO sensor wet.
~he
water, and to keep
After DO data was taken once for all the stations in
the estuary, it was evident that the estuary was well aerated at all depths
and the DO readings were discontinued for the remainder of the study.
Current data was taken simultaneously with the water quality measurements.
Current speed was measured and manually recorded for a variety
of depths at each station.
The instrument used was a Gurley current meter,
6
which was weighted to maintain it in a horizontal attitude and lowered
over the side of the boat.
The taped one-foot intervals on the instru-
ment cable were found to be inaccurate, and instrument depths should
be considered to be accurate to only
~
1 foo,t.
Except when the instru-
ment was near the surface and hence visible, the current direction was
not known.
GEOGRAPHY AND GEOLOGY
The estuary is about 1.09 km 2 (270 acres) in area; reported surface
and tideland areas are given in Table 1
(Percy,~
mately 60 percent of the surface area is tidelands.
al.; 1974).
Approxi-
The remaining 40 per-
cent comprises the main channel of the Salmon River which is well defined
and strongly prismatic from the head of tide, located approximately at Otis
6.9 km (4.3 miles) from the estuary mouth, to about river kilometer 3.6
(Figure 1).
Farther downstream, the channel is poorly defined and large
tide flats predominate.
Table I
Reported Surface Areas of Salmon River Estuary
Reference Surface Area
km 2
1
0.69
2
1.77
.3
0.83
0.32
Measured At
Tidelands
km2 percent
Submerged Lands
2
km
percent
1.01
0.51
0.76**
0.32
HW
*
~
MLT
*Specified as the area affected by tidal action
**Interpreted by authors of this report
(1)
(2)
(3)
Johnson, J.W. (1972)
Marriage, L.D. (1958)
Oregon Division of State Lands (1973)
57
62
43
38
7
The estuary volumes for low and high tide and the resultant tidal
prisms for 20 segments (Figure 3) were computed in the following manner.
Discrete (in time and space) depth measurements and a single longitudinal
bathymetric profile coupled with 2 tidal elevations records made it
possible to estimate the mean high and low water depths for each segment.
Linear interpolation between the tidal records produced segmented tidal
ranges.
The low water depths was estimated as the measured depth minus
the height relative to the predicted low water.
The high water depth
was taken as the sum of the low water depth and the tidal range.
Finally,
volumes were computed from these mean depths and planimetered surface areas.
The bathymetry of the estuary is characterized by extreme shallowness
and low relief except near the mouth where the waterway abutts the steep
flanks of Cascade Head.
Channel depths at MLLW vary from less than 0.6 m
(2 ft) to as much as 2.3 m (7 ft) between the mouth and kilometer 4.0.
Farther upstream, channel depth was much more uniform, about 2.3 m (7 ft)
although depths in excess of 3 m (10 ft) were
notable
shallowness-~less
encounte~ed.
Three zones of
than 0.6 m (2 ft) at MLLW--were also identified.
, The longest shoal reach lies between stations 1.92 and 2.68.
The second
reach occurs between the bend at kilometer 4.0 and station 4.24 and the
third at approximately station 5.28.
Finally, a relatively dense and
apparently stable log jam at kilometer 5.6 forms a dam-like structure to
river and tidal flow as well as to sediment movement.
The sediments underlying the Salmon River flood plain downstream of
Otis are characteristically silty sands which are competent enough to support
near-vertical river banks as far downstream as kilometer 3.5.
Tideflat
sediments are similar to these sediments but the river channel sediments are
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29
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$CALl
.,ijiii"-~.,.;--.--=~-
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Figure 3.
Average cross-section depth by segments.
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9
characteristically coarse silty sands upstream of kilometer 1.5, approximately.
Farther downstream than the Lincoln County boat ramp, the quartz
sands from the north-trending sand spit and the dark igneous cobbles and
gravels from the erosion of Cascade Head form a sharp contrast.
The white
sands are restricted to the spit by strong tidal currents which flow along
its margin.
The channel bed is composed principally of dark cobbles and
gravels although extensive, well-formed dunes are present at kilometer 0.7
(Plate 1).
The sand spit's encroachment upon Cascade Head has produced a welldefined estuary "throat" or channel constriction at kilometer 0.3 and a
shallow sand bar at the recurved mouth of the estuary.
The estuary mouth
and the sand spit are partially exposed to wave attach from the west and
southwest but are protected from northerly waves by Cascade Head.· Wave
attack and other factors have led the Oregon State Conservation and Development Commission (1974) to classify the sand spit as only "partially
stabilized" with "critical erosion" occurring on its seaward side.
However,
it has been estimated (Oregon State University, 1971) that the estuary as
a whole is receiving some 12,730 metric tons (14,000 tons) of river sediment
per year.
HYDROGRAPHY
2
The Salmon River watershed comprises an area of some 200 km 2 (78 mi )
which annually yields an estimated 555 million m3 (450,000 acre-feet) of
runoff (Oregon State Water Resources Board, 1965).
charges have been tabulated in Table II.
Average monthly dis-
Summer flows
are·-~extremely
weak,
the lowest recorded August flow having been only 0.61 ems (21.7 cfs).
Winter discharges however, are sufficiently strong to cause the periodic
11
flooding of approximately 1000 acres of lands bordering the estuary.
During
the study period, the river flow at tidewater was estimated at 3.4 ems
(120 cfs), a level substantially below the estimated May average of 8.6 ems.
Table II
Averag_e Estimated Discharge of the Salmon River
(cubic meters per second)
Oct.
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May
June July
Aug.
Sept.
8.1
24.3
32.3
28.9
33.1
23.2
15.8
8.6
4.2
1.2
2.0
2.2
Mean annual discharge 15.23
TIDES
Previous to this study little was known concerning the nature of this
estuary's tides.
The mean tide range was described-as 1.8 m (5.8 ft) with
a diurnal range of 2.3 m (7.6 ft) and an extreme range of 3.9 m (13.0 ft)
(Percy~
al., 1974).
The Corps of Engineers had determined the head of
tide to occur slightly downstream of Otis at river kilometer 6.9 (mile 4.3).
Figure 4 shows the tidal data collected at Stations 0.70, 1.92 and 4.86
as well as the times and heights of the predicted (NOAA, 1976) ocean tide.
Unfortunately, an apparent malfunction of the bubbler system used at Station
0.50 has invalidated the tide height data from that station.
of high and low tide are shown.
Only the times
The elevations for Station 3.02 are refer-
enced to the 1929 mean sea level datum via a uss·PR bench mark in the new
U.S. Highway 101 bridge over Salmon River which gives an elevation of 5.50 m
(18.33 ft).
The tidal data for Station 1.92 is referenced to a surveyors
nail in the Lincoln County boat ramp.
No levels were encountered for this
point and hence only relative elevations are given for Station 1.92.
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Station 1.98.
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....,
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Times of tidal extrema at Station 0.5
9
...,
•
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C'O
0
...,,.,
•ri
1
CD
>
>
0
0
0
0
May 15~
0
0
0
0
0
0
\0
0
0
0
C\1
.-t
0
0
. .-t
0)
~May 17
Time in Hours
Figure 4.
Observed tides, Stations 0.5, 1.92,4.86.
......
N
13
The tidal data tabulated in Table III indicate several important points.
Firstly, the tidal predictions for Nestucca Bay--which lies only 9 miles
to the north of Salmon River--are not applicable to Salmon River.
The tides
observed at Station 0.28 occurred up to 41 minutes earlier than predicted
for high tides and as much as 71 minutes later for low tides.' Secondly,
the lag between Stations depends greatly upon location and whether the tide
is flooding or ebbing.
High tide lags between Stations 0.50 and 1.92 were
substantially longer than for low tide whereas between Stations 1.92 and
4.86, the opposite is true.
Such contrasting behavior indicates that the
estuary has at least two hydraulically distinct sections.
The exaggera-
tion of the low tide lag between Stations 1.92 and 4.86 is presumably due
to the extremely shallow reach of channel between kilometers 1.92 and 3.6.
Table III
Predicted Tides (NOAA, 1976), and Observed Lags and Ranges
Day Tide
14
HL
Predictedt
Time
(PDT)
1947
Lag RelaLag Rela·
tive to
tive to
Predicted
Sta 0.70
(min) at
(min) at
Station*
Station
0.70 1.92 4.86 .!.:21 1.:!2.
26
98
-
Range
Lag Sta
1.92 to
Sta 4.86
(min)
Pred.
62
Observed Sta
0.70 1.92 4.86
-
1.92
15
16
HH
0123
4
18
14
LL
0850
126
192
136
LH
1512
-18
-7
HL
2058
-2
-5
57
-41
-4
HH
0210
-37
19
30
11
-3
59
62
0939
-
2.01
1.83
-3.11
-2.23 -1.83
2.50
1. 711 1.34
-1.25 (-2.81) 1.46 1.34
1.77
(1.62) 1.92
36
-2.96 ( -2. 56)
71
53**
*Negative values indicate earlier occurrence than predicted.
**Low tide at mouth bar observed at 1033.
()Suspect tide ranges
tPredictions for Nustucca Bay based on Humboldt Bay.
LL
(M)
14
Tidal range were observed to decrease with distance up river as would
be expected to occur from frictional effects.
However, despite the obviously
strong influence of the shallow reach mentioned above in slowing the low
tide, relatively little tidal energy is dissipated over that stretch since
the ranges observed at Station 4.86 were on the average, only 15% lower than
at Station 1.92; the greatest dissipation (23%) was associated with the range
from higher high to lower low tide.
The physical form of the tide curves (Figure 3) is of interest in that
they also reflect the influence of the shallow bathymetry of the river
estuary. Whereas most tides produce smoothly changing sinusoidal waves, the
low-water portion of these records shows an anomalous "shark fin" or ttkeel"
shape.
The tide level drops smoothly.to a low-water level which may be sus-
tained over a considerable period of time as at Station 4.86 for the range
from higher-high to .lower-low water.
The onset of the flooding tide is
remarkably abrupt and the rising leg of the curve is almost linear.
Such
/'
behavior of the flooding tide is suggestive of a tidal bore or flood wave.
It should also be noted that whereas the low water levels of Station
1.92 differed by as much as 0.2 m (0.8 ft), the low water level at Station
4.86 was uniform over the entire study period.
The "bottoming out" of the
upstream tide curve serves to emphasize the control that the downstream section of shallow channel has on the tidal dynamics of this estuary.
Leveling
of the tide recorder with reference to a benchmark on the Highway 101 bridge
gave a low water stand at Station 4.86 of 0.15 m (0.5 Jt) below the 1929
sealevel datum.
This level would approximately correspond to the riverbed
elevations of the shallow reaches previously noted.
Thus, in addition to
delaying the arrival of the low tide, the shallow section acts as a natural
'
T
15
I
spillway.
In this case, the flow through the shallow reach should be appro-
ximately equal to the river flow during periods of very low flow.
Dynamically, the tide observed at Station 0.5 has characteristics of a
semi-standing wave in that tidal maxima and minima occur within one to two
hours of slack water.
For. a purely standing wave slack water and tidal ex-
trema would occur-simultaneously.
Unfortunately, the uncertainties associa-
ted with the tidal data from Station 0.7 prevented the determination of the
reflection and damping coefficients via the cooscillating tide method.
It
should be noted that the variation of depth along the length of the estuary
violated a basic assumption of this method.
Whether this method can be modi-
fied to take into account such variations remains to- be--seen.
An attempt was made to apply the Harleman-Thatcher (1973) one-dimen-
sional numerical tide model using bathymetric and flow data obtained from this
survey.
Reasonable agreement for tide ranges and lag times between Stations
1.92 and 4.86 but only temporal agreement of Station 0.70 was achieved,
again due to the limitations of the data at that Station.
The relation of the
estuarine tide to the ocean tide is still unknown since no data was obtained
outside'of the estuary and the predicted times did not agree well with those
observed.
Manning coefficients of 0.03 and 0.05 were required, reflecting
the importance of frictional damping during wave propagation.
This model could
be utilized to predict the effects of altering the channel characteristic,
e.g. through dredging, or of an increase or decrease in river flow.
CURRENTS
Flows along much of the estuary were characteristically weak, commonly
less than 0.3 mps (1 fps), and data was collected sporadically in time and
space over most of the estuary.
Stations 0.5 and 0.7 however, provided a
16
reasonably continuous record of surface (Station 0.5) and bottom (Station 0.7)
currents.
As shown in Figure 5 the surface current achieved values as high
as 1.5 mps (4.5 fps) on both flooding and ebbing tides.
These values are
less than the maximum currents which were observed to exceed 2 mps (6 fps).
These peak velocities were. restricted to the margin of the sand spit and undoubtedly have a great influence on the stability of that deposit.
The boat
and current meters were moored out of the zone of maximum velocity due to
unstable anchoring conditions.
Bottom velocities are naturally lower than surface velocities and reflect in part a greater channel cross-section at Station 0.7 than at Station
0.5.
The velocity profile is characterized by a peculiar oscillation which
occurs approximately midway in time between tidal extrema.
This oscillation
may be related in some manner to the influence of the shoal region or regions
in the estuary which lie at or near sea level.
Such effects may be due to a
· relatively sudden effective lengthening or shortening of the estuary as water
flows over or is blocked by the shoal.
Despite the shortness of the record, the graphic data and Table IV show
that current reversals at Station 0.5 and 0.7 are not simultaneous.
flows tended to lag the surface on the change to a flooding tide.
particularly evident towards the end of the record.
Bottom
This is
Such behavior is abnormal
in that one generally idealizes estuarine flow by an earlier flooding bottom
flow than surface flow.
One unlikely~~xplanation for this behavior may be
that the density of the estuary water exceeded that of the incoming sea water.
A more probable explanation is that the flooding current favored the seaward
side of the estuary whereas there still existed a downstream flow along the
inland portion of the channel where the current meter was positioned.
Surface Current
--- Bottom Current
1.5
"0
0
01.0
.-t
1.1..
0.5
-
..0
..0
(J.J
1.0
1.5
Figure 5.
Surface and bottom currents.
Stations 0.5 and 0.7.
18
Table IV
Observed Tidal and Current Characteristics
Day
May
Tide Stage
Sta 0.50
Time
Current Stage
Time
Time
Surface
Bottom Difference
Sta 0.50 Sta 0. 71
(min)
14
H Slack
Peak Flood
2100
0000
15
L Slack
Peak Ebb
H Slack
Peak Flood
H Slack
Peak Ebb
L Slack
0200
0420
1052
1400
LH
1435
HL
2056
HH
0129
LL
1050
16
Peak Flood
H Slack
Peak Ebb
L Slack
0207
0528
1028
1050
1100-1400
1500
1730*
2100
25
0030
0230
1030'*
61
4
HS = High Slcak Water
LS = Low Slack Water
HH = Higher High Tide
LH = Lower High Tide
HL = Higher Low Tide
LL = Lower Low Tide
*Secondary minimum ebbing flow present.
For the limited data available, it was observed that high slack water
occurred approximately 30-60 minutes after high tide but that the low slack
occurred at about the same time as
lo~
tide.
Such behavior again may re-
fleet some influence of the shoal reaches of the estuary since a flooding tide
would experience an increasingly longer estuary as time progressed and the
flood would proceed although the maximum elevation had already been reached
downstream.
Conversely, the low tide and slack water would be expected to
coincide as less water would need exit since the estuary would continually be
"shortening" with time.
19
WATER QUALITY
The temperature, salinity, dissolved oxygen and pH data observed by the
continuous monitoring system at Station 0.5 are presented graphically in
Figures
~, __?_and
8 and in Appendix A and are tabulated in Appendix B.
Fig-
ure 6 demonstrates the inverse relationship between temperature and salinity,
the flooding ocean water being up to 5°C cooler and in excess of 15% more sa/
line than the ebbing estuarial waters.' Figure 7 implies a semi-hysteresis
type curve for temperature and salinity, the T-S curves tending to close
back upon themselves periodically.
However, both ebbing curves are for
approximately the same tide range, yet the average slope of each curve is
distinct, perhaps reflecting solar warming of ocean and/or estuarial waters
over one day's time.
Such thermal influence is also suggested by the off-
set of the ebb tide DO vs temperature curve of Figure 8 which also shows
that the flooding ocean water is slightly richer in oxygen than the estuarial
wate1:s.
In gaining a familiarity with the Salmon estuary it is informative to
take a look at the profiles of water quality parameters over depth, time, and
distance along the estuary's length.
At the mouth of the estuary (Station 0.50) a
mixe~_c::o~~ition
existed at
1350, 15 May 1976, on a rising tide with a constant salinity of 29.5%; the
current increased logarithmically over the depth of 1.5 m to about 0.12 m/sec
at the surface.
Similarly, station 0.92 registered a __ :lll!.X~___co~~i:~i~_n 10
minutes later, about 1.5 hours before high tide, with a constant salinity
of 32.0%; temperature decreasing slightly from 9.0°C at the bottom to 8.5°C
at the surface.
The depth at this station was approximately 2.1 m.
Further up the estuary the existence of a
sa.~."!~ge
begins to show over
13
30\
\
\
1-2
25
'
/
/
\
/
I
I
11 -20
.....
0..
0..
( /)
I
15th
10
u 15
-
0700
-
1300
1700
I
>'
0
~
I
9,
8
1200
/r
(OC)
-,.,.,. /
I
\
I
T (Oc)
I
I
l
5
I
';
.. ",
'•
'-, ........,. /
7
0
Figure 6.
N
0
Tempera.ture and salinity vs. time~
Station 0.5.
21
13
• • • Flooding Tide
;
•
+ + +Ebbing Tide
12
11
_lo
CJ
)
•
9
8
7
10
Figure?·
1~
s
~emperature
20
25
30
(ppt)
vs.
~~linity
at Station 0.5.
22
13
~~
,,
12
.
..
~
~+­
;
. I
•
,+
, +
I
16th 1 ·
I
11
-
I
+
u
~
!
10
l-
I
J..t..
,..
9
4 ,
, r·
I
J
I
+\
+···
•
;.p
\'
.·~
.
+
.
I·
T--+--~-- -~.,:ft• •
8
.
·•
.
/
·/
Ebbing
Flooding
/'/.
,. .
t+:::--~
--·
7
•.
7
8
. 9
...·.
10
11
12
DO (ppm)
Figure 8.
Temperature vs. dissolved oxygen.
Station 0.5.
23
a two hour period.
At Station 1.92, 40 minutes prior to high tide, a
slight vertical salinity gradient appears of about 0.7% per meter,
the depth being 1.8 m, whereas two hours later a gradient of about 7.5%
per meter over a depth of 1.9 m exists.
Corresponding to this change,
an increase from 9.5°C over the depth to a constant 13.5°C occurs, signaling a greater influence of fresh water.
Further analysis of the data shows
that the tide has reached slack water and is beginning to go out, since in
the mixed condition the salinity is 29.5% at 0.3 m below the surface, while
as the tide goes out the salinity at the same relative depth to the surface
is 18.5%, thus suggesting that the riverine water is starting to override
the sea water.
The salinity toward the bottom is also less in the wedge
situation, an indication that mixing and river flow occur over most of the
·---·"··--·-......---·--·--··.
depth. A look at the current velocity shows a maximum of 0.06 m/sec at the
--~.·
~ ... ·.-~
surface.
At just after high tide Station 2.68 exhibits a mixed condition with a
fairly constant salinity and temperature profile of 28-29% and 10.5°C respectively over the 2.9 m depth.
Two hours after high tide the salt
wed~e
at
this Station is strong, with a maximum gradient of about 28% per meter between
depths 0.7 and 1.3 m from the bottom (total depth of 1.6 m).
Further up-
stream the gradient is about 17% per meter between 1.0 and 2.2 m from the
bottom at Station 3.41 (total depth of 2.5 m).
As
would be expected, the
less saline surface water occurs at the upstream location, 5.5% as opposed
to 11.5%.
A~-~--S.~~p
salt gradient occurs so does a sharp temperature gra-
dient over the same depths.
Bottom temperatures are about ll°C with surface
temperatures about l4°C, suggesting a high correlation, inverse, with
salinity and temperature.
The time between these two samplings is only 5-10
•
24
minutes so that a comparison is reasonable.
At these two Stations, as with
Station 1.92, the magnitude of the current is greatest at the surface with a
much lower constant velocity extending through more than half the depth.
The rest of the Stations. sampled occur where the width of the estuary has
decreased substantially.
Besides the width decreasing, the depth of the estu-
ary at this point ·is very shallow.
Station 3.66 shows a salt wedge both at
high tide with a depth of 1.4 m and at two hours after high tide with 0.9 m,
with ranges in salinity of 22.5% and 16.0% respectively.
noticeable about the water temperature; 1)
Two things are
at a low tide with more river
influence a higher bottom temperature occurs, 12.0°C as opposed to ll.0°C;
2)
the surface temperature is cooler at this point in the tidal cycle being
14.0°C, compared to 15.5°C.
The explanation of this temperature comparison
is the fact that under the influence of the sun at 1500 the surface waters
are noticeably warmed whereas the river influenced profile of 1700 is less
influenced by the sun.
Station 3.96 is the first location which exhibits a stagnant hole area.
The depth ranges from 3.8 - 5.1 m over the three samples taken; the samplings
represent low tide, high tide, and two hours after high tide.
The hole is
represented by salinities greater than 30% at an interval from the bottom
to 1.7 m above the bottom, whereas the other samples have salinities of greater
than 27% from 3.3 - 3.6 m above the bottom.
has the highest salinity in the hole.
Oddly enough, the lowest tide
The relative height of the wedge
definitely increases on the high tide, with the surface water recording a
higher salinity than the low tide profile, 4.5% compared to 1.5%.
As to-
wards the mouth, the temperature is inversely related to the salinity distribution.
A profile of the current shows zero velocity for the last 40-60%
of the depth, depending on the profile.
The current at the surface ranges
25
from 0.09-0.24 m/sec.
It is, worthwhile noting that even in the holes the
dissolved oxygen is high, not going lower than 8.1 ppm, suggesting that
there is ciruclation in the holes even though small (also inferred from the
salinity variance over time) and/or there is little or no oxygen uptake in
this somewhat stagnant region.
Further upstream at low tide the depth is very shallow; Station 4.24
having 0.8 m of water around low tide, registering 2.6 m just after high tide.
At low tide the salinity is constant at 1.5% with the temperature also constant
at ll.0°C.
High tide shows a sharp salinity gradient, a change of 19.5%
in 0.1 m, with half the depth occupied by river water and the lower half
by salt water.
The corresponding temperature change is just as sharp.
Two
hours after high tide the gradient is just as steep however the saline water
occupies only 20% of the total depth.
The current relating to the river
is much greater than that associated with the salt wedge.
The next Station again exhibits tendencies of trapping saline water.
With samplings at the same basic intervals as Station 4.24, Station 4.78
has a maximum surface salinity of 1.5% at high tide.
remain between 28-30% over the entire cycle.
The bottom waters
The relative depth of the
wedge again is responsive to the fluctuating tide.
It is becomimg apparent
that the salt wedge at this approximate location from the mouth is only
---·· -·-. ·-·-- ..,...,
exerting a partial influence over the water column at any one moment of time.
The currents associated with the wedge are negligible except in the case of
high tide.
The last regular sampling Station, Station 5.09, shows the trend of a
hole.
The depth is fairly deep here, with 2.9 m recorded at about 30 minutes
prior to high tide and about 4.4 mat 1.75 hours after high tide.
The sa-
linity of the surface water never exceeds 0.5% and the salt wedge is a fairly
26
constant feature.
With three samples taken, the relative depth of the
wedge does change, suggesting influx and efflux of saline water over a
tidal cycle.
The current again is concentrated to the river dominated
flow near the surface.
To summarize, the water quality parameters are distributed as expected:
The salinity decreases upstream with the exten9 of the wedge fluctuating
with the tides (the averaged salinity as a function of distance from the mouth
is plotted in Figure 9);at high tide the lower portion of the estuary is
completely mixed with stratification occurring during the lower tides; the
upper portion of the estuary is stratified over the whole cycle, except for
the shallow sills when only fresh water flows over on low tide; the temperature
is inversely related to the salinity at this time of the year when air temperatures begin to warm; the dissolved oxygen throughout the estuary is high,
being a low at 8.0 ppm; the pH of the sea water is slightly higher than the
fresh water with both being on the basic side of neutral.
FLUSHING PREDICTION
To compute the flushing time in the Salmon River. estuary various methods
were employed.
The following is a description of the models and the results
obtained when these models were applied.
The methods are the Classical Tidal
Prism, Ketchum's Modified Method, Fraction of Freshwater Approach, and the
Dyer-Taylor Segmented Approach.
The Classical Tidal Prism approach approximates the number of tidal
cycles it takes for the tidal prism to completely replace an equivalent
amount of the high tide volume.
This approach assumes a completely mixed
estuary and does not deal with river inflow in any way, therefore a low number would be expected.
Ketchum's Modified Method takes into account a river inflow, breaking the
27
35
30
20
....,
-
Q.
c.
....,>-
15
.....
....c
"""'
IV
U)
Q)
0
10
,...
IV
Q)
~
5
0
1
2
3
4
5
Distance from ~outh (km)
Figure 9.. Average sa1ini ty vs. distance from mo.uth.
6
28
estuary up into segments of defined length and computes an exchange ratio
from which a flushing time for each segment may be determined.
The flushing
time is the sum of the individual times, which can be translated as the
time required to completely replace the accumulated river water in the
estuary.
The first segment is defined by that location where the cumulative
tidal prism equals the river flow per tidal cycle.
Each successive length is
defined by an excursion length over a tidal cycle.
This method, as the
previous one, assumes a completely mixed situation, which again will yield a
low number.
The Fraction of Fresh Water Approach is similar to Ketchum's Model, calculating the time to replace the fresh water in the estuary.
This is achieved
by saying that the river flow will replace the average fresh water concentration, i.e.· there is no tidal motion.
The estuary is broken up into segments
arbitrarily and the fraction of fresh water in each segment is calculated
from the known salinity distribution in the estuary.
To get an average
fraction, the salinity at various stations is averaged over the cross-section
and over a tidal cycle.
As with the other models, a mixed condition. is
assumed.
The Dyer-Taylor Model is an off-shoot of Ketchum's, with some modifications.
These investigators state that Ketchum's definition of
segment defies continuity considerations.
~he
first
In their model, they define the
first segment by the location where the cumulative tidal prism equals the
river flow over a flooding tide, in other words, half the river flow over a
tidal cycle.
The rest of their segments take into account that the assumption
of complete mixing in the other models is unrealistic, and so a correction
factor is applied.
/
'
The problem with this method is that the segments are very
small at the head of the estuary, which tends to stretch the accuracy of the data.
29
that is usually available for flushing predictions, and causing round off
errors to be important, all which tends to give a high flushing time.
The low tide volume (4.5 x 10 5 m3) and the tidal prism volume (9.5 x
5 3
10 m ) of the estuary are used to calculate the flushing time in the
Classical Tidal Prism Method.
The result is a flushing time of about 1.5
tidal cycles.
Ketchum's Modified Method is used with the cumulative bw tide volume and
the cumulative tidal prism to produce another estimate.
of the method are presented in Table
v.
The calculations
The high river flow combined with
the small low tide volumes in the estuary allow its division into only two
segments, the second of which extends 0.5 miles beyond the mouth (about 0.1
of the segment length).
The calculated flushing time is 3.6 tidal cycles.
The quality of this estimate is seriously degraded over that possible with
this method
due to the small number of segments used.
The number of segments in the Fraction of Freshwater Method was arbitrarily chosen as 11, with all segments equal up to the end of the salt
wedge, the last segment covering the rest of the estuary up to the head of
tide.
An average salinity for each segment was determined from the salinity
distribution in the estuary (Figure 9).
The calculations are tabulated
in Table VI with the sum of the flushing times
equali~g
2.0 tidal cycles.
Applying the Dyer-Taylor Model suggested the use of 2 to 3 segments.
The second segment fell 1800 m short of the estuary mouth, whereas the third
segment went about
4350 m out into the ocean.
tabulated in Table VII, using an a
= 0.8.
The calculated values are
The flushing time for the estuary
comes to be between 4.6 and 5.7 tidal cycles depending on whether the third
segment is added in.
As was suspected, this approach gives the highest value
for flushing time of all the prediction methods. All predictions indicate only
Table V
Modified Tidal Prism Method Calculations
Distance
From 11ead
2
(10 m)
Segment
Segment
Number
0
35.6
1
90.1
Cum Low
(10 m )
Loc Tidal
Prigm 3
(10 m )
1.52
2.20
1.52
3. 72
.• 409
2.45
23.58
3.72
22.06
25.78
.856
1.17
(10 m )
5
Cum Tidal
Prigm 3
(10 m )
35.6
2.20
54.5
5.92
Len~th
(10
Tide V~l
m)
Local Low
5
Tide v~l
Local H
5
Tide V~l
(10 m )
Exchange
Ratio
Flushing Tim
(cycles)
EFm = 3.62 cycles
River Flow, R = (
120
( 3 • 281 )3ft3
3 3
3
ft m ) (12.4 hrs. ) (3600 sec) = 1. 52 x 105 _m~­
cycle
sec
cycle
hr
•
'~
..
Table VI
Station
(m)
Segment
Sta-Sta
Cum.VL
3
(10 4m )
4 3
(10 m )
Fraction of Fresh Water
Cum P
4 3
(10 m )
10.2
6810-5000
5000
10.2
4500
13.6
4000
17.0
3500
20.4
3000
23.8
2500
27.2
5000-4500
30.6
1500
34.0
1000
37.4
2000-1500
44.2
0.750
2.0
4.4
12.4
0.635
0.184
3.1
4.9
13.0
0.618
0.199
4.6
5.7
19.0
0.441
0.165
6.2
6.5
23.9
0.297
o. 127
25.3
0.256
0.123
7.3
9.9
8.3
26.3
0.226
0.123
12.5
9.6
27.6
0.188
0.119
14.7
10.7
29.5
0.132
0.093
17.5
12.1
30.7
0.097
o. 077
20.7
13.7
32.5
0.044
0.040
80.9
3.4
0
1.0
63.4
40.8
0
0
48.7
3.4
500
11.4
T
n
(tidal cycles) .
36.2
3.4
1000- 500
(%)
2.5
7.9
3.4
1500-1000
n
26.3
3.4
2000
f
18.4
3.4
2500-2000
sn
12.2
3.4
3000-2500
VMTLn
{104m3)
7.6
3.4
3500-3000
n
4 3
(10 m )
4.5
3.4
.4000-3500
p
2.5
3.4
4500-4000
500-
vLn3
100.8
E
= 2.0
tidal cycles
....
~
...
tN
1-'
-
I,
Table VII
Segment
Dyer-Taylor Segmented Tidal Flushing Method
Cumulative
Station
Prism
Boundaries
5 3
10 m
m
Exchange
Ratio
r
Flushing
Time
Tidal Cycles
0
6810-4035
1.67
1.67
0.76
0.76
0 .313
3.19
1
4035-1800
3.19
1.52
4.11
3.35
0.688
1.45
2
1800-
7.38
4.19
44.69
40.58
0.908
1.10
I:
R/2
(l
= 0.76
= 0.8
5
3
x 10 m
= 5.74
33
a few tidal cycles for flushing.
/
ESTIMATION OF THE DISPERSION COEFFICIENT
Various methods for estimating the dispersion coefficient have been
proposed and were applied to the survey data.
Results are summarized in
Table VIII.
Taylor estimates the dispersion coefficient E from E (f~~~~c)
where n
u
= Mannings
= mean
n, here taken as 0.05
·----· ......
velocity
= 77nuR5/ 6
...
R = hydraulic radius.
Dispersion coefficients computed along sections of the estuary varied
markedly although a value between 0.14 and 0.18 m2/sec occur commonly (Table
IX). An arithmetic mean of 0.28 m2/sec is obtained for the estuary with a
maximum of 0.47 m2/sec near the mouth.
Harleman estimates the coefficient over a length, L, of the estuary via
EL = 100 numaxR516 , where umax is the difference between the maximum tidal
----···-----·
velocity and the mean river flow. Other parameters are as previously defined.
------
Between the mouth and 1.9 km upstream a umax of 1.4 mps (4.8 fps) was determined using current data recorded at Station 0.5. This velocity and the
resulting high dispersion value of 5.8 m2/sec are necessarily limits for
the parameter in the estuary and are most likely only applicable to points in
or near the estuary throat.
Time series records of currents at other points
in the estuary were not available for use in computing a more representative
umax and dispersion coefficient.
Greeney and Bella use the expression D = 16.0 lVI, where Vis in meters
per second.
Using the average velocity of 0.5 mps for the reach from the
2
mouth to kilometer 1.9 yields a dispersion coefficient of 240 m /sec.
34
The Rotterdam waterway equation of E
tudinal distance from mouth and L
= total
= 13,000
(l-X/L) 3 with X = longi-
length of estuarine intrusion was
judged inapplicable in this study because salt intrusion was severely
modified by very shallow conditions.
It would appear that the Greeney and Bella figure of 2.4 m2/sec may be
relatively representative of average conditions during the period of study if
only becuase it lies between the other two estimates.
Altered tidal conditions
or stream flow would necessarily alter this coefficient.
Table VIII
Estimated Dispersion Coefficients
Method
Averaged or
Estimated Value
(m2/sec)
Taylor
0.28
Harleman
5.76
2.40
Greeney and Bella
Table IX
Taylor Dispersion Coefficients
Section Range
'km)
0-1.9
1.9-3.5
3.5-4.0
4.0-4.2
4.2-4.7
4.7-5.1
u
(ft/sec)
Mean Depth
(ft)
R
(ft)
E
(ft2/secJ
E
(m2/sec)
0.5
0.17
0.02
0.07
0.17
0.10
3.3
3.0
0.5
12.5
2.8
8.9
3.2
2.9
0.5
10.7
2.7
7.3
5.17
1.60
0.04
1.94
1.50
2.06
0.47
0.14
0.004
0.18
0.14
0.18
Average dispersion coefficient E
= 0.28
2
m /sec.
35
POLLUTANT PREDICTION
An attempt was made to predict the distribution of a pollutant that
was dumped into the Salmon estuary at a constant rate.
Realistically
speaking, any outfall in this estuary that was serving a community would be
very impractical because of depth considerations.
Even though the flushing
time is low, the shallowness of the estuary at low tide would mean that the
pollutant discharge could be a significant portion of the flow.
For the ex-
ercise of predicting pollutant dispersion however, an example was calculated
making certain assumptions.
The pollutant was defined as domestic sewage with the following characteristics.
Population:
30,000 people
BOD Discharge:
0.2 lb/day per capita
For simplicity, the fraction of fresh water approach was applied.
It
must be noted that this method is for a conservative pollutant and BOD is a
non-conservative pollutant.
In this respect the distribution will be con-
servative, predicting a distribution that is higher than if decay of the waste
is considered.
Another justification for the approach is that the flushing
time of the estuary is only about 2 tidal cycles, so that the waste will not
have decayed too much by the time it has left the estuary.
For example,
with a decay coefficient of 0.318 per day (base e, and at l2°C), approxi-
-----
mately 75% of
t~e
waste will be remaining after two tidal cycles.
For the computations the same segments as used for the flushing prediction with the Fraction of Freshwater Method were used, and three outfall locations were studied (x
= 500-lOOOm,
x
= 1000-1500
m, and x
= 1500-2000
m).
The results are tabulated in Table X, with the distributions plotted in
Figure 10.
The results show that downstream from the outfall the distribution
36
.....
2.0
-.....
'-e
1.~
Outfalls
0
c:
Segment 3
0
...,
~
..-4
...,...
ttl
1.0
2
c:
CD
u
c:
0
u
Q
ss
0.5
0
Figure 10.
1
2
3
Distance from Mouth (km)
4
5
BOD vs. distance from mouth for hypothetical pollutant
discharges located at various points along the estuary.
37
is insensitive to where the outfall is placed, however the concentrations
upstream from the outfall increase as the outfall is moved upstream.
Table X
Freshwater Fraction and Pollutant Concentration for the
Fraction of Freshwater Method
Segment
ch (mg/1)
fn
Outfall
2
1
2
0.044
0.097
3
4
5
6
0.132
0.188
0.226
0.256
0.297
0.441
0.618
7
8
9
co
cn
0.40
0.89
0.85
0.80
0.76
0.73
0.41
0.90
1.22
1.14
1.09
1.05
0.69
0.55
0.38
0.99
0.79
0.53
p
R
co
fn
fo
C = 1-fn C
1-fo o
n
4
0.40
0.89
1.21
1.73
1.65
1.58
1.50
1.19
0.81
p = 0.069 lb/sec
=- fo
=
in Segment
3
downstream
3
R = 3.40 m /sec
upstream
INLET STABILITY
O'Briens relationship was used to compare the estimated inlet crosssectional area of the Salmor. River estuary with that predicted for inlet
stability by A= 4.69 x l0- 4P0 • 85 , where A is the minimum cross-sectional area
2
of the inlet in ft and P is the spring range tidal prism in ft 3 Since
the ocean tides during the field study were near their spring range, the
•
•
38
estimated tidal prism of the field study, 33.61 x 106 ft 3 , was used in the
relation.
The calculated minimum area is
A = 4.69 X 10• 4 (33.61 X 10 6) 0 • 85
= 1170
ft 2
The actual cross-sectional area of the inlet was estimated from the width
(200 ft) and mean depth (3·.5 ft) determined during the field study, to be
2
approximately 750 ·ft • This value agrees reasonably well with the predicted value.
O'Brien also presented the relationship
A =2
X
-s
10 P
for unimproved inlets.
The calculated minimum cross-sectional area from
this relation is
A =2
X
10
-5
(33.61
X
6
10 )
= 672
ft
2
which shows good agreement with the estimated actual value.
Both calculated
values for the inlet area are within the range of accuracy of the actual
estimated area of 750 ft 2 , and indicates that the inlet cross-sectional
area is in equilibrium with the estuarine tidal prism.
CLASSIFICATION
There are many classification schemes in use, each of which has been
developed for a particular prupose, e.g. whether to describe the· salinity
stratification, frequency of inundation, geography, etc.
A brief considera-
tion of some of the more applicable classification schemes follows.
Geomorphically, the estuary would be classified as a coastal plain
estuary or drowned river valley.
The main channel is characteristically
meandering and shallow in comparison to its width.
Vertical cross-sections
increase in area in the downstream direction as a consequence of the increasing tidal prism as well as of the increasing tide flat area •
39
The salinity structure of the estuary is that of a moderately mixed
to stratified estuary between the mouth and kilometer 1.92 but is more that
of a stratified estuary upstream of kilometer 2.5.
~~uch
subdivision of
the estuary is necessitated by the intervening shoal region which divides
the estuary into a downstream section where strong tidal mixing occurs and
a deep upstream region where salt water is trapped below the sill depth.
Seasonal peak runoff may however flush the upstream and much of the downstream region of the estuary, creating a salt wedge condition near the mouth.
Such runoff may force the wedge out of the estuary at low tide.
Applying the Simmons(l955) criteria for the observed river flow and
tidal prism, a flow ratio (the ratio of the total river flow over a tidal
cycle to the tidal prism) of 0.15 was calculated.
This figure indicates
partially-mixed to well-mixed condition in the estuary and agrees with the
subjective appraisal of the data.
For the maximum winter flow of 43 ems
(1,169 cfs) a flow ratio of 1.45 is generated, indicating highly stratified
conditions.
It is probable that during such flows the salt wedge is driven
out of the estuary.
True highly stratified conditions would be expected
to exist within the estuary for smaller flows.
MODELING OF THE SALMON RIVER
In modeling the Salmon River a distorted model should be used because
of the extreme flatness of the topography.
Most estuary models built today
are distorted geometrically with a larger linear scale for heights than for
horizontal dimensions.
Extra roughness generally will have to be added to
bring the delays within proper scale relationships.
Ippen has suggested an upper bound on the vertical scale of
H 314 = H /3.5(VT ) 1/ 2
r
p
p
40
H
H = ~ model height.
r
H prototype he~ght
p
v-= viscosity = 1.5 x 105 .ft 2/sec
T
p
= tidal
period
= 12.42
x 60 x 60
= 44,640
= 1 ft minimum depth in channel axis at low tide
Hr 3/ 4 = 1/3.5 (1.5 X 10 5x 4.464 X 104) 1/ 2 = 1/28
H
p
Hr
= (1/28) 413 = ~0117 = 1/85
Selection of 1:50 for the vertical would seem practical.
of 3 m would then become 6 em.
model tidal period, Tm.
i.e. 14 m approximately.
The tidal range
Similarly one can derive the remaining
H would be reasonable as 1:500 for a minimum,
rx
So T = T IH 2;H
m
p rx rz
'
= 10.2
minutes.
.
41
APPENDIX A
Water Quality Data-Graphical Presentation
FiS?re
Parameters
A1
Salinity vs pH
A 2
Temperature vs pH
A 3
Dissolved Oxygen and pH vs Time
A 4
Water Quality Profiles
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