-----------------~~
------------------
---
---- --- - -- - - - - -
---
---
J;
HYDROLOGIC STUDY FOR
SOUTH SLOUGH ESTUARINE SANCTUARY,
I
'.
, COOS BAY, OREGON
i
j
David vI. Harris
William G. McDougal
William A. Patton
Nasser Talebbeydokhti
.'
CLASS PROJECT FOR CE 527 APPLIED HYDROLOGY
Profe~sor
~-~---c---:r--.-,
Peter C. Klingeman
SUBMITTED TO THE
SCIENTIF'ICjTECHNICAL
ADVISORY GROUP
.'
OF THE
SOUTH SLOUGH ESTUARINE SANCTUARY MANAGEMENT COMMIS~ION
13 May 1979
WATER RESOURCES RESEARCH INSTITUTE
OREGON STATE UNIVERSITY
CORVALLIS, OREGON 97331
i!
..
-.--
TABLE OF CONTENTS
Title
List of Figures
ii
List of Tables
ii
I.
Introducti on
II. Freshwater Runoff
2
Basin Description
2
Data Assembly. .
2
Calculation -Methods
6
Results of Analyses
8
IlL Estuarine Flushing
Estuary Description
14
Tidal Prism Volume
14
Calculation Methods
14
Results
17
Flushing and Mixing
17
Anticipated Conditions and Calculation Approach
17
Results
21
IV. Conclusions
V.
14
References
22
23
Appendix I:
Precipitation and Streamflow Data Used
1-1
.Ll.ppendix II:
Tidal Prism and Flushing Calculations
II -1
LIST OF FIGURES
Number
Title
'. Page
1.
South Coast Drainage Basin, Including South Slough
3
2.
Pertinent Streams, Hydrolcigic Stations and Average
Annual Precipitation Near South Slough . . . . . .
5
Method for Deriving Monthly Runoff From South Slough
Drainage Basin. . . . . . . . . . . . . . . . . . .
7
3.
4.
Estimated Average Monthly Precipitation and Equivalent
Depth of Runoff for South Slough Drainage Basin
. . . 10
5.
Flow Duration Curve for South Slough Drainage Basin
Di scharge
...................
13
6.
South Slough Estuary, Including Station Locations
15
7.
Relative Concentration of a Conservative Tracer as a
Function of Tide Cycles . . . . . . . .
. . . 19
Longitudinal Relative Concentration Profiles Based on
the Flushing Number . . . . . . . '. . . . .
. . . 19
8.
LIST OF TABLES
Number
Title
1.
Available Hydrologic Data
2.
Coefficients for South Slough Drainage Basin Monthly
Runoff Equations .
. . . . .
.........
4
9
3.
Fresh Water Budget for South Slough Drainage Basin
11
4.
Analysis of Boyce (1977) Field Measurements
16
ii
I. INTRODUCTION
No hydrologic study has previously been made of the South Slough
Estuary drainage basin. Yet, since freshwater runoff is paramount to an
estuary, it would seem that such a study is vital to the proper understanding -- and hence management -- of the estuary. Therefore, a brief
hydrologic analysis of the South Slough basin has been conducted as part
of a class project at Oregon State University. The results are presented
on the following pages. This analysis consists of two major parts:
(1) The freshwater streamflow that enters the estuary from the drainage
basin; and (2) the mixing of that fresh I'/ater within the estuary.
It should be noted that hydrologic data for the South Slough basin
are made conspicuous by their absence. For this reason, data from nearby
collection stations outside the basin have been used in the analyses made
for South Slough. This has permitted an estimate of precipitation and
runoff. But the results presented here can in no way take the place of
the analysis of data collected in the drainage basin itself. Nor should
these results, based on monthly averages of precipitation, be compared
indiscriminately with measured daily values.
II. FRESHWATER RUNOFF
Basin Description
For analytical purposes, the nor~hern boundary of the South Slough
Basin \vas chosen to be at the Charleston highway bridge across the estuary
mouth. This closely represents the natural basin directly affecting tHe
slough. It inlcucJes the entire boundary of the South'Slough Estuarine
Sanctuary. Figure
shovJS a map of the Sough Coast drainage area 'llithin
which South Slough is located.
The area of the land surface that drains into South Slough is approximately 31.0 square miles. The basin is generally forested. Drainage is
accomplished chiefly by means of small streams, most of which enter the
slough from the east or south.
Data Assembly
hydrologic data are available on the contribution of fresh water
runoff to South Slough from tributary streams. Hence, recourse vias made to
the development of an empirical relationship between precipitation and runoff for the drainage basin. Precipitation and streamflow data from two nearby
drainage basins \vere employed for this purpose. Coefficients for the relationship were determined for each month of the year.
~o
The available hydrologic data at different stations and the corresponding periods of record are shown in Table 1. Figure 2 shows the locations of
all hydrologic stations used and other relevant information, such as contour
lines of average annual precipitation (isohyetal lines).
The two listed gauging stations for streamflow were chosen for their
proximity to Coos, Bay. Other, more distant stations also exist. The drainage basins above the Millicoma River and Coquille River gauging stations are
relatively small, being 45.0 and 73.4 square miles in area, respectively.
This fits the need to simulate streamflovls from relatively small tributary
areas to South Slough. It is also assumed that the soil and vegetatiotl itl
these two basins are similar to those found in the South Slough basin.
2
I
UMPQUA
BA.5lN
·.
. - - ---.----
FIGURE 1,
SOUT~
COAST DRAINAGE BASIN,
3
I~CLUDING
SOUTH SLOUGH
TABLE l.
AVAILABLE HYDROLOGIC DATA
STATION
DATA TYPE
Bandon
Precipitation.
Temperature
Coquille City
Prec i pita t i on
Temperature
Dora
Precipitation
Temperature
Fairview
Precipitation
Temperature
North Bend FAA AP
REFERENCE
YEARS OF
RECORD *
1919
- pre$ent
1972
- present
1969
- present
1
1974
- present
Precipitation
Temperature
1
1902
- present
Sitkum
Precipitation
Temperature
1
1944 - 1969
West Fk Mi11icoma
near Allegheny
Streamflow
2, 3
1954
North Fk Coquille
near Fairview
Streamflow
2
1964 - present
1
- present
*Note that streamflow records are kept by "vlater Yearll, which extends
from October 1 to September 30 and is identified by the calenda\~ year
in which the water year ends.
4
LEGENO
o
6
P"IIMARY Pi"£CIPITATION
STATION
.jTREAM GAf..}$ING
-STATION
-70- PR£C:PfT,...17ICN,
IN /NCH£".S
~~.-.--'
~.
~
FIGURE 2,
.
:"-;;U,,,-C-
PERTrr~ENT STREAJ~SI HYDROLOGIC STATIONS AND AVERAGE
ANNUAL PRECIPITATION NEAR SOUTH SLOUGK
5
Calculation Methods
A schematic representation of the method us'ed to drive fresh \vater
runoff values for South Slough is shown in Figure 3. Precipitation and
discharge data for the two nearby basins (identified as x and y in Figure
3) were combined and used to determine ~oefficients for the runoff equations. These monthly runoff equations, along with derived values of precipitation, were used to predict the average monthly runoff from the South
Slough drainage basin.
Monthly precipitation values were estimated for each basin by use of
the Normal Weighting Method (4),
P
x
=1\
3
~ Pa
\N
!..a
+ Nx p + Nx p
Nb b
N
c
1
Eq. 1
c-1
in which
Px = average monthly precipitation over drainage basin x
Pa := monthly precipitation at gauging station a
P
b
:=
monthly precipitation at gauging station b
Pc = monthly precipitation at gauging station c
Nx := normal annual precipitation over drainage basin x
Na , Nb , Nc = normal annual precipitation over dl~a i nage basins
a, b, and c, respectively.
For this study, station a was chosen to be Bandon (N a := 59.8 inches),
station b to be North Bend (N b = 61.7 inches), and station c to be Sitkum
during 1960-69 (N c = 74.7 inches) and Dora during 1969-76.(N c = 66.6 inches).
Monthly precipitation for that period 1957-1976. Note that during that period
1957-1959 only two stations (Bandon and North .Bend) were used. Equation 1
was adjusted accordingly ..
The. values fo'r Nx and Ny (y replacing x in the above equation) were
determined from the isohyetal lines for each basin (see Figure 2). Two
sets of monthly precipitation values were thus obtained, one for each of
the two nearby drainage basins.
Monthly data for streamflow were available for the West Fork of the
Millicoma near Allegany and the North Fork of the Coquille near Fairview.
All precipitation and streamflow data used are sU~TIarized in Appendix I.
6
NORMAL
\---':::»-=-+1
WEI G H T IN G
EQUATIONS
~z l ~~NOFF----I
~
EQUATION
,--=-IP""'r------~ COEFFICIENTS
~y-- ) ,-[
- L----l-·~-··-.
"",
tJ
\1
'-l
RUNOFF
MONTHLY. FLOWS
EQUATIONS
FOR SOUTH
FQR EACH MONTH
SLOUGH BASIN
-------P
- m.onthly precipitation at index stations
(Bandon, North Bend, Sitkum/Dora)
P
- derived monthly precipitation over
x, y~ Z
d
'
l
'
( W. Fk . Ml"1'
ralnage
JaSlnS
llcoma, N. Fk. Coquille, South Slough)
Q
- rnonthly strean'l£low at "known" gaging stations
x, y
(Wo Fk, Millicoma, No Fk. Coquille Rivers)
a, b, c
F:]:GURE 3.
METHOD FOR DERIVING MONTHLY RUNOFF FROM
SOUTH SLOUGH DRAINAGE BASIN
----.--.~------~--~~~-....,-'"
dlf
q
9*&
,x;
1f3?-
,\f¥!i)\M
1-1,\
iC\'t!RHij1f+'1¥W¢,,*
%"
:;i<¥ff
%},*,
;;:;
The precipitation-streamflow data sets Ivere arranged by individual
month (January, etc.). They were then subjected to least-squares regress i on ana lyses to fit the data with power r::qua ti ons of the form:
= a[P .mo.. ' DA]b
1
Eq. 2
1
in which
average monthly di scharge for the ith month, in cfs
p
== monthly average precipitation for the ith month, in inches
mOi
DA
== drainage basin area, in square miles
a,b = coefficients to be determined.
QmOl.
==
Results of Analyses
The 12 sets of coefficients obtained from the combined data of both
drainage basins are shO\vn in Table 2. VaJu,es of r.2, the coefficient of determination, are also presented. Low values indicate a poor "fit" of the
power equation whereas high values indicate a better "fit".
Average monthly precipitation values were derived for the South Slough
drainage basin by use of the Normal Weightinn Method (see Appendix I). Normal annual precipitation over South Slough 5asin were estimated as 55 inches
(see Figure 2). No areal variation or precipitation over the basin was
assumed.
runoff flows were then obtained by use of the deri ved South
Slough basin precipitation values and the runoff equation (see Appendix I).
These values represent the sum of contributions from'all drainage basin
sources into South Slough.
t~onthly
Estimated average monthly values of precipitation and runoff for the
South Slough drainage basin for the period 1957-1977 are shown in Figure 4.
Runoff is represented as an equivalent depth, in inches.
A fresh water budget for South Slough drainage basin is shown in Table
3. Overall, precipitation is in excess of runoff, as expected. Losses, presumably through evapotranspiration, represent 22% of the total annual precipitation. However, the magnitude of precipitation in excess of runoff for
the months August to January, and runoff in excess of precipitation during
Feb\~uary, April, June, and July cannot be fully explained byevapotranspiration. Probably, soil moisture and groundwater recharge and depletion take
place on a yearly cycle and account for the above patterns.
8
TABLE 2.
·
COEFFICIENTS FOR SOUTH SLOUGH DRAINAGE BASIN MONTHLY
RUNOFF EQUATIONS (EQUATION 2)
MONTH
b
a
r2
January
0.4545
1.0864
0.7963
February
2.5818
0.8400
0.7076
March
1.5054
0.8927
0.8201
Apr; 1
3.2846
0.7428
0.5872
May
1.2654
0.8580
0.6388
June
12.1942
0.2683
O. 1998
July
11 .8854
0.0798
0.0805
August
4.9419
0.1466
0.2248
September
2.9523
0.3072
0.2853
October
0.0053
1.5729
0.7304
November
0.4672
0.9951
0.5604
December
0.3376
1.1221
0.8122
9
10. 0
.-----
9.0
-.-------------~---.---
-,
....
________ PRECIPITATION
. ··-··--------·~EOUIVAJ.::,ENT­
8. a
DEPTH OF
7.0
------ ..
___ g .~LNQ.f F ... ___ _
-----~
---' ..
6. a
.04
5.0
4.
a
3.
a
2.
a
1.
a
"-I
o
-~----.'
o. a
A
S
o
N
D
.----
.-~-
J
F
----
M
-----
A
-----_._---_._ .....
M
J
J
Month
FIGURE 4.
ESTI~1ATED AVERAGE ~10NTHLY PRECIPITATIDN i\ND [QUIVALENT DEPTH
OF RUNOFF FOR SOUTH SLOUGH ORAl Ni\GE BAS IN
1.0
'
.
TABLE 3.
MONTH
FRESH WATER BUDGET FOR SOUTH SLOUGH DRAINAGE BASIN
AVG DEPTH OF
PRECIPITATION
IN INCHES
AVERAGE
RUNOFF
IN CFS
EQUIV DEPTH
OF RUNOFF
IN INCHES
LOSS THROUGH
ABSTRACTI ONS
IN INCHES
January
9.26
215
8.00
1. 26
Febl~uary
6.94
232
7.80
-0.86*
March
7.26
188
6.99
0.27
April
3.77
110
3.96
-0.19*
May
2.52
52
1. 93
0.59
June
0.97
28
1. 01
-0.04*
July
0.31
14
0.52
-0.21*
August
0.86
6
0.22
0.64
September
1. 50
9
0.32
1.18
October
3.70
10
0.37
3.33
November
8.65
120
4.32
4.33
December
9.08
192
7.14
1. 94
54.82
98
42.58
12.24
ANNUAL
* Represents net gain of water from source other than precipitation (e.g., from ground water
base flow) Abstractions include interception, evaporation, transpiration, infiltration.
_ _ _ _ _ _ _.---,..
"tw "q;,
,i,
"f
MJ14JifA
\J A
wp,
rrt
i!fii'%W
¥Alii 1t,*WiR"¥*iN,,Q,,YI¥'¥CYX;iK¥ (8 -1 \1Qj UM j\MfP,i,,*i!f-O/?fj\itI\\4<Y,,iiLV!4' h f , « M¥'ii{)i'W!'fiiA ¥t"b,
'1h if)SF - ¥w «
'''7~~''-r"0;~'\W0'J'1HlfN';''A\'f
'I¥Zff'if'lib;:'.1Jii\'{lff-.y*t*'1Y'i('t ¥li'fi\\f;d.,\W
§
,,{''IN'%'
A flow-duration curve based on the monthly values of runoff for the
South Slough drainage basin is shown in Figure 5. The median flow (exceeded
50 percent of the time) is about 50 cfs. The mean flow of 98 cfs (see Table
3) is exceeded about 40 percent of the time. The shape of the curve indicates that the basin is characterized by moderately high seasonal flm'/s and
a low-flow regime that is poorly sustained at the end of the dry season.'
"12
300
200
100
70
V1
4U
50
C
'rn
30
Q)
CJ)
$-
rO
..c
20
U
V1
'r-
o
10
7
5
3
2
o
1Q
20
3Q
40~
50
60
70
80
90
100
Percent Time Indicated Flow is
Equaled or Exceeded
FIGURE 5.
FLOW DURATION CURVE FOR SOUTH SLOUGH DRAINAGE BASIN DISCHARGE
13
III. ESTUARINE FLUSHING
Estuary Description
The South Slough estuary is fairly long and narrow, with its long axis
running mainly north-south, as shown in Figure 6. Its mouth opens onto Coos
Bay approximately one mile upstream from the mouth of Coos Bay at the Pacific
Ocean. The surface area of South Slough south of the bridge at Charleston
is 2.04 square miles and the mean tide range is 5.7 feet (6). Fresh water
enters the slough from several small streams, mostly flowing from the east
<:lnci south.
Tidal Prism Volume
Calculation Methods
The tidal prism volume and the flushing and mixing characteristics are
each calculated here in three ways. The results are then compared.
The first method of finding the tidal prism volume involves the assumption that the sides of the estuary are steep. In this case, the prism
volume is simply the product of the plan area of the estuary and the tide
range.
The second method is based on a trapezoidal approximation. Boyce (2)
presented field data on the mean cross-sectional depth at several stations
in South Slough. The station locations are identified in Figure 6. By
means of these measurements a trapezoidal approximation for the prism volume
is obtained. These data are presented in Table 4.
The third method is based on a two-dimensional, non-linear circulation
model developed by the Corps of Engineers (3). The volume flow rate is
calculated across several cross sections. Integrating the volume flow rate
at the entrance to South Slough over a rising or falling limb of the tide
gives the volume of the tidal prism. This flow rate was integrated over
four limbs and averaged. An 8.2 foot tide was used in the numerical model.
Therefore, this was linearly scaled to the 5.7 foot mean tide range for use
with South Slough in this study.
are planar.
The
14
scali~g
is accurate if the tide flats
KEY
,i
-'...1
! '0£.
1-/-1
r- CJ, S
/
"'/\ -?.::t
L0
i
'.r'\.
I
"'<
--
I
-.,4>
~-~"'l
'\~ L'D-,f "',-
..
.i
.
/.
,
"
\
\
I,/'
iI
I
I
/
I
;
13
I
I
I
J
- -=sc: 07
FIGURE 6,
SOUTH SLOUGH ESTUARY, INCLUDING STATION LOCATIONS
[From State of Oregon Division of State Lands Map
(1973)]
15
I
'.':~
TABLE 4.
Station
ANALYSIS OF BOYCE (1977) FIELD MEASUREMENTS
HI-I Depth
LW Depth
6.6 ft.
0.9 ft.
-
w
x
L
2750 ft.
4800 ft.
6850 ft.
2
21. 3
15.6
3100
8900
3600
3
6.2
0.5
1350
12000
2550
4
16. 1
10.4
2750
14000
5
n.2
5.5
1700
18500
VLW
Vp
17.0 (10 6 ) ft 3
174.1 (10 6 )
107.4 (10 6) ft 3
63.6 (10 6 )
3250
1.7 (10 6 )
93.0 (10 6 )
19.6 (10 6 )
50.9 (10 6 )
7710
72.1 (10 6 )
74.7 (10 6 )
3,58 (10 8 )
3.16 (10 8 )
--'
0\
.•.
Results
The three independent techniques all give similar results for the
volume of the tidal prism. These are:
tidal p ri sm.
method
3.25 x 10 8 ft3
steep sides
'l
field data
3. 16 x 10 8 ft 3
3.46 x 10 8 ft3
numerical model
The calculations \A/ere based on a volume of the estuary at ,low \'/ater (V LW )
of 3.58 x 10 8 ft 3 (see Table 4).
Th~ good agreement increases the confidence iri the estimates.
A rep-
resentative value for tidal prism was selected from the above comparison
to be used in flushing calculations~ This is
Vp = 3.3 X 10 8 ft 3
v/here
Vp = tidal prism volum~.
Flushing and Mixing
Anticipated Conditions and Calculation Approach
South Slough ;:s very long with respect to its width. This suggests
that there is little lateral variation in salinity. The depth is generally
shallow. These conditions, combined with significant winds and tides,
suggest that little vertical stratification will be found.
The river inflow is small, so longitudinal gradients should also be
small. Furthermore, the length of the bay is small with respect to the
tide wave length so there is little phase lag, if friction is neglected.
ft,ll of the above suggest that a IlvJell-mixed box model
pri ate .
II
may be appro-
. For a well-mixed-box estuary, the mass of a conservative tracer refllainth
ing in the basin at the n tide cycle after an initial injection is given
by:
Eq. 3
17
in v,rhich
Mo
=
the initial mass of tracer
Mn
=
the remaining mass of tracer after the nth tidal cycle
n
=
the number of tidal cycles
Vp
=
estuary tidal prism volume
LW = estuary lOll/-water volume.
V
For South Slough, using the representative values for Vp and VLW
tained above, substitution into Equation 3 gives:
ob-
M
Mn
Mn.1M 0
= [O.52]n
o
is plotted against the number of tide cycles, n, in Figure 7.
A technique proposed by Arons and Stommel (1) offers a second method
for examining the mixing or longitudinal stratification in an estuary. The
downstream advection of a tracer is balanced by its upstream diffusion.
Stratification is a function of the flushing number, F, where F is given by
- 2
F = uh
2BA2WL
Eq. 4
o
in which
u- = mean velocity due to stream inflow
h mean depth
B = dimensionless numerical constant
A = tide amplitude
o
W= tidal frequency
L estuary length.
For South 5lough this becomes (see Appendix 11)
F = O.000129Q
in whi~h Q in the streamflow in ft 3/second. To apply this we need only select
representative values for streamflow. Use of the extreme monthly flows shows
the likely range for the flushing number. The maximum and minimum freshwater
inflows occur during February and August, respectively. The corresponding
average monthly values are:
QFeb
QAug
=
=
232 cfs,
6 cfs.
18
I.e
O.O~--.~
__~~J~~~
o
~
3
~=-=-=-=-~=-==~====~~~~~
,
5
&
_____
7
n
FIGURE 7.
RELATIVE CONCENTRATION OF A CONSERVATIVE TRACER AS A
FUNCTION OF TIDE CYCLES
KEY
--- South Slough
February
8
s
~1easurelllents
(\1illialllson (9))
05
0-5
It
FIGURE 8.
LONGITUDINAL RELATIVE CONCENTRATION PROFILES BASED ON
THE FLUSHING NUMBER
19
For these extreme flows the flushing numbers are
FFeb
FAug
0.030
= 0.001.
=
In Figure 8 the longitudinal relative concentration profile of the'
tracer is given as a function of the flushing number. The abscissa shows the
relative position along the estuary (A = x/L) and the ordinate shows the,
relative salinity compared to that of the ocean Cs = local salinity/ocean
salinity). South Slough data show that the majority of the estuary has
near-ocean sal inity. By the shape of the F curves it is inferred that the
sal inity would only be r'educed locally at points of fresh water inflow.
has been indicated above, the So~th Slough is well mixed. Based on
this assumption, a third method of estimating mixing can be tried, involving
fresh water inflow. The concentration of tracer in the estuary is assumed
to be proportional to the ratio of the volume of fresh water in the estuary
to the volume of sea water. This ratio is a function of the fresh water in
f1 01'1 rate.
A~
For South Slough, this fresh water inflow rate is:
V
Feb. -- -VFW
-
=
3.05%
P
VFW
Aug. -- -V-=
0.09%
P
The small ratio of fresh water volume to tide prism over the range of encountered conditions indicates that the fresh water flow is of lesser importance to flushing.
The relative concentration of sea water, S is given by the total volume
ratios:
Eq, 5
in which
FW = volume of fresh water.
For South Slough.
V
SFeb
S
Aug
=
98.09%
99.95%
20
The salinity of North Pacific water is around 34 parts per thousand
(0/00). Using this and the above information, differences in salinity in
the estuary due to fresh water inflow, (>"s, would be
(>"sFeb = 0.649 0/00
(>"sA ug
=
0.017
0/ 00
Resul ts
All three of these methods seem to imply the same thing regarding flushing and mixing in South Slough: mixing is fairly thorough and, in fact, the.
effect of the fresh water inflow is very small.
The box model shows that the fresh water is quickly carried out of the
estuary. The mixing length theory of Arons and Stommel shows that the longitudinal gradients of salinity are small. Incidently, this method assumes
that all fresh water entered at the estuary head. But for South Slough the
inflow occurs at several points, which ~ould seem to imply that stratification
is even less than that calculated. Finally, the method of a well-mixed estuary
indicates that the fresh water inflow is much less important to flushing
than is tidal flow. It is also seen that even at the peak of fresh water
runoff the change of sal i nHy is 1ess than one part per thousand.
21
IV. CONCLUSIONS
Average annual freshwater runoff from South Slough draihage basin was
estimated to be 98 cfs. Monthly average values ranged from 6 cfs, i~
August, to 232 cfs, in February. An annual average precipitation of 54.82
inches resulted in 42.58 equivalent inches of runoff. Evaporation presu;nably accounts for the remaining 22%. Based on analysis of 20 years of data,
the median monthly freshwater flow was estimated to be 70 cfs. Extreme
values of monthly runoff were 1 cfs and 445 cfs, respectively.
These hydrologic data and results were used to characterize the degree
of mixing and flushing of fresh water in South Slough. Three indopendent
methods were used to estimate the volume of the tidal prism, yielding close
agreement and a representative value of 3.3 x 108 ft 3 . Mixing was also
described in three ways: 1) an exponenential-decay, relative-concentration
method, wnicn showed that the concentration of a tracer is halved every tide
cycle; 2) a longitudinal stratification, flushing number technique, which
yielded extreme values of flushing numb~rs of Fc, e b = 0.030, FA ug = 0.001
(low values indicate little stratification); and 3) the ratio of fresh water
volume per tide cycle to tidal prism, which gave extreme values of 3.05%
and 0.09% fresh It/ater for Feliruary and August, respectively.
calculations were made it was assumed that the salinity outside
the entrance to South Slough was that of the open waters of the North Pacific.
If, however, the sa 1i nity is 1ess than oceani c due to freshwater fl ow into
Coos Bay, the salinity in South Slough will drop correspondingly; but the
effect of fresh water runoff directly into South Slough should remain small.
t~here
In general, it seems from this analysis that mixing is very thorough
and that flushing is very quick: the effect of fresh water appears to be
minor. In fact, the nutrients, pollutants or sediment associated with the
fl~esh \</ater may be more impol"tant to the estuary than the fresh I-later itself.
22
V. REFERENCES
(1)
Arons, A. B. and H. Stommel (1959). A mlxlng length theory of tidal
flushing. Trans. Amer. Geop. Un., 32:419-421.
(2)
Boyce, A. R. (1977) Interstitial fluid mixing in estuarine benthic
sediments. Masters Thesis,O.S.U. 296 pp.
(3)
Butler, H.L. (1978) Numerical simulation of Coos Bay- South Slough
complex. Army Corps of Engr., TRH-78-22.
(4)
Linsley, R. K., Jr., M. A. Kohler and J. L. H. Paulhus., Hydrology
for Engineers, McGraw-Hill, Inc., New York (1975).
(5)
Climatological Data, Oregon, NOAA, Environmental Data Service,
Ashevill e, N. C. ( 1957 -T9T6T.
(6)
Tide Tables 1972:. West coast of North and South America including
the HiHlfaiian Islands, NOAA, U. S. Dept. of Commerce.
(7)
vlater Resources Data for Oregon, USGS l~ater Data Reports, prepared by
USGS in cooperation with Oregon Dept. of l~ater Resources and other
agencies (1961-1977).
(8 )
Water Supply Paper 1738, Compilations of Records of Surface Water of
the United States through Octobe~ 1950.
"An Examination of some Physical and Biological Impacts of Dredging
in Estuaries
GI 34346, Division of Environmental Systems and
Resources, National Science Foundatinn, Washington, D. C. December
1974.
ll
,
23
APPENDIX I:
PRECIPITATION AND STREAMFLOW DATA USED
Monthly Precipitation Data for Bandon . .
1-2
North Bend
I -3
II
II
II
II
II
II
II
II
Sitkum .
1-4
II
II
II
II
Dora 2W
1-4
Monthly Streamflow Data for North Fork Coquille River Near Fairview
1-5
Monthly Streamflow Data for West Fork Millicoma River Near Alleghany
1-6
Monthly Precipitation Estimates for South Slough Drainage Basin
1-7
Monthly Runoff Estimates for South Slough Drainage Basin
1-8
1-1
·
\s.~3:9-95)1tl.3 '3,.39
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APPENDIX II:
A.
TIDAL PRISM AND FLUSHING CALCULATIONS
Tidal Prism
(1) Steep walled estuary
Vp::: (Area) (Tide Range)
::: (2.04 mi 2) (5280 ft/mi)2 (5.7 ft)
::: 3.24 x 10 8 ft 3
(2) Field measurements
See Table 4
8
3
Vp ::: 3.16 x 10 ft
VLW ::: 3.58 x 108 ft 3
B.
Flushing
(1) Box model
es tua r,Y __L~J1_gJh ___ :::
tide wave length
L
gh T :::
23960 ft
T6ftT32~.-=2-f=t--;/-se-c-c'.~;-r)-;:--;]!"-2-r(-':12;C-.-=2-;h-r-s'"")--'(r~3:-;:6~OO;:--s-ec-'-)
hr
L - 0.039
rr-
n
1
n
:::
M
Mn ::: [0.52]n
o
(2) mixing length method (Arons and Stommel (1))
F :::
o h') 2
2BA~WL
in which
o ::: mean velocity due to stream inflow
h = mean depth
B ::: dimensionless numerical constant
(a value of 0.36 was determined from
l1i11iamson (9)1
Ao = tide amplitude
W=-tidal frequency
L = estuary length.
11-1
fi~ld
data taken by
"i
By continuity,
- - Q
-' Q
u - Area - hw
in which
IV ::: mean
then F :::
\lJl
dth of estuary
Q,_h_ _
2BA2 VI w L
o
:::
Q (6 ft)
2(1)(5.7 2 ft)2
[(l~.;
hrs) [3600 secT] (2300 ft) (23960 ft)
~
F ::: 0.000129 Q
(3)
well mixed model
Feb.
3
VFW _ Q (duration)
::: (232 ft jsec) (12.2 hrs) (3600
8
3
8
3
3.3 x 10 ft
3.3 x 10 ft
v;- -
sec
~)
:: 3.05%
~.
3
sec
FI.J _ Q (duration) ::: (6 ft jsec) (12.2 hrs) (3600~)
3. 3 x 10 8 ft 3
3.3 x 10 8 ft 3 .------
V
v;- -
::: 0.09%
I 1-- 2