SEPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Corvallis OR 97330
Research and Development
EPA-600/S3-81-039 Sept. 1981
Project Summary
Effects of Planform
Geometry on Tidal
Flushing and Mixing in
Marinas
R. E. Nece, E. P. Richey, J. Rhee, and H. N. Smith
Physical models for rectangular
marinas were tested to determine how
various geometric designs affect tidal
flushing and internal circulation in
small harbors. The models were scaled
to have surface areas, water depths,
and tide ranges comparable to proto-
type marinas in the Pacific Northwest.
Various geometric parameters were
investigated and results were pre-
sented in terms of average tidal flush-
ing coefficients and by contour draw-
ings of equal exchange coefficients.
Emphasis was placed on planform
geometry and aspect ratio, the two
variables which designers have the
most control over when designing a
marina. The report shows that the
optimal flushing and internal circula-
tion occurs when the basin length to
width ratio lies between 0.5 and 2.0,
the corners are rounded, and the
single entrance is centrally located in
the breakwater on the seaward side of
the harbor.
This Project Summary was develop-
ed by EPA's Environmental Research
Laboratory, Corvallis, OR, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Procedures
This research focuses primarily on
the relative exchange of water due to
tidal flushing of the marina basin. Tidal
flushing characteristics are expressed
in terms of an "exchange coefficient."
The laboratory tests were conducted in
an 8 x 12 foot basin 18 inches in depth.
Constant period tides were produced by
a tide generator. Tide ranges, water
levels, and tidal periods could be
adjusted to simulate a variety of real-
world situations. A photo densitometer
was used to measure dye concentra-
tions. Photos were taken by a camera
mounted approximately 7 feet above the
center of the marina basin. Dye density
values were measured directly from 35
mm black-and-white negatives.
All experiments were conducted on
model marina basins of the same plan-
form area and same uniform depth at
mean tide. The variables included tide
range, entrance width, entrance loca-
tion(s), and the rounding of interior
corners of the rectangular basin
The equivalent "prototype" dimen-
sions were as follows. The planform
area was 1.25 x 106 square feet (2.87
acres). This is larger than the area of
most Pacific Northwest marinas, but is
exceeded by some. On the basis of the
limited boat density values described by
other investigators, the "prototype"
tested could accommodate approxi-
mately 1,000 boats. The mean depth
within the basin was 16 feet, taken at
mean water level. The three tide ranges
used were neap, 3 feet, mean, 6 feet;
spring, 12 feet. These values are, repre-
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sentative of marinas in the Pacific
Northwest and in Puget Sound in partic-
ular. The range of aspect ratio used in
the tests varied from 0.21 to 4 80, more
than spanning the usual range found in
small-boat basins with single entrances
aligned with one side wall. A single
250-foot radius was selected to investi-
gate effects of rounding the interior
corners of the rectangular basin. The
three entrance widths were 125, 250,
and 500 feet. Results of tests using a
variety of variables are represented
graphically in the final technical report.
Conclusions
The conclusions apply only to the
hydraulic, or tidal flushing, performance
of the marina. Tidal exchange in itself is
not an index of water quality, although
in general as the exchange improves,
the quality of the water within the basin
approaches that of ambient water. In
judging the water quality of a marina,
relative rather than absolute standards
should be employed because water
quality in the marina cannot exceed that
of the exterior water with which it
exchanges.
Comparisons presented of different
planform geometries are based on
results for the 6-foot tide range. From a
design standpoint, this particular value
was chosen because it is typical of mean
ranges m the Pacific Northwest.
1. Planform geometry of aspect
ratio. The study confirmed earlier
conclusions of other investigators that
for basin length (L)/basin width (B) less
than 1 /3 and L/B greater than 3, multi-
ple circulation cells (gyres) exist in
rectangular basins with single
asymmetric entrances. When multiple
cells exist, the gross exchange de-
creases and spatial variability of local
exchange increases.
When a single gyre exists, the
exchange is lower in the center of the
basin than it is near much of the peri-
meter, due to the residual circulation
when the gyre is established. This may
be a positive result from a fisheries
standpoint, as juvenile migrant salmon
that reside temporarily in marinas tend
to remain in the relatively shallower
water near the basin perimeter where
local exchanges may be greater than the
gross exchange for the basin.
The oval-shaped manna with an
asymmetric entrance that has become
popular in the Pacific Northwest, pro-
duces a single-gyre circulation pattern
and good overall flushing performance.
Since this study indicates the exchange
would tend to be greater than the aver-
age around the perimeter, such oval
basins should be favorable from a fish-
protection Standpoint.
2. Ratio of entrance cross-sectional
area, a, to basin planform area, A. The
range of discrete entrance widths, w,
from 125 feet to 500 feet provides a
four-fold variation in a/A, but for a con-
stant tidal range, A, the differences in
the spatial average per-cycle exchange
coefficient, E, are no larger between the
various curves of w = constant than they
are between various L/B ratios for the
same w. Because such wide variations
in performance do occur for a/A and H
constant as planform geometry varies, it
is concluded that the a/A ratio is not a
governing factor.
3. Effect of rounding of corners in
the basin interior. Rounding of interior
corners apparently has little effect on
overall flushing, but it has been quali-
fied that the rounding of corners does
indeed produce a greater uniformity in
local exchange throughout the basin.
"Hot spots" of poor local exchange are
mostly eliminated.
4. Orientation and location of single
entrances. On the basis of the limited
experiments performed, it appears that
a single center entrance results in
better flushing than does a single
corner-related asymmetric entrance.
This result was obtained for rectangu-
lar, square-corner basins only; presum-
ably, the same result would hold for
basins with rounded corners. This result
can be attributed partially to the fact that
the jet entering the basin on the flood
tide is able to circumnavigate a greater
length of basin perimeter than it could in
a basin with an asymmetric entrance,
all other geometric parameters being
the same.
One precautioning statement must be
made. In the experimental program, the
entrance was designed so that the flood
tide inflow entered as a uniform flow in
a direction normal to the other face of
the marina. In a more typical field situa-
tion, the entrance would more likely be a
gap in a breakwater. Consequently, in
the presence of longshore currents, the
inflow would enter the basin with some
momentum parallel to the shoreline
and, therefore, the flow patterns in the
basin would not possess the symmetry
sought in the laboratory tests. However,
the results of a previous theoretical
study by another investigator (D. R.
Askren), indicate that for circulation
induced in basins by a steady, non-tidal
longshore current past entrances i
frontal breakwaters, a central locatio
was the optimum site for a single er
trance. Thus, the two sets of result:
obtained for different boundary cond
tions, lead to the same conclusion.
The experimental data, in particule
the exchange contours, show that for a
elongated basin with a single asym
metric entrance the uniformity of flush
ing, and in particular the exchange i
the innermost part of the basin, is bette
when L/B is less than 1 than it is whei
L/B is greater than 1. Again, this be
havior can be linked to the penetratioi
distance of the inflow jet. The recom
mendation with respect to desigi
criteria is that when a basin is elongatei
(say, with an aspect ratio exceeding ar
absolute value of 2.0) and a singU
asymmetric entrance is used, the en
trance should be aligned so that the
inflow direction is parallel to the lone
axis of the basin.
5. Effects of two entrances versus &
single entrance. Results are quite
limited. In general, two-entrance basins
would be very sensitive to the effects ol
persistent longshore currents which
were deliberately avoided in the labora-
tory experiments. Short-circuiting could
exist if the longshore currents are
primarily unidirectional. Generaliza-
tions are dangerous for multi-entrance
basins because the interior circulation
patterns are sensitive to local head
levels which result from the physical
configurations of the entrance and
near-shore and longshore current pat-
terns. However, on the basis of the
limited results, it appears that more
uniform flushing is obtained when the
entrances are of equal, or at least com-
parable, width.
On the basis of the results presented,
although the particular configuration
was not tested, the test design for a
rectangular basin for optimum tidal
flushing would incorporate an aspect
ratio L/B between 0.5 and 2.0, rounded
corners, and a center entrance. Asym-
metric entrance basins within the same
L/B range also possessed satisfactory
flushing action, particularly those with
rounded corners. As noted, these
results would indicate that basins with
oval planform and an asymmetric
entrance, such as have been built
and/or proposed at a limited number of
sites in the Pacific Northwest, should
possess satisfactory exchange charac-
teristics.
This study provides guided estimates,
but not precise values of gross flushing
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coefficients for various harbor plan-
forms having entrance coefficients
comparable to those tested. Center en-
trances not normal to the outer face of
the marina still should be investigated
because they would produce at least
two equal gyres within the basin.
R. E. Nece, E. P. Richey, J. Rhee, andH. N. Smith are with the Department of Civil
Engineering, University of Washington. Seattle. WA 98195.
Richard J. Callaway is the EPA Project Officer (see below).
The complete report, entitled "Effects of Planform Geometry on Tidal Flushing
and Mixing in Marinas," (Order No. PB 81-219 537; Cost: $9.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
200 S. W. 35th Street
Corvallis, OR 97330
it U S GOVERNMENT PRINTING OFFICE, 1981 — 757-012/7324
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