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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