UW Model

Puget Sound Model

John H. Lincoln
..January 1974

1. Introduction

The first Puget Sound model was constructed in 1951 under the direction of Prof. Clifford A. Barnes for the Department of Oceanography, University of Washington, with funds provided by the Office of Naval Research. Construction of the model for the Pacific Science Center Foundation was started in 1971 with support from the Washington Sea Grant Program. This model is essentially a duplicate of the original with respect to the basin modeling and the representation of oceanographic processes.

1.1 Construction

This model was fabricated of fiberglass, using the wood patterns made for casting the original model in 1951. The patterns had been carefully formed by laminating wood cut to represent water volumes of successive contour intervals and then hand-carved and shaped to the precise bathymetry. The individual pattern sections were reassembled in correct orientation to form the mold for the fiberglass. Fabrication of the fiberglass basin was done by Holiday Fiberglass Boat Co., Seattle.

1.2 Model Scales

The scales used in the model are listed in Table 1. The horizontal and vertical scales were selected originally on the basis of available space, construction cost, and on the requirements for representation of oceanographic behavior adequate for the studies anticipated. The vertical scale (depth) is exaggerated by a factor of 35, that is, depths in the model are 35 times greater than an equivalent horizontal distance. This was necessary to provide sufficient depth in the principal channels for turbulent flow to occur during all but slack-water periods, and to reduce the effect of surface tension in the shoal areas such as tide flats. All other scales are determined as functions of the horizontal and vertical scales in accordance with the Froude modeling laws.

1.3 Limitations of Model Observations

No small-scale model can exactly duplicate the behavior of the prototype. This is because of parameters that cannot be scaled and because of the scale distortions. One of the principal factors that must be neglected in the model is wind. Wind effects cannot be simulated properly, mainly because of surface tension effects and the effects of land topography. This is an important limitation because winds and waves contribute to mixing, and wind drag on the surface can modify surface transport, water exchange processes, and to some extent the tide height. Surface tension cannot be eliminated, thus water movement in shallow areas and close to the shoreline is unreliable. Reduction of surface tension by the addition of a surface-active agent or detergent is insufficient to justify other undesirable side effects. Viscosity cannot be scaled, thus flow through very small channels is somewhat impeded, and small-scale turbulence is reduced. It is impractical to use a liquid of lower viscosity in the model.

The vertical distortion in the model, while essential to its operation, imposes additional limitations. Horizontal flow in the model can be considered to be quantitative. Vertical motion, however, can be considered only qualitatively. For example, where upwelling occurs in the prototype, upwelling should occur in the model but the magnitude or flow speed will not be correct. In the case of vertical eddies induced by flow over or past a topographic feature, the flow within the eddy will be nearly circular. For proper representation in the model, the flow would have to follow an elliptical path because of the vertical distortion, and this cannot occur. In the model, then, such behavior is only indicative of the presence of a similar flow in the prototype.

The above factors make it necessary to verify the model observations by comparison with field observations made at comparable times and locations. With such comparable data available, the model can be used effectively for interpolation of field data in both space and time, and to some extent the prediction of processes resulting from a given set of conditions. The model also provides a means of guiding the selection of the most suitable locations and times for field observations.

1.4 Operating Equipment 1 Tide Generation

Tides are generated in the model by displacement of water in the headbox resulting from vertical motion of a plunger controlled by the tide computer. The computer is a simplified version of the mechanical Kelvin Tide Prediction Machine used by the U. S. Coast & Geodetic Survey until 1966 for computing the published Tide Tables. The tide computer constructed for the Puget Sound model provides continuous summation of six cosine functions representing the six major tidal constituents that describe the effects of the orbital characteristics and astronomical relationships of the Earth, Moon and Sun. These constituents and their characteristics are listed in Table 2.

The tide machine provides the capability to generate tides in the model corresponding to natural tides for any specific time period within the accuracy limitations imposed by the summation of only six constituents rather than the 37 used in preparing the published predictions. Provision is also made to generate tides representing a particular day or type and to repeat the diurnal sequence. This enables one to investigate flow characteristics at multiple locations under identical tides.

Table 1. Puget Sound Model Scales

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Scale Parameter;Ratio;Prototype;Model Scale

; Value; Value

Horizontal distance;1:40,000 ;1 naut. mile;1.824 inch

;1 stat. mile;1.584 inch

;1 kilometer;25.000 mm

Vertical (depth) 1:1,152 ;1 fathom ; ;0.0625 (1/16")

1 foot ; ;0.0104 inch

1 meter ; ;o.878 mm

Speed 1:33.94 ;1 knot ; ;0.597 in. sec

1.52 cm sec

Time 1:1,178.5 ;1 hour ; ;3.055 sec.

1 day (solar) ;73.32 sec.

1 day (tidal) ;75.86 sec.

Volume 1:1.843 x10 ;1 cubic mile ;210.7 cubic"

0.913 gal.

3.456 liters

1 cubic Km ; ;0.543 liter

Flow 1:1.564 x 109 1000 cfs ; 0.0011 in.3 sec.-1

l.o88 cc min. -1

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Total volume below MLLW 26.5 mi.^3 ; ;24.3 ;gal.

Total water area

;At MHW 766.9 mi.^2 ; ;17.7 ;ft.^2

;At MELW 679.1 mi.^2 ; ;15.7 ;ft.^2

Mean Tidal Exchange 1.26 mi.^3 ; ;1.15 ;gal.

 

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Table 2. Tidal Constituents for the Puget Sound Model

Symbol ; ;Effect ; ;Type ; ;Speed ; ;Period

( / hr.) ;(hours)

Principal Lunar ; ;Semidiurnal ;28,984 ;12.420
K1 ; ;Lunisolar Declinational ;Diurnal ;15.041 ;23.940
O1 ; ;Lunar Declinational ;Diurnal ;13.943 ;25.820
N2 ; ;Larger Lunar Elliptic ;Semidiurnal ;23.439 ;12.650
P1 ; ;Solar Declinational ;Diurnal ;14.959 ;24.076
S2 ; ;Principal Solar ; ;Semidiurnal ;30.000 ;12.000
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Updated 24, May 2000