PS Map

THE PUGET SOUND MODEL Summary

By John H. Lincoln
Sr. Oceanographer
University of Washington
Department of Oceanography

Introduction

Puget Sound is a complex estuarine system of inter-connecting basins having one major and one minor connection with the Strait of Juan de Fuca. The major connection is through Admiralty Inlet and the minor connection through Deception Pass. Flow through Deception Pass amounts to about 2% of the total tidal exchange. Puget Sound includes three major fjord-like basins: the main basin extending from the main entrance south to Commencement Bay, Hood Canal, and South Sound. The principal entrance sill is at the northern end of Admiralty Inlet where depths are about 40 fathoms. From near Possession Point south to Tacoma, the depth of the main basin is near 100 fathoms. The Tacoma Narrows constitutes a secondary sill separating the South Sound basin and the main basin. Sill depth at The Narrows also is about 40 fathoms, with the interior depth quickly dropping to near 100 fathoms. The entrance sill at the northern end of Hood Canal also is approximately 40 fathoms and inner depths of the basin exceed 100 fathoms. These basins, together with Port Susan, Saratoga Passage, and Skagit Bay comprise the Puget Sound System.

The Puget Sound System extends approximately 100 statute miles from Deception Pass south to Olympia. A shoreline of 1,33? statute miles encloses a water area of 1,020 square miles at Mean High Water. Fresh water inflow from rivers amounts to a yearly mean of 41,000 cfs (cubic feet per second), ranging between a peak of about 367,ooo cfs to a minimum of about 14,000 cfs.

Tides are of the mixed type in which two highs and two lows occur each tidal day of 24 hours 50 minutes, with the greater difference generally occurring in the heights of the low waters. Bathymetry, basin configuration, and interconnections result in a progressive increase in the diurnal range of tide, the difference in height between the Higher High tide and the Lower Low tide during a tidal day, and in the delay in time of tides toward the south from Port Townsend. Diurnal range at Port Townsend is o.3 ft., and at Olympia it is 14.4 ft. Time lag is variable, depending on the tide range and type, but approximates one hour, 20 minutes for highs and about two hours 15 minutes for low tides from Port Townsend to Olympia.

The principal physical characteristics of the Puget Sound System are summarized in Table 1.

Origin of the Model

The original model was constructed at the Department of Oceanography, University of Washington in 1950 - 1991 with funds provided by the Office of Naval Research. It was designed for research studies of tidal currents, water exchange, and flushing in Puget Sound, particularly within the Admiralty Inlet section. This model is still in use as a teaching aid for oceanography classes for applied oceanography studies, and for research.

Acquisition of an oceanographic model for the Pacific Science Center was planned by Dr. Dixy Lee Ray, then Director. An attempt to obtain a model of southern Puget Sound, built by the Department o! Oceanography, that was scheduled to be destroyed because the building housing it was to be razed, was unsuccessful. It was not feasible to move the model because of its size and construction, and an attempt to make a copy mold failed. In 1971, the Pacific Science Center received a grant from the Washington Sea Grant Program to partially fund a duplicate of the original Puget Sound model. Construction was started that year. The original wood patterns prepared for casting the original model in concrete were used in construction of this fiberglass mode. Refinements in operating equipment and instrumentation have been incorporated in this model.

Table 1. Physical Characteristics of Puget Sound.

Water area at Mean High Water.................1020 sq. St. miles
Length of shoreline......................................1332 statute miles
Total volume below MHW............................26.5 cubic miles
Mean tidal exchange....................................1.26 cubic miles
Average depth...............................................205 feet
Maximum depth (off Point Jefferson)........930 feet
Maximum current speed
..........Deception Pass....................................9 - 10 knots
..........Tacoma Narrows..................................5.5 knots
River Discharge
..........Maximum monthly average...................367,000 cfs
..........Yearly mean.............................................41,000 cfs
..........Minimum monthly average....................14,000 cfs

Tidal characteristics:

Location Time Difference Tidal Range from Port Townsend (feet)

(minutes later)

....................................High Tide.....Low Tide.....Mean.....Diurnal

Port Townsend........... ---.................---.................5.1..............8.3
Union, Hood Canal.....37................52................8.0.............11.7
Seattle...........................48................48.................7.6.............11.3
Bremerton....................55.................60................8.0.............11.7
Tacoma.........................55................54.................8.1.............11.8
Olympia.........................79................94...............10.5.............14.4
Shelton........................120...............142.............10.6.............14.2
Everett...........................39.................37.................7.4.............11.1
Cornet Bay....................63................76.................6.6.............10.2

 

Purpose

The primary purpose of the original model was for research studies to investigate tidal currents and circulation characteristics in the Admiralty Inlet section of Puget Sound Because tidal behavior and other oceanographic properties related to tidal action are affected by the system as a whole, the entire Puget Sound system was modeled. Although the first model was primarily a research tool, the major purpose of this model is to provide an opportunity for the general public to become acquainted with the kinds of knowledge and interactions among scientific disciplines that characterize the field of oceanography. In addition, it will provide them with a better understanding of the Puget Sound marine environment as a dynamic system and as a valuable resource that requires careful and knowledgeable planning for proper use now and in the future. It was also planned that the model would be made available to school classes, to advanced students, and to visiting scientists as a facility to carry out studies related to a wide range of problems concerned with the marine environment.

For use in research or applied oceanographic studies, a physical model can be considered as an analog computer in which critical parameters can be controlled. From this standpoint, the principal features of the model include the following:

Size:

The relatively small scale of the model makes observations over the entire system or large areas for synoptic studies feasible. Large-scale features of circulation and tidal flow can be observed readily and behavior in different areas easily compared. However, the small size increases the relative effect of non-scalable parameters that impose limitations on representation. Thus small-scale features of tidal flow and behavior, particularly in shoal areas, small bays and inlets, and in areas of very weak currents are not represented accurately.

Time:

The time scale is such that, in this model, an hour in nature becomes only about three seconds, or a tidal day (24 hours, 50 minutes) occurs in 76.0 seconds. Dynamic processes that require days or weeks in nature can be observed within relatively short observing periods.

Equally important is the fact that, in the model, time is not a one-dimensional parameter. The model can be operated to provide representation of specific calendar periods for investigation of past events, or with equal ease can be used for prediction of processes related to tidal action. Conditions for a particular period can be repeated at will, thus enabling observations to be made at multiple locations under identical conditions to simulate simultaneous measurements.

Control of Parameters:

Certain natural variables can be controlled or altered as desired to investigate their effect either singly or in combination. For example, river discharge may be set to study the consequence of extreme runoff, tidal action may be set for specific epochs, or anomalous conditions can be simulated. Some factors cannot be scaled. These include gravity, water viscosity, and surface tension. Consequently, some factors such as wind effects cannot be shown properly. These factors impose limitations of representation, and as a result, model behavior must be subject to verification on the basis of prior field observations and interpretation on the basis of the known limitations.

Model Construction:

The model basin was formed by making a fiberglass lay-up over the accurate wood patterns made for casting the original University model in 1951. The patterns were made by first contouring charts of the area at model scale. These contours were then transferred to pine boards planed to the thickness of the contour interval. After being sawed to the contour line, the pieces were laminated and excess wood removed by hand carving. Constant reference to the charts was made to assure accuracy of the reverse representation of the bathymetry. The patterns thus represented the volume from an elevation of 150 feet above mean high water to depth. The inverted patterns were then surveyed into correct position on a special platform by reference to a latitude-longitude grid, and secured to the platform. All joints between the pattern sections were filled and smoothed. Mold release material was applied and the fiber-glassing done with careful attention paid to proper application over the fine detail. "Egg-carton" type bracing was installed and a rigid polyurethane foam added to ensure adequate strength and rigidity of the structure. The complete model basin was then lifted off the patterns. After being moved to its present location, operating equipment was installed. Instrumentation and demonstration equipment suitable -for the initial use acquired. Additions and improvements to this latter equipment will be made as required by development of demonstration and study programs.

Scales

The scale ratios of the model are listed in Table 2. As this model is a duplicate of the original University model, the scales were determined by the requirements of that model. The horizontal and vertical scales were selected 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. This vertical distortion 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 shoal areas such as tide flats. All other scales are determined as functions of the horizontal and vertical scales.

Function

Tide Computer

Tidal action is the principal driving force of the dynamic oceanographic processes occurring in Puget Sound, and thus must be represented accurately in the mode]. Model tides are computed continuously by a Kelvin-type tide machine similar in principle to the machine used by the U. S. Coast & Geodetic Survey until 1966 for computing the published Tide Tables. The tide machine for the model provides summation of six cosine functions representing the six major tidal constituents that describe tidal effects of the orbital characteristics and astronomical relationships of the Earth,

Table 2. Puget Sound Model Scales
-------------
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) inch
............................................1 foot.....................0.0104 inch
............................................1 meter.................o.878 mm
Speed..................................1:33.94...................1 knot...........0.597 in. sec^-1
............................................1.52 cm sec^-1
Time.................................l:1,178.5.................1 hour..............3.055 sec.
...........................................1 day (solar) ..........73.32 sec.
...........................................1 day (tidal).............75.86 sec.
Volume.............................1:1.84x10^121......210.7cubic in....0.91 gal.
...........................................................cubic mile
...............................................................................3.456 liters
...........................................1 cubic Km................0.543 liter
Flow..................................1:1.564 x 10^9.........1000 cfs..........0.0011 in.^3 sec.^-1..........1.088 cc min.^-1
----------------
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 MLLW................................679.1 mi.^2..........15.7 ft.^2
Mean Tidal Exchange....................1.26 mi.^3..........1.15 gal.
----------------

 

Moon and Sun. Tides for any specific calendar period can be computed with an accuracy governed by the limitation of only six constituents (3'I constituents are used in computing the published tide tables). The error averages less than 0.5 foot, although it can amount to near 1.5 foot occasionally. .

Tides in the model are generated by displacement of water in the headbox by a plunger whose vertical movement is controlled by the tide machine The plunger is shaped to compensate for the tidal prism resulting from the change in "flooded area" between high and low tide caused by the beach slope.

Three tidal characteristics are important and must appear in the model as they appear in nature.

1. Type of tide

Within Puget Sound, tides are of the mixed type in which two highs and two lows occur each tidal day. Heights of low waters generally have the greater variability. Thus each tidal day there occurs a Higher High Water (HHW), a Lower Low Water (LLW), a Lower High Water (LHW), and a High Low Water (HLW). Heights and ranges of the tides vary between Spring Tides (larger tides) and Neap Tides (smaller tides) over a period of 14.3 days. the tides never exactly repeat, but the closest repetitive cycle is approximately 18.5 years, the period of the rotation of the Moon's node (the intersection of the plane of the Moon's orbit with that of the ecliptic, the Earth's orbital plane).

2. Range of tide

The range of the tide varies within the system, approximately doubling in going from Port Townsend to Olympia. Tidal range variation within Puget Sound is due to the effect of the bathymetry, length of the system, and configuration of the basins and interconnecting channels. The diurnal Range is the difference between mean HHW and mean LLW

3. Time of tides

Tides are essentially wave phenomena. Progression of the wave along the length of the system results in a time difference in occurrence of high and low tides at various locations. Puget Sound tides are several hours later than the corresponding tides on the ocean coast, and later at Olympia than at Port Townsend. This time difference, as with the tide heights, is a function of the tide character and of the physical characteristics of the system.

Because tidal flow characteristics are dependent on bathymetry and the configuration of the basins and channels, modeling must be as accurate as possible to obtain reliable representation of these tidal characteristics at all locations. In general, model tides are within about 0.4 scale feet of predicted heights although a deviation of up to 1.5 feet can occur. The deviation from prediction is a combination of the effect of neglected constituents in the model tide computation and modeling effects. It is of interest to note that in nature, meteorological effects can cause deviations from prediction that are of the same magnitude.

Puget Sound receives a volume of fresh water each year from river discharge amounting to about 20 of its total volume. Since this low has continued over geological time and Puget Sound has not become fresh, it is apparent that there is a process operating that maintains the salinity. The strong tidal currents and turbulence mix the fresh water and sea water. The inflowing river water must escape to the ocean and in doing so, as a result of the mixing, carries about nine or ten times its volume of seawater with it. To compensate for dilution and maintain the salt budget, there is an inflow of more saline water from the Strait of Juan de Fuca. The flow balance is thus nine parts saltwater inflow plus one part river water to produce ten parts of mixed outflow. Since the mixed water is of lower salinity and therefore of lower density, a net outflow occurs at and near the surface and a net inflow occurs at depth. Surface salinities and the vertical salinity gradient varies over a wide range with both location and time.

Water exchange and flushing of the basins are largely dependent on the above factors and thus proper salinity control and river discharge must be provided in the model. Fresh water inflow is provided at the locations of the eleven principal rivers discharging into Puget Sound.

PS River Controls Major Rivers Feeding into Puget Sound
Cedar
Duwamish
Nisqually
Skokomish
Hama Hamma
Duckabush
Dosewallips
Skagit
Stillaguamish
Snohomish
Puyallup




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Copyright Pacific Science Center 2000
Questions or problems to: David_Taylor@pacsci.org
Updated 24, May 2000