The Puget Sound Model

Puget Sound is the southern arm of the inland waterway system that also includes the Strait of Juan de Fuca and Strait of Georgia. Puget Sound is a complex estuarine system of four major interconnecting basins, three of which have fjord-like characteristics. The principal entrance from the Strait of Juan de Fuca is at the northern end of Admiralty Inlet, with a much smaller connection through Deception Pass. The three major fjord-like basins include the Main Basin, with an entrance sill located at the northern portion of Admiralty Inlet near Port Townsend; Hood Canal, with its entrance sill at the northern end near the location of the Hood Canal floating bridge; and Southern Sound, which is a complex of two large inlets and numerous smaller inlets and connecting channels. The Southern Sound entrance sill is at The Narrows. In each case sill depths are approximately 73 meters (40 fathoms) and basin depths generally are near or exceed 180 meters (100 fathoms). The fourth basin is the Whidbey Basin, comprised of Possession Sound, Port Susan, Saratoga Passage, and Skagit Bay. Deception Pass is the northern terminus of the Whidbey Basin.

The Puget Sound System extends approximately 160 kilometers (100 statute miles) from Deception Pass south to Olympia at the southern end of Budd Inlet. A shoreline of 2,144 kilometers (1,332 miles) encloses a water area of 2,642 square kilometers (1,020 square miles) at Mean High Water. Freshwater inflow from rivers amounts to a yearly mean of 41,000 cubic feet per second (cfs), ranging form a peak near 367,000 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. The tidal range between high and low waters varies markedly with the greater variability occurring in the heights of the low waters. Bathymetry and the System configurations result in a progressive increase in the range of the tides and the delay in times of high and low water toward the south from Port Townsend at the northern end of Admiralty Inlet. The diurnal range, the difference in height between Higher High tide and Lower Low tide, averages 8.3 feet at Port Townsend and 14.4 feet at Olympia. The time difference is variable, depending on characteristics of the tidal sequence, but approximates one hour and twenty minutes for high tides and about two hours and fifteen minutes for low tides between Port Townsend and Olympia.

The principal physical characteristics of Puget Sound and some representative tidal data are summarized in Table 1.

TABLE 1.  Physical Characteristics of Puget Sound

Water area at Mean High Water..............................................................       1,020   sq st. miles

Length of shoreline..................................................................................       1,332   statute miles

Total volume below Mean high Water....................................................         36.5   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   knots

            The Narrows................................................................................           5.5   knots

River Discharge (total)

            Maximium monthly average........................................................   367,000   cfs

Yearly mean.............................................................................................     41,000   cfs

            Minimum monthly average..........................................................     14,000   cfs

Tidal Characteristics:

Model Construction

The model was constructed in 1950 with funds provided by the Office of Naval Research and under the direction of Professor Clifford A. Barnes to investigate the tidal currents in Admiralty Inlet. The extreme complexity of tidal flow in this area makes a detailed understanding of their characteristics very difficult, if not almost impossible, by field observations alone. A physical model was considered to be the most appropriate method for obtaining the desired information, provided that acceptable representation could be achieved.

Prior to the start of construction of the present model, a detailed theoretical study was made to investigate suitable model scales. The primary requirement for proper representation is that flows must be turbulent. Normally, turbulence is a function of both channel dimensions and speed of flow. However, in Puget Sound the channels are irregular and flow speeds are variable from near zero to several knots, depending on tide characteristics and time of tide. To be conservative, straight, uniform channels were assumed in this theoretical study. Additional considerations were the ultimate size of the model, space available, and cost of construction. These studies resulted in selection of a horizontal scale of 1:40,000 and a vertical scale of 1:1,152. From these two scales all others are derived by mathematical relationships. The model scales and their prototype equivalents are shown in Table 2. It will be noted that the horizontal and vertical scales are not the same. The vertical scale (depth) is exaggerated by a factor of 34.7. This was necessary to provide sufficient depth for turbulent flow to occur in the principal channels except during periods of slack water, and to reduce the effect of surface tension in shoal areas such as tide flats.

TABLE 2.  Puget Sound Model Scales

Tidal behavior is strongly affected by the interaction of the physical characteristics of the entire system, e.g., tidal exchange volume, dimensions, bathymetry, and configuration of the system, with the tidal periods and amplitudes. Thus, although investigation only of Admiralty Inlet was the initial objective, it was considered desirable to model the entire system. This has permitted extension of studies to include a major portion of the system.

Following the theoretical studies and before starting construction of the present model, a "pilot model" of the Admiralty Inlet area was made. This model, which provided for only a one-way flow to simulate different flood tides, was made to evaluate suitability of the selected scales and to verify, qualitatively, critical flow behavior. This model indicated that the selected scales were satisfactory for the planned studies and would result in a model meeting the space and cost limitations. Construction of the Puget Sound Model was started in the spring of 1950, and the basin was completed by the end of that year. Design, construction, and installation of control equipment and preliminary instrumentation were accomplished the following year. Initial trial runs for testing and verification of representation were then commenced. Improvements in control equipment, instrumentation, and operating techniques have continued to the present time.

The model basin was formed by making a concrete casting from hand-carved wood patterns. Charts of the area were contoured and the contours transferred at the proper scale to pine boards planed to a thickness corresponding to the contour intervals. After being sawed to the contour lines, the sections were laminated. The excess wood was carefully removed, with frequent reference to the charts to ensure accuracy, to produce the patterns corresponding to the water volume and air space to 150 feet above Mean High Water. A total of 26 individual pattern sections were required. The patterns were inverted and surveyed into position with reference to a latitude-longitude grid established on the model-support platform and secured. A form was built around the assembled patterns, and sheet metal was placed between the pattern sections. This enabled the entire basin to be cast at one time but in sections that could be handled and would fit together properly. After the concrete had hardened, the individual sections were separated, righted, the patterns removed, and the model reassembled.

Tides

Tidal action is the principal driving force on the dynamic oceanographic processes occurring in Puget Sound, and thus must be represented accurately in the model. The tide computer constructed for the model is the Kelvin type, similar in principal to the machine used until 1966 by the U.S. Coast & Geodetic Survey for preparing the published Tide Tables. The computer for the model provides summation of six cosine functions representing the six major tidal constituents as listed in Table 3. Tides for any specified calendar period can be computed with an accuracy governed by the limitation of only six constituents, while at least 37 are used in computing the published tide predictions. The error generally is less than 0.5 scale feet, although it can amount to a theoretical maximum ear 1.5 scale feet. It may be noted that in nature meteorological effects can cause deviations of actual tides by as much as 2.5 feet from the predictions.

TABLE 3.  Constituents of the Puget Sound Model Tide Computer

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

The three tidal characteristics are important and must occur in the model as they occur in nature. These are:

1. Type of Tide

Within Puget Sound, tides are the mixed type, having two highs and two lows each tide day. Heights of the low water 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 Higher Low Water (HLW). Diurnal ranges vary between Spring tides (larger tide ranges) and Neap tides (smaller tide ranges) over a period of 14.3 days, but never exactly repeat. The closest repetitive cycle is approximately 18.5 years, the period of the revolution of the Moon's nodes.

2. Range of Tide

The range of tide varies within the system, nearly doubling over the distance from Port Townsend to Olympia. Tidal range variation is the result of the effect of bathymetry, length of the system, and configuration of the basins and interconnecting channels. The Diurnal Range is the difference between HHW and LLW, generally given as the mean difference. The Tidal Datum to which tide heights are referred is Mean Lower Low Tide.

3. Time of Tides

Tides are essentially a wave phenomenon. Progression of the wave along the length of the system results in a time difference in occurrence of Highs and Lows at various locations. Puget Sound tides are several hours later than on the ocean coast and later at Olympia than at Port Townsend. This time difference as well as the tide heights) is a function of the physical characteristics of the system.

Because of the critical importance of tidal behavior to accurate representation, and its dependence on bathymetry and system configuration, the model was made as accurately as possible to duplicate these tidal characteristics at all locations.

Transport and Flushing

Puget Sound receives a volume of fresh water each year from river discharge, amounting to about 20% of its total volume. The strong tidal currents and turbulence mix the fresh water and seawater. The inflowing river water must escape to the ocean and in doing so, as a result of mixing, carries about nine or ten times its volume of seawater with it. To compensate for the loss of salt and seawater, and to maintain the salt budget, there is an inflow of more saline water from the Strait of Juan de Fuca. The flow balance is thus: 9 parts salt water inflow + 1 part river runoff = 10 parts mixed outflow. Because the mixed water is of lower salinity and therefore of lower density, a net outflow occurs near the surface and a net inflow at depth. A wide range of surface salinities and vertical salinity gradients occurs that varies with both location and time.

The flushing of waters in and out of Puget Sound is strongly affected by the sill at Admiralty Inlet (Fig. 1), where the water depth is only 64 m (in contrast to depths exceeding 150 m on either side). Tidal mixing of waters over this sill is intense and causes mixing downward of some of the seaward flowing surface water into the landward flowing deep water (Ebbesmeyer and Barnes, 1980). This recycled or old water may represent 1/3 to 2/3 of the total deep water inflow. Such recycling lengthens the retention time of waters and associated chemicals (including contaminants) in the Puget Sound water column. According to Geyer and Cannon (1982), the inflow of new bottom water is enhanced fortnightly during periods of neap tides, when mixing over the sill is at a minimum. Pulses of new deep water are characterized by higher salinity and higher concentrations of resuspended bottom sediments than other deep waters.

Water exchange and flushing of the basins are largely dependent on the above factors, and thus proper salinity control and correct river discharge must be provided in the model. Fresh water inflow is provided at the locations of the 11 principal rivers discharging into Puget Sound. River flow is manually controlled with discharge rates of each river indicated by individual flow meters. A separate tank simulates the ocean as a source of salt water. The "seawater" is continually circulated between the ocean tank and the model headbox. Conductivity of the ocean water, as a measure of salinity, is monitored and compared with a standard. A concentrated salt solution is added by automatic control as required to compensate for dilution by river discharge and to maintain the ocean salinity at the control level to within about ±0.01‰.

Limitations

No small-scale model can exactly duplicate 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 correctly simulated mainly because of surface tension effects and the effects of land topography. This is an important limitation because winds an wave contribute to mixing, and wind drag on the surface can modify the tides, surface transport, and water exchange processes. Surface tension cannot be eliminated; thus, water movement in very 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 so 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 because of the requirement for density gradients generated by the intermixing of seawater and river discharges.

The above factors make it necessary to verify model observations by comparison with field observations made at comparable times and locations. With such comparable data available, the model can be used effectively to interpolate field data in both space and time and to some extent predict processes resulting from a given set of conditions.

Applications

The model is used for basic and applied research and as a teaching aid for classes relating to the marine sciences. During the past several years, the model has been used extensively in studies associated with marine environmental problems. These have included investigations of the behavior of effluent plumes from existing outfalls, probable dispersion and flushing from proposed outfalls, and the selection of outfall sites. In addition, the model has been of great interest to boaters and fishermen. In response to this interest, the model was used as the basis for preparing Tide Prints, a publication depicting surface tidal flows within Puget Sound. The model also has been used to some extent in studies relating to oil spills. However, because of its limitations, particularly with regard to wind effects and the difficulty of properly scaling the properties of oils, its use in this application has been limited.

Operational Characteristics

For the purpose of oceanographic studies, the 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 makes possible observations over the entire system for synoptic studies. Large-scale features of circulation and tidal flow can be observed readily.

Time:

The time scale is such that in this model an hour in nature becomes only about three seconds; therefore, a tidal day (24 hours 50 minutes) passes in 76 seconds. Dynamic processes that require days or weeks in nature can be observed within relatively short observing periods and can be repeated at will. Specific calendar periods can be represented and model observations timed to within less than 15 minutes of a specific time for comparison with field observations.

Equally important is the fact that in the model, time is not a one-dimensional parameter. The model can be operated to investigate past events, or with equal ease can be used to predict processes related to tidal action. Conditions for a particular period can be repeated at will, thus enabling observations made at multiple locations under identical conditions to be treated as having been made simultaneously.

Control of Parameters:

Natural variables can be controlled or altered as desired to investigate their effects either singly or in combination. For example, river discharge can be set to study the consequences of extreme runoff, or tidal action can be set for specific epochs, or anomalous conditions can be simulated.

OBSERVATION GUIDE FOR THE PUGET SOUND MODEL

Puget Sound is characterized by very complex bathymetric features and dynamic oceanographic processes. The following section is provided as an introduction and guide to observe some of the more significant and interesting features and dynamic behavior.

BATHYMETRY

Sills

Sills are relatively shallow areas separating deeper basins from the deeper water seaward. This feature is characteristic of fjords. The principal sills in Puget Sound are:

• entrance sill in Admiralty Inlet in the vicinity of Port Townsend
• entrance sill to Hood Canal near its northern end
• the Tacoma Narrows separating Southern Sound from the Main Basin

Numerous secondary sills occur within the system.

Deep Basins

The major deep basins are:

• the Main Basin, extending from Possession Point to Commencement Bay at Tacoma. The deepest water in Puget Sound, 284 meters (155 fathoms or 930 feet), is found west of Point Jefferson.
• Hood Canal and Dabob Bay
• Port Susan and Saratoga Passage
• South Sound, south of Toliva Shoals and including the major portion of Carr Inlet

Shallow Basins

The principal shallow basins are Skagit Bay and the smaller inlets of Southern Sound.

Channels

Channels are in most cases the interconnections between basins and are narrow in comparison with their length. Tidal currents in these areas generally are appreciably stronger than those in the basins. Some of the more interesting channels in the system include:

• Deception Pass
• The Narrows
• Colvos Passage
• Rich Passage
• Hammersley Inlet
• Pickering Passage
• Dana Passage

Tide Flats

The major tide flats in Puget Sound are located in Skagit Bay and the northern end of Port Susan. Smaller tide flats are located at the deltas off the mouth of the Nisqually River and the Skokomish River. Quite extensive tide flats also are found at and near the heads of many of the shallow inlets in South Sound.

Points

A large number of points or promontories may be found in Puget Sound. These points are of considerable importance because of their effect on tidal flows and the mixing processes. Some of the more prominent points in the Main Basin are:

• Bush point
• Point No Point
• West Point
• Alki Point
• Pulley Point (Three Tree Point)
• Point Robinson
• Point Defiance

Deltas

Deltas are present at the mouths of all the major rivers represented in the model. They range in size from that of the Skagit River, which covers Skagit Bay, to very small deltas off the mouth of the smaller Hood Canal rivers, such as the Hamma Hamma or Dosewallips.

TIDAL FLOW CHARACTERISTICS

High Velocities and Turbulence

Strong currents and accompanying turbulent flow occur in the channels. In most cases, maximum currents occur near mid-tide when the water level is changing at its maximum rate. Deception Pass, however, is an exception. Currents in Deception Pass are hydraulic, caused by a difference in water level between its eastern and western ends. The height difference is the result of both the difference in times of high or low tide at either end and of the difference in the character of the tidal sequence at the two locations.

The strongest currents found in the system occur at Deception Pass where speeds in excess of eight knots can occurs. The strongest currents in the Narrows are about 5.5 knots. Although Admiralty Inlet is broad in comparison with other channels in the system, the combination of the large tidal exchange by volume that must transit this area and the presence of the entrance sill results in tidal currents of several knots and every strong turbulence in this area.

Strong currents may be observed in Dana Passage and in Hammersley Inlet, both in South Sound.

Large-Scale Eddies

Large-scale eddies develop in Admiralty Inlet but do not persist because of strong flow and turbulence. Other large-scale eddies are found farther south in the Main Basin. These large eddies are produced by the tidal flow past the prominent points such as those mentioned earlier. Because tidal currents in this area are not as strong, the eddies can develop more fully and are more persistent. Note that an eddy forms downstream of a point, with the nearshore side moving in a direction opposite to the mid-channel flow. When the tide flow reverses, another eddy will form on the opposite side of the point, while the first eddy may be carried toward mid-channel by the main tidal flow. Such eddies are often referred to as "back eddies." Others may be found in the embayments along the main Basin. The back eddies often are of particular interest to boasters, because they may be able to take advantage of them when the main channel currents are against the direction that they wish to sail.

Circulation

Two quite different types of circulation occur in Puget Sound. The first is the general circulation of net surface outflow and net inflow at depth in which density gradients are the primary driving force. Second are the more localized circulation patterns that are the result of the combination of tidal flows and system configurations producing net flows through certain channels.

The first type of circulation is mentioned above tin the section on "Transport and Flushing" and discussed in more detail in the following section.

An interesting example of the second type is the clockwise circulation around Vashon Island. On a flood tide, deep water in the vicinity of Commencement Bay is upwelled and together with the surface water is carried into and through The Narrows where it is mixed by the turbulent flow. On the following ebb tide, the water exiting The Narrows is directed largely into Colvos Passage by the configuration, bathymetric features, and orientation of the channels. This produces a strong northerly flow through Colvos Passage during ebb tides. As the northerly flow passes, Blake Island and the northern end of Vashon island, the front heads for Alki Point or slightly south of that point. Part of this water is deflected south and will make another circuit. During flood tides, Blake Island and the submarine ridge between Blake and Vashon islands, combined with the configuration of the main channel east of Vashon island, directs southerly flow almost entirely through East Passage. As a result there is a net northerly flow through Colvos Passage and a net southerly flow through East Passage. This behavior is of major significance to the flushing of the Main Basin. The Narrows system acts as a pump drawing both deep and upper layer water south through East Passage. The mixed water is then discharged through Colvos Passage where it becomes part of the outward surface transport.

A net outward transport also is found through Deception Pass. This flow is a combination of Skagit River discharge and deep water entering Skagit Bay from Saratoga Passage.

Differential Flow

A net outward flow of less dense, less saline water at and near the surface and a net inflow of denser, more saline water at depth exists in the system. This flow is superimposed on the much stronger oscillating tidal currents. On the average, the deeper water tends to penetrate farther into the system with each successive large flood tide, while the surface water moves a little farther seaward on each successive larger ebb tide. The net outward flow is more rapid because it is confined to the shallower upper layer and also includes the total freshwater inflow from the rivers.

This behavior is most readily observed by injecting a dye solution into the surface water and a second injection at depth in the area south of Point Jefferson in the Main Basin or near the northern part of Hood Canal. Several tidal days are required for the differential flow to become easily apparent. Although the differential flow occurs in Admiralty Inlet, it is not easily seen because the stronger currents and turbulence cause the dye to become too dispersed quite rapidly.

It is of interest to compare the net transports in the Main Basin with those of the Whidbey basin. Although nearly 70% of the freshwater inflow into Puget Sound enters the Whidbey Basin, the "pumping" action of The Narrows results in a far more rapid flushing of the Main Basin.

Upwelling

Perhaps the most dramatic upwelling is that between Point Defiance and the southern end of Vashon Island, which has been mentioned above. Another location where upwelling may be observed is at the shoal area south of Possession Point, where tidal flows carry water up and over the shoals.

Other more localized areas where upwelling may be seen include:

• West Point
• Illahee, near Bremerton
• North of Budd Inlet, where flood flow through Dana Passage is upwelled

When observing upwelling, it must be remembered that vertical motion of water in the model is only qualitatively represented. Where upwelling occurs in nature, upwelling occurs in the model, but the magnitude of the motion in the model is not correct because of the vertical distortion.

Estuaries

During a flood tide, the rising water level will result in a wedge of salt water penetrating upstream from the mouth of most rivers. On the following ebb tide, the salt water, together with the freshwater that has been impounded, will flow upward to form a prominent plume in the basin. This action is variable, depending on the tide range, river discharge rate, and the character of the river mouth. One of the most dramatic plumes is formed at the mouth of the Puyallup River in Commencement Bay.