lbcurry at u.washington.edu
PDF copy of "Dissipation of Internal Waves in Port Susan, Puget Sound"
Poster Presentation From Ocean Sciences Meeting, 2006
Table
of Contents:
Cover Letter
Non-Technical
Summary
Abstract
Introduction
Methods
Results
Discussion
Conclusion
Acknowledgments
References
Table
and Figure Descriptions
copyright
Lauren Curry 2005
Dear Limnology and Oceanography,
I have enclosed a manuscript title “Dissipation of Internal Wave in Port Susan, Puget Sound”. The Puget Sound is a diverse and interesting estuarine environment and has increasing numbers of people moving to the region. The natural system is important to understand and should be expressed to the greater scientific community. This manuscript discusses mechanisms of dissipation for a tidally linked internal wave observed in Port Susan on April 8th 2005. Port Susan is a small but fast growing region in the northern Puget Sound with Indian lands and recreational facilities on its boarders. The internal wave was seen to propagate towards a major town with a marina and naval base, but the wave dissipated soon after reaching the end of Port Susan. I have observed that the likely mechanisms of dissipation are Bathymetry changes and the loss of stratification in the water column as the wave propagates.
Please consider this manuscript for publication in your journal, and I am available for comments or questions at lbcurry@u.washington.edu
Thank you for your consideration,
Lauren B. Curry
Every tidal cycle, a packet of waves with amplitudes greater than 10 meters travel through Port Susan, Puget Sound, almost unnoticed. These waves travel just a few meters under the surface of the water and, except for a series of surface slicks, are invisible to the casual observer. The water column in Port Susan is strongly stratified; that is, buoyant fresh waters float on the salty marine waters. This creates an interface for the internal solitary wave (ISW) to propagate along within the water column. During the ebb flow, fresh water off the Stillaguamish River delta displaces salty marine waters in Port Susan. This initiates an ISW that propagates to the mouth of the basin. After the internal waves have exited the Port Susan channel they had been observed to decay, but what caused the wave to decay was unknown. My investigation focused on finding the mechanism causing the decay.
Internal waves produce a surface feature from converging and diverging water masses that appear as slicks. Salinity and temperature were recorded by yo-yoing a CTD in the water column in these features to determine the structure of the ISW and attempt to observe its decay. Measurements were taken at seven stations as the wave propagated south along the axis of Port Susan and into the channel between Everett and Gedney Island.
A preliminary cruise found the interface the wave travels on becomes mixed at the channel between Gedney Island and Everett, just south of Port Susan. It was observed that when the wave moves past Camano and Gedney Islands its wavelength significantly lengthens and then starts to dissipate. It is no until the wave reaches a region of well-mixed water that the wave was no longer showing any surface expression, and choppy water with larger winds made continued sampling too difficult to continue.
Internal waves have been observed in Port Susan, Puget Sound. Their timing with tides, speed and amplitudes were well documented in a previous study, but the reason they decay remained unknown. On April 8th 2005 the R/V WeeLander was taken to Port Susan with a Seacat CTD to observe the decay of the internal wave. Internal waves create regions of convergence and divergence that are observable as surface slicks. The CTD was yo-yoed at seven stations in these surface features. We will show observations that demonstrate that the internal waves respond to changes in the channel width and to decreases in the stratification of the water column as the wave propagates out of Port Susan. At the surface, we observed a strong foam front in Port Susan, where the foam disintegrates after reaching the end of Camano Island and spreads over a much larger area. Wave amplitudes are seen to dramatically decrease in size from 9 meters to 4 meters over the course of our observations, while the wavelengths increase with distance. The final end of the wave is a region of well mixed water south of Gedney Island in Port Gardner. The loss of stratification here is theorized to be the cause of the decay of the internal wave, because the interface on which the wave propagates no longer exists.
Background
Internal solitary waves (ISW) are waves that propagate on the interface between two density stratified layers. These waves can have large amplitudes due to the small density difference between the two stratified layers; this causes an effect called reduced gravity. The reduced gravity lessens the pressure gradient force on the wave which reduces the restoring force and acceleration (Knauss 1996); thus large amplitude, slow waves can exist with small differences between density layers.
The effects of internal waves on the environment can be substantial. Internal waves could be a large source for dissipating tidal energy by mixing stratified environments (Garret and Munk 1979). Sandstorm and Elliott (1984) found that the Scotian Shelf had internal waves that were the main source for mixing nutrients to the euphotic zone. In 1997 Bogucki et al. found internal waves that caused resuspension of particulates in coastal waters off California without large currents. This could be very dangerous because it was “the nation’s largest ocean dumping ground of DDT.” Port Susan is not a toxic dumping ground, but there are water quality factors for the recreational use of the parks and Indian lands on the estuary. Some estuaries in Puget Sound have seen water quality declining and habitat shrinking (Puget Sound Action Team 2005). Knowing the physical characteristics of Port Susan is important; it will help us see how human activities may be affecting estuaries with changes in the greater Puget Sound region.
| The
Port Susan ISW Port Susan is a fjord in the northern end of Puget Sound (Fig. 1). At the north end, there is a large delta generated by sediment inflow from the Stillaguamish River. The Stillaguamish River is the fifth largest river discharging into the Puget Sound. It releases an average of 280m3s-1 of fresh water into Port Susan, and this stratifies the entire basin. The Snohomish River is located just outside of Port Susan but has unknown effects on the basin. The fjord is relatively shallow reaching only 120 meters at its greatest depth, and has a characteristic sill near the mouth of the basin (Fig. 1). The first study of the ISW in Port Susan was accomplished by Jeffrey Harris. He theorized that the mechanism initiating the ISW at Port Susan occurs at max ebb. At this time fresh water on the Stillaguamish Delta pours into the basin, displacing marine water and causing an ISW. In his observations he found the wave speed to be 0.63 ± 0.04 m s-1, slightly slower than this investigation observed. The amplitudes in both studies reached 10 meters for a packet of seven waves. |
Figure
1. |
c = 2/pi *Nh (1 + 2/3*a/h)
| where c is the wave speed, N is the buoyancy frequency associated with the interface, h is the depth of the surface layer, and a is the amplitude of the wave (Fig. 2). The buoyancy frequency N, also know as the Brunt–Väisälä frequency, is defined in the AMS glossary as the natural frequency of vertical oscillation for a water parcel in a stratified fluid. It is influenced by the stratification and density change in the water column. Harris did not find this equation to explain the speeds and amplitudes he was observing in Port Susan and suggests an equation for the wave speed at Port Susan to be: | Figure 2.  |
c2 = 4/pi2 * N2(h2 +4/9 * a2)
Harris notes that this equation does not agree with the small amplitude KdV wave theory, but seemed to explain the wave speeds observed at Port Susan.
The Port Susan wave has not been completely characterized in its form and type. The mechanisms that generate the ISW is still speculative. In Harris’ work, observations of the wave were not made until low tide, a few hours after their theorized generation. Based on the wave speed, Harris tracked the ISW back to the Stillaguamish Delta near the time of maximum ebb.
The Decay of an ISW
Internal waves have a few ways they can dissipate; the most common is an unstable wave that breaks, somewhat like a wave breaking on a beach. There is also the possibility that the wave’s energy propagates from a point source and spreads out to the point where viscosity and diffusion will break up the wave. Here I will discuss how these potential outcomes affect the ISW in Port Susan.
When an ISW reaches a ‘critical amplitude’ they can decay from shear instabilities (Bogucki and Garrett, 1993). The Richardson number is a stability indicator, the larger the value, the more stable the water column. The critical amplitude occurs when the Richardson number falls below 0.25, at which time the wave becomes unstable and will break. The critical amplitude is a function of the height of the free surface. Bogucki and Garrett found that amplitudes of 0.82 times the height of the surface layer should be the critical depth. Harris, nor I found this to be the case, and Harris suggests background flow to be a factor in the decay of the wave. Observations of the ISW where taken when the wave propagated out of the basin while currents moved into the basin during flood tide. During Harris’ observations the waves started to dissipate after peak flood when the background flow of the tides decreased. This would mean the waves generation and final collapse are tidally linked. In my observations the wave dissipated almost an hour before a change in tidal currents
(Fig
3),
suggesting another mechanism.The change in bathymetry can also be a potential cause for the wave’s disappearance, or energy dispersion. The waves initiate at the northern end of the Port Susan Basin where the channel has a relatively constant width, at the end of Camano Island there is some very shallow bathymetry keeping deeper water in the channel, but not containing the internal wave that is closer to the surface. The beginning of Gedney Island the channel refocuses into a very narrow point, and reaches a sill within the mudflats of the Snohomish River delta. The narrow consistent cannel in northern end of Port Susan confines the ISW, potentially with little energy loss due to dispersion. Once the wave has exited the enclosed space of Port Susan it is free to spread into a much larger region. It diverges in the area between Camano and Gedney Island, but refocuses when it reaches Gedney.
The energy associated with an ISW is a significant factor in how much the wave can mix the water column. It can be affected by the radial propagation of the wave as it moves away from a point source, the bottom shear stress from bathymetry or shear in the water column itself. The energy of internal waves is described in Knauss (1996) as:
E = .5 sigma g' a2
where g' = g (sigma1 - sigma2)/sigma2
and E is the energy in the system, ρ is the density of the water with ρ1 the density of the surface layer and ρ2 the density of the deep water. Gravity is represented by g and the reduced gravity is g’
(Fig. 2). Another important factor for this ISW is consideration of how the wave propagates. The disappearance of a strong gradient between surface fresh water and deep marine water alters the wave properties at it propagates.
Figure 3
Figure 4
Figure 5
Three cruises were carried out to investigate the Port Susan ISW; the first on board the R/V Thomas G. Thompson and the second two on the R/V WeeLander. The first cruise was designed to find the depth and extent of the fresh water lens and the general structure of Port Susan. The first Weelander cruise focused on finding the wave, and recording the wave associated temperature and salinity changes with a CTD as the wave exited Port Susan. The Third cruise attempted to use an ADCP to observe underlying currents created by the ISW.
The time on the Thompson was spent taking samples from a transect through the basin to find the stratification of Port Susan. This cruise took place on March 23nd and five stations were used for sampling: two PRISM sites (PRISM 2004), and 3 new sites (Figure 4; Table 1). Two CTD systems were used for data collection. The first was a Seabird Seacat that was used on the later Weelander cruises; the other was the Thompson’s onboard Seabird CTD system. Both CTDs were attached to a rosette and recorded data at the same time with simple single casts at each station. Seabird processing software was used to process and extract CTD data. Salinity triplicate samples for calibrating the CTD units were also taken from station LC3. Analyses were run on Guideline Autosals, models 8400A & B, and calibration was with IAPSO Standard Seawater.
The Seabird Seacat CTD that was used on the Thompson and Weelander cruises was calibrated with the salinity samples collected from LC3. A linear correlation between the lab analysis and CTD measurements of the form Saladj = α(SalCTD) + β was assumed where Saladj is the adjusted value of salinity from the analysis, and SalCTD is the salinity measured from the CTD. A best fit was found with α = 1.0236; and β = -0.76596; nearly a one to one relationship.
Finding the Internal Waves
April 8th was the first Weelander cruise where the ISW was observed. Samples were taken from a transect down the axis of Port Susan starting at LC3 (Fig. 3, Table 2). While the original sampling period estimates predicted the wave arriving at the first station by 13:45 PST, the wave was faster than expected and arrived on the first station at 12:10. The last station ended by 15:20, when sampling was too difficult in choppy conditions to continue. In this span of time seven stations were taken.
Internal waves produce an easily observable surface feature from converging and diverging water masses that appear as slicks (Fig. 2). Water near the front of the wave will be pulling surface water into the wave, and the wave will be moving itself forward. This creates an area of converges; and in this study it was an obvious foam feature seen in front of the leading internal wave, and at some points on the following waves (Fig. 4). When there was no foam present the convergent region is filled with small capillary waves that are easily contrasted with the divergent regions at the back of the wave. These divergent regions are characterized by a very smooth surface caused from water movement away from the center of the internal wave.
Stations were held when the foam front would reach the Weelander and held until the wave had at least two fronts move through the station. The Seabird Seacat CTD was attached to a battery powered winch mounted on the Weelander. At each station the CTD was yo-yoed down to 25 meters; this was based on the fresh water lens depth determined from the Thompson cruise data and the amplitude of the ISW seen by Harris. Sampling started just before the surface expressions (Fig. 5) of the ISW reached the boat as to obtain isopicnal displacements for the entire wave. The first station was the only station held through every front seen, most others were held through the first two fronts. There were problems with the CTD and winch system having too slow of a yo-yo speed, and the battery for the winch ran out, so we frequently hand pulled the CTD. At the fourth station the CTD turned itself off on an up cast due to a mechanical failure. Most of the data for this station was never recorded by the CTD. After the cruise, data collected on the CTD was transferred to a computer and processed in Seabird, then exported into Matlab for more processing.
Second Wave Finding Trip
April 22nd was the final Weelander cruise. This cruise we brought out a 300 kHz Workhorse ADCP that was attached to the side of the Weelander, and the same Seacat CTD and set up. We headed farther north to search out the wave and found a weak and broken surface expression. We followed it south until it appeared to stop moving. There were three CTD sampling locations (Fig. 3) where simple once-down casts were done to approximately 30 meters. CTD data was processed similar to previous cruises. The ADCP ran through several front-like signatures during the first sighting in northern Port Susan and through the end, but never obtained a clean signal. WinADCP was used to export data into Matlab for visual interpretation.
Figure 6
Figure 7
Figure 8
Figure 9
Figure 12
The data from the first cruise aboard the Thompson shows the water structure in Port Susan. There was a thin surface freshwater lens, about seven meters deep (Fig. 6). The lens was thickest in the middle of Port Susan at station LC3. It was also visually observed from deck that there was “muddier” water at this location. The fresh water lens is very stratified at the northern end and becomes more mixed as you exit Port Susan (Fig. 7). The water column is almost entirely mixed just outside of Port Susan at LC5. Also there was an interesting deep warm water mass seen at LC1 - LC4. This water was the saltiest and densest in the system and occurred at the bottom (Fig. 8).
In the first Weelander cruise to record the ISW, the wave moved faster than expected. Harris data shows the wave moving at 0.63 ± 0.04 m s-1, while our wave speeds averaged 0.73 ± 0.06 m s-1. This could be due to heavy rains on the day preceding the observations. Of interest were the very slight changes in speed as the wave propagated (Fig. 9). The average wave speed before the wave reaches the end of Camano Island is very slightly slower than after it has passed the island.
| Figure 10A | Figure 10B | Figure 10C |
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From the data (Fig. 10A; Fig.10B; Fig.10C) the entire wave form was not seen in the earlier stations with the CTD yo-yos. The waves had very short wavelengths compared with the slow casting intervals, thus the data collected from the first stations is a partial sampling from many waves. The amplitudes that were observed came from later stations, and were about 9 meters. In our first observations seven distinct convergence zones were counted, and by the last station only three were seen, this was very similar to Harris’ report. In this cruise however, at the middle stations it was difficult to identify convergence zones due to a lengthening wavelength and disturbances in the foam convergence zone (Fig. 11A; Fig. 11B; Fig. 11C; Fig. 11D).
| Figure 11A | Figure 11B | Figure 11C | Figure 11D |
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 |
 |
During this cruise max ebb was around 8:30 (PST), this is the estimated time of origin for the ISW based on Jeffery Harris’s previous observations. Low tide was around 11:45 (PST), just before the first observations of the ISW at LC3. Max flood was around 15:00 (PST), which was when we ended the survey of the Port Susan ISW. The entire sampling period was a flood tide into Port Susan, while the ISW moved out of Port Susan.
Prior to the end of Camano Island a thick foam convergence zone was observed, but very soon after the wave propagated past this “funnel” the foam was very weak and broken. It also did not follow the actual convergence zone, making it very hard to time waves to determine their velocity (Fig. 9). Again, soon after the wave reached Gedney Island, the foam convergence was reestablished. At the last station the surface waves were starting to be large enough that white camps were forming in the convergence zones, and large swells were in the divergent areas.
During the Thompson cruise the stratification in Port Susan became less distinct south of the entrance, this was also the case on the Weelander cruise. The density stratification very slowly decreases moving out of Port Susan; along with it the amplitude of the wave significantly decreases from 9 to 4 meters and the wavelength increased (Table 3, Fig. 12).
On the second Weelander cruise no strong signal was seen. There was a thin foam line that was not consistently moving down the channel of Port Susan, instead the southern side was moving much faster than the northern side; unfortunately we were unable to quantify this. This trend continued the entire length of Port Susan until the end of the Camano Island, when the wave was no longer propagating at all. Tides and stratification were very similar to the previous Weelander cruise.
The internal wave packet observed on April 8th aboard the Weelander had strong surface signal changes that gave hints into the decay near the mouth out Port Susan. Two main observations have been made that are believed to cause the decay of the ISW: Bathymetry and stratification. Shear induced decay due to changing tides is not seen as a strong link because the speed of change seen in the dispersion of the fronts is fast compared to changes in the tides.
Bathymetry
The bathymetry seemed to have a great impact on the surface feature of the ISW. Based on surface observations, energy dispersion from leaving the constraints of Camano Island are seen as a main dispersion mechanism. As seen in the picture series (Fig. 11A; Fig. 11B; Fig. 11C; Fig. 11D) there is a strong foam front leading the wave out of the initial channel of Port Susan. As this wave leaves the confined area of Port Susan it enters a region where the wave can disperse over a larger range. The fronts now dispersing at a rate of area squared instead of staying confined and only losing energy to viscosity. Between Camano Island and Gedney Island the wave front makes this dramatic change from a thick foam front to almost no surface feature. After the wave has propagated past this place the wave seems to recover by reforming a weak foam front.
| Figure 11A | Figure 11B | Figure 11C | Figure 11D |
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| Stratification The end of the stratification present in the channel between Gedney and Everett maybe the main factor for the final decay of the ISW seen in Port Susan. Larger tidal currents near the large delta of the Snohomish River could be the main mixers of the area. In the higher energy area outside of Port Susan even the Snohomish River outflow has little effect on the stratification (Fig. 7). The March 22th the range of densities for the entire water column was between 1022 and 1023 kg m-3 at the station just outside Port Susan, where inside the rage was between 1017 kg m-3 at the surface and 1023 kg m-3 at depth. This loss of the surface lens implies that an internal wave can not propagate here, so it will decay as it enters these waters. There is a slow decline in the Brunt–Väisälä Frequency that has a similar pattern to the decreasing amplitude of the wave (Fig. 12 and Table 3). The first station’s amplitude readings are probably underestimated due to the speed of the oncoming waves and the slow yo-yoing speeds. Other than this point the wave amplitudes are seen to decrease along with the Brunt–Väisälä frequency. |
Figure 7 Figure 12 |
Tides
The shear induced by tides was originally thought to be the main cause of the dispersion of the ISW seen at Port Susan. In many cases an unstable wave with a height greater than 0.82 times the height of the surface layer will cause the wave to collapse. It was believed that tides moving against the direction of propagation helped stabilized the wave, and when the direction changed the wave decayed (Harris 2003).
The wave started to increase its wavelengths considerably at station 4 based on surface observations, this was over an hour before the tide was to change
(Fig.
3), and this timing correlated
to exiting the
channel of Port Susan and
Brunt–Väisälä frequency
changes. April 22nd: The missed wave
On the second Weelander cruise the signal for the wave was not nearly as strong or consistent as on April 8th. At CTD sampling sites density profiles were very similar to previous measurements taken on April 8th, so a newly mixed interface is not the cause for the inconsistency of the wave. Tides were also very similar to April 8th, although a slightly smaller, less than 0.01 m s-1, currents. Another option was the fairly strong winds could have broken up the front so even though a wave was propagating, its surface expression was broken.
The internal waves seen in Port Susan slowly decay due to stratification loss and topography changes, most likely not shear induced by tides. Surface observations of the front literally disintegrating after reaching the end of Camano Island show a strong link to topography. The final breakup of the wave occurs in waters that have little to no stratification at the mouth of Port Susan.
A study of how much the waves mix the water outside of Port Susan is the next step. It will help us to understanding why these waves are important to the area, and what role they play in the ecosystem. In a sense these wave could be causing their own demise by helping to mix the water column at the mouth of Port Susan creating an environment that future waves cannot propagate through.
I would like to thank Seelye Martin and David Thoreson for helping me enormously with two cruises on the Weelander. Professor Martin had to yoyo a CTD up and down for hours, and Mr. Thoreson had the patients and insight to look for the wave on days I would not have thought it would be visible. I would also like to thank Parker McCready and Ryan McCray for the use of their ADPC in my project. Ryan helped on the second Weelander Cruise with working the ADCP, and Parker spent an entire day setting up the ADCP in the Weelander, and worked to untangle the data when we got back.
American Meteorological Society. 2000. Glossery of Meteorology.
http://amsglossary.allenpress.com/glossary/search?id=buoyancy-frequency1, March 25, 2005.
Bogucki, D. and C. Garrett. 1993. A simple model for the shear-induced decay of an internal
solitary wave. J. Phys. Oceanogr. 23: 1767-1776.
Bogucki, D., T. Dickey, L.G. Redekopp. 1997. Sediment Resuspension and Mixing by
Resonantly Generated Internal Solitary Waves. J. Phys. Oceanogr. 27: 1181-1196.
Finlayson, David. 2005. Combined Bathymetry and Topography DEM of The Puget Lowlands.
http://www.ocean.washington.edu/data/pugetsound/index.html#psdem2005; May 1, 2005.
Garrett, Christopher and Walter Munk. 1979. Internal Waves in the Ocean. Annual Review of
Fluid Mechanics. 11: 339-369.
Harris, Jeffrey C. 2003. Internal Solitary Waves in Port Susan, Puget Sound. unpublished.
Knauss, John A. 1996. Introduction to Physical Oceanography. Prentice-Hall, Inc.
PRISM. 2004. Hydrographic Survey Data Interfaces.
http://www.psmem.org/CD?action=Manage, March 1, 2005.
Puget Sound Action Team. 2005. State of the Sound 2004.
http://www.psat.wa.gov/Publications/StateSound2004/State_Sound_base.htm, March 1, 2005.
Sandstorm, H., J. and A. Elliott. 1984. Internal tide and solitons on the Scotian Shelf: A nutrient
pump at work. J. of Geophysical Research. 89, C10: 6415-6424.
Table 1. Locations for the five stations from the transect held on the March 22nd cruise on board the Thompson. Two PRISM stations and three new stations were used to help increase the resolution of the temperature and salinity structure in the Port Susan. The data from this cruise was used to determine surface fresh water lenses depth for yo-yoing the CTD on the later WeeLander Cruises.
Table 2. Location for the seven station transect in the April 8th Weelander cruise. The first station corresponds to the LC3 station from the March 22nd cruise aboard the Thompson. This transect is the actual internal wave observation set, and is referenced as the table headers for table 3.
Table 3. A summary of characteristics for the most complete data set for a wave from each station. Station 4 data was lost before a complete wave was seen, so the wavelength and period are incomplete. Amplitudes and the Brunt–Väisälä frequency are seen to decrease over time, and wavelength and period are observed to increase. Energy and speed were calculated from the equations found in Knauss (1996) and Bogucki and Garrett (1993), respectively.
Figure 1. Bottom topography map of Port Susan (Finlayson 2005). Highlights important topography and bathymetry feature affecting the ISW.
Figure 2. An example of a single internal solitary wave showing the location of the foam front for a wave (the “surface slick”). The direction of currents (arrows) of the wave helps to show the structure of the wave at different depths and how water is moved by the ISW. The “background flow” is the direction of the flood tides in Port Susan relative to the direction of propagation for the ISW. The wave speed is shown as c, h is the depth of the surface layer, a is the amplitude of the wave and the surface and deep water densities are shown as ρ1 and ρ2, respectively.
Schematic of ISW based on Knauss (1996) and Bogucki, D. and C. Garrett (1993)
Figure 3. April 8th station sampling period times over the tide data. (A) Shows the predicted currents and (B) shows the tidal height. Maximum ebb was around 8:30.
Figure 4. Map of Port Susan with CTD stations labeled from three cruises. The two major rivers in the area are also marked, the Stillaguamish River to the north and the Snohomish River near Everett.
Figure 5. Picture of surface slicks seen from the Weelander on April 8th with convergent and divergent surface layers labeled. These regions are caused by the water masses being pulled into or pushed away from the internal wave. Areas of convergence mark the front of a new wave.
Figure 6. Plots showing the density structure from the Thompson’s cruise data focusing on the top twenty meters of the five casts.
A. The surface lens is clearly visible in blue, and is thickest at the middle station. It was observed from the ship that the water was muddier here, potential from a previous river surge caused by a storm.
B. Shows a direct comparison of each station with the whole. Station LC3 is the freshest at the surface, LC1 has the strongest gradient, and LC5 has the weakest gradient.
Figure 7. The complete density structure data from the Thompson cruise with a very mixed water column seen at station LC5, and stratified water columns within Port Susan. The think surface lens is stilly clearly visible.
Figure 8. Temperature structure of Port Susan from Thompson cruise data. There is a temperature inversion at the bottom of Port Susan that is seen here. The surface fresh water is also visible, but is dwarfed by the changes seen at the bottom of the water column. The warmer water intrusion in the northern basin could be mixed up from the bottom, or from sediment laden river water sinking. Our measurements did not bring us close enough to the delta to see water column properties there
Figure 9. A plot of distance over time of the foam front. Every location the wave front was seen the latitude, longitude and time were recorded. The distance was calculated as distance away from a starting location at the end of the Stillaguamish delta (the 0m mark); the theorized starting location. Maximum ebb was 8:30.
Figure 10. Density data collected at the first, second and last stations held on the April 8th Weelander cruise. The wave is easily visible in the last station (C), but the first station (A) does not have the time resolution to distinguish the seven wave patters seen from the surface. The second station (B) is very interesting as it does not show a sinusoidal patter, where the last station does. The 1015 kg m-3 1017 kg m-3 and 1020 kg m-3 density contours are marked to help visualize the wave patterns.
Figure 11. Pictures illustrating the disintegration of the front between the Islands of Camano and Gedney.
A. The initial front before the end of Camano Island, very thick, strong, coherent and consistent.
B. and C. the disintegrating wave seen on both sides of the WeeLander. Patchy and thin characterized the portion of the propagation time between Camano and Gedney Island, this made the wave harder to visualize from the surface.
D. The time the wave spent between Gedney and the mainland it started to refocus and build a consistent front again. This foam front was not a thick as the original, but was quite easy to observe from the surface.
Figure 12. Comparison plots of wavelength (A) and wave amplitude (B). It is seen that the trend is for the wave’s amplitude to decrease as the wavelength increases.