COHSTREX 2006 In Situ Measurements

During the past year, our primary focus has been on planning and preparing for the major field experiment With a significant effort on the part of all parties involved at the University Washington and Stanford University, in addition to cooperation with colleagues from Woods Hole Oceanographic Institution, we had an extremely successful experiment with approximately ~95% data recovery for all in situ instrumentation. The analysis of the large datasets will be the primary focus in the upcoming year. The collected data should prove useful for the modeling effort as well as provide important details of the hydrodynamic setting under which the coherent structure fields were observed.

In order to adequately quantify the hydrodynamics that dominate the complex Snohomish estuarine system, we expanded our originally proposed experimental setup to include nearly twice the instrumentation in the region of expected coherent structure generation. These additional instruments were obtained by leveraging matching funds with internal funding within Stanford's School of Engineering and also borrowing instrumentation from other field projects at Stanford's Environmental Fluid Mechanics Lab.

In order to resolve the cross-channel current variability and turbulence fields, two pairs of high resolution, fast pinging ADCPs, along with surface and bottom mounted CTDs and a nearbed high frequency Acoustic Doppler Velocimeter (ADV) were deployed at two river cross sections surrounding the area of focus for measuring coherent structures (Figure 3). In addition, we deployed additional instrumentation to provide boundary condition information for the broader reaches of the numerical modeling domain of the Snohomish (M4 and M6 in Figure 3)

Figure 3: Summary of locations of in situ instrumentation for July 2006 experiment. Aerial photograph with superimposed bathymetry showing the location of the moorings (red triangles).

A significant amount of effort was put into the design the moorings. For example, we needed to design a mooring that allowed measurement of the density over the entire tidal range, yet protected the instruments (in particular the CTDs) from damage from pumping in air or sediment, and maintaining horizontal position without the surface instrumentation being submerged in the presence of significant currents (over 2 m/s in some locations). The final design is shown in Figure 4. Using proper floatation and a tidally-adjusting mooring system for the mid-water depth and surface CTDs we were able to measure density as at multiple depths with great temporal resolution (Figure 5).

Figure 4: Vertical CTD mounts ready for deployment (7/5/2006—left panel). M3A CTD buoy in place with additional flag and light for visibility (right panel).

Figure 5. CTD time series, mooring M3A (low pass filtered 1 minute data). Entire records shown in upper panel and subsets for representative spring and neap tides are shown in the lower panels.

All of the fixed mooring data from the experiment has been downloaded, backed up, and assessed initially for quality. The quality of the in situ data looks excellent and with the exception of some minor instrument malfunctions, the data return was extremely high

The velocity data from the ADCPs is also of very high quality. The mean time series recorded by the ADCPs show the strong semi-diurnal tidal signal as well as the spring-neap variability (Figure 6). We expect to soon analyze the single ping data collected from all four of the fast sampling ADCPs to compute Reynolds stresses and assess the turbulence levels as a function of time, depth, and cross-channel location (see recent work of Nidzieko, et al., 2006 for the methodologies that will be employed). We will compare these profiles of stress with that derived by the near bed ADVs to form a complete water column profile of turbulence and be able to quantify the bottom roughness and drag in the estuary system both upstream and downstream of the region of interest for the coherent structures as well as quantify any cross-stream variability and temporal variability that may exist.

Figure 6. ADCP time series (10 minute averaged) collected at mooring M3A. Entire record shown in upper panel and representative spring neap cycles shown in lower panels.

High resolution spatial maps of velocity and density were also captured during two 30 hour experiments within the one month experiment. Considerable effort will be made in the upcoming year to process the large velocity dataset into a useable form which maps the velocity evolution at fixed spatial slices over the evolving tidal cycles in the region of focus as well as phase adjusting the density (i.e., salinity and temperature profiles from the along estuary CTD casts) information in time to create synoptic snapshots of along-estuary density.

Finally, we successfully deployed the REMUS autonomous underwater vehicle (AUV) to measure both salinity and temperature variability as a function of space and to evaluate benthic structure and bedforms using its sidescan sonar (Figure 7). Both large features and fine scale bed features are resolved in detail as sand waves ~10 cm high are resolved as well as larger features such as sunken logs. To our knowledge, this is the first use of the REMUS AUV in a shallow, highly energetic tidal system. While the measurements weren't without difficulty, we were able to capture some of the spatial variability in the density field (including part of the salt wedge evolution) as well as map the bottom in great detail. These measurements will be useful in understanding the bottom conditions leading to the turbulent field measured as well as providing feedback to the REMUS development team and AUV community at large on the limitations that must be overcome in order for AUVs to be successfully deployed as the primary hydrodynamics measurement method in such challenging environments.

Figure 7. Sidescan sonar image near M3A and the navigation channel (7/17/06). Sand waves ~ 10cm high and logs are clearly visible in this individual image.

In summary, the in situ field measurements were very successful. The datasets collected should provide a detailed look at the hydrodynamics at the field site and be invaluable to better understanding and characterization coherent structures and provide relevant and valuable information for the numerical modeling efforts.

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