COHSTREX 2005 Bathymetric Survey and Velocity Tansects


Raw bathymetric data were collected from approximately 6 km upstream of the study site to the downstream river mouth. All bathymetric data were collected using a RDI Workhorse Monitor ADCP (Acoustic Doppler Current Profiler) and Bathymapper software (Aaron Blake, USGS). Tidal variability was removed using tidal elevation information from the NOAA Seattle Tidal Gauge (station number 9447130), which was the most successful of several tidal models and gauges tested to account for the tides. The corrected bathymetry data were interpolated using a weighted averaging program to fit the data onto a grid. A spacing of two meters in both the northing and easting directions was used for the study site. The grid resolution in this area is limited by the accuracy of the GPS system used during data collection. Other areas of the Snohomish in which data were collected were interpolated to grid resolutions of five and ten meters. The resulting bathymetry map of the site in Figure 4 was created from the interpolated bathymetric data and shows details of the location and extent of the sill and surrounding scour holes.

Figure 4: Bathymetry measured during preliminary experiment referenced to mean low low water based the NOAA Seattle Tidal Station. Additional transect sections were located upstream of this site. Location of transect sections and cement factory turn may vary for any given transect due to variability in the tidal cycle.

Although the primary purpose of the first year's field work was to collect bathymetry data, a preliminary velocity survey was also undertaken to estimate the mass balance for the study area and to examine the flow over the sill throughout the tidal cycle. Velocity data were collected in a circuit consisting of five major transects. Three transects parallel to and both up- and downstream of the sill (Transects 3-5 in Figure 4) will be used to study the flow over and around the sill. This information was collected starting before peak ebb and concluding after peak flood. The flow over the sill from Transect 3 during peak ebb is shown in Figure 5a. The gap creates a pronounced jet flow and eddies on either side, all of which are observed in the velocity transects. The flow over the sill from Transect 3 during peak flood is shown in Figure 5b. At this point in the tidal cycle the flow in the main channel has reversed and the flow near the sill is very slow. Further analysis of the velocity transects will focus on the plunging ebb jet in the gap in order to provide a comparison with the surface images. Transects across the bypass northwest of Jetty Island and across the main channel upstream of Jetty Island will be used to study the complex flow upstream of the study area and to determine the mass balance.

Figure 5: Subsurface velocity data from transect section 3 (see Figure 4) at (a) peak ebb tide and (b) peak flood tide relative to the distance from the cement factory directly across the river from the N. tip of Jetty Island. The gap is located at approximately 240 m.

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