COHSTREX 2006 Biosonics/ADCP Measurements

The in situ observational effort that was focused on the generation of coherent structures involved two measurement platforms (Figure 8). The first consisted of ADCP and CTD boat transects in the region immediately upstream and downstream of the sill. The second consisted of ADCP and acoustic backscatter measurements from a set of tripod-mounted upward-looking instruments located immediately downstream of the sill.

Figure 8: Bathymetric map of the study site showing the location of ADCP transects, CTD casts and the Biosonics/ADCP tripod.

As in the 2005 study, the main sill-parallel ADCP transects capture the structure of the flow incident on the sill. They were repeated two to three times per hour throughout the spring and neap 30 hr studies. A circuit was also sampled downstream of the sill (labeled Sill transect in Figure 8) during the major ebb period of one tidal cycle in order to observe the reattachment of the flow with the bottom. The flow is very complex in this region and it is hoped that these transects will help to understand the details of the flow in which the coherent structures propagate. The data will help with interpretation of the remote sensing data as well as to connect measurements near the sill with measurements made by Geyer and Trowbridge (WHOI) using the MAST further downstream of the sill. Finally, CTD casts were made during a separate ebb cycle in the region immediately downstream of the sill in order to determine the density stratification. The surface velocity in this region during ebb is high so that the boat drifted 10-20 m during one cast. Nonetheless, the data will provide a reasonable measure of the average vertical stratification in the region of boil production.

The tripod configuration included three Biosonics echosounders for imaging the subsurface signature of coherent structures located within the field of view of the remote sensing platform. The exact location of the Biosonics transducers was marked with surface floats and imaged with the remote sensing platform so that the two data sets can be exactly compared. At this point, however, the geo-referencing is still underway and such a comparison can not be made. A 1200 kHz ADCP was also included on the tripod to measure the advection speed of the structures. Although this method for imaging the structures was somewhat experimental, it turned out to be very successful. Typically, one transducer was used at a time and sampled at 25Hz. We observed structures in the top 1-2 m of the water column clearly in the return (Figure 9a) at times when structures were also observed on the water surface. We also observed that the core of the structures contained low-return water at times when cold boils were observed in the infra-red imaging. Since the underlying fluid is generally characterized by lower acoustic return than the surface fluid, we are hopeful that we will be able to differentiate between cold and warm boils based on the Biosonics data. This hypothesis will be tested once the location of the Biosonics beams in the IR field of view has been determined.

Previous studies suggest that the frequency of the boils will depend on the flow speed and flow depth according to a Strouhal relationship. When boils begin to appear the flow is roughly 1 m/s and the water depth above the sill is 1 m, which predict a frequency of approximately 0.1-0.2 Hz. Spectral analysis of the near-surface return from the Biosonics transducer shows that the frequency associated with the boils varies between 0.1 and 0.5 Hz over the course of the ebb (Figure 9b and 9c). By using the Biosonics to determine the exact water depth and the ADCP to determine the mean surface velocity at each time, we will test the above scaling in order to explain the observed variation in boil frequency.

Figure 9: a) Acoustic backscatter intensity from the Biosonics transducer during a period of active boil production. The return is plotted on a log scale and dark blue corresponds to the lowest return. b) Tidal height from the NOAA Seattle gauge during the measurement period in question. The black box indicates the intense boil production period. c) Boil frequencies based on spectral analysis of the near-surface acoustic backscatter during the intense boil production period.

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