COHSTREX 2006 Infrared Remote Sensing Measurements


The preliminary experiment in 2005 showed that the occurrence and appearance of coherent structures in the river are highly dependent on the tidal forcing. Furthermore, the nature of the coherent structures varies over relatively large areas because of the variable bathymetry and geometry of the river. Therefore, the primary requirements for remote sensing in this application are the ability to gather long time series of images over a wide area. During the 2006 COHSTREX deployment, stationary measurements from the lift on the barge provided the long dwell time and aircraft-based measurements provided the large area coverage.

A tentative finding from the 2005 preliminary experiment was that the temperature of the boils generated by flow over the submerged sill is determined by the presence or absence of the tidally-driven salt wedge. Validation of this result was a primary focus of the long time series coverage provided by the barge-based measurements. A new image acquisition system was designed to acquire continuous measurement for several hours at a time during ebb and flood tide. In addition to the remote sensing instruments, we deployed CTD sensors on a mooring with cable connections back to the barge. These data provide stratification information in real-time so that we could correlate changes in the boil temperature with the location of the salt wedge. The time series of temperature, salinity, and pressure in Figure 10 show the typical behavior associated with the passage of the salt wedge on both ebb and flood tide.

Figure 10. Time series of temperature (T, top), salinity (S, middle), and pressure (bottom) from a mooring approximately 40 m upriver from the sill. The red lines indicate times when the salt wedge was traveling upstream on flood tide, showing an abrupt drop in T and rise in S, followed by stratification. The blue lines indicate times when the salt wedge was traveling downstream on ebb tide, when T increases, S decreases, and both become uniform with depth.

The 2006 measurements confirm the hypothesis that the thermal signature of the boils is determined by the vertical stratification. Furthermore, we found that the boils were generated by flow over the sill on both ebb and flood tide and that their temperature changed accordingly. (The 2005 measurements focused on flood tide, so the ebb tide finding is new.) The three images in Figure 11 span a period of about 40 minutes during ebb tide during which the temperature of the boils changed from cool to warm, corresponding to the passage of the salt wedge downstream. The time series of T, S, and D in Figure 12 cover this period and confirm the correlation.

Figure 11. Sequence IR images of boil field generated downstream of submerged sill during ebb tide (lighter grey is warmer). The images are labeled A-C and correspond to the times indicated by letter in the time series of T, S, and D in Figure 12. At time A, the vertical stratification is strong enough that the boils appear cool because they bring up colder water from the bottom. By time C, the stratification has weaked to the point where the boils appear warm because they are disrupting the cool skin.

Figure 12. Time series of T, S, and D covering the period of the images in Figure 11. The times A-C correspond to the images in Figure 11.

The area-extensive coverage provided by the aircraft measurements show that the cold signature of the boils persists well downstream of the generation site. Figure 13 compares coincident images from the aircraft and barge at two times, showing how the surface disturbances grow and persist. Finally, Figure 14 shows an example of co-registered event in the aircraft and barge imagery and rectification of the barge imager.

Figure 13. Comparison of coincident IR imagery of evolving boil field from aircraft (top) and barge (bottom) during ebb tide. The right images were taken 45 minutes later than the left. The same individual boils can be identified in both the airborne and barge-based images. The airborne images show the downstream growth and persistence of the surface disruptions.

Figure 14. Co-registration of features and image rectification. Top row compares airborne (left) and barge (right) with a number of features marked (+) in both views. The barge-based image in the bottom row (right) has been rectified and its outline is shown in the magnified aerial image (bottom left).

Our preliminary analysis has confirmed that the salt wedge can be detected from the boil temperature. We successfully combined airborne surveys and fixed time series to produce a high quality data set that is rich in variety of coherent structures.

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