COHSTREX 2005 Infrared Measurements

A significant result of the preliminary experiment was the safe and effective deployment of the remote sensing instrumentation from the aerial lift on the barge near the N. tip of Jetty Island (see Figures 2 and 4). The equipment was installed on a 20' x 40' spud barge, which has two pilings (spuds) on either end that allow the barge to remain on station while riding up and down with the tide, which has a maximum range at the study site of approximately 5 m. A 65' aerial lift secured to the barge deck was used for elevating the sensor package (Figure 2). The sensors were mounted on a pan and tilt head, which allowed us to survey a wide area to determine the variability of the thermal features. The data acquisition systems were housed in two 8' x 10' portable offices, which also served as sleeping quarters. Power was provided by a portable diesel generator. The barge was located as close as possible to the N. tip of Jetty Island without causing significant interference of the river flow.

The two general kinds of thermal variability observed were variations due to different water types and the signature of surface disruptions that bring different temperature water up from below. A striking example of the interaction of two different water types is the convergence of warm river water and cold seawater from the bypass through the gap formed by the sill and Jetty Island shown in Figure 6a. The river flow is much faster than the bypass flow, resulting in a strong shear line along which a vortex street is formed. The main flow and accompanying secondary recirculation patterns are indicated by the blue arrows in Figure 6a. Surface velocities from a DPIV algorithm applied to this type of IR imagery are shown in Figure 6b. The main flow through the gap is clearly shown and the recirculating flow is apparent in time lapse animation. The maximum velocity in the gap of approximately 1.5 m s-1 is consistent with the velocities from the ADCP (Figure 5a) and RiverScat (see Figure 10, below). These preliminary findings suggest that DPIV applied to IR imagery may be a useful method for measuring the mean surface flow.

Figure 7. (a) IR image of cold boils generated by flow over submerged sill. (b) IR image of warm surface features generated by flow over submerged rocks at the tip of Jetty Island.

The thermal signatures due to surface disruptions included a wide variety of both warm and cold features. A cold signature produced by boils generated by flow over the submerged sill is shown in Figure 7a. A warm signature produced by flow over submerge rocks off the end of Jetty Island is shown in Figure 7b. These and other observations of surface disruptions will be used in conjunction with the bathymetry and velocity measurements to guide the design of our observation strategy for the main experiment in July 2006.

The incidence angle necessary to cover the entire area of interest ranged from roughly 35 to 70. Since the emissivity varies significantly at incidence angles greater than 45, a significant spatial gradient due to emissivity variation is present in the raw imagery. This effect can be seen in the IR image of flow through the gap shown in Figure 8a, which exhibits a relatively uniform vertical gradient. The result of a preliminary algorithm to correct for the variation of emissivity with incidence angle is shown in Figure 8b in which the jet is much more clearly delineated.

Figure 8. (a) Raw IR image of flow through the gap showing vertical gradient due to variation of emissivity with incidence angle. (b) Same image corrected for emissivity variation with incidence angle.

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