Nearshore Bar Mapping Using Portable Marine Radars

Our recent radar services provided to the Virginia Institute of Marine Sciences, demonstrates the ability of a marine radar to image surface expression of enhanced wave breaking over an offshore sand bar. A similar method has been applied to video imagery by ocean scientists at Oregon State University for many years, under the direction of Professor Rob Holman. (See, for example, Holland, K.T, R.A. Holman, T.C. Lippmann, J. Stanley and N. Plant. Practical use of video imagery in nearshore oceanographic field studies. IEEE Journal of Oceanic Engineering, 22(1), 81–92, 1997.) Using that method, one sums a sequence of video images of ocean waves. The mean intensity of this sum of images eliminates individual wave patterns, and features a whitewater enhancement due to wave breaking at the water’s edge, and also over the offshore sand bar.

The radar echo for a horizontally polarized marine radar operating at X-band is likewise enhanced due to wave breaking both at the water’s edge and over the nearshore bar. This enhanced echo process has been studied in great detail for purpose of understanding target detection in sea clutter (See, for example, Trizna, D.B., and D. Carlson, Studies of low grazing angle radar seascatter in nearshore regions, IEEE Trans. Geosciences and Remote Sensing, 34, pp. 747-757, 1996.)

If one sums a sequence of images, similar to the set that were used for 3D-FFT analysis, one finds that individual wave patterns disappear and the regions of enhanced wave breaking are featured. We have studied in the past this effect from a fixed marine radar roughly ten meters above sea level at the U.S. Army Corps of Engineers Field Research Facility (FRF) pier at Duck, NC. However, recent collaboration with Professor Jesse McNinch, of the Virginia Institute for Marine Sciences, demonstrated for the first time that the bar could be imaged using a small 36” radome radar on a portable beach vehicle from just a few meters above mean sea level. This allows portability and thus the ability to do long beach surveys of sand bar morphology. Video methods require (a) high, fixed and very stable platforms, (b) daytime operation, and (c) good viewing weather (rain drops on camera lenses distort the scene). Portable marine radars are not limited by these requirements, offering portability, 24-hr operation, and are impervious to weather conditions.

Results of this portable experiment are shown below. The radar was operated at four different sites along the beach, spaced roughly 500 meters apart, and data were collected for ten minutes at each location. The collected images were summed for each site, and the figures below represent the raw data format for each location. The straight strong echo is the FRF pier, for reference.

ISR Sensing: Nearshore Bar Mapping Using Portable Marine Radars

Professor McNinch merged these four images using specialized software and the following image resulted. The intensity of the signal follows a rainbow color pattern, and these colors are draped over a 3-D rendition of the intensity level as well. The results are quite dramatic, showing apparent shore-normal structure at the 300-m location, for example. The peak at the 900-m location is an instrument pod sticking out of the water.

ISR Sensing: Nearshore Bar Mapping Using Portable Marine Radars

These radar results were then compared with Argus video results for the similar time frame and are shown below, the radar data at the bottom. The similarity is striking. Both systems appear to show regions where the bar may be cut, an area where rip currents might be expected to occur. To a degree, the marine radar image appears to show such areas more clearly, although added experiments are necessary to verify the accuracy of the location of such breaks in the bar.

ISR Sensing: Nearshore Bar Mapping Using Portable Marine Radars

While more inter-comparisons are also necessary to address the sensitivity to wave breaking strength, range-offset as a function of tidal cycle, and other environmental parameters, the marine radar has apparently come into its own as a sensor for measuring sand bar morphology.

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