Optical Imaging of the Nearshore

Transcription

1 Optical Imaging of the Nearshore Rob Holman COAS-OSU 104 Ocean Admin Bldg Corvallis, OR phone: (541) fax: (541) Award #: N LONG-TERM GOAL The long-term goal of nearshore processes research has been to develop a predictive understanding of the fluid dynamics of a random wave field shoaling over the complicated bathymetry of a natural beach, and the response of the beach to those overlying wave and current motions. Success requires advances in our understanding of nearshore physics, improvement in our capabilities for numerical modeling, and development and testing of methods for the rapid collection of the data required for model initial and boundary conditions. OBJECTIVES Our program continues to follow three primary themes: a) studies of nearshore fluid dynamics, b) studies of the morphodynamic variability of the nearshore fluid-sediment system at longer time scales, and c) development of new or better data collection methods and methods to integrate these remotely sensed data with numerical models. The first two components address deficiencies in our understanding of the basic physics of the nearshore and of the nonlinear dynamic system composed of nearshore fluid motions and nearshore bathymetry. The third addresses the critical need to provide sufficient input data such that the accuracy of nearshore predictions will be limited by physics or models issues, not simply by input data scarcity. APPROACH The primary sampling method used by the Coastal Imaging Lab has been optical remote sensing from land-based camera systems called Argus Stations. The history and capabilities of Argus have recently been described in Holman and Stanley [in press]. Applications of Argus to Rapid Environmental Assessment have been described in [Conley, et al., in press] and to Coastal Zone Management in [Davidson, et al., 2006]. Argus has become a basic research tool for many nearshore applications, as reflected by the following example publications [Becker, et al., in press; Elko, et al., 2005; Holman, et al., 2006; I.L.Turner, et al., 2006; Plant, et al., 2006; Stockdon, et al., 2006]. We have also been involved in some research, joint with NATO, on the application of Argus methods to small UAVs. WORK COMPLETED Work has been completed on an extensive study of the longshore current system at Duck, NC. The use of optical sampling methods allowed greater resolution of the surf zone current structure than was

2 possible with previous in-situ arrays and the results showed that the surf zone current system was objectively classified as being alongshore variable for 71% of 218 data runs [Chickadel and Holman, 2006], in contrast to previous findings of typical alongshore uniformity. In addition, offshore deviations of breaking patterns, often seen in time exposure images and usually interpreted as being rip channels, were shown to be related to rip currents (computed from the alongshore convergence field of longshore currents; Figure 1). We are also in final edits on a paper testing the ability of optical methods for the estimation of highresolution directional wave spectra. Tests from the NCEX field experiment in 2003 show a good comparison of a lag-array-based optical method with a series of in-situ PUV sensors (e.g. Figure 2). Of interest is the nature of the physics relating the optical signal to the component waves that are being viewed, a relationship known as the modulation transfer function or MTF. A paper was also published describing the statistical nature of a natural rip current system at Palm Beach, Australia [Holman, et al., 2006], showing that rip current spacing is much more irregular and variable than that which would be compatible with most existing rip generation models (and, indeed, that actual generation of a new rip system is about two orders of magnitude less common than evolution of an existing rip). A paper by Becker et al [Becker, et al., in press] compared optical observations of a field of bottom ripples seen over one year in Argus images with existing ripple theories, showing that ripple movement in these observation is overwhelmingly shoreward. Work has been published by Plant et al [Plant, et al., 2006] examining a very simple model of variability of a sand bar system under time-varying forcing. It was shown that the onshore movement of a sand bar is statistically linked to an initial development of alongshore variability in the bar crest position (and, by assumption, 2DH circulation). Thus, it may be that 2DH circulation is a required component for modeling onshore bar movement, in contrast to the bulk of current models. We have also been involved with research on wave-driven boundary layers, finding that pressure gradients may be an important forcing term for sediment transport under steep-fronted bores [Foster, et al., 2006].

3 Figure 1. Examples of eight Argus images from Duck, NC, showing estimated 2D horizontal currents superimposed on rectified time exposure images. Offshore bulges in bar crest position are seen to correspond to rip current locations.

4 optical in situ a b c d Figure 2. (a) Examples of optical and (b) in situ (puv) spectra from 31 October, 0900 PST. Spectral densities have been normalized by peak values. (c) The total frequency and (d) directional distributions of spectral density are similar between both spectral estimates, including similar peak directions and frequencies. IMPACT/APPLICATION Argus has an ever-widening impact on the world s community of nearshore researchers. Beyond the many publications listed here (directly linked to CIL efforts), there are many research efforts not reported that arise from the approximately 25 non-cil Argus stations. The regular Argus user group meetings are well attended and the latest ICCE conference in San Diego (2006) was just one of many recent conferences that have featured Argus heavily. Optical remote sensing is an obvious solution to the large data needs to nearshore observing systems. TRANSITIONS Argus technology or spin-offs have been at the heart of many current Navy research efforts into practical optical remote sensing. This has included the VISSER program by Dr. Todd Holland at NRL-SSC and connections through previous LRS efforts and follow-ons. Recently this work has expanded to explore the applicability of Argus methods to moving platforms such as small UAVs. This work is collaborative with NRL-SSC as well as the NATO Forward Eyes program run by Dan Conley. We continue collaboration with the U.S. Army Corps of Engineers both through Bill Curtis at Vicksburg and through the FRF on a variety of Argus issues and with the USGS in both research and

5 applications-based work. Argus was the focus of the recently completed CoastView European Union program featuring the integration of Argus into standard Coastal Zone Management practice. Argus has been transitioned to commercial availability through transition agreements between OSU and Northwest Research Associates (for North America) and Delft Hydraulics (for the rest of the world). RELATED PROJECTS 1 - Joint work with Dr. Todd Holland, NRL-SSC 2 Collaboration with the WSC at Navoceano on nearshore remote sensing 3 LRS follow-on efforts, particularly UAV development 4 EU CoastView Program ( ) 5 Numerous collaborations with the Field Research Facility 6 Three month sabbatical at the NATO research center in La Spezia (09-12/06) REFERENCES Becker, J. M., U. L. Firing, J. Aucan, R. A. Holman, and M. Merrifield (in press), Video-based observations of nearshore sand ripples and ripple migration, Journal of Geophysical Research. Chickadel, C. C., and R. A. Holman (2006), Alongshore variability of longshore currents at a barred beach, Journal of Geophysical Research, in review. Conley, D. C., A. Trangeled, G. Zappa, L. Gualdesi, P. Guerrini, and R. A. Holman (in press), REA in the nearshore, Journal of Marine Science. Davidson, M. A., S. G. J. Aarninikhof, M. V. Koningsveld, and R. A. Holman (2006), Developing coastal video monitoring systems in support of coastal zone management, paper presented at Journal of Coastal Research, Itajai SC, Brazil. Elko, N. A., R. A. Holman, and G. Gelfenbaum (2005), Quantifying the rapid evolution of a nourishment project with video imagery, Journal of Coastal Research, 21, Foster, D. L., A. J. Bowen, R. A. Holman, and P.Natoo (2006), Field evidence of pressure gradient induced incipient motion, Journal of Geophysical Research, 111, 1-8. Holman, R. A., and J. Stanley (in press), The history and technical capabilities of Argus, Coastal Engineering. Holman, R. A., G. Symonds, E. B. Thornton, and R. Ranashinghe (2006), Rip spacing and persistence on an embayed beach, Journal of Geophysical Research, 111. I.L.Turner, S. G. J. Aarninkhof, and R. A. Holman (2006), Coastal imaging applications and research in Australia, Journal of Coastal Research, 22, Plant, N., K. T. Holland, and R. Holman (2006), A dynamic attractor governs beach response to storms, Geophysical Research Letters, 33. Stockdon, H. F., R. A. Holman, P. A. Howd, and J. A.H. Sallenger (2006), Emprical parameterization of setup, swash and runup, Coastal Engineering, 53,

6 PUBLICATIONS (all refereed, ONR-Supported in total or part) Becker, J. M., U. L. Firing, J. Aucan, R. A. Holman, and M. Merrifield (in press), Video-based observations of nearshore sand ripples and ripple migration, Journal of Geophysical Research. Chickadel, C. C., and R. A. Holman (2006), Alongshore variability of longshore currents at a barred beach, Journal of Geophysical Research, in review. Davidson, M. A., S. G. J. Aarninikhof, M. V. Koningsveld, and R. A. Holman (2006), Developing coastal video monitoring systems in support of coastal zone management, paper presented at Journal of Coastal Research, Itajai SC, Brazil. Foster, D. L., A. J. Bowen, R. A. Holman, and P.Natoo (2006), Field evidence of pressure gradient induced incipient motion, Journal of Geophysical Research, 111, 1-8. Holman, R. A., and J. Stanley (in press), The history and technical capabilities of Argus, Coastal Engineering. Holman, R. A., G. Symonds, E. B. Thornton, and R. Ranashinghe (2006), Rip spacing and persistence on an embayed beach, Journal of Geophysical Research, 111. I.L.Turner, S. G. J. Aarninkhof, and R. A. Holman (2006), Coastal imaging applications and research in Australia, Journal of Coastal Research, 22, Plant, N., K. T. Holland, and R. Holman (2006), A dynamic attractor governs beach response to storms, Geophysical Research Letters, 33. HONORS/AWARDS/PRIZES SECNAV/CNO Chair in Oceanography, OSU Alumni Association Distinguished Professor Award, 2006