Rip current spacing in Southern Monterey Bay via digital imagery John E. Woods LT, US Navy, Naval Postgraduate School, Monterey, California, USA Introduction Rip currents are approximately shore normal seaward directed jets that originate within the surf zone and broaden outside the breaking region (MacMahon, 2004)(Fig 1). Rip currents can reach velocities of 1-2 m/s and are a major concern in the United States due to the large number of drownings associated with them. Over 100 drownings per year are blamed on swimmers being swept to sea by rip currents. More people fall victim to rip currents in the state of Florida every year than hurricanes, lightning, and tornadoes combined (Lushine, 1991;Lascody,1998). Understanding the dynamics and variability associated with the highly complex rip Fig 1 A schematic of a rip current current system is a major tasking for scientists. Monterey Bay is an ideal laboratory for the study of rip currents. Most wave energy approaches the beaches shore normal, which is condusive for rip current generation. wave energy. Rip current spacing is highly correlated with As wave energy increases, the spacing between the rip currents increases, and as wave energy decreases, rip currents spacing decreases. A large energy gradient is dominant from south to north along the southern half of Monterey Bay creating this difference in the rip current spacing in the alongshore direction. A large submarine canyon causes a focusing of energy on either side of Fig 2 Refraction over a it (Fig 2) with the increased energy canyon being focused at the northern end of the data set. Background A number of studies have taken place in Southern Monterey Bay attempting to understand the numerous rip currents that are present year round. Aerial photography is an excellent method to obtain data that clearly depicts rip currents. Photographs have been compared dating back to 1968 to find correlations of rip current spacing to recent data sets. More recently shore-based video imaging platforms have been erected along the coastline to obtain continuous monitoring of the nearshore along the southern half of Monterey Bay. The study site (Fig 3) contains 18 kilometers of shoreline beginning at Monterey’s Fisherman’s Wharf #2 and Fig 3 Study Site 18 km in the alongshore along the Southern Half of Monterey Bay extending northward to the Salinas river mouth. Method and Data The digital images data sets were acquired using a 5.0 mega pixel Olympus C5050 zoom digital camera. A downward facing mount was attached to the CIRPA Twin Otter aircraft to obtain a 0 degree angle of view. Pictures were manually taken every 10 seconds to obtain a 2 kilometer field of view with 1 kilometer of overlap between frames. A total of 29 images were acquired covering 15 kilometers of shoreline starting at kilometer 3 in the alongshore direction and continuing north to kilometer 18 at the mouth of the Salinas river. The images were compared and navigated to a 1980 data set that was ortho-rectified in ArcView GIS. Similar features were compared between the 1980 photos and the current data set to obtain a scale for the 2005 data set (Table 1). This scale was then used to determine pixel size on the 2005 data set. Table 1. Distances between features compared from 1980 data set to 2005. Image # 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 Dist(m) 588 423 425 454 485 164 830 63 60 190 53 53 47 408 722 336 182 24.5 38.5 38.5 270 56 386 92 15 56 139 No data 27 69 2.5 # pixels 1309 958 865 948 1022 335 1827 131 112 360 118 117 105 848 1610 728 373 45 90 87 584 134 845 203 33 113 282 Pixels/meter 2.2 2.2 2.0 2.0 2.1 2.0 2.2 2.1 1.9 1.9 2.2 2.2 2.2 2.1 2.2 2.2 2.0 1.8 2.2 2.2 13-15 2.2 14 2.4 14-16 2.2 15 2.2 15-17 2.2 16 2.0 2.0 16-18 17 9 .222 .200 10 .181 11 .222 9 DistAlongshore(km) 3-5 4 4-6 5 5-7 6 6-8 7 7-9 8 9-11 10 10-12 11 11-13 12 12-14 13 7 .285 7 .285 6 .333 7 .285 12 .200 11 .181 12 .166 16 .125 # Rips 18 .111 Spacing (km) Table 2. Avg. rip spacing in alongshore direction Rip current spacing was determined by taking the average number of rip currents every 2 kilometers with a 1 kilometer overlap. alongshore distance was divided by the number of rip currents to obtain The the spacing between rip currents (Table 2). Rip current locations were determined by the visual signal associated with rip channels and the offshore jet flow of water. In aerial photographs, rip channels appear as dark areas with a distinct absence of breaking waves. Shore based data collection was via the Naval Postgraduate School’s Oceanography Departments surf camera located at Sand City. Two Eltec mini HiPerCam cameras are mounted on an 18.7 meter tower and acquire images every 20 minutes. An ortho-rectified time exposure is generated every 20 minutes along with a single snap shot and a variance image. The rectified image shows distinct rip channels that appear as dark patches in the image. Wave breaking appears as white and is distinctively absent in areas of the rip channels. The variance image is similar to the time exposure with the rip channels appearing as dark regions. Variance images gain the advantage of identifying regions that are changing in time (like the sea surface), from those that may be bright, but are unchanging (like the dry beach) (Aaraninkhof, 2003). A rectified image as well as a snap shot and variance shot were obtained at approximately the same time as the aircraft digital image overhead the Sand City site (Fig 3). Results Rip current spacing increased as the alongshore distance increased along the study site. Order of 100-200 meter spacing was found in the vicinity of Sand City and the rip spacing increased to order 300 meters towards Fort Fig 4 Rip Spacing in the alongshore direction and wave energy spectrum Ord and Marina. The wave energy spectrum agrees with these results with increasing wave energy coinciding with increased rip spacing. Good correlation is seen between the 2005 photo data set and previous aerial photo sets and a 2003 jet ski bathymetry survey (Fig 4). Correlation begins to fall off toward the northern end of the study site, and this is attributed to a lack of confidence in being able to determine exactly where certain rips begin or end. Large, highly turbulent rips are present north of approximately 14 kilometers alongshore leading to the possible error in estimating the exact spacing between the rips. At the Sand City site there is strong evidence that the same feature was present in all four images. The magnitude of the rip currents width matched between the aerial photograph and the rectified image with an approximate width on order 50 meters. images also coincided very well. The locations in the The nearest rip current feature was also easily discernable in the variance and snap shot images, but the farthest feature is lost to the background. Discussion The strong correlation between two historical aerial photo data sets, one bathymetric survey, and the current photo data supports the argument that rip current spacing is highly dependent on wave energy. Four distinctly different wave events spaced over 35 years apart produced rip spacing on the same order of magnitude. Using shore based video imaging techniques is a highly effective method for determining the locations of rip channels. Compared with the aerial photographs, the location and spacing of the rip channels were clearly determined from the rectified plan view image. This has many operationally significant uses, since understanding wave breaking locations can give a good understanding to the underlying bathymetry. Bathymetry is vital for all near-shore Fig 5 Currents overlayed onto rectified plan view image operational models and using video imaging techniques to obtain that bathymetry has proved highly effective. Commander Bruce Morris has validated the Delft3D near shore model using currents and bathymetry obtained from video imaging during the NCEX Experiment at Blacks Beach in 2003 (Fig 5). Other operational uses can be applied to intelligence for any type of amphibious landing or assault. Near real- time imagery can easily be obtained from either manned or unmanned aerial vehicles, and from that data a rough estimate of the nearshore bathymetry can be obtained. Also, useful wave climate and beach data can be briefed just before the actual insertion. Conclusions Understanding rip current systems has far reaching effects. Civilian interests in being able to forecast or at least understand where rip currents occur will arm them with data to help save lives. Understanding the underwater bathymetry for a given area is vital to running high resolution nearshore models. Littoral military forces rely greatly on these models for mission planning and depend on a good product. Digital imaging is an effective tool for understanding nearshore bathymetry. Besides obtaining a good understanding of the seafloor bottom, digital imaging aids in the understanding of the rip current system. Shore-based stations add the extra advantage of time exposure capability that enhances the opportunity to track rip currents continuously. Future works should include rip current morphology and movements to help aid the researcher in understanding these complex systems. Acknowledgements I would like to thank Todd Anderson for coordinating the CIRPAS flight and taking the digital photos. Rob Wyland Mark Orzech for processing the data and camera assistance. for programming help, and Ed Thornton and Jamie MacMahon for their knowledge base on the subject matter. References Aarninkhof, S., Nearshore Bathymetry from video imagery, PhD dissertation, 2003. Lascody, R.L., East central Florida rip current program, National Weather Digest pp.13-19, 1991. Luschine, J.B. A study of rip current drownings and weather related factors. National Weather Digest pp.13-19. MacMahon, J.H., E.B. Thornton, and A.J. Reneirs, Rip current kinematics; An overview, Elsevier Science, 2004.
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