Journal of Coastal Research 24 4 867–875 West Palm Beach, Florida July 2008 Principal Component Analysis of Morphology Change at a Tidal Inlet: Shinnecock Inlet, New York Frank S. Buonaiuto Jr. †,‡ , Henry J. Bokuniewicz † , and Duncan M. FitzGerald § † Marine Sciences Research Center State University of New York at Stony Brook Stony Brook, NY 11794-5000, U.S.A. fbuonaiu@notes.cc.sunysb.edu ‡ Department of Geography Hunter College City University of New York New York, NY, U.S.A. § Department of Earth Sciences Boston University Boston, MA 02215, U.S.A. ABSTRACT BUONAIUTO, F.S., JR.; BOKUNIEWICZ, H.J., and FITZGERALD, D.M., 2008. Principal component analysis of mor- phology change at a tidal inlet: Shinnecock Inlet, New York. Journal of Coastal Research, 24(4), 867–875. West Palm Beach (Florida), ISSN 0749-0208. Principal component analysis was used to identify dominant patterns of change in morphology of five recent LIDAR surveys of Shinnecock Inlet. The first principal component accounted for 55% of the ebb shoal variability over a period of 6 years (21 June 1994 through 3 July 2000) and identified three accretionary features: (1) an updrift bar complex, (2) a downdrift bypass bar, and (3) shore perpendicular bars along the downdrift barrier. The evolution of these features, accounted for in the first principal component, appears to be the result of both a natural deflection of the main ebb channel and realignment of the channel by dredging. Evidence of channel migration is seen in comparison of 14 sequential historical photographs of the inlet from 24 September 1938 to 22 April 1997. This study also docu- ments that sand waves (length = 100 to 200 m) within the outer channel region were displaced because of variations in incident wave climate. ADDITIONAL INDEX WORDS: Inlets, bypassing, morphology. INTRODUCTION Some geomorphic characteristics of inlets are determined by tidal parameters whereas others are shaped by waves driving both cross-shore and longshore transport. Qualitative classification schemes have been based on a mix of wave and tidal processes (e.g., HAYES, 1979). More quantitative cate- gorization is difficult not only because the driving forces op- erate at different temporal scales but also because the mor- phological response is dependent on the spatial scale. Simply put, smaller inlets are more sensitive to episodic changes in wave attack whereas geomorphic changes around larger in- lets, requiring the net movement of greater volumes of sand, will be controlled by long term conditions (KRAUS, 2001). De- spite scale dependent differences, inlet morphology typically approaches dynamic equilibrium so that the integration of waves and tides, over a wide range of temporal scales, con- verges to particular scale-dependent morphologies. The configuration and morphology of an inlet are continu- ally adjusting to changing conditions. Littoral sediments sup- plied to the inlet from updrift and downdrift barrier beaches are transported by currents derived from wave transforma- tion and breaking processes. This sediment is deposited and recycled through the sand reservoirs that comprise the inlet complex. Changes in inlet morphology are expected to occur in response to (1) long-term average wave energy and tidal conditions as well as rates of sand supply, which determine DOI: 10.2112/06-0739.1 received 6 August 2006; accepted inrevision 18 January 2007. the idealized equilibrium form of the inlet and its associated ebb shoal; (2) seasonal or interannual weather cycles such as periodic episodes of increased storm intensity and occurrence; (3) individual storm events; and (4) anthropogenic alter- ations, such as jetty construction or channel realignment. Of course, the response time of the observed morphology to any particular event will depend on the scale, with larger features requiring events of longer duration or stronger intensity to measurably adjust the morphology. Although determining the interaction of the morphologic features and the pathways of sediment movement are difficult to quantify, the dominant pattern of morphology change can be qualitatively deter- mined from aerial photographs and various bathymetric sur- vey techniques. At tidal inlets the complex movement of sediment due to wave, current, and water level variations, make them some of the most difficult systems in the coastal environment to quantify. Sediment bypassing, the process by which sand is transported from the updrift to the downdrift side of the inlet, controls the rate, location, and composition (LIU and HOU, 1997) of sand nourishing downdrift beaches. The dominant variables controlling the processes and rates of inlet sand by- passing include tidal prism, inlet geometry, wave and tidal energy, sediment supply, spatial distribution of backbarrier channels, regional stratigraphy, slope of the nearshore, and engineering modifications (FITZGERALD,KRAUS, and HANDS, 2001). In general, sediment bypassing is achieved through movement of individual sand grains along the perimeter of the ebb shoal by wave action and through tidal channels.