Interannual evolution of multiple longshore sand bars in a mesotidal environment, Truro, Massachusetts, USA

Multiple longshore bars, 0.30–1.5 m in amplitude, are present in the nearshore zone off Truro, MA in Cape Cod Bay. These bars exist in a mesotidal environment with a tidal range of 3.0 m. A detailed interannual investigation of these bars over a 16-year time period using orthorectified vertical aerial photography has been carried out to characterize the behavior of multiple bars forming in a mesotidal environment over a temporal scale of observation not generally achieved by direct field study. This investigation reveals that the bars are dynamic over the time interval studied with offshore extents of up to 660 m and wavelengths of 45–150 m, generally increasing in the seaward direction. At any one location 4 to 10 bars are present, with an apparent addition of bars at the southern end of the study area. In addition, the bars are consistently parallel to offshore bathymetric contours and comparison of more recent photographs with a 1960 image reveals a ∼0.5-km alongshore zone without bars that has persisted and grown since 1960. The characteristics of the Truro multiple bar field are consistent with several existing theories of multiple bar formation, including formation by breaking waves, standing infragravity waves, and edge waves. The demonstrated dynamic nature of the Truro multiple bar field over a long temporal scale suggests that field experiments designed to distinguish between possible formation theories may provide an explanation for the continuing modern creation and modification of this and similar multiple bar fields occurring in mesotidal environments.