ABSTRACT

At first glance many sandy beaches appear to consist of sand that is all the same size. Most numerical models of the changing shape of the beach also assume uniform grain size across the model domain. However, even small changes in the size of the grains can have a large effect on whether or not the sand will move and how much sand can be transported. The hypothesis for the present study is that small grain size variations can determine whether and where a beach is more likely to erode. As part of this research, the variability of sand grain size will be measured, both in space on a natural beach and in time through storms and calm periods. Correlations between the spatial and temporal variability of sand size and changing beach morphology will be examined, with the goal of determining whether variations in grain size are important in the development of patterns of erosion and accretion. For example, coarser sediments are often observed in rip channels while finer sediments are found on the shoals between channels. It has also been observed that finer sediments are found high on the intertidal beach and offshore in sand bars, whereas grain sizes in the surf zone are much coarser. In addition, this coarse-sediment region moves up and down the beach with the rise and fall of the tides. To make measurements of sand grain size, a digital imaging system (DIS) will be used that consists of a digital camera with a macro lens in an underwater housing. Images of sand are processed to give estimates of mean grain size. With the DIS, many more measurements of grain size can be obtained than is logistically feasible with traditional sediment sampling techniques. As part of this study, high-resolution grain size surveys will be conducted regularly. These observations will be compared with predictions of the motion of different sand size classes from state-of-the-artmodels of waves, currents and beach changes. The results will be used to explore the physical processes moving different size classes, to assess the importance of spatial and temporal grain size variability in making accurate predictions of sediment transport and to improve prediction of beach morphodynamics for a wide range of environmental conditions.

The present research will contribute to improved knowledge and prediction of coastal changes. Predictions of this type are presently used by federal, state, and local governments to provide coastal planning and management, for military operations, and by scientists to better understand the physical processes responsible for moving sand and other materials in the coastal region. In addition, the resulting improved model will lead to better predictions for sediment transport and shoreline changes on coasts threatened by extreme waves and sea level rise. A large percentage of the world's population lives within the coastal region and a better understanding and predictive capability of the physical processes affecting the coasts will allow planning, management and mitigation for safer coasts worldwide.

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