CHANGE ANALYSIS OF NEW HAMPSHIRE’S BEACHES FROM MULTIPLE AIRBORNE LIDAR COLLECTIONS, HISTORICAL CHARTS, AND ORTHOPHOTOGRAPHY

In order to assess the stability (landward or seaward migration) of the New Hampshire coastline and assess volumetric changes of the beaches, short-term changes (years) were analyzed using lidar data and long-term trends (decades) were analyzed using shorelines drawn from charts and orthophotography. Multiple vintages of airborne lidar spanning the last decade and a half were analyzed to detect changes in volume of sand and gravel beaches of New Hampshire’s coast using a simple DEM of Difference (DoD) method. Due to the short length, the entire coast was analyzed at a fine (1-2m) spatial resolution. All beaches showed variability in trends, but most beaches had a net loss of sediment resulting from both more time steps with loss and bigger losses in those time steps. The two largest beaches in the state (Hampton Beach and Seabrook Beach) show similar variability to the other beaches, but with more gains than losses. Using each time step for the lidar surveys with the uncertainty removed showed a positive change of 19,170m³ over the period. However, analysis of the oldest versus the newest time steps indicated a negative difference of 70,424m³. This difference between methods resulted from the majority of time steps being negative, resulting in more negative volumes being thrown out. Beach nourishment and other anthropogenic changes have not been accounted for at present. In addition to the volumetric analysis, shorelines were delineated from charts and orthophotography dating back to the late 1800s and early 1900s, and the trend of the shoreline position was determined for shore-perpendicular transects using the Digital Shoreline Analysis System (DSAS). The large, southern beaches show net seaward movement (accretion) and the smaller northern beaches show a net shoreward movement (erosion), similar to the pattern seen in the lidar data. By combining the two datasets, the long-term and short-term trends of sediment budgets in New Hampshire can be summarized. A break in process seems to occur between the large southern beaches and the generally smaller northern beaches. Such data can provide insights for coastal managers to help focus beach management strategies (e.g., nourishment).