Application of geomorphological maps and LiDAR to volumetrically measure coastal geomorphological change from Hurricane Sandy at Fire Island National Seashore

Abstract

The geomorphological map offers a snapshot of the landforms generated through topographical response to physical process drivers within a given landscape. A comparison of geomorphological maps with consistently and objectively identified landform boundaries allows for the assessment of spatiotemporal variations in process-response dynamics that represent the fundamental nature of geomorphological evolution, and form a basis to derive quantitative metrics of landform development. Two geomorphological maps of Fire Island National Seashore created from LiDAR datasets and orthophotography collected prior to and immediately following Hurricane Sandy were compared according to consistent and objective criteria. The mapped geomorphology and the LiDAR elevation data facilitated the use of regional-residual separation methods to assign sediment volumes to specific processes and landforms. This led to the calculation of sediment volumes deposited as beaches, foredunes, and washover deposits before and after the storm and to the quantification of the changes associated with the storm across the entire 50 km stretch of barrier coastline that comprises Fire Island. The temporal comparisons indicated that different portions of Fire Island exhibited variable geomorphological responses to a major storm. Notably, the central and western portion of the island, from 6 km to 36 km from the mouth of Fire Island Inlet, experienced substantial losses to coastal dunes — on the order of 2.9 × 106 m3 of sediment. The beaches in this segment of the coastline gained volume (~86,000 m3) in the storm indicating that the dunes provided a source of sediment to the beach, both buffering erosion and supplying sand to the littoral system. The eastern third of the island experienced beach erosion (~540,000 m3) and erosion of the dunes (~350,000 m3), as well as a significant amount of overwash deposition (~250,000 m3) that represents a transfer of sediment from the beach/dune reservoir to the backbarrier. This mode of geomorphological evolution adds elevation to the backbarrier at the expense of sediment available to buffer the shoreline position. Two 6 km strecthes of Fire Island coastline, Bellport/Old Inlet and Talisman/Watch Hill, exemplify the geomorphological response of the eastern and central potions of the island to the impact of Hurricane Sandy: 1) at Bellport/Old Inlet, overwash deposition was partially sourced from beach deposits adding elevation to the backbarrier at the expense of the shoreline position; and 2) at Talisman/Watch Hill, sediment stored in dune systems was transferred to the beach to slow storm induced shoreline retreat, and the dune buffer and its associated resilience was reduced.In quantifying the system response of a barrier island to a major storm event like Hurricane Sandy, we demonstrate the feasibility and utility of deriving metrics of geomorphological change from geomorphological maps. This approach of quantifying process-response outcomes through mapping is broadly applicable to a variety of geomorphological settings and timescales given a consistent means of boundary determination and using regional-residual separation.