Cape Henlopen Spit complex and recent evolution of Breakwater Harbor, Lewes, Delaware

The Cape Henlopen Spit is one of the most recognizable features along the Delaware coast and has been the focus of many studies in the past, especially at the University of Delaware by Dr. John C. Kraft and his students in the 1970's and 1980's. The spit has grown considerably since the arrival of the Dutch in the early 1600's and is still evolving through the various stages of development. The purpose of this thesis project was to document the sediment distribution and sedimentary processes occurring in the Cape Henlopen Spit area, including Breakwater Harbor. The aim was to investigate and define current geologic conditions in this area through the use of acoustic imaging equipment including RoxAnn acoustic, chirp subbottom, and multibeam bathymetric systems. The acoustic data was integrated with ground-truth information about bottom and subbottom sediments obtained from grab samples and vibracores.; Bottom sediments, critical for habitat mapping as well as natural resource management, were defined and surveyed with a RoxAnn bottom sediment classification system. Over 256 km of RoxAnn and chirp tracklines were collected in the area. To ground-truth the RoxAnn acoustic data, 22 bottom sediment samples were collected using a petite ponar grab sampler. The extent of coarse and fine sediment deposits was used to infer depositional environments as well as evaluate areas of accretion and erosion. An interesting result of a repeat RoxAnn survey was the change in sediment character from finer- to coarser-grained sediments landward/inshore of the Inner Breakwater. This change, which occurred over a two-week interval, is most likely due to the effects of a coastal storm that struck during this time causing the finer-grained sediments to be resuspended.; Chirp subbottom profiles were collected to track sedimentary layers, including paleochannels through the study area, and to document the propagation of the Cape Henlopen Spit. To aid in the interpretation of the chirp data, 10 vibracores that were collected in the survey area during the time of this project were made available for study. The chirp profiles were used to constrain changes in sediment thickness off of the tip of the spit providing information on the general trend of spit growth allowing for prediction of future spit growth and areas of rapid shoaling in Breakwater Harbor.; In addition to the RoxAnn and chirp data, two select areas were surveyed with a multibeam bathymetric imager in order to gain high definition images of the sea floor bathymetry. A deep scour feature off the west end of the Inner Breakwater and the channel between the Inner Breakwater and Cape Henlopen Spit were mapped with the multibeam. These maps reveal that there is extensive erosion on the western side of the Inner Breakwater indicating that ebb tidal currents are removing sediment and eroding the inshore side of the breakwater.; By integrating these three acoustical imagining instruments, a base study of the geologic setting in Breakwater Harbor was established and this study can be used for comparison with future studies to better constrain the processes of erosion and deposition in Breakwater Harbor, as well as predict the future of the Cape Henlopen Spit complex.