We determined rates of shoreline change in the New River Estuary and Intracoastal Waterway adjacent to Marine Corps Base Camp Lejeune over the past 50 years. We investigated the relationship among shoreline habitat type, sediment supply rate, and long-term erosion rates. These data will help the base make informed decisions regarding the use and long-term protection of their shoreline and provide a broadly applicable framework for understanding erosion along sheltered coasts.
Why We Care
Camp Lejeune, the largest Marine Corps base on the East Coast, is committed to sustainable management of its coastal ecosystems and shoreline. Marine Corps Base Camp Lejeune specializes in expeditionary force readiness and amphibious vessel training is central to their mission. Shoreline erosion and loss of ecologically important intertidal habitat as a result of both natural and man-made forces may negatively impact the integrity of the Camp Lejeune coastal ecosystem.
Understanding the rates and primary causes of shoreline erosion in the New River Estuary will help Camp Lejeune to develop a shoreline management plan that protects the value of this system for the military training mission. Further, by providing information about the forces that drive erosion in estuarine regions, these data will help other military installations and coastal communities to conserve and protect their shorelines.
What We Did
Shoreline mapping – Working from a small boat, we documented the type and extent of shoreline habitat (swamp forest, marsh, sediment bank) present throughout the New River Estuary, mapped the location of man-made structures, and identified the species present in most plant communities.
Shoreline change rate determination – We used aerial photography from 1956, 1989, and 2004 to calculate shoreline change rates. We also used aerial photography to examine shoreline impacts associated with military training exercises.
Sediment Budget – By combining the results of our shoreline mapping and shoreline change rate determination efforts we were able to estimate the volume of sediment that is liberated by erosion of New River Estuary shorelines. This information was used to evaluate the importance of shoreline sediments in maintaining downstream marshes.
Assessment of boat wake vs. wind wave energy – Because waves are the primary cause of shoreline erosion, it is pertinent from a management perspective to determine whether most waves are from natural (wind) or man-made (boat) sources. We monitored wakes created by passing boats and wind driven waves to compare the effects of boats wakes and naturally generated waves in different parts of the estuary.
What We Found
New River Estuary Erosion – The average erosion rate for the entire estuary from 1956–2004 was -0.3 m/yr. Using this average, one could predict that any given point on the shoreline would have receded 15 meters (49 feet) over the past half century. It is important to keep in mind that this number is the average of all regions within the New River Estuary. Some regions receded at twice this rate, while others experienced net gains (accretion) over the same time period. It is also important to note that there may be significant temporal variability in this erosion rate, with long periods of little to no erosion interspersed with episodic periods of high erosion.
Impact of Shoreline type on Erosion Rates – Some shoreline types experienced greater net loss than others. Sediment banks void of vegetation experienced the highest erosion rate, and marshes experienced the least. However, sediment banks bordered by even a narrow strip of saltmarsh vegetation experienced less erosion than those without. This finding is in agreement with previous studies which show that the presence of marsh vegetation can reduce overall erosion.
Atlantic Intracoastal Waterway – analysis of the region of this channel that runs through Camp Lejeune shows that it has doubled in width (from 60–120m) since 1956. Waves produced by passing boats were in many cases of equal magnitude or greater than those generated by wind. Several of the military splash points along the channel had elevated erosion rates, although the impacts of training exercises overall were found to be minimal.
Sediment delivery – Our estimates indicate that erosion of sediments from New River Estuary shorelines provides on the order of 35,000 m3/yr of sediment to downstream regions. Erosion of these shorelines likely plays a significant role in sustaining the growth of downstream saltmarshes.
Understanding the forces that cause one shoreline to expand while another one erodes is the key to devising sustainable shoreline management policies. Results of the work described above are important for guiding decisions regarding development of coastal parcels within the New River Estuary and Camp Lejeune. Further, these data help to identify which shorelines are candidates for shoreline stabilization and what type of stabilization is necessary.