Analyzing Estuarine Shoreline Change in Coastal North Carolina

Abstract

With continued climate change, sea-level rise, and coastal development, concern about shoreline dynamics has expanded beyond oceanfront areas to encompass more protected coastal water bodies, such as estuaries. Because estuaries are critically important ecosystems, understanding coastline changes in these areas is necessary for evaluating resource risks. Throughout the recent decades various methods have been developed to calculate shoreline change and multiple parameters have been hypothesized to correlate with estuarine erosion, including fetch, wave energy, elevation, and vegetation. A transect-based approach is commonly used to quantify shoreline change on linear (i.e., ocean) shorelines; however, due to the complex morphology of the estuarine environments, a point-based approach was developed and applied in this study. Shoreline-change rates and additional parameters (i.e., wave energy and shoreline composition) were determined using 1958 and 1998 aerial photography and available datasets. From these data the average shoreline change of Cedar Island, NC is determined to be -0.24 m yr[superscript]-1, with 88% of the shoreline eroding. Of the parameters analyzed, shoreline composition appears to have an important control on shoreline erosion along Cedar Island, whereas wave energy is not significantly correlated with shoreline-change rates. The point-based approach was applied to the trunk of the Neuse River Estuary to analyze parameters associated with estuarine erosion at two contrasting scales, regional (whole estuary) and local (estuary partitioned into 8 sections, based on orientation and exposure). With a mean shoreline-change rate of -0.58 m yr[superscript]-1, the majority (93%) of the Neuse River Estuary study area is eroding. Although linear regression analysis at the regional scale did not find significant correlations between shoreline change and the parameters analyzed, trends were determined at the local scale. Local-scale analysis determined higher erosion rates, higher elevation, and lower exposure and fetch up-estuary. Erosion rates, fetch, and wave exposure increase, while elevation decreases moving eastward, down-estuary. The general trends found at the local scale highlight the importance of the spatial distribution on shoreline-change rates and parameters analyzed within a complex estuarine system, like the Neuse River Estuary. Linear regression analysis between mean fetch and mean shoreline-change rates at the Local Scale determined an equation to predict shoreline-change rates. Predicted shoreline-change rates overestimate erosion on extremely high fetch shorelines and underestimate erosion on shorelines classified as sediment bank. Overall, the model is conservative in predicting shoreline-change rates by underestimating erosion and accretion within the Neuse River Estuary. Further analysis of mean fetch by specific vegetation type may offer additional insight into the influencing forces on estuarine shoreline change.