*3.1. Shoreline Change*

Analysis of subsequent shoreline pairs between 1883 and 2014 shows that the rate and magnitude of shoreline change on the Napatree varied over time, particularly following a major storm event that removed the foredune and overwashed the entire barrier. Shoreline change rate was lower and even slightly accretional during a subsequent period of foredune recovery. The results here show the changes between transects 20 and 50 located on the barrier and outside the influence of the groins. The average changes, maximum change, minimum change and annualized rate of change are summarized in Table 4. Transects 51 to 60, while located on the barrier, were not considered in the analysis, as the width has been altered by the installation of the groins and the construction of a concrete seawall on the bayside of the barrier adjacent to the parking lot.

**Table 4.** Average change in shoreline position, maximum change, minimum change, average annualized rate, positional uncertainty values (Upair) and rate uncertainty values (Ur) for subsequent shoreline pairs on the Napatree barrier for transects 20–50.


The shoreline change data (1883–2014) (Table 4) show that retreat of the Napatree barrier varied over time, with little change between 1883 and 1939, followed by landward shoreline migration between 1939 and 1975 and another period of little to no net change in position between 1975 and 2014. Figure 6 shows the shoreline positions for the entire Napatree Barrier over time. Figure 7 represents the average position of the barrier. Average landward migration of the shoreline here is reported as the change in distance from the baseline rather than the annualized rate. The error bars shown represent the standard deviation of the distance from the baseline for an individual shoreline between transects 20 and 50. These standard deviations are similar in magnitude to the shoreline positional uncertainty (see supplemental Table S2 for standard deviations).

The average retreat of the Napatree barrier between 1883 and 1939 was −9.4 m, which falls within the positional uncertainty of the shoreline pair (+/−13.4 m) (Table 4). Individual transects reported values ranging from essentially no change (+/−<3 m) to more than −20 m of change near the western end of the barrier (Figure 6). Examining the sequential shoreline pairs beginning in 1939 shows the highest overall amount of landward migration occurred between 1939 and 1948, with an average of −24.1 m of change along the barrier (−2.7 m yr<sup>−</sup>1) and individual transects recording changes of more than −30 m. This was followed by average retreat of −13.4 m between 1948 and 1951 (the same value as the uncertainty of the shoreline pair), although some transects showed >22 m of retreat. A groin was installed between 1948 and 1951 on each side of the barrier at the east end (Figure 1) and appears to have impacted the shoreline position between transects 51 and 56 (Figure 6). The shoreline continued to retreat between 1951 and 1963. The average migration was 18 m (−1.5 m yr−1). Western portions of the barrier retreated > −35 m. Retreat of the shoreline continued between 1963 and 1975. The average change was −10.1 m, and parts of the barrier retreated > 15 m. Overall, between 1939 to 1975 the shoreline retreated an average of −69.5 m, and some transects saw > −77 m of change. The average annualized shoreline change rate was −1.9 m yr−1. These rates are substantially higher than other previously published rates of shoreline change (Table 1) [23–25]. The 1975 shoreline represents the most landward shoreline recorded. The shoreline position has fluctuated between 1975 and 2014 (largely within the uncertainty); however, the 2014 shoreline remains seaward of the 1975 shoreline along most of the barrier (Figures 6 and 7), suggesting the shoreline has been neutral to slightly progradational since 1975. These results match the reported short-term change between 1975 and 2000 (+0.28 m yr−1; Table 2) derived from aerial photographs and airborne LiDAR as part of the national shoreline assessment [25].

**Figure 6.** (**Top**) Digital orthophotograph of the Napatree Barrier showing the location of the transects used in this study. (**Bottom**) Distance from the onshore baseline for transects 15–60 for Napatree Point. The barrier begins at transect 20. Note the migration of the shoreline between 1939 and 1975, and the lack of change in position between 1975 and 2014. The apparent progradation of the shoreline at transect 54 represents the installation of the small groin between 1948 and 1951. The impact of the groin on shoreline position appears to be limited to transects 51 to 56.

**Figure 7.** Average position of the southern shoreline of Napatree Barrier 1883–2014 expressed as distance from the onshore baseline. Triangles show the average position of the barrier (transects 20 to 50). Error bars are based on the standard deviation in distance from the baseline for that shoreline reported in Table 4. The dashed blue line represents a linear best fit through the average positions (1883–2014). The dashed gray line represents the continuation of the 1883–1939 trend.
