DISCUSSION
The data within
the coastal vulnerability index (CVI) show variability at several
spatial scales. However, the physical process variables maintain the
most consistency over the extent of the shoreline (Figure 11). The
value of the relative sea level variable is constant for the entire
study area. The significant wave height vulnerability is very high
for the outer cape and then decreases to moderate risk within Cape
Cod Bay where fetch length decreases. Tidal range rankings are mostly
moderate with a small high vulnerability section north and south of
Coast Guard Beach.
The geologic variables show the most variability and thus have the
most influence on the CVI value (Figure
11). Geomorphology in the park includes low vulnerability medium
glacial cliffs, very high vulnerability sandy barrier beaches, as
well as moderate and high vulnerability landforms (Figures
3-8). Vulnerability due to shoreline change along the seashore
varies from low to high (Figure 9 A-C).
The outer Cape (north of Coast Guard Beach to Head of the Meadow Beach)
is labeled here as moderate vulnerability which would suggest a stable
shoreline position such that all erosion/accretion rates fall within
1m/yr, however, this region has historically experienced erosion rates
just under 1 m/yr. Thus, accretion is not a common process in this
part of CACO, even though the moderate shoreline change ranking might
suggest this. Regional coastal slope varies from very low vulnerability
adjacent to Wilkinson Basin to very high vulnerability within Cape
Cod Bay (Figure 10 A-D).
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There are four separate regions of relative coastal vulnerability
within CACO as determined by CVI analysis. The highest (very high
vulnerability) vulnerability region is in the most southern portion
of CACO starting around Coast Guard Beach. Geomorphology is the variable
that controls the CVI here, but relatively high rates of shoreline
change and low coastal slopes also make this area more vulnerable.
Regions of high vulnerability are distributed within the park, but
the most consistent area of high vulnerability exists within Cape
Cod Bay. High vulnerability within the park is a result of spit morphology
combined with low coastal slopes and moderate wave energy. Moderate
vulnerability shoreline is concentrated around the Provincetown spit
system, and the CVI here is mostly controlled by the coastal slope
and geomorphology. The lowest vulnerability shoreline is on the outer
cape from Head of the Meadow Beach south to Marconi Beach. Here vulnerability
is equally controlled by glacial cliff morphology and steep coastal
slopes.
The most influential variables in the CVI are geomorphology and regional
coastal slope; therefore they may be considered the dominant factors
controlling how CACO will evolve as sea level rises. In most cases
geomorphology reflects coastal slope such that the highest vulnerability
landforms often have the lowest coastal slopes. Shoreline change,
significant wave height, and tidal range have mostly large-scale (>20
km) secondary effects on the spatial variability of the CVI value.
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