DISCUSSION

Calculated CVI values suggest that the Gulf Islands in Mississippi may be at a greater vulnerability to predicted sea-level rise than the Gulf Islands of Florida (Figure 9A-B). The physical process variables are constant over the extent of the park with the exception of relative sea-level rise, where the Mississippi islands are experiencing a slightly higher rate of relative sea-level rise (Figure 9A-B). The sea-level vulnerability increases from low, measured at Pensacola, to moderate based on the Dauphin Island station.

For the Gulf Islands, the geologic variables are most important in controlling the difference in vulnerability between the two sections of shoreline. The GUIS shoreline is relatively uniform, consisting of low elevation washover-dominated barriers with small sections of slightly lower vulnerability dune ridges. The geologic variability along the national seashore is most apparent in the shoreline change and coastal slope variables. Calculated shoreline change rates from 1855-2001 are much higher for the Mississippi islands. The strong westward longshore transport system produces very high erosion rates on the east end of the barriers and decreasing erosion to the west. The shoreline change data for Santa Rosa and Perdido Key over the last 150 years suggest that these shorelines are relatively stable (Figure 7 D-E). Regional coastal slope vulnerability changes significantly from Mississippi to Florida as well. The Mississippi barriers are backed by a shallow wide bay (Mississippi Sound) and the shoreface slopes very gently (Figure 8 A). In contrast, Florida has a steeper sloping shoreface and the island's backbarrier lagoon is narrow and quickly approaches higher elevation mainland (Figure 8 B).

Within the Gulf Islands National Seashore, the Mississippi barriers islands may be at the highest vulnerability to sea-level rise. Areas of moderate to low vulnerability are mostly concentrated along Santa Rosa Island and Perdido Key in Florida, however, there are a few low vulnerability areas in Mississippi where rates of shoreline accretion are high. It is important to mention that CVI results for Perdido Key were influenced by a large nourishment project in 1989-1990 that placed over 4 million cubic meters of sand on the eastern 7.5 km of Perdido Key and an additional 3 million cubic meters in an offshore bar (6 meters water depth). A program that monitored the stability of the nourishment revealed that after eight years Perdido Key had maintained 56% of the original 4.1 million cubic meters that was placed on the beach, and the beach was an average of 53 m wider than pre-nourishment (Browder and Dean, 2000). Figure 11 shows how the sand from the 1989 nourishment project impacted the results of our shoreline change calculation. The long term effects of beach nourishment on mitigating erosion are unknown.

Cultural resources at GUIS can be used to illustrate the difference in coastal dynamics from the Mississippi islands to Florida. Civil War-era Fort Massachusetts is located on West Ship Island in Mississippi. The bay shoreline fronting this fort has been experiencing severe erosion which has led to a number of management and renourishment projects to preserve this historic cultural resource (Figure 12). Conversely, Fort Pickens was built only 150 yards from the bay in 1834, however spit progradation on Santa Rosa has resulted in accretion in front of the fort.

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