the rates of shoreline change vary west to east from 5 m of erosion, where the spit 

 attaches to the erosional headland, to near stability with accretional and erosional 

 fluctuations, adjacent to Caminada Pass (Figure 10). This pattern of shoreline change 

 reflects the increasing sediment abundance in the nearshore zone, downdrift toward 

 Grande Isle. The Caminada Pass spit has been breached several times in this century by 

 hurricane landfall; the major breaches were associated with Flossie in 1956 (Figure 16) 

 and Betsy in 1965. These breaches were unstable, infilling rapidly because of the 

 sediment supply from the Bayou Lafourche headland. Farther downdrift at Grand Isle, 

 the characteristic flanking barrier pattern of updrift erosion/downdrift accretion occurs, 

 as observed at Timbalier Island. Prior to 1972, Grand Isle had historically eroded on its 

 western end at Caminada Pass, and had accreted downdrift on its eastern end at 

 Barataria Pass. With construction of the jetty system on the western shore of Caminada 

 Pass, the western-end erosion stopped and minor accretion began, averaging 

 approximately 5 m/yr. Along the central shoreline of Grand Isle, erosion rates of less 

 than 5 m/yr are common. Farther downdrift, toward Barataria Pass, this erosional trend 

 again turns to accretion of 5 to 10 m/yr. Prior to jetty construction at Barataria Pass in 

 1958, the eastern end of Grand Isle accreted 3 to 6 m/yr, which is considered usual for 

 the downdrift end of a flanking barrier island. After 1958, sedimentation in this region 

 accelerated, producing accretion rates in excess of 10 m/yr. The U.S. Army Corps of 

 Engineers (1972) estimated that this jetty system traps approximately 230,000 m of sand 

 per year. 



A time series of the total area of Grand Isle again indicates the importance of the 

 impact of coastal structures compared to the impact of hurricane landfall on flanking 

 barriers and the strategic importance of the location of the shoreline structure within 

 the sediment dispersal system (Figure 17). Following the placement of coastal structures 

 and the initiation of beach nourishment after 1950, Grand Isle increased in area from 7.8 

 km^ in 1956 to 8.8 km^ in 1979. This pattern is analogous to that observed in the 

 Timbalier Islands and indicates a marked sensitivity to coastal structures and the active 

 sediment dispersal system of the erosional headland and its flanking barriers. Placement 

 of the structures updrift of flanking barriers results in severe erosion of marginal spits 

 and reduction in flanking barrier island area. Placement on the downdrift ends of 

 flanking barriers leads to localized accretion. 



Figure 16. The ebb surge of Hurricane Flossy breached the Caminada Pass spit 

 in 1956. Note the scour features along the shoreline formed by gulfward flow 

 across the spit undergoing a hydraulic jump. 



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