drought, pioneer plant species are colonizing to such an extent that in a 

 relatively few years the large dune field that existed in the 1960 's on the 

 Laguna Madre side of Padre Island will disappear. This rapid revegetation is 

 possible because the north end of Padre Island National Seashore is no longer 

 grazed by livestock and recreational use is limited to managed areas. 



V. CONCLUSIONS 



Hurricane Allen's impact on north Padre Island dunes was confined 

 primarily to eroding the face of both the natural and experimental dunes 

 leaving vertical cliffs. It breached only one experimental dune, the 

 335-meter bitter panicum dune. During Hurricane Allen part of the eroded sand 

 from the beach was transported farther inland around the ends of existing 

 dunes or through breaches in dunes. Also, much of the beach sand was 

 transported temporarily into the gulf. Apparently, the sand deposited in the 

 gulf was quickly redeposited on the beach, as the cross-sectional surveys 

 revealed a near-normal beach elevation 7 months after the storm. 



The hurricane's impact on north Padre Island beaches appeared much less 

 severe than previous major hurricanes, such as Hurricane Car la and Hurricane 

 Beulah in the 1960's. This conclusion was reached because elevations in 1969 

 on the backshore, where the experimental dune plantings were made, were only 

 1.4 meters above MSL. Similar locations 7 months after Hurricane Allen had 

 elevations of more than 1.5 meters above MSL. 



Sand accumulating on the beach and foredune 199 meters (distance inland) 

 continues to accumulate at about 11.5 cubic meters per linear meter of beach, 

 which is near the rate reported by Dahl and Goen (1977) for the 1975-76 

 monitoring period. Both the natural and experimental dunes continue to widen 

 1.8 to 2.4 meters per year. The base widths of all the experimental dunes now 

 exceed 30 meters (elevations 2.4 meters above MSL), which may not withstand 

 the erosion attributed to Hurricane Carla in 1961 when the natural dunes of 

 this width were destroyed. However, these experimental dunes would be more 

 than adequate to withstand major hurricanes comparable to Hurricane Allen. 

 Naturally formed dunes have basal widths more than 76 meters. Apparently the 

 dune-width extension dune, with an initial 15-meter planting in 1969, followed 

 in 1973 by another 15-meter planting seaward, can provide an effective barrier 

 to hurricane erosion. This dune width is now 50 meters compared with only 30 

 to 40 meters for dunes resulting from a single planting. 



Naturally forming dunes, such as the unplanted control area monitored, 

 will require a 25-year storm-free interval to provide protection equivalent to 

 the double width experimental dune. 



Invasion of unplanted species into the experimental foredunes continues 

 to be extremely slow due to the rapid sand accretion and plant vulnerability 

 to salt spray. For example, the back (landward) dune of the dune-width 

 extension planting (Table 9) had 10 species compared with essentially the 

 planted species on the front (seaward) dune. The ground cover was much 

 greater when protected from salt spray (80 percent versus 34 percent on the 

 back and front dunes, respectively). This is further evident by noting the 

 well-developed grassland landward of the foredunes. The ground cover averages 

 56 percent behind the experimental foredunes with 18 species commonly 

 occurring. The unplanted control area did not have the protection of a 



48 



