elevations. The planted dunes rise abruptly at about 2.4 meters above MSL; 

 therefore, the width of the dune was recorded between the area where 

 elevations rise above 2.4 meters MSL on the seaward side and where they drop 

 below 2.4 meters MSL on the bay side of the dunes (Table 5). This showed that 

 the dunes continue to widen at about 1.8 to 2.4 meters per year. The 

 naturally formed dune, north of the Ranger Station access road, also 

 apparently grew in width at about the same rate. 



The unplanted control section grew in a different way. Because no 

 uniform line of plants existed naturally, the sand was not trapped in a narrow 

 strip, but accumulated over a broad base of about 91 meters. Consequently, 

 accumulating sand was spread over almost the entire 91-meter width. In March 

 1976 few of the elevations exceeded 2.4 meters MSL. By 1977 the dune width 

 over 2.4 meters above MSL increased to about 30 meters. By 1981, the full 91 

 meters had elevations 2.4 meters above MSL or higher (Table 5), except for 

 about 9 meters in the middle of two of the transects. When this section 

 becomes a mature dune it will have a broad base which is similar to other 

 naturally formed dunes. 



The planted experimental dunes have a base width from 37 to 53 meters 

 (Table 5). Naturally formed dunes in the area have a base width over 80 

 meters and probably most are more than 91 meters. Though the planted dunes 

 have narrower bases, there are advantages to providing a uniform sand-trapping 

 field immediately following dune erosion as occurs during severe storms such 

 as Hurricane Carla in 1961 or Hurricane Allen in 1980: 



(1) A dam is rapidly built to help stop future storm waters 

 from crossing the island to flood the mainland areas. 



(2) Highly mobile sand is rapidly confined to one area of 

 accretion, hence it is not lost to the beach system. 



(3) The resultant wall of accumulating sand prevents inland 

 movement of saltwater from annual storm surges of moderate 

 intensity. At the same time, the accumulating sand acts 

 as a dam for rainwater providing a mesic environment that 

 is free from saltwater on the seaward side of the 

 plantings so that salt-intolerant vegetation can become 

 rapidly established. 



(4) After moderate accumulation of sand, little salt spray 

 penetrates beyond the forepart of the planted dune, 

 further hastening the establishment of the island 

 vegetation intolerant of salt spray. 



During the experimental plantings from 1960 to 1974, the 366-meter bitter 

 panicum and the dune-width extension plantings were specifically made to find 

 the most effective way to widen the base of dunes constructed from vegetation 

 plantings. Techniques for increasing the base width of the planted dunes are 

 described in Dahl and Goen (1977). 



