exist. The major difference between this area and the planted 

 dune areas is that the planted dunes present a solid wall of 

 resistance to sand moving inland, so migrating sand from the beach 

 accumulates on the dune face. On the unpl anted area, the front 

 "wall" is not solid so migrating sand penetrates through and over 

 a broader base. The result is a relatively high "floor", around 

 7.6 feet (2.3 meters) MSL among the scattered pimple dunes. In 

 contrast, the floor elevation behind and among the dunes of the 

 planted study areas is only about 5.6 feet (1.7 meters) MSL. 

 The missing 2 feet (0.6 meter) of sand from the floor of the 

 planted areas has accumulated in the high elevation dunes formed 

 as a result of sand trapped In the vegetated segments from the 

 intentional plantings (Fig. 18) . 



Although constructed dunes should have a broader base than 

 80 to 90 feet (24 to 27 meters), which is the base width of the 

 planted dunes, waiting for the broader base to occur naturally is 

 not always useful. There are advantages to providing a uniform 

 sand-trapping field immediately following denudation, such as occur 

 during hurricanes of the severity of Hurricane Carla in 1961. These are: 



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

 accumulation, hence it is not lost to the beach 



system. 



(b) The resultant wall of accumulating sand prevents 

 storm surges of moderate intensity that occur annually 

 from saltwater moving inland, and at the same time 

 the accumulating sand acts as a dam for rainwater 



to provide a mesic environment free from saltwater 

 on the bayward side of the plantings so that invading 

 vegetation intolerant to salt can become rapidly 

 established. 



(c) After moderate accumulation of sand, little salt 

 spray penetrates beyond the fore part of the planted 

 dune, further hastening the establishment of 

 island vegetation intolerant of salt spray. 



d. Crest Elevations of Experimental Dunes . Longitudinal 

 surveys which parallel the beach were made along the east and 

 west crests of all planted dunes. No definable dune existed in 

 the unplanted study area so no longitudinal surveys were made 

 there. Figures 19 to 23 graphically show the east crest survey 

 data. The longitudinal figures are more revealing than the cross- 

 section figures for ascertaining effective height of dunes. 

 It is also easier to show where relatively more sand is accumulating; 

 e.g., the south 700 feet (213 meters) of the sea oats dune is 

 4 to 5 feet (1.2 to 2.5 meters) higher than the north 500 feet 



38 



