The rate of spread for American beach grass has averaged about 3 feet 

 per year on the landward side of the dune and 7 feet per year on the sea- 

 ward slope of the dune as long as sand was available for trapping. (See 

 Figures 6-35 and 6-36.) The rate of spread of sea oats is considerably 

 less, 1 foot or less per year. 



Figure 6-35 shows an experiment to test the feasibility of increasing 

 the dune base by a sand fence in a grass planting. The fence was put in 

 the middle of the 100-foot wide planting. (See Figure 6-39.) Some sand 

 was trapped while the American beach grass began its growth, but afterwards 

 little sand was trapped by this fence. Figure 6-36 shows how the central 

 part of the planting had a limited increase in elevation due to the reduced 

 amount of sand reaching it. The seaward edge of the dune trapped nearly 

 all of the beach sand during onshore winds. The landward edge of the dune 

 trapped the sand transported by offshore winds blowing over the unvegetated 

 area landward of the dune. 



6.447 Trapping Capacity . Periodic cross-section surveys were made of the 

 plantings to determine the volume of trapped sand, and to document the pro- 

 file of the developing dune. The annual average rate of sand trapped is 



2 to 3 cubic yards per linear foot of beach by American beach grass in 

 North Carolina, (Figures 6-35, 6-36), and by sea oats (Figures 6-37 and 

 6-40), and panic beach grass in Texas. The annual average rate of sand 

 trapped by sea oats in North Carolina is 1 cubic yard. (See Figure 6-38.) 



European beach grass annual trapping rate on Clatsop Spit, Oregon, has 

 averaged about 5 cubic yards. Although surveys were not taken until nearly 

 30 years after planting (Kidby and Oliver, 1965), the initial trapping 

 rates must have been greater. (See Figure 6-41.) 



These rates are much less than the rates of vigorous grass plantings. 

 Small plantings (100 feet square) of American beach grass trapping sand 

 from all directions have trapped as much as 16 cubic yards per linear 

 foot of beach in a period of 15 months on Core Banks, North Carolina. 

 (Savage and Woodhouse, 1969.) While this figure may exaggerate the vol- 

 ume of sand available for dune construction over a long beach, it does 

 indicate the potential trapping capacity of American beach grass. Similar 

 data for sea oats or panic beach grass are not available. However, obser- 

 vations on the rate of dune growth on Padre Island, Texas following Hurri- 

 cane Beulah (September 1967) indicate that the trapping capacity of sea 

 oats and panic beach grass is greater then the 3 cubic yard annual rate 

 observed for the planted dunes. This suggests that dune growth in most 

 areas is limited by the amount of sand transported off the beach rather 

 than by the trapping capacity of the beach grasses. 



6.448 Dune Elevation . The crest elevation of the constructed dunes is 12 

 feet MSL on Ocracoke Island (Figure 6-35), 13 feet MSL on Core Banks (Fig- 

 ure 6-36), 10 feet MSL on Padre Island (Figure 6-37) and 37 feet MSL on 

 Clatsop Spit (Figure 6-41). This is an increase of nearly 8 feet for dunes 

 in North Carolina, 6 feet on Padre Island, and 27 feet on Clatsop Spit. 



6-51 



