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

 Small plantings of 10 meters square (100 feet square) of American beachgrass 

 that trap sand from all directions have trapped as much as 40 cubic meters per 

 linear meter (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 volume of sand available for dune construction over 

 a long beach, it does indicate the potential trapping capacity of American 

 beachgrass. Similar data for sea oats or panic grass are not available. How- 

 ever, observations on the rate of dune growth on Padre Island, Texas, follow- 

 ing Hurricane Beulah (September 1967) indicate that the trapping capacity of 

 sea oats and panic grass is greater than the 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. 



The average annual vertical crest growth, as indicated in Table 6-5, 

 shows some variation over the range of test sites. However, in all cases the 

 dune crest growth has been sufficient to provide substantial storm surge 

 protection to the previously unprotected areas in back, of the dune. This was 

 evidenced on North Padre Island during Hurricane Allen in 1980. The storm 

 surge at the location of the experimental dune building site has been 

 estimated to be between 2 and 3 meters (8 and 10 feet) . Although a 

 substantial part of the dunes had eroded, they still provided protection from 

 flooding in the areas landward of the dune. This area is undeveloped on North 

 Padre Island (National Seashore), but the value of a healthy dune system can 

 be readily appreciated. 



g. C ost Fa ctors. The survival rate of transplants may be increased by 

 increasing the number of culms per transplant. This increase in survival rate 

 does not offset the increase in cost to harvest multiculm transplants. It is 

 less expensive to reduce plant spacing if factors other than erosion (such as 

 drought) affect the survival rate. 



Harvesting, processing, and transplanting of sea oats requires 1 man-hour 

 per 130 hills, panic grass requires 1 man-hour per 230 hills. For example, 

 a 15-meter-wide , 1 .6-kilometer-long planting of sea oats on 60-centimeter 

 centers requires about 500 man-hours for harvesting, processing, and trans- 

 planting if plants are locally available. Using a mechanical transplanter, 

 from 400 to 600 hills can be planted per man-hour. 



Nursery production of transplants is recommended unless easily harvested 

 wild plants of quality are locally available. Nursery plants are easier 

 to harvest than wild stock. Commercial nurseries are now producing American 

 and European beachgrasses, panic grass, and sea oats. Some States provide 

 additional information through their departments of conservation or natural 

 resources. The Soil Conservation Service routinely compiles a list of commer- 

 cial producers of plants used for soil stabilization. 



V. SAND BYPASSING 



The construction of jetties or breakwaters to provide safe navigation 

 conditions at harbor entrances or tidal inlets along sandy coasts usually 

 results in an interruption of the natural longshore transport of sand at the 

 entrance or inlet. The resulting starvation of the downdrift beach can cause 



6-53 



