lost into the inlet. A supplementary structure (as a groin) may be needed 

 to reduce material movement into the inlet caused by either tidal currents 

 or change in longshore transport. 



The nearly continuous interception of littoral materials on the up- 

 drift side of an inlet and mechanical transportation of the materials to 

 a point on the downdrift shore (sand bypassing) constitutes a form of 

 stockpiling for artificial nourishment to the downdrift shore. In this 

 type of operation, the stockpile or feeder beach will generally be small 

 in size as the stockpile material will be transported downdrift by nat- 

 ural forces at a rate about equal to or greater than the rate of deposi- 

 tion. For the location of the stockpile or feeder beach for this type 

 of operation, see Section 6.5, SAND BYPASSING. The need for a jetty or 

 groin between the stockpile or feeder beach and the inlet to prevent 

 return of the material to the inlet must be evaluated where such struc- 

 tures do not already exist. 



5.4 SAND DUNES 



5.41 FUNCTIONS 



Sand dunes are an important protective formation. The dune ridges 

 along the coast bar the movement of storm tides and waves into the area 

 behind the beach. Dunes prevent storm waters from flooding the low in- 

 terior areas. Dune ridges farther inland also protect, but to a lesser 

 degree than foredunes. Well-stabilized inland ridges are a second line 

 of defense against water erosion should the foredunes be destroyed by 

 storms. Use of native vegetation may be desirable to stabilize dune sand 

 that might migrate over adjacent areas and damage property. (See Figure 

 5-4.) Stabilizing dunes also prevents the loss of their protection. At 

 locations with an adequate natural supply of sand, and which are subject 

 to inundation by storms, a belt of dunes can provide protection more 

 effectively at a lower cost than a seawall. (See Section 6.4, SAND DUNES.) 



Sand dunes near the beach not only protect against high water and 

 waves, but also serve as stockpiles to feed the beach. Sand accumulating 

 on the seaward slope of a dune will extend or build the dune toward the 

 shoreline. This sand, once in the dune, may be returned to the beach by 

 a severe storm and thus nourish the beach. Figure 5-5 is a schematic 

 diagram of storm wave attack on the beach and dune. As shown, the initial 

 attack of storm waves is on the beach berm fronting the dune. IVhen the berm 

 is eroded waves attack the dune. If the wave attack lasts long enough, the 

 dune can be overtopped by waves with resultant lowering of the dune crest. 

 Much of the sand eroded from the berm and dune is transported directly 

 offshore and deposited in a bar formation. This process not only helps 

 to dissipate incident wave energy during a storm, but offshore deposits 

 will normally be transported back to the beach by swells after the storm. 

 Onshore winds transport the sand from the beach toward the foredune area 

 and the dune building proceeds on another natural cycle. This dime build- 

 ing, however, is generally at a very slow rate linless supplemented by 

 fences or vegetation. 



5-21 



