located at the updrift end of the problem area. Until the stockpile material 

 is transported by littoral processes to the beach area downdrift of the 

 stockpile location, that beach may be expected to recede at the same rate as 

 determined from historical survey data. If economically justified, stockpiles 

 may be placed at points along the problem area, which will decrease the time 

 interval between stockpile placement and complete nourishment of the area. 

 Stockpile lengths from a few hundred meters to a kilometer have been employed 

 successfully. If the plan involves a feeder beach just downdrift of a coastal 

 inlet, wave refraction and inlet currents must be considered to locate the 

 feeder beach so that a minimum of material is transported into the inlet. A 

 supplementary structure (such as a groin) may be needed to reduce the material 

 movement into the inlet caused by either tidal currents or a change in 

 longshore transport. 



The nearly continuous interception of littoral material on the updrift 

 side of an inlet and the mechanical transportation of the material to a point 

 on the downdrift shore (sand bypassing) constitute a form of stockpiling for 

 artificial nourishment to the downdrift shore. In this type of operation, the 

 size of the stockpile or feeder beach will generally be small; the stockpile 

 material will be transported downdrift by natural forces at a rate about equal 

 to or greater than the rate of deposition. For the suggested location of the 

 stockpile or feeder beach for this type of operation, see Chapter 6, Section V 

 (SAND BYPASSING). The need for a jetty or groin between the stockpile or 

 feeder beach and the inlet to prevent the return of the material to the inlet 

 should be evaluated if such structures do not already exist. 



IV. SAND DUNES 



1. Functions. 



Sand dunes are an important protective formation. The dune ridges along 

 the coast prevent the movement of storm tides and waves into the land area 

 behind the beach. Dunes prevent storm waters from flooding the low interior 

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

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

 defense against erosion should the foredunes be destroyed by storms. The use 

 of native vegetation may be desirable to stabilize the dune sand that might 

 migrate over adjacent areas and damage property (see Fig. 5-6). Stabilizing 

 dunes also prevent the loss of their protection. At locations that have an 

 adequate natural supply of sand and are subject to inundation by storms, a 

 belt of dunes can provide protection more effectively at a lower cost than a 

 seawall (see Ch. 6, Sec. IV). 



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

 but also serve as stockpiles to feed the beach. Sand accumulation on the sea- 

 ward slope of a dune will either build or extend the dune toward the shore- 

 line. This sand, once in the dune, may be returned to the beach by a severe 

 storm and thus nourish the beach. Figure 5-7 is a schematic diagram of a 

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

 storm waves is on the beach berm fronting the dune. Waves attack the dune 

 when the berm is eroded. If the wave attack lasts long enough, the waves can 

 overtop the dune, lowering the dune crest. Much of the sand eroded from the 

 berm and dune is transported directly offshore and deposited in a bar forma- 

 tion. This process helps to dissipate incident wave energy during a storm, 



5-24 



