sufficient excess width to provide for net losses of material from the 

 area between maintenance periods. Still additional width should be 

 provided for such seasonal changes as offshore and onshore movement of 

 material. 



136. The width of beach required to provide for net losses out of 

 an area can be estimated after the annual rate of recession has been 

 determined from comparative beach profiles. (A comparison of rates of 

 drift into and out of an area can be used but is not as reliable unless 

 all other losses and gains are accounted for.) The annual loss of material 

 is computed either from the profiles or directly from contours. The total 

 quantity of material required may be computed as the annual loss times the 

 number of years between maintenance periods. Protective beach width re- 

 quired can then be roughly estimated from the equation 



, W - Q/L (18) 



where W a required width of beach, in feet 



Q s total quantity of material lost from the area in the period 



between maintenance operations in cubic yards 

 L = length of beach to be filled, in feet. 



137. It should be noted that this method takes no account of 

 possible seasonal or other short term beach losses. Seasonal changes up 

 to 1$0 feet are not uncommon. Accordingly the additional width of beach 

 required for protection against short term losses must be based on ex- 

 perience at the site in question. 



138. Size of Feeder Beaches . - A feeder beach is designed to pro- 

 vide for the net losses out of the feeder area only. The required size 01 

 a feeder beach may be determined by multiplying the annual net rate of 

 littoral loss by the number of years between maintenance periods. Beach 

 width may be determined in the same marmer as for beach fills. By 

 regular augmentation of supply of beach material to the extent of the wave 

 competence to move it, erosion or other wave damage to the downdrift shore 

 line may be prevented. 



139. Sand Dunes . - Sand dunes perform the function of beach fills in 

 preventing waves from reaching the upland, but are located landiifard of the 

 beach itself. A belt of sand dunes will provide effective protection to 

 improved upland property as long as the tops of the dunes remain above 



the limit of wave uprush. At those locations which have an adequate 

 natural supply of sand and which are subject to innundation by storm tides 

 and high seas, a belt of sand dunes may provide more effective protection, 

 and at lower cost, than either a bulkhead or seawall. 



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