Rj = 7^3" or 0.022 



The results indicate that the project requires 1.15 cubic meters of this 

 borrow material fill to satisfy each cubic meter required by the project 

 dimensions. If the beach requires periodic renourishment , the renourishment 

 must only be provided 0.022 times as often from the borrow material as from 

 nativelike material in order to maintain the desired beach profile. Please 

 note that very low Rj values, as in this example problem, should be 

 applied in design with caution. A conservative approach is recommended, or 

 initially using an Rj equal to unity in these cases for planning the first 

 renourishment and then later adjusting the value in accordance with the 

 results of monitoring the performance of the project. 



*************************************** 



The location of the borrow source is also a factor to be considered in 

 project design. In the past, readily available sources have frequently been 

 bays, lagoons, and onshore sites. Onshore sites generally require less 

 sophisticated material-handling equipment than for offshore sites but the cost 

 per cubic meter of land-derived material is often very high, which makes these 

 sites unattractive borrow sources. Bay and lagoonal sediments are generally 

 finer and more poorly sorted than native beach sand. Although these textural 

 differences often result in volumes of borrow material several times that 

 required by project dimensions, these sources are still often selected as the 

 most cost effective due to the proximity of bays and lagoons to project sites 

 and because of the shelter they provide to dredging equipment. Few bays and 

 lagoons are currently available as sources because of environmental consid- 

 erations. The development of more seaworthy and innovative dredging plants 

 has made offshore sources of borrow material more attractive, and to date, 

 offshore sources have generally provided fill materials that are initially 

 more compatible with native beach sands. 



Hobson (1977) evaluated two borrow areas for beach fill at Oak Island, 

 North Carolina — the Yellow Banks area on the mainland and the middle-ground 

 shoal at the mouth of the Cape Fear River. U.S. Army Engineer District, 

 Wilmington (1973), found it practical to account for the proportion of grain 

 sizes finer than sand, which are considered unstable on the beach, by 

 increasing the fill factor using the following formula: 



% = ^A ^ T^ (5-4) 



where R^ is the modified fill factor. Comparisons of the two borrow areas 

 are shown in Table 5-2. 



For this particular project, the estimated mobilization-demobilization 

 expenses and cost per cubic meter of fill estimates, used in the original 

 General Design Memorandum (U.S. Army Engineer District, Wilmington, 1973), 

 favor the Yellow Banks area even when renourishment is considered. However, 

 as the use of offshore borrow sites becomes more commonplace and the tech- 

 niques of their exploitation better understood, the costs of offshore sedi- 

 ments are likely to become more economical when compared with conventional 



5-19 



