VI. EFFECT OF PROJECT ON SEDIMENT BUDGET 



1. Preproject Sediment Budget. 



To develop an operating plan and establish bypassing needs for a proposed 

 weir jetty project, a thorough understanding of inlet processes at the project 

 site is necessary, particularly the prevailing sand transport conditions. A 

 preproject sediment budget for the inlet should be constructed from all avail- 

 able data, including (a) aerial photography; (b) beach profiles updrift and 

 downdrift of the inlet; (c) beach nourishment records; (d) inlet dredging 

 records; (e) hydrographic surveys of the inlet, shoals, and ocean bar; 

 (f) results of dye and drifter studies; (g) wave data (to determine longshore 

 sand transport rates); and (h) transport rates measured at nearby locations. 

 Wave data which include direction may be available from hindcasts, Summaries 

 of Synoptic Meteorological Observations (SSMO); or visual observations. Sev- 

 eral alternative postproject sediment budgets may subsequently be evaluated to 

 establish an optimum operating procedure for the system. 



The development of a sediment budget is best illustrated by an example. 

 Jarrett (1976) constructed a sediment budget for Bogue Banks, Shackleford 

 Banks, and Beaufort Inlet, an area just west of Cape Lookout in Brunswick 

 County, North Carolina. Data available and developed for his beach study were 

 surveys of the inlet dating back to 1939, beach recession rates, beach nour- 

 ishment records, inlet dredging records, hydrographic surveys of inlet shoals, 

 SSMO wave data, refraction analyses, and information on local sea level rise. 

 A conceptualized layout of the area is shown in Figure 22. Jarrett developed 

 information on sand volume changes occurring in the three elements of the 

 sediment budget (Bogue Banks, Shackleford Banks, and Beaufort Inlet). The 

 conservation of sand for each element was expressed as 



AV- 



Q<n - Qn„f = — < 15 ) 



At 



<in ^out 



where 



Q. = volumetric sediment inflow rate 



Q . = volumetric sediment outflow rate 



A-V - = change of sediment volume contained in the element 



At = time interval over which the change took place 



The inflow of sediment to each element, Q. At, less the sediment carried out 

 of the element, Q out At, must balance the change in sediment volume contained 

 within the element AV-. The sources of sediment gain and loss must be iden- 

 tified. For Bogue Banks (28,000 feet long), measured beach profiles shifted 

 landward at an average rate of 4.2 feet per year between 1936 and 1974, the 

 beginning and ending times of the example sediment budget. Data on beach 

 changes were obtained from surveys and analyses of aerial photos. The 4.2- 

 foot average beach recession rate converts to a change in volume equal to -4.2 

 feet per year x 28,000 feet x 1.3 cubic yards per square foot = -153,000 cubic 

 yards per year. The factor 1.3 cubic yards per square foot (specific for this 



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