There has been some concern about possible "sanding-in" of short weir 

 sections. Sanding-in was believed to be possible during severe storms when 

 large amounts of sand might be transported along the updrift beach to the weir 

 (but not carried over the weir) and cause rapid deposition adjacent to the 

 updrift side of the weir. The landlocked weir would then cease to pass sand, 

 requiring excavation of the accumulated sand before the weir could again 

 function. Because of this concern, existing weir sections have been extended 

 to more than 1,000 feet long. However, experience with existing weir jetties 

 has indicated that sanding-in may not be a significant problem; no existing 

 weirs have sanded-in. In fact, the shoreline of the updrift beach has not 

 moved seaward much beyond the landward end of the weir section although the 

 profile adjacent to the weir may have flattened. Since most of the transport 

 appears to be over the landwardmost end of the weir, lengths in excess of 500 

 feet are probably unnecessary; however, in regions where large volumes of sand 

 are known to be transported during intense storms, longer weirs may be indi- 

 cated. Other factors such as cost and protection of dredges in the deposition 

 basin also influence the selection of weir length. Longer weir sections gen- 

 erally cost less because less materials are required to construct the rela- 

 tively low weir section; however, longer weirs afford less wave protection to 

 vessels navigating the channel and to a dredge in the deposition basin. 



IX. POSTCONSTRUCTION PROFILE ADJUSTMENT 



After construction of jetty systems at inlets, initial profile adjust- 

 ments occur because of modification of tidal current patterns by the jetties. 

 Before construction, spreading ebb tidal currents work to keep the ocean shoal 

 offshore. The shoal exists in a state of dynamic equilibrium; waves which 

 tend to move the shoal onshore are balanced by ebb currents. Jetty construc- 

 tion directs ebb currents into a narrow channel where they no longer act over 

 the entire shoal. The shoals updrift and downdrift of the jetties, although 

 no longer acted on by ebb currents, continue to be acted on by waves. There- 

 fore, much of the sand stored in the shoal may move onshore under the influ- 

 ence of waves. Following construction, the sand formerly in this part of the 

 shoal contributes to the fillets between the jetties and updrift and downdrift 

 beaches. On the updrift side of the project, the additional sand may even- 

 tually find its way into the deposition basin and lead to increased dredging 

 requirements for several years after completion of construction. The rate at 

 which sand moves onshore depends on the wave climate, particularly wave con- 

 ditions during the immediate postconstruction period. The quantity of sand 

 stored in the offshore bar can be estimated by comparing preconstruction 

 profiles which extend through the shoal area near the inlet with profiles 

 taken updrift and downdrift of the inlet at a distance away from the influence 

 of tidal currents. Profile changes near the weir at Murrells Inlet are shown 

 in Figure 34. The two surveys plotted in the figure were taken about 1 year 

 apart and show a decrease in the size of the offshore shoal. Figure 35 shows 

 two profiles taken about 11,100 feet updrift (northeast) of Murrells Inlet at 

 the same time as the profiles were taken in Figure 34. Changes in these 

 updrift profiles are less than changes occuring at the inlet. The first 

 survey shown in Figure 35 was taken about 7 months after construction started 

 on the north jetty. Figure 36 shows the updrift profile superimposed on the 

 profiles near the inlet. The area between the profiles represents the amount 

 of sand stored offshore which can potentially be transported to the beach and 

 deposition basin by wave action. Additional closely spaced profiles in the 

 vicinity of the inlet may be used in conjunction with the updrift profile to 

 determine the volume of sand contained in the bar. 



53 



