simulation of Field Profile Change 



484. The numerical model was used to simulate beach profile change 

 measured at the FRF in Duck, North Carolina. The FRF is operated by CERC at 

 the US Army Engineer Waterways Experiment Station. Beach profile surveys 

 along four shore-normal lines and associated wave and water level measurements 

 have been obtained regularly for more than 4 years at the FRF. Measurement 

 procedures and a listing of the data are given by Howd and Birkemeier (1987). 

 Surveys are carried out at approximately 2-week intervals, statistical wave 

 parameters are calculated from gage data every 6 hr, and water level is 

 recorded every 6 min. 



485. Five time periods were chosen for model simulation, each distin- 

 guished by two profile surveys between which erosional conditions prevailed 

 (storm events). Erosional conditions were characterized by offshore movement 

 of one or two bars, whereas shoreline position in most cases was very stable 

 and no retreat was noted. Anomalous stability of shoreline position is 

 characteristic of the FRF beach and may be caused in part by the presence of 

 coarser sediment that produces an armoring effect on the foreshore, thus 

 requiring a larger amount of wave energy to move the material . 



486. Figure 77 shows the sediment size distribution across the profile 

 on 17 March 1981 as given by Howd and Birkemeier (1987). The median grain 

 size was more than an order of magnitude greater on the foreshore compared to 

 the seaward region. Because of the large grain size, the slope of the 

 foreshore at the FRF is usually very steep, allowing waves to break directly 

 on the beach face . 



487. Although profile data from the FRF are unsuitable for evaluating 

 shoreline change and the prediction of eroded subaerial voliame, movement of 

 the bar may still be simulated with reasonable confidence. Furthermore, beach 

 changes may be highly three-dimensional in the field, making it essential to 

 identify profile change in the record for use here that was likely minimally 

 affected by longshore transport and rip currents. Howd and Birkemeier (1987) 

 documented a wave event 821013 - 821015 (notation: year-month-day) during 

 which time closely spaced profiles showed very different responses, with a bar 

 moving onshore on one survey line and offshore on the other line, illustrating 



204 



