96 



parameters are known. An additional consideration is how to monitor the 

 variation of current flow across and along the surf zone. Because of the 

 extreme difficulty of obtaining data from the surf zone, neither the cross-shore 

 variations of currents nor the temporal changes in longshore currents are well 

 known. 



Longshore (or littoral) drift is defined as: "Material (such as shingle, 

 gravel, sand, or shell fragments) that is moved along the shore by a littoral 

 current" (Bates and Jackson 1984). Net longshore drift refers to the differ- 

 ence between the volume of material moving in one direction along the coast 

 and that moving in the opposite direction (Bascom 1964). Along most coasts, 

 longshore currents change directions throughout the year. In some areas, 

 changes occur in cycles of a few days, while in others the cycles may be 

 seasonal. Therefore, one difficulty in determining net drift is defining a 

 pertinent time frame. Net drift averaged over years or decades may conceal 

 the fact that significant amounts of material may also flow in the opposite 

 direction. 



Because net longshore currents may vary greatly from year to year along a 

 stretch of coastline, it would be desirable to deploy current meters at a site for 

 several years in order to obtain the greatest amount of data possible. 

 Unfortunately, the cost of a multi-year deployment could be prohibitive. Even 

 a long deployment might not detect patterns which vary on decade-long scales, 

 such as the climatic changes associated with El Nino. At a minimum, near- 

 shore currents should be monitored at a field site for at least a year in order to 

 assess the changes associated with the passing seasons. Coastal scientists must 

 be aware of the limitations of field current data and recognize that long-term 

 changes in circulation patterns may remain undetected despite the best field 

 monitoring efforts. 



Figure 37 is an example of current data from offshore Fort Walton Beach, 

 Florida. The current directions and velocities were calculated from wave 

 orbital velocities measured by a wave gage in the 10-m water depth, 400 m 

 offshore. Because the gage was outside of the surf zone, the currents reflect 

 the combined influence of tides, winds, waves, and possibly offshore influ- 

 ences like the Gulf of Mexico Loop Current (Morang 1992). Nevertheless, 

 the isobaths in this area are parallel to the shore and the directions of the 

 nearshore currents are likely to be the same as those of the longshore currents 

 within the surf zone. 



Flow in tidal channels and inlets 



An inlet is "a small, narrow opening in a shoreline, through which water 

 penetrates into the land" (Bates and Jackson 1984). Inlets range in size from 

 short, narrow breaches in barrier islands to wide entrances of major estuaries 

 like Chesapeake Bay. Many geologic and engineering studies concern flow 

 through tidal inlets in sand-dominated barriers, particularly when the inlets 

 serve as navigation channels connecting harbors to the open sea. 



Chapter 5 Analysis and Interpretation of Coastal Data 



