the time interval is the time between successive surveys. Such a volume 

 change may be an important quantity in the sediment budget of the litto- 

 ral zone. 



A fourth source of error comes from assuming that the measured beach 

 profiles (which are only an instantaneous picture), represent a long-term 

 condition. Actually, beach and nearshore profiles change rapidly in re- 

 sponse to changing wave conditions, so that differences between succes- 

 sive surveys of a profile may merely reflect temporary differences in 

 bottom elevation caused by storms and seasonal changes in wave climate. 

 Such fluctuations obliterate long-term trends during the relatively short 

 time available to most engineering studies. This fact is illustrated for 

 nearshore profiles by the work of Taney (1961a, Appendix B) who identified 

 and tabulated 128 profile lines on the south shore of Long Island that had 

 been surveyed more than once from 192 7 to 1956. Of these, 47 are on 

 straight shorelines away from apparent influence by inlets, and extend 

 from Mean Low Water (MLW) to about -30 feet MLW. Most of these 47 pro- 

 files were surveyed three or more times, so that 86 separate volume 

 changes are available. These data lead to the following conclusions: 



(a) The net volume change appears to be independent of the time 

 between surveys, even though the interval ranged from 2 months to 16 

 years. (See Figure 4-19.) 



(b) Gross volume changes (the absolute sums of the 86 volume changes) 

 are far greater than net voliome changes (the algebraic sums of the 86 vol- 

 lome changes). The gross volume change for all 86 measured changes is 8,113 

 cubic yards per foot; the net change is -559 cubic yards per foot (loss in 

 volume) . 



(c) The mean net change between surveys, averaged over all pairs of 

 surveys, is -559/86 or -6.5 cubic yards per foot of beach. The median 

 time between surveys is 7 years, giving a nominal rate of volume change 

 of about -1 cubic yard per year per foot. 



These results point out that temporary changes in successive surveys 

 of nearshore profiles are usually much larger than net changes, even when 

 the interval between surveys is several years. These data show that care 

 is needed in measuring nearshore profiles if results are to be used in 

 engineering studies. The data also suggest the need for caution in inter- 

 preting differences obtained in two surveys of the same profiles. 



The positions of beach profiles must be marked so that they can be 

 recovered during the life of the project. The profile monuments should 

 be tied in by survey to local permanent references. If there is a long- 

 term use for data at the profile positions, the monuments should be ref- 

 erenced by survey to a state coordinate system or other reference system, 

 so that the exact position of the profile may be recovered in the future. 

 Even if there is no anticipated long-term need, future studies in any 

 coastal region are likely, and will benefit greatly from accurately sur- 

 veyed, retrievable benchmarks. 



4-58 



