Spacing error is the difference between the actual volume of a segment of 

 shore and the volume estimated from a single profile across that segment. 

 Spacing error is potentially more important than sounding error, since survey 

 costs of long reaches usually dictate spacings between nearshore profiles of 

 hundreds of meters. For example, if a 3.2-k.ilometer (2-mile) segment of shore 

 1,220 meters (4,000 feet) wide is surveyed by profiles on 305-meter (1,000- 

 foot) spacings, then the spacing error is about 23 cubic meters per meter (9 

 cubic yards per foot) of beach front per survey, according to the data of 

 Saville and Caldwell (1953, Fig. 5). This error equals a major part of the 

 littoral budget in many localities. 



Closure error arises from the assumption that the outer ends of nearshore 

 profiles have experienced no change in elevation between two successive 

 surveys. Such an assumption is often made in practice and may result in 

 significant error. An uncompensated closure error of 0.03 meter (0.1 foot), 

 spread over 305 meters (1,000 feet) at the seaward end of a profile, implies a 

 change of 9.3 cubic meters (3.7 cubic yards) per time interval per meter 

 (foot) of beach front where the time interval is the time between successive 

 surveys. Such a volume change may be an important quantity in the sediment 

 budget of the littoral 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 response 

 to changing wave conditions, so that differences between successive 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 engineer- 

 ing studies. This fact is illustrated for nearshore profiles by the work, of 

 Taney (1961, App. B) , who identified and tabulated 128 profile lines on the 

 south shore of Long Island that had been surveyed more than once from 1927 to 

 1956. Of these, 47 are on straight shorelines away from apparent influence by 

 inlets and extend from mean low water (MLW) to about -9 meters (-30 feet) 

 MLW. Most of these 47 profiles were surveyed three or more times, so that 86 

 separate volume changes are available. These data lead to the following 

 conclusions : 



(1) 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 Fig. 4-27). 



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

 are far greater than net volume changes (the algebraic sums of the 86 

 volume changes). The gross volume change for all 86 measured changes is 

 20,351 cubic meters per meter (8,113 cubic yards per foot); the net change 

 is -1,402 cubic meters per meter (-559 cubic yards per foot) (loss in 

 volume) . 



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

 surveys, is -1,402 (-559)/86 or -16.3 cubic meters per meter (-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 -2.5 cubic meters per year 

 per meter (-1 cubic yard per year per foot). 



These results point out that temporary changes in successive surveys of 



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