terrace (seaward from the foreshore, if the low-tide terrace is absent) 

 are the longshore troughs and longshore bars. 



4.514 Profile Accuracy . Beach and nearshore profiles are the major 

 source of data for engineering studies of beach changes; sometimes lit- 

 toral transport can be estimated from these profiles. Usually, beach 

 and nearshore profiles are measured at about the same time, but differ- 

 ent tecliniques are needed for their measurement. The nearshore profile 

 is usually measured from a boat or amphibious craft, using an echo sounder 

 or leadline, or from a sea sled. (Kolessar and Reynolds, 1965-66; and 

 Reimnitz and Ross, 1971.) Beach profiles are usually surveyed by standard 

 leveling and taping techniques. 



The accuracy of profile data is affected by four types of error: 

 sounding error, spacing error, closure error, and error due to temporal 

 fluctuations in the sea bottom. These errors are more significant for 

 nearshore profiles than for beach profiles. 



Saville and Caldwell (1953) discuss sounding and spacing errors. 

 Sounding error is the difference between the measured depth and the 

 actual depth. Under ideal conditions, average sonic sounding error may 

 be as little as 0.1 foot, and average leadline sounding error may be 

 about twice the sonic sounding error. (Saville and Caldwell, 1953.) (This 

 suggests that sonic sounding error may actually be less than elevation 

 changes caused by transient features like ripples. Experience with suc- 

 cessive soundings in the nearshore zone indicates that errors in practice 

 may approach 0.5 foot.) Sounding errors are usually random and tend to 

 average out when used in volume computations, unless a systematic error 

 due to the echo sounder or tide correction is involved. Long-period 

 water level fluctuations affect sounding accuracy by changing the water 

 level during the survey. At Santa Cruz, California, the accuracy of 

 hydrographic surveys was ±1.5 feet due to this effect. (Magoon, 1970.) 



Spacing error is the difference between the actual volume of a seg- 

 ment 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 near- 

 shore profiles of thousands of feet. For example, if a 2-mile segment of 

 shore 4,000 feet wide is surveyed by profiles on 1,000-foot spacings, then 

 the spacing error is about 9 cubic yards per foot of beach front per survey, 

 according to the data of Saville and Caldwell (1953, Figure 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 0.1 

 foot, spread over 1,000 feet at the seaward end of a profile, implies a 

 change of 3.7 cubic yards per time interval per foot of beach front where 



4-57 



