For example, if AZ = 0.15 m and AZ(ave) = 0.94 m, then MLE is: 



0.30 m n _,, _ n 



= 0.32 = 32 percent 



0.94 m 



Note that this is for a Class 1 survey; many offshore surveys are not 

 conducted under such tight specifications. If AZ = 0.46 m, then MLE for the 

 above example = 97 percent. Under these circumstances, it becomes mean- 

 ingless to say that an area has changed in volume by a certain amount plus or 

 minus 97 percent. 



The size of the polygons used in the calculation of AZ(ave) can influence 

 the MLE. A particular polygon that covers a large area may average AZ of 

 only 0.3 or 0.6 m, but water depths from spot to spot within the polygon may 

 vary considerably more. Therefore, by using smaller polygons, AZ will 

 typically be greater and MLE correspondingly less. However, the use of 

 smaller polygons must be balanced against the fact that positioning errors (AX 

 and AY) become correspondingly more significant. 



More research is needed to quantify errors associated with the various 

 types of offshore surveys and to identify how these errors are passed through 

 computed quantities. They must not be neglected when analyzing geologic 

 data, particularly if management or policy decisions will be based on 

 perceived trends. 



Sources of error in beach and nearshore surveys 



Repetitive surveys of beach and nearshore profiles are commonly used to 

 compute volume changes along the shoreline. Several sources of random 

 error must be considered: 



• Survey scheduling. If there is a time lag between the onshore and 

 offshore surveys, the profiles may not join vertically because of genuine 

 sediment changes (assuming that the mismatch is not due to incorrect 

 tide, wave, datum, or other survey corrections). 



• Seasonal changes. The profile may change seasonally because of storm 

 or fair weather patterns. These temporary changes may mask long-term 

 trends. 



• Yearly variations. From year to year, the profile may change because 

 of varying global climate or oceanographic conditions. Again, the long- 

 term trend may be masked. 



• Variations in regional sediment input. Unusual sediment inputs, such as 

 a flood on a nearby river, may mask the overall trend. 



A more fundamental limitation in using widely spaced profiles is that major 

 morphological features between the survey lines are not included in the 



Chapter 5 Analysis and Interpretation of Coastal Data 



119 



