102 



in Figure 40, or all measurements from one site can be plotted together (top, 

 middle, and bottom). 



In summary, current data can be displayed in the form of instantaneous 

 snapshots of the current vectors or as time series curves of individual stations. 

 Many plots are usually needed to display the data collected from even short 

 field projects. It may be advantageous to present these plots in a data appen- 

 dix rather than within the text of a report. 



Error analysis of current data 



Error analysis of current records can be broadly divided into two 

 categories. The first concerns calibrations of the actual current sensing 

 instruments. A user needs to know how closely the numbers reported by a 

 particular instrument represent the water motions that it is purported to be 

 measuring. This information is important for both evaluation of existing data 

 sets and for planning of new field experiments, where some instruments may 

 be more suitable than others. 



The second broad question pertains to whether the measurements that have 

 been gathered adequately represent the flow field in the inlet or channel that is 

 being examined. This second problem is exceedingly difficult to evaluate 

 because it raises the fundamental questions of "How much data do I need"; 

 and, "Can I afford to collect the data that will really answer my questions?" 

 The user is typically tempted to respond that he wants just as much data as 

 possible, but this may prove to be counterproductive. For example, if the 

 currents in an inlet are being measured to determine variations in the tidal 

 prism over time, will a dense gridwork of sampling stations in an inlet 

 provide more useful data? Or might the excess data reveal unnecessary details 

 about turbulence and mixing in the inlet? These are intrinsically interesting 

 questions, but may not be germane to the engineering problems that must be 

 addressed. Although the dense grid pattern of data can be used to evaluate 

 overall flow, the collection, analysis, and management of the excess data can 

 be costly and time-consuming. The money used on management of this data 

 might be better spent extending a simpler sampling program for a longer 

 period at the site. 



Possibly a statistical approach could be used to plan the placement of 

 instruments in an inlet. In order to plan the optimum deployment of current 

 meters in the 1973 North Atlantic Mid Ocean Dynamics Experiment 

 (MODE-73), Bretherton, Davis, and Fandry (1976) applied the Gauss-Markov 

 theorem to minimize the expected interpolation error between instruments and 

 map the expected error. These errors depended upon the statistics of the field 

 array and not upon the measurements themselves. Wunsch (1978) and 

 Wunsch and Minster (1982) used inverse theory to determine the circulation 

 of the Atlantic Ocean by modeling the conservation of various properties and 

 then comparing the resulting models to the flow field actually measured by 

 instruments. They concluded that there were serious shortcomings to the 



Chapter 5 Analysis and Interpretation of Coastal Data 



