interpretation and drawing of all contour lines will be unbiased for all charts. 

 For the purpose of this report, machine plots were traced for better reproduction 

 and readability. Each month requires about four minutes of plotter time or about 

 fifty minutes per location per year. 



DISCUSSION 



This graphic display of water level data can be described as a plan view 

 of monthly water level fluctuations contoured in time-space. Consider the 

 common portrayal of water-level fluctuations in graphic form where height is 

 usually taken as the ordinate and time forms the absicca. A simple water-level 

 curve so constructed might look something like a sine curve (Figure 2). 



If sequential daily water level curves are placed adjacent to one another 

 in three dimensions so that the zero hour for each falls along a third axis, then 

 that third axis represents day numbers. The three dimensions of this coordinate 

 system are then hours, days and height. The adjacent daily water-level fluctua- 

 tions now form a surface (Figure 3). This surface represents the time variations 

 in water leveLand we need only to find a useful way of looking at the surface. 

 Lines drawn through points of equal height are contours of water level and may 

 be interpreted in plan view (Figure 4). 



The water level for any time throughout the month can be found by linear 

 interpolation of either the hour scale or between contours. Detailed examples are 

 presented in Appendix A. Interpolation between days is not meaningful. A 

 regular trend in water levels is apparent and reflects the difference between the 

 lunar and solar days. Since tides more closely follow the moon, a lag of about 

 one hour per day in similar water levels is observed. 



As an example of the utility of this display, note how quickly the highest 

 and lowest water levels for a month can be found and their date and time deter- 

 mined. Similarly, the natural technique of visual comparison allows easy detection 

 of sustained high or low water levels by locating relatively widely spaced contours 

 along the day lines. 



Since the charts represent monthly time histories of water level fluctua- 

 tions, important natural parameters affecting operations may be shown on the 

 contour charts. For example, hours of darkness may be shaded in according to 

 sunrise and sunset tables (Figure 5). An operation requiring a combination of 

 minimum visible light and maximum water depth can be scheduled by examining 

 the monthly chart for periods of dark hours and times of extended high water 

 (Figure 6). 



355 



