generally best to select a standard unit. A number of types with different 

 options are available. 



If a digital recorder is chosen, the sample rate should be such that 

 at least 15 to 30 data points are taken over each important long-wave 

 period. The level resolution should be less than one-tenth the long-wave 

 height. Because a digital recorder makes only an instantaneous measurement 

 at each sampling interval, it is especially important to design the still- 

 ing well to eliminate high-frequency or short-period noise. 



An analog recorder should have a chart speed fast enough so each impor- 

 tant long wave is long enough on the chart for an easy and accurate meas- 

 urement. However, too fast a speed for longtime periods will lead to large 

 volumes of paper and frequent maintenance checks to replace the chart paper. 

 The height scale should adequately record the important waves and still 

 allow sufficient space at the top and bottom of the chart paper to record 

 any extreme events which may occur. Some recorders offer a reversing pen 

 when the chart paper width is exceeded. Analog recorders are available 

 with either strip charts or drums. Strip charts are best for long-term 

 operation; drums may be used for a short operation on the order of 1 day. 

 Because analog recorders record the water level continuously, some water 

 level fluctuations shorter in period than those of interest can be allowed 

 to propagate into the well and this noise can be eliminated when digitiz- 

 ing or analyzing the data. 



IV. A SAMPLE DESIGN 



The design of a stilling well at Pentwater, Michigan, is considered in 

 measuring long-period waves potentially important to inlet hydraulics. A 

 study of the Pentwater harbor indicates that waves with periods of between 

 1 to 2 hours will cause the largest reversing currents in the inlet 

 (Seelig, Harris, and Herchenroder, 1976). Observation of the inlet also 

 revealed that the water reversals have a period of about 1.5 hours. Figure 

 4 shows that a well with D^ = 0.83 foot (10 inches) and an orifice pipe 

 of Dp = 0.0208 foot (0.25 inch) should have an Lp of about 7 feet to 

 record 90 percent of a wave period, T = 1 hour, and about 11 feet for 

 T = 1.5 hours. The well was constructed with an intake pipe length of 20 

 feet and several drainage tests were run. As predicted, the drainage test 

 showed that this length dampened too much of the waves in the 1- to 2-hour 

 range. The plastic orifice pipe was then cut back after several tries to 

 a length of 15 feet. The drainage test showed that this well had a time 

 constant, T^ = 500 seconds; this well was selected for the final design. 

 The predicted response characteristics of the well from equation (7) are 

 shown in Figure 6. 



Note that for wind waves with a period of 10 seconds (0.003 hour) the 

 amplitude response is 0.0003 which means that only 0.03 percent of wind-: 

 wave amplitudes are propagated into the well. 



16 



