g = the acceleration of gravity (32.2 feet per second squared) 



D„ = the inside diameter of the orifice pipe (feet] 



T = the wave period (seconds) . 



The theoretical dimensionless response characteristics of the linear 

 stilling well as functions of N and 32 ^^^ shown in Figure 3. It is 

 frequently best to design a well with a value of N greater than 5 because 

 the well can be tested using a drainage test to determine the actual 

 response characteristics of the well (Noye, 1974b). Wells with a value of 

 N less than 5 are more difficult to test. A value of N = 0.33 gives the 

 sharpest distinction between measured and dampened waves; however, this 

 type of well is difficult to build of common materials and even more dif- 

 ficult to test. It is desirable to have <^ 32 1 0.4 for the long-period 

 waves to be measured so that the long-wave amplitude in the well is approx- 

 imately equal to the long-wave amplitude outside the well (Fig. 3) . At 

 the same time the value of 32 should be 10 or greater for the short- 

 period wind waves and other noise for these to be thoroughly dampened by 

 the well. 



Simplifying equations (1) and (2) : 



^, 3.975 X 10-10 Lp D^2 



P 



and 



32 = 6.244 X 10-5 ^£_^ (4) 



U-p 1 



for English units. 



Solving for the length of pipe in feet, Lp, from equation (4): 



6 D ^ T 

 L = 0.160 X 105 2 P^ — . (5) 



The theoretical length of pipe as functions of the inside pipe diameter 

 and the well diamater is given in Figure 4. This design is for 90 percent 

 of the forcing wave with a period of 1 hour measured by the well. Since 

 Lp is linearly related to period from equation (5) , the pipe length from 

 Figure 4 can be multiplied by the period (in hours) to estimate Lp re- 

 quired for waves other than 1 hour. To obtain 95 percent or more of a 

 1-hour wave, reduce the pipe length in Figure 4 by about one-half. 



If the period of the important long waves is unknown, an alternate 

 method of well design is to dampen wind waves and record any longer period 

 waves. Curves designed to dampen 95 percent of the amplitude of a 10- 

 second wave are presented in Figure 5. The pipe length can also be 



