multiplied by the period (in seconds) and divided by 10 to estimate Ip 

 required for other than 10-second waves. The recorded long waves of inter- 

 est can then be corrected for the particular well damping that occurred. 



The drainage test is recommended to determine the actual hydraulic 

 characteristics of a well designed to measure a specific long wave. If 

 the alternate well design is used, a drainage test is unnecessary. The 

 drainage test should be conducted after the stilling well and its com- 

 ponents are assembled, but before the well is installed. To perform this 

 test, partially fill the well and allow it to drain until the flow stops. 

 Close off the orifice, then fill the well with a head of water, H (in 

 feet), above the orifice pipe level of approximately: 



_ 8000 v^ Lp _ 2.48 X lO"'^ Lp 



(6) 



but not higher, to assure laminar flow throughout the system. Tests by 



Noye (1974b) show that the time constant of the well, T^, is the time 



that it takes for the head inside the well to fall to 0.37 (37 percent) 

 of it's initial head, H. 



One way of determining when 37 percent of the head has been lost is 

 by measuring the volume of water coming out of the orifice pipe with a 

 premeasured bucket or beaker. The value of T^ is then used to deter- 

 mine the amplitude response, a2 (well amplitude divided by forcing 

 amplitude) , of the well from: 



1.0 ,_, 



"2 = / , (7) 



and the phase lag from: 





Eo = arctan I — ^ — I (8) 



In building a well, first determine the theoretical orifice pipe length, 

 then build the well with a longer pipe length. Test the well several times, 

 using the drainage test to determine the actual hydraulic characteristics 

 of the well. If the well dampens too much of the important waves, cut off 

 some of the orifice pipe and re-run the drainage test until the desired 

 response characteristics are obtained.' 



The drainage test can be run at any time during the life of the well 

 to determine if corrosion or other fouling has impaired the function of 

 the well. 



III. DESIGN OF THE WATER LEVEL RECORDER 



Care must be taken to select a level recorder compatible with the 

 stilling well and the waves to be measured. For economic reasons, it is 



15 



