STILLING WELL DESIGN FOR ACCURATE WATER LEVEL MEASUREMENT 



by 

 W-illiam N. Seelig 



I. INTRODUCTION 



Coastal engineers and scientists have a frequent need for accurate 

 measurements of long-period water level fluctuations (periods longer than 

 about 5 minutes) . Important long waves may include astronomical or meteor- 

 ological tides, seiching of lakes and harbors, and tsunamis. The approxi- 

 mate distribution of ocean surface waves is shown in Figure 1. 



A problem in measuring water level has been that the long-period wave 

 of interest, the signal, is often of much smaller amplitude than the short- 

 period wind waves that act as noise. For example, on the Great Lakes a 

 seiche important to inlet hydraulics may have an amplitude on the order of 

 0.1 foot, while wind waves may be several feet high. In this type of 

 situation where the signal-to-noise ratio is small, a carefully designed 

 system is needed to dampen or eliminate noise while recording the important 

 long waves. 



This paper presents a method of designing a water level recording 

 system for accurately measuring water level fluctuations of interest by 

 dampening or eliminating undesirable short-period fluctuations. The unique 

 aspect of this design is that water level fluctuations inside the well are 

 linearly related to fluctuations outside the well, so no nonlinear water 

 level amplifications occur. 



The linear stilling well design presented requires that the well be 

 free from fouling. Even a small piece of debris in the orifice will dis- 

 rupt the response characteristics of the well, so this type of well is 

 recommended for short-term operation in clear water areas. 



II. LINEAR DAMPING STILLING WELLS 



Noye C1974a), using laboratory tests, showed that the conventional 

 stilling well, consisting of a well and orifice, will respond to a broad 

 spectrum of waves and a response of this type may confuse the record of 

 interest. Cross (1968) reported that this type of well system may intro- 

 duce higher harmonics of waves into the well and that wind waves, through 

 nonlinear effects, can cause a net displacement of the mean water level 

 inside the well as compared with the mean water level outside the well. 



A recommended stilling well system for accurately measuring long waves 

 is the linear damping stilling well which has a pipe or tube as an orifice 

 (Fig. 2) (Noye, 1974b, 1974c). This system can be designed to record the 

 important long waves and eliminate undesirable noise. 



A disadvantage of the system is the critical diameter of the intake 

 pipe. Dirt or other foreign matter entering the pipe will disrupt cali- 

 brated response characteristics. For this reason, the intake to the well 

 should be carefully sited, preferably in an area where fouling will not 

 affect the operation of the well. 



