The well is especially useful for short-term operations in measuring 

 harbor response to long-wave forcing and long-wave conditions outside the 

 harbor. On the Great Lakes, forced .pumping mode oscillations near or 

 longer than the natural pumping mode ("Helmholtz mode") of the inlet-harbor 

 system are generally the most important in producing reversing inlet cur- 

 rents and harbor oscillations. If the gage is to be placed in a harbor 

 and the geometry of the harbor and inlets are known, a formula can be used 

 to predict the Helmholtz period (Seelig and Sorensen, 1976) or a simple 

 numerical model can be used to show which long waves will be important to 

 the harbor (Seelig, Harris, and Herchenroder, 1976) . Typical Helmholtz 

 periods for Great Lakes harbors range from 10 minutes to 5 hours . Once 

 these potentially important periods have been determined, the stilling 

 well system and recorder can be designed to record the waves. 



The linear stilling well consists of two basic components: (a) A well 

 of inside diameter, D^, which provides the Stillwater level to be meas- 

 ured; and (b) an orifice consisting of a pipe of length, Lp, and inside 

 diameter. Dp. Both the well and pipe should be smooth and free of ob- 

 structions. Common materials are plastic or metal. 



The bottom of the well is sealed so that the water can only enter the 

 well through the orifice pipe. Friction in the pipe due to laminar flow, 

 in conjunction with the continuity of flow between the small orifice pipe 

 and the relativly larger well, determines how the stilling well will 

 respond to long-wave forcing. 



The variables that can be changed in well design are the diameter of 

 the well, the diameter and length of the intake pipe, and the depth of 

 the orifice pipe entrance below the water level. Noye (1974b, 1974c) 

 has theoretically and experimentally shown that two dimensionless param- 

 eters (S>2 and N) can be used to design a linear stilling well. ^2 i^ ^ 

 dimensionless frequency; N describes the amplitude modulation and phase 

 lag of the long wave inside the well as compared with the wave outside the 

 well. The parameters are given by Noye as: 



i 



and 



where 



^ ^W^ \ 2^ 



CD 



C2) 



V = the kinematic viscosity of water (about 10"^ feet squared per 

 second) 



the inside diameter of the well (feet) 



10 



