Stilling well

 

Most tide stations require a stilling well that is connected to the sea through a hole located below the low tide level, sometimes equipped with a pipe whose length depends on the topography of the site. 

 

Exemple de puits de tranquillisation : Schéma de l'observatoire de Brest (Crédits SHOM, 2008)

 

 

Stilling wells are necessary for several reasons. The main purpose is to filter out the fluctuations in the sea level due to waves, swell or seiches in order to obtain a horizontal surface whose height is identical to that of the external sea level, averaged over a period of time equal to the measurement sampling time. Moreover, the shelter mounted over the stilling well protects the measuring equipment from the weather.

However, the hydraulic system consisting of the stilling well has the disadvantage of not being a linear filter. It is therefore important to examine the errors associated with the internal response of the system based on variations of the external sea level. This issue is discussed in Appendix C of the book La Marée Océanique Côtière by B. Simon (2007). 

 

Other errors related to wells

 

In addition to the intrinsic error (independent of the measurement system) related to the nature of the response of the stilling well hydraulic system, other errors (also independent of the measurement system) of different origins may occur and bias the response. The system must be monitored carefully for blockages in the hole and the pipe due to siltation and concretions (related to the development of marine life). 

Bouchon du puits de tranquillisation envasé et nettoyé à Nouméa (Crédits SHOM, 2010)

 

 

This initially results in a phase difference without a substantial impact on the amplitude. It is often undetectable, but seriously compromises the quality of the measurements over time. The pipe must be inspected and cleaned yearly to avoid this problem.

 

  • Errors due to density differences

 

The density difference between the outside and inside of the well is another source of error which must be taken into account for accurate measurements. During a tidal cycle, the temperature and salinity of coastal waters can vary significantly, particularly in summer and near the mouths of rivers. The well is connected to the ocean through a small hole. The water in the well is never completely renewed. Because the hole is located at the bottom of the well, the water that enters as the tide is rising is often denser than the water outside at equal depth. As a result, the level in the well is lower than the level outside. Conversely, if lower density water has entered the well at low tide and is not renewed, the opposite effect occurs: the water in the well is lighter and