Standing Waves

Standing waves are wave energy that is sustained in a stable position relative to the bathymetry (underwater terrain) of the river bottom, channel bottom, ocean floor, etc. The following photograph is taken in Active Pass while travelling on a BC Ferry from Victoria to Vancouver (eastbound). 

The wave form depicted in the photo is a standing wave that was self sustaining.  The wave form existed prior to the ferry's entry into the area and was not yet affected by the ferry's passing. There was a light to moderate wind of about 10 knots blowing from east to west (left to right) which strengthens, but did not create, the standing waves.  (There is a clue to the wind direction in a small toppled wave in the photograph.)  This situation is complex as there is residual Ebb tide flowing from east to west (left to right in the photo). Due to the narrow pathway through Active Pass, the ebb tide has not reached equilibrium on both sides of the passage before the flood tide has started  to flow against the ebb tide.

This photograph also shows an eddy line just below the shoreline in the darker shiny water area.  The eddies in this area will be revolving clockwise due to the shearing of the water between the shore and the mid-stream current.  The water adjacent to the shoreline cannot move as fast as the mid channel current due to shoreline and bottom friction resistance.  The high velocity of the mid-channel current and the stability of the shoreline water  creates a rip or shear zone somewhere between the two. This shear line creates eddies which revolve and actually reverse the current adjacent to the shoreline.

The photograph was taken with the camera facing West. The tide was very low on this day and had started to rise.  The tide was flooding, which means rising, and was starting to pour into active pass as indicated by the blue arrow in the top right.  There is wave energy visible in the photo from left to right, as indicated.

The current from the tidal flood approaches 8 knots as indicated on the following map diagram. This counter current against the wave energy forms a tidal rip and adds to the wave height.  The waves are said to be standing as the energy from the tide counters the energy from the wave and the wave does not propagate past the standing wave.

The map shows a distinct funnel shape at the entrance to active pass, accelerating the current entering the pass by convergence. In the map an underwater ridge is visible off the marker shown in the photo right underneath the standing wave.  Just west of the pass, the map shows the bottom depth exceeding 300 feet where the ridge is only about 30 to 60 feet in depth.  This also creates a three dimensional divergence further accelerating the current and forcing an upwelling as the water is forced upward along the rising bottom.  Surface boils of water indicate where the ridge impacts the current. These boils were visible in the approximate location of the blue arrow indicating the tide direction.

Mountain Waves are atmospheric internal gravity waves and were discovered 1933 by German glider pilots above the Riesengebirge. Wind driven air parcels hitting a mountainlike obstacle are being deflected upwards and will, in a stably stratified atmosphere, return to their initial height setting up an oscillatory up-down motion. Such a wave system, which often exhibits typical phenomena as interference and wave breaking, is frequently made visible by stationary, lens shaped clouds, so called lenticular clouds (Altocumulous Lenticularis Ac len). {footnote}Mountain Wave Project of OSTIV at http://mwp.flightplanner.info/Beschreibengl.htm{/footnote}

Quite simply put Mountain Waves are Rapids in the air. As indicated elsewhere in the introduction to waves, wave science is a universal concept, only the medium through which the wave propagates is different.

You will even notice a breaking wave in the accompanying diagram. Sometimes this is the reason for the Clear Air Turbulence that unexpectedly injures many airline passengers.

Important Note:

Look at the standing wave in Active Pass and notice the similarity to the Mountain Wave in the adjacent photo. The bathymetry of Active Pass that generates the standing wave is very similar to the mountain terrain that generates the adjacent Mountain Wave.