Current Prediction Error

[WallyD]

I would like to report an error in the algorithm used to calculate currents for Malibu Rapids which is the entry point for Princess Louisa in British Columbia.

On July 29, 2012 CNP reported low tide would be at 0929 (Z -7). This calculation agrees with other publications. Current was reported to be slack at approximately 1100. I decided to time my departure from Princess Louisa for 1015 when CNP predicted the current would be about 2 kts heading 240. What I encountered was a current of at least 3 kts heading 060 – in essence a 5 kt error. Timing of slack current was off at least an hour, possibly a little more. Perhaps the error is associated with incorporation of daylight savings time.

After the passage I discussed this observation with a friend who also uses CNP. He stated that he had heard the same observation about Malibu Rapids from others. CNP is the only place I have seen current published for Malibu Rapids. It has the potential to be a great asset but, if it is not correct, it can be a serious problem.

[HappySeagull]

Thanks for that observation,Wally D!

Another one to "watch out" for is Whirlpool Rapids (in BC Canada).... note that the time diff' is shown as Z+09.....when it should be Z-07....
( added...them as wants to fix it(Whirlpool Rapids, I mean) can put a negative sign "-" ahead of the "8:0" in Harmonic idx)

[cagney]

Happy!

If you check the 20111230-nonfree.tcd in 3.1.802, there are a few BC tidal stations included,... any improvement? This is supposedly the latest data available to O.
Download harmonics-dwf-20110410-nonfree.tar.bz2 from ftp://ftp.flaterco.com/xtide.

Thomas

[HappySeagull]

Hi, Thomas.
No, that particular one doesn't include Malibu Rapids or Whirlpool Rapids or very much at all around here... the error WallyD mentions is also present in my WxTides databases. I think Paul Getman supplied this in the original HARMONIC and his tides are usually excellent so I'm looking for a glitch. Malibu Rapids aren't available as an online pdf from the Canadian Hydrographic Service so I'll have to look a little harder. If anyone out there has an offset time from a Primary Current Station, or from the tide levels, that would be interesting....

[WallyD]

Happy Seagul

Tidal height for Malibu rapids has a published time offset based on Point Atkinson. High water slack occurs about 24 minutes agfter high water at Point Atkinson, and low water slack occurs about 36 minutes after low water at Point Atkinson.

WallyD

[HappySeagull]

Hi, WallyD. thanks for that... a really wild guess for the current change to ebb at the rapids might be less than an hour after high water given the topography in Princess Louisa inlet itself? ie the tidal surge up and down..there's always some delay comparing current vs tide height. I was just looking at the BCPilot and didn't find any help there to base the current change per HW/LW there-abouts....Sorry, I haven't been up there in a long while...so I don't have any observations of my own to offer.

[HappySeagull]

...And the snow-melt in summer is going to "push" the ebb too...so even my bad guess is pretty useless.