Mike’s Bi-Weekly quiz…Week 2: Tides

  • Creator
  • #19011
    Mike Gilbert

    Missed last week so the quiz goes from weekly to bi-weekly.

    This quiz is all about tides.  A few questions; Some definitions; a rule (or two); a couple of problems to solve.  Have fun and enjoy.

    1. Questions: See if you know what the following mean without looking them up – and then look them up!

    a. What two bodies exert the greatest influence on the vertical movement of our oceans and seas?

    b. Explain why tides move ahead (for the time they occur during our day) by 50 minutes every 24 hours?

    c. How many different kinds of tides (cycles) may be found and explain what differentiates each?

    d. Which is the most prevalent form of tidal cycle found on our west coast?

    e. What is the difference between a Tsunami and a Tidal Wave?


    2. Definitions: Can you define these off the top of your head?

    Amphidromic Point;  Apogee;  Chart Datum (Canada);  Drying Height;  Ebb;  Fathom;  Flood;  Higher High Water Large Tide (HHWLT);  Lowest Normal Tide (LNT);  Neap Tide;  Perogee;  Spring Tide


    3. Rule of 12ths

    – what is the “job” of the Rule of 12ths?

    – how many equal time periods is the tidal cycle broken up into by the Rule of 12ths?

    – what is the simple pattern a tide cycle models under the Rule of 12ths?

    – why is it so much easier to work in feet vs centimetres for the Rule of 12ths?

    – as kayakers, name 5 different practical uses for the Rule of 12ths (when would it be handy to know and use when paddling?)?


    4. Problem #1:  “Lunch time” – You and your group of friends land on a lovely, very wide sandy beach in order to stop for lunch.  You plan on resting there for about 2 hours.  You arrived at noon exactly and want to know how far to bring your kayaks up the beach so you do not have to be disturbed and move them again while enjoying the sun.

    The beach has a level and continuous  7.13 degree slope to it rising up to the forest edge and some tent platforms.

    The Low tide of 3 feet happened at 1000 hours so you missed scouring the lower tidal pools.

    High tide is expected to reach its maximum of 9 feet at 1600 hrs.  OMG!  How high will the water get up the beach?


    5.  Problem #2:  When paddling and camping overnight, what is a really good habit to get into doing with your kayak prior to going to bed?  What method do you use?  Can you remove a middle kayak from the group without untying the rest?



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  • Author
  • #19047
    Sarah Cummins

    Hi Mike ,

    thanks for posting the quiz . It’s handy to revise and learn stuff , even if I get to it late ! Here are my answers :

    1A) moon and sun  , moon has greater pull.   B) lunar day is 24 hours and 50 minutes which is the time moon takes to travel its orbit . High and low tide therefore 50 minutes later each day .

    1C) Tidal Cycles . If the earth had no land masses so that water could move unobstructed , tidal bulges caused by the gravitational force of the moon and sun would move constantly westward, and every place on the planet would have 2 high and 2 low tides of equal size on each lunar day . Continents and land masses get in the way . Free  vertical rise and fall of water is strongly influenced by distribution of land masses , topography of the shore lines and by ocean currents , and different tidal cycles exist in different parts pf the world  , they are diurnal ,semi-diurnal and mixed semi diurnal .
    Semi diurnal tidal cycles consist of 2 nearly equal high tides and low tides a lunar day. Most places on earth have this type of tidal cycle .
    Diurnal = only one high and one low tide per lunar day , found in Gulf of Mexico . Victoria BC is positioned such that there is a lot of flooding and ebbing water happening simultaneously , so although it is mixed semi diurnal it looks like diurnal tidal cycles .
    Mixed semi diurnal , 2 high and 2 low tides per lunar day , and of different sizes , “ diurnal inequality “ .

    1D)Mixed semi diurnal is found predominantly along West Coast .

    1E)Tidal Wave describes the vertical movement of the oceans caused by the gravitational pull of the moon and sun .
    Tsunami is a wave caused by a submarine earthquake or volcano . It can travel for vast distances and can build to huge heights over shallower water . It has a very long wave period , so when it hits the land the wave just keeps going and going and going ,

    2 Amphidromic point …a no tide point ( dynamic theory of tides ) water sloshes backwards and forwards between continents and cancels out to zero tide amplitude . Happens in several places in Pacific .
    Apogee. Moon is afar away at its furthest point from the earth and has less gravitational pull.
    Chart datum . Depths on a chart are shown from a low water level called chart datum . Selected so that water level will very seldom fall below it .
    Drying height  . Vertical distance of seabed exposed by tide and above the lowest tide .
    Ebb. Horizontal movement of water caused by falling tide .
    Fathom . Unit of depth .
    Flood . Horizontal movement of water caused by rising tide .
    Higher high water large tide . Average of highest high waters taken over 19 years .
    Lowest normal tide . Average of all the lowest tides over 19 years .
    Neap tide , happens when the sun is at a 90degree angle to the moon cancelling out some of moons gravitational force . Smaller tides during these phases .
    Perigee . Moon is proximal to earth nearest to earth in its elipitical orbit ,exerting stronger gravitational force .
    Spring tide . Sun and moon in alignment exerting strong magnetic force .

    3) Rule of 12’s

    a) rough assessment of speed , heights and  timing of tidal changes .
    b) 6 time periods .
    c)diurnal .
    d) tidal volume divided into portions of 12 , working in base 12 old imperial measurements is easier to apply rather than decimal base a 10 .
    e)1) when current is strongest and weakest , plan trip accordingly . 2) j of tide relative to launching and landing 3) how long to high and low tides .  Timing , heights ,current  pertaining to trips ,

    4) the lunch problem ,  I am not sure how to calculate this , so the questions I would ask is what time is it now ? How long to high tide ? What sort of tide neap or spring ?is the wind blowing onto the beach that might make the water higher than expected and where is the high tide seaweed mark . As it seems the group is pretty laid back wanting to stop and sunbathe for 2 hours ,I might opt to pulling the boats up above the high tide seaweed mark , and get away before it falls  too low .Good watch with alarm to wake up snoozers and a weather eye to the water level aided by stick marker might help .!


    5) tying boats up for the night …….don’t know the best answer for this although I have used my tow line before . Haven’t been able to get the middle boats out without untying everything though . Could you show me how to do this sometime ? Thanks   Sarah .


    Clark Perry-Bater

    I’ll be up all night figuring this one out!!

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