Reply To: Mike’s Bi-Weekly quiz…Week 2: Tides

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 .