2011Rijkswaterstaat
Generation of tides
• main drivers are Moon and Sun• explaining the tides in 11 steps
2011Rijkswaterstaat
Step 1: gravity force of moon
• No continents, just water (equilibrium tide)• Only influenced by attraction of the moon
moonearth
2011Rijkswaterstaat
Step 2: centrifugal force
• If this was the only force, the moon would crash on the earth• Also centrifugal force due to rotation around centre of gravity
2011Rijkswaterstaat
Step 4: twice a day?
• twice a day would mean every 12 hour• however, we see 12 hour and 25 minutes• where is this ‘error’ of 50 minutes a day ?• moon travels around earth in 27.32 days• after one full earth rotation• moon travels also 13 dgr
t=24h5023h56
t=o
13º
54 min.
2011Rijkswaterstaat
Step 5: the sun
• sun acts the same as moon• amplitude ca 25% of moon• rotation of earth: 23 h 56 minutes
t=o
(by definition)t=24h00
1º = 4 min.23h56
eg. at 12h, sun in South
after 23h56 + 4 min, sun in South
2011Rijkswaterstaat
Step 6: the sun moon interaction
• sun and moon work together: Spring Tide• during full moon and during new moon
2011Rijkswaterstaat
Step 6: the sun moon interaction
• sun and moon work opposite: Neap Tide• firts quarter and last quarter of the moon
2011Rijkswaterstaat
Step 7: diurnal inequality
• inequality due to the declination of earth axis
23,5ºdeclination
equator
rotationof earth
h2
h1
equator
rotationof earth
2011Rijkswaterstaat
Step 8: variation in driving forces
• distance moon to earth varies (ellipsoid, 27,55 day)• direction ellipsoid varies in 8,85 yr.• distance sun to earth varies• plane of moon varies 5 degrees around 23,5 (18,61 yr.)• plane of sun varies with the precession (25.765 yr.)
2011Rijkswaterstaat
Step 9: continents & coriolis
• the equilibrium tide runs around the southern hemisphere• further it travels around the world with a delay• tides in the north sea are more than 2 days old• the coriolis force (due to rotation of the earth) influences the tides
2011Rijkswaterstaat
Step 10: non-lineair effects
• due to bottom friction the tide will deform• allthough the driving forces consist of 2 forces, called M2 (moon) and S2 (sun), at our coast a set of nearly 200
components are needed to reconstruct the tide
2011Rijkswaterstaat
Step 11: wind effects
• due to meteorological influences the actual tide differs• surge bij NW storm• negative surge by Easterly winds
2011Rijkswaterstaat
Forecast of tides
• there are two ways of forecasting (predicting)• 1: harmonical analyse and composing• observed tide > decomposing > reconstruction• 2: modelling• calibrating model > meteo input > result