Week 6 - Tidal Flashcards
What is tides and what causes them?
- Water surface on the Earth subjected to two bulges, one towards the Moon and another on the opposite side.
- Caused by the effect of gravity in the Earth-Moon-Sun system, and the movement of those three bodies.
- The rise and fall in sea level is caused by the Earth rotating on its axis underneath these bulges (Bulges staying still while earth moving).
- There are two tides a day because it passes under two bulges for each rotation (24 hours). This is called the lunar tide.
- The Sun also creates two bulges of water called the solar tide and is about a third of the size of the lunar tide.
- About the common center, two forces are acting: Gravity towards the Moon and a rotational force (centrifugal force) away from the Moon - they are perfectly in balance.
- The ‘tide generating’ force is the difference between these two forces.
- On the surface of the Earth nearest the Moon, gravity is greater than the rotational force, and so there is a ner towards the Moon causing a bulge towards the Moon.
- On the opposite side of the Earth, gravity is less as it is further away from the Moon, so the rotational force i dominant. Hence there is a net force away from the moon. It is this that creates the second bulge away from the Moon.
- On the surface of the Earth, the horizontal tide generating forces are more important then the vertical forces in generating the tidal bulges.
What is Neap tide?
Neap tide is when the Sun and the Moon are at 90 degrees, they effectively cancel each other.
What is Spring tide?
With the earth, Moon and Sun aligned they reinforce each other to produce large tides.
What are the main tidal cycles and what are they governed by? What other factors manifest themselves as a tidal cycle?
Main cycle is the semi-diurnal cycle arising from attraction of Moon and the rotation of the Earth which results in 12h 25min cycle.
Next is spring-neap cycle arising from relative position of the Sun and the Moon, approximately 14 day cycle.
Other cycles arise from the Moon’s elliptical and out of plane orbit as well as Sun’s elliptical orbit.
What is the semi-diurnal, diurnal and mixed cycles?
Semi-diurnal: Two high and two low tides per day.
Diurnal: One high and one low tide per day.
Mixed: A combination of the two.
What is ‘Highest astronomical tide’ (HAT)?
The height of the highest occurring water level under tidal forcing.
What is ‘Mean high water Springs’ (MHWS)?
The height of an average Spring tide high water level.
What is ‘Mean high water Neaps’ (MHWN)?
The height of an average Neap tide high water level.
What is ‘Mean sea level’ (MSL)?
The mean water depth at the site.
What is ‘Mean low water Neaps’ (MLWN)?
The height of an average Neap tide low water level.
What is ‘Mean low water Springs’ (MLWS)?
The height of an average Spring tide low water level.
What is ‘Lowest astronomical tide’ (LAT)?
The height of the lowest occurring water level under tidal forcing. LAT usually corresponds to the chart datum (CD) on hydrographic charts.
When does tidal resonance occur?
Resonance occurs when the length of a estuary is a fraction of the wavelength.
L = 1/4 * n * wavelength (where n is an odd integer 1, 3, etc).
For an estuary to be resonant its dimensions L/sqrt(h) = 36000.
where h is depth.
What is a tidal range?
Use of artificial enclosures to harness energy in the rise and fall of the tides. Commonly refferd to as ‘tidal barrage’ but ‘tidal entrainment’ also used.
What is a tidal barrage?
A low head hydropower scheme built across an estuary to form a lagoon.
Uses axial flow turbines to generate power from head difference.
Main components are Ship lock, Turbines, Sluices and Causeway.
Typically carries roadway.
