waves quick key notes

Question 1

breaking waves

Answer 1

at shallow depth of 1/2 wavelength, deepest circling water molecules come in contact with seafloor.
Friction between seafloor + water changes speed, direction + shape of waves.

1. waves slow down as they drag across bottom.
2. wavelength decreases, + successive waves start to bunch up.
3. deepest part of wave slows down more than top of wave.
4. wave begins to steepen as crest advances ahead of base.
5. Eventually, when water depth less than 1.3x wave height, wave topples over + breaks against shore. It is only at this point that there is significant forward movement of water as well as energy.
6. After wave has broken, water moves up beach as swash, driven by transfer of energy that occurs when wave breaks.
7. speed of this water movement will decrease the further it travels due to friction + uphill gradient of beach.
8. When it has no more available energy to move forward, water drawn back down beach as backwash
   energy for this movement comes from gravity + always occurs perpendicular to coastline, down steepest slope angle.

Question 2

constructive waves

Answer 2

Low height, long wavelength, low frequency (typically around 6 to 8 per minute)

break by spilling forwards, + strong swash travels long way up the gently sloping beach.

Due to long wavelength, backwash returns to sea before next wave breaks, + so next swash movement is uninterrupted + thus retains its energy.
key feature of these waves - swash energy exceeds backwash energy.

Question 3

destructive waves

Answer 3

greater height, shorter wavelengths, higher frequency (often 12 to 14 per minute)

tend to break by plunging downwards and so there is little forward transfer of energy to move water up steeply sloping beach as swash.

Friction from steep beach slows swash + so it does not travel far before returning down beach as backwash.

With short wavelength, swash of next wave is often slowed by frictional effects of meeting returning backwash of previous wave.
Key feature of these waves - backwash energy exceeds swash energy

Question 4

tides

Answer 4

periodic rise + fall of sea surface + produced by gravitational pull of Moon (+ Sun to lesser extent)

Question 5

when do highest tides occur

Answer 5

when Moon, Sun + Earth all aligned + so gravitational pull is at its strongest
- happens 2x each lunar month + results in spring tides with high tidal range.

Question 6

when do lowest tides occur

Answer 6

2x month, Moon + Sun are at right angles to each other + so gravitational pull at its weakest, producing neap tides with low tidal range