Block 2 (Temporal Variations And Their Influence On Coastal Environments) Flashcards
What are temporal variations?
Changes over time
What are tides?
Regular rising and falling of the sea, due to the gravitational interaction of Moon, Sun + Earth in orbit
How do tides form?
- Water on side of Earth closest to Moon is pulled towards Moon by gravity, to create a bulge (high tide)
- Water on side of Earth furthest from Moon has the Earth pulled away from it, to create bulge (high tide)
- Sides perpendicular to Moon have least water, no bulge (low tides)
What are the two types of tide?
- Semi-diurnal
- Diurnal
What is a semi-diurnal tide and where experiences it?
- 2 high and 2 low tides across a 24hr period
- High (6hrs), low (6hrs), high (6hrs), low (6hrs)
- Experienced by most coastlines
What is a diurnal tide and where experiences it?
- 1 high and 1 low tide across a 24hr period
- High (12 hrs), low (12hrs)
- Rarely experienced - by areas with little surrounding landmass - Antarctica
What is a mixed semidiurnal tide?
- Semidiurnal tide (2 high 2 low over 24hrs) but the high and low tides differ in height
- E.g one high tide (side closest to Moon) is higher than the other high
What is a spring tide and when does it occur?
When Sun, Moon + Earth are in a line, so water movement is exaggerated to produce the greatest tidal range.
- High tides higher, low tides lower
- (Approx 15 m above average tide)
- Happens twice per month (Full Moon + New Moon)
What is a neap tide and when does it occur?
When Sun + Moon are perpendicular, so water movement is largely cancelled out to produce the lowest tidal range.
- High tides lower, low tides higher
- Happens twice per month (Half Moons)
What is a tidal range?
Difference between high + low water levels over monthly cycle
What areas have high tidal ranges?
Areas where water is largely self-contained
- E.g. Atlantic (surrounded by landmass)
What areas have low tidal ranges?
Areas where water struggles to move in/out
-E.g. Mediterranean (has to pass in + out through narrow Gibraltar Strait)
What are the three types of tidal range + their definitions?
- Macrotidal = >4m
- Mesotidal = 2-4m
- Microtidal = <2m
What is the Inter-Tidal Zone (ITZ)?
Stretch of coastline between Spring High Tide + Spring Low Tide, where coastal processes (morphology) occur
What are currents?
Steady, predictable movement of a fluid within a larger body of that fluid (e.g water movement within Sea)
What are the two broad classifications of currents?
- Onshore currents
- Offshore currents
What are onshore currents?
Any currents flowing TOWARDS the shore
What are offshore currents?
Any currents flowing AWAY FROM the shore
What scale do currents occur on?
Local scale
What are the 4 specific types of currents?
- Tidal currents
- Shorenormal currents
- Longshore currents
- River currents
Describe tidal currents
- Flood tide moves towards shore, entraining sediment + depositing it (onshore current)
- Once high tide is reaches, current reverses
- Ebb tide moves away from shore (offshore current)
(Velocities are low at start of each flood/ebb tide, peak in middle)
Is a flood tide and onshore or offshore current?
Onshore
Is an ebb tide an onshore/offshore current?
Offshore
Describe shorenormal currents
- Waves approach shore with crests parallel to coastline
- Water carried straight onshore
- Water moves along shore for short distance
- Water moves offshore in fast (up to 1m/s), powerful RIP CURRENTS
Describe longshore currents
- Waves approach shore at oblique angle to coastline
- Water carried onshore at an angle
- Water moves along shore for short distance
- Water moves offshore in fast (up to 1m/s), powerful RIP CURRENTS
Are rip currents onshore/offshore?
Offshore
Describe river currents
- River flow can become very energetic in estuary (e.g. after inland rain)
- River currents pass into ITZ
- Can cause very disturbed conditions
What is an onshore flow?
Movement of any weather feature TOWARDS the shore
What are the weather features we look at when observing onshore/offshore flows?
- Hurricanes
- Sea breeze/land breeze
Describe how hurricanes can act as onshore flows
- Winds blow out of SE part of hurricane
- Winds force ocean water + precipitation shields TOWARDS coast, in a STORM SURGE
What is a storm surge?
Rise in seawater levels (that can cause flooding), solely due to storm
Describe how sea breeze can act as an onshore flow
- In afternoon
- Water cool, LAND WARMER
- Warm air over land rises vertically
- Cool ocean air moves onshore to fill empty space
Creates: onshore wind, afternoon showers/storms (as warm air rises)
What is an offshore flow?
Movement of any weather feature AWAY FROM shore
Describe how hurricanes can act as offshore flows
- Winds blow out of NE part of hurricane
- Wind forces ocean water + precipitation shields AWAY FROM shore
Describe how land breeze can act as an offshore flow
- In evening
- Land cool, OCEAN WARMER
- Warm air over ocean rises
- Cool air over land moves offshore to fill empty space
Creates: offshore wind, showers + storms (warm air rises)
Case Study of onshore flow (causing storm surge) + details
North Sea Storm Surge - 1953
When: 31st Jan - 1st Feb 1953 (overnight)
Where was affected: E coast of England/Scotland, Netherlands, Belgium
What: High spring tide in semi-enclosed N.Sea + severe European windstorm, caused storm surge 3.35m above av + extensive flooding
What increased losses: no national warning system, at night, poor qual sea defences (after WW2)
Effects: Princess Vic ferry sank (Scot - N.Ire) (133 deaths)
307 total deaths in E(1836 in Neth)
140,000 acres land flooded in E (340,000 in Neth)
32,000 people evacuated in E (100,000 in Neth)
24,000 properties damaged in E (47,000 in Neth)
46,000 livestock killed in E (30,000 in Neth)
Responses:
UK - National ‘Storm Tide Warning Service’
- Completion of Thames Barrier (1984) to protect London from surges (plans to upgrade after 2030 - sea level rise)
Netherlands - ‘Delta Works’ (large scale protection scheme, e.g. dams, locks, barriers)
What is a wave?
Disturbance of the surface of the sea/ocean, due to the transfer of energy from the wind to the surface (via friction)
What is fetch?
Distance the wind blows over the sea.
- Larger fetch gives more powerful waves
- Largest fetch in UK = SW from S.America
What is duration?
Length of time the wind lowers over the sea
- Longer time = larger, more powerful waves
What is wave length?
Distance crest to crest
What is wave height?
Distance crest to trough
What is wave steepness?
Ratio between length + height (L:H)
What is wave energy?
Length x Height squared
What is wave power?
Wave energy x Wave velocity
What is a wave period?
Time interactional between crests (in s)
What is swash time?
Time interval between wave break + swash reaching highest point on beach
(Time waves spend pushing material up beach)
Describe how waves form
- Air blows across water surface
- Friction causes transfer of energy between atmosphere and sea
- Waves gain kinetic (particle movement) + GPE (wave height)
- Individual water particles move in circular orbit within wave, so waves appear to ‘bob’
- Diameter of circular orbit decreases as water depth increases
Describe how waves break
- Waves reach shallow water
- Friction with seabed
- Water particles’ orbits become elliptical (forwards movement faster than backwards)
- Wavelength + velocity decrease
- Wave height increases (conservation of energy)
- Waves become very steep
- When wave steepness reaches 1:7, orbit is broken + wave breaks
What are the three ways that waves break?
- Spilling
- Surging
- Plunging
What is spilling? Where does it occur? And what type of waves spill?
Waves break at a distance from shore, creating lines of surf up shore
- Seabed is GENTLY SLOPING
- Waves are generally CONSTRUCTIVE
What is surging? Where does it occur? And what type of waves surge?
Waves break (often partially) close to shore, water slides up shore
- Seabed is STEEP
- Neither wave specifically
What is plunging? Where does it occur? And what type of waves plunge?
Waves are steep fronted, curl over and plunge vertically onto shore
- Seabed is moderately steep, then suddenly becomes steep
- Waves are generally DESTRUCTIVE
What are the two types of wave?
- Constructive
- Destructive
What is a constructive wave?
Lower energy wave that has a stronger swash than backwash, so builds up the beach
Where/when are constructive waves typically found?
- Gently shelving beaches
- Calm, Summer conditions
What are the properties of constructive waves?
Height: low (often <1m) Length: long (often up to 100m) Steepness: gentle Period: long Frequency: low (6-8 per min) Breaker: often spilling Material movement: up beach (berm formed) Beach gradient: gradual Swash/backwash strength: swash>backwash
What is a destructive wave?
High energy wave that has a stronger backwash than swash, so breaks down the beach
Where/when are destructive waves typically found?
- Suddenly shelving beaches
- Stormy/windy Winter conditions
What are the characteristics of destructive waves?
Height: high (often >1m) Length: short (often < 20m) Steepness: steep Period: short Frequency: high (10-14 per min) Breaker: often plunging Material movement: down beach Beach gradient: gentle lower beach, steeper up beach (storm beach) Swash/backwash strength: backwash>swash
What are the characteristics of a sandy beach?
- Wide
- Gentle gradient
- Form in calm conditions - sand particles moved by low energy
- Easily eroded in higher energy conditions - low percolation, small transportable particles
What are the characteristics of a shingle beach?
- Narrow
- Steep gradient
- Require high energy conditions to form - large particles
- High percolation rates
Describe sediment sorting
- Largest calibre sediment deposited at back of beach
- Smallest calibre sediment deposited closest to Sea
Because, constructive waves only have enough energy to transport large calibre during their strong swash
What is a strand line?
Non-permanent marker, made of driftwood/seaweed/non-natural material, that marks the last high tide
What are berms?
Seaward-sloping ridges of sand/shingle/pebbles, deposited by tides
Case study of how waves+tides influence geomorphology of coasts +details
The Gower
Location:
- S.Wales,
- Peninsula, acts as sub-cell boundary in Sediment Cell 8 (St David’s Head-The Severn)
- Swansea city
- Borders Bristol Channel (N.Atlantic)
Beaches: E.g Rhossili Bay (3 miles wide, flat)
- Waves - High energy waves form Storm Beach
- Low energy cause sediment sorting
- Tides - Tidal range impacts area exposed to geomorphology
- Strand line + berms are left
Spits: E.g Whiteford Point (across Burry Estuary)
- Waves: Cause LSD, as waves approach at oblique angle
Mudflats/saltmarshes: E.g Llanrhidian sands + llanrhidian marsh
- Waves: sheltered area causes waves to deposit mud, then colonised by halophytic veg
- Tides: Tidal range splits saltmarsh into 2 zones (mudflats - covered at normal high tide + sward zone - covered at spring high tide)
Sand dunes: E.g Oxwich Burrows (at Oxwich Bay)
- Waves: Cause LSD, providing sediment
- Tides: Also bring sediment + the larger the tidal range, the more sand dries out and can be blown to form dunes
