Lecture 14 - 26.pdf

Lecture 16: Coastal processes and environments 1: Coastal systems controls 1.1: Morphodynamic Systems external forcing refers to weather, water fall, climate etc 1.2: Geology The first order control on coastlines is geology and is strongly related to plate tect...

Lecture 16: Coastal processes and environments 1: Coastal systems controls 1.1: Morphodynamic Systems external forcing refers to weather, water fall, climate etc 1.2: Geology The first order control on coastlines is geology and is strongly related to plate tectonics: 1. Collision coasts (coastlines that are sitting on the boundary of a tectonic plate) more mountainous coastlines Beaches aren’t as well developed 2. Trailing edge coasts (coastlines that are not on a tectonic boundary and instead in the middle of a plate such as the east coast of South America and the whole of Australia flat coastal planes Geological control is also regional: Includes sediment such as bed rock Sand Clay etc etc 1.3: Sea level Sea level is the 2nd order control on coastlines - strongly related to climate change Quaternary (geological) period dominated by cycles of ice ages and warmer inter-glacial. Last glacial maximum was 18,000 years ago and sea level was approximately 120 m lower than today. This was followed by a large and rapid rise in sea levels that ended in 6500 years ago (post glacial marine transgression - PGMT) Types off sea level change: 1. Ecstatic (Global) - A volumetric change in the global sea level (eg. Ice melting) 2. Isostatic (Local) - A chnage in land elevation resulting in change in sea level 1.4: Wave climate Wave climate is the 3rd order control on coastlines and is strongly related to (atmospheric) climate. large storm waves transport sand offshore (erosion) and ‘normal waves’ Transport it shoreward (accretion/deposition) Eg. Sydney has an average wave climate of 1.6m and a period of 8 seconds between waves, with most waves coming gin from the south east —> this can change due to various climatic differences such as, a big storm event. Big storm event can cause the sand to erode away, while no storm event in a long time, will allow the beach to have a lot of sand 1.5: Daily processes Daily processes (waves, currents, tides, sediments transport) are the 4th order control on coastlines eg. Strong currents and waves, leads to changes in sand etc 2: Waves 2.1: theory Wave: a disturbance through a medium (eg. Water) Wave form and water movement are different. water particles move in a circular motion, with teh top of the wave called a crest and the bottom called a trough Definitions: Wavelength (L): Distance from one crest to another Wave Height (H): Vertical distance between the top of the crest to the bottom of the trough Wave period (T): time it takes for two waves to pass a single point (interval between each wave) Wave depth (h): depth of the mean surface to the bottom ( As waves move form deep to shallow water the following things will happen; water depth (h) — decreases Wavelength (L) — Shortens Speed (C) — Decreases Wave height (H) — increases Wave period (T) — stays the same 2.2: Generation Waves are generated (disturbed) by wind Wave height (H) and wave period (T) are governed by: WInd velocity Wind duration Fetch Wave heights are limited either by the available fetch or the duration of the wind/storm event. *Significant wave height (Hs) = highest of 1/3 of waves* 2.3: types of waves Waves are differentiated based on their period (T) and can be described by a spectrum of wave energy most waves are ‘gravity waves’ with periods of T = 1-30 secs Wind waves (‘sea’): waves that are in the process of generations (locally generated), has a short period with messy waves with T = 2-8 seconds Swell waves: Waves that have left the zone of generation and have travelled long distances; they have large wavelengths and are very regular, transitions from ‘wind/sea’ to ‘swell’ occurs due to viscous damping and dispersion 2.4: Wave Shoaling Waves start to shoal when h = L/2(wave base) and their shape becomes asymmetric changes shape due to wave refraction in response to the underlying topography 2.5: Types of breaking waves Plunging: related to rapid shoaling from deep to shallow waves occur on moderate-steep beaches, sand bars, reef platforms Wave barrels due to sudden change in underlying topography Wave breaks in one spot Spilling: Related to slow shaping on Getty sloping beaches found on gently sloping, and flat beaches Top of the waves, kinda crumbles and spills down Safer as the energy is dissipated over a greater area Surging: Small waves on moderately steep beaches rush up the beach and back out very quickly and suddenly kinda like a miniature tsunami Collapsing: Long wave periods with steep beaches very dangerous 2.6: Wave reflection Also known as ‘clapotis’ found near rocks, headlands, steep beaches and structures Where the wave bounces off a structure, but hits the oncoming wave behind it Very turbulent Very erosive phenomenon 3: Nearshore currents 3.1: Forcing Surf zone currents are directional flows of water that are forced by; spatial and temporal variability in wave breaking Generally stronger with larger H (height) and breaking intensity Stronger the current the large the breaking waves, and leaves the circular current (within wave) water level rises a little bit when water breaking than non broken water, (set-up official name), water will flow from a high water set to a low water set up When waves break, water is released and increases the water level locally. This is called ‘set-up’. Water will flow from regions of high setup to low set-up/ 3.2: Longshore currents Shore-parallel (or longhorn) currents are created by; Oblique angle of wave approach, (eg. If you have a long beach, the water rushes up at an angle but recedes in a straight line, thus creating a long shore current. Strong winds in the along shore direction longshore currents flow fast and move a lot of sediments Flow speeds typically 0.3 - 0.6 ms^-1 3.3: Rip currents Strong, narrow seaward flowing currents which usually occupy deep channels between sand bars. Internal component of nearshore cell circulation. Typical flow speeds 0.5 - 1 ms^-1 Are characterised by pulsating flow (> 2ms^-1) and usually flow fastest around low tide seeder currents - a ·shoul pr when two feeder currents meet, they will be pushed offshore and create and rip, creating a rip neck and rip head. Creates a near shore cell circulation Three main components of rips: 1. Feeder current, 2. Rip neck, 3. Rip head Rips are forced by spatial and temporal variability in wave breaking and essentially exist to bring water carried shoreward by breaking waves, back offshore. has two types of behaviours, exit and circulatory, which can be alternated even within minutes of each other Exit behaviour: when the rip current will go beyond the surface zone Circulatory behaviour: When the rip circulation gets trapped in the surf zone and doesn’t go past the surf zone Measuring: Rip flow quite variable over space and time which can be measured using GPS Drifters 3.4: Types of Rip currents 1: Channelised rips: Occurpy deeper channels between sand bars and can be persistent in location for days, weeks or months most commonly observed and can occur on both low/high wave energy beaches Looks like green gaps in the beach 2: Boundary Rips: Flow nect to physical structures like headlands, reefs, structures persistent in location and usually occupy a deeper channel. Often flow faster and further offshore than channelised rips Deflected offshore due to structure/topography of the beach 3: Flash Rips: Are controlled only by the wave characteristics and by ‘vorticity’ caused by wave breaking common rips that are short-lived and variable in location (ie. mobile in space and time); Very few measurements available Unlike channelised rips, which are green channels and persistent, flash rips are while bubbly, and aerated, they are also not persistent flash rip ↓ * 4: Mega Rips: Form during large storms and cause significant beach erosion massive waves during storms easts up the sand. 4: Tides 4.1: Forcing Tide: Periodic rise and fall of water level produced by the gravitational attraction forces of the earth-moon-sun system eg. Very long wave kinda. Get about 2 high and 2 low tides a day (but some places can only get 1 depending on the location 4.2: Tide range Tide range is the difference between sea level at high and low tide Tide Range: the range of tide height between certain period of time for example on full moons the tide is at its highest range When the moon and sun is right angle to each other, you get a smaller tide range as they forces contradict each other Types of tides: Microtidal - if tide range < 2m Mesotidal - if tide range > 2 - 4m Macrotidal - if tide range > 4m Tide range is affected and amplified by; water depth Side adn shape of ocean basins and continents/ land masses