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MARS2001
Module 2
| Question | Answer |
|---|---|
| What is significant wave height? | the average top third of all waves; used because it is directly comparable to visual estimates of wave height; closely describes swell and the visual observations of wave height |
| Ways to measure tides | tidal gauges (problems can occur when comparing between sites); satellite observations (preferrable for global measurements as they can be globally consistent in vertical datum) |
| When are local measurements of tides necessary? | the site of interest is too far away from a tide gauge; or the study site is located in a complex environment (e.g. estuary, bay) |
| Detecting Tsunami's | the differentials between predicted and measured tides can give us an indication if a Tsunami has been produced in the ocean; can predict tidal heights very accurately so any deviation will likely be due to some other event |
| How are local tides measured in Moreton Bay? | usually by using a pressure transducer (tidal gauge) or buoy deployment |
| Measuring waves | attempts to make sense of the surface expression and energy transference in the ocean through time; two main approaches: surface tracking (primarily using buoys), and pressure recordings (using pressure transducers) |
| How are wave models predicted? | by using wave forecasting formulae (usually based on wind) and numerical modelling |
| Two ways of measuring waves directly | using pressure transducers or wave rider buoys (motion sensors) |
| Pressure transducers | measure air pressure + pressure of weight of water column; limited to shallow water due to pressure attenuation with depth; cheap, accurate option to monitor wave conditions; when deployed in transect can determine dissipation of wave energy across site |
| Wave rider buoys | deep water deployments; surface tracking buoys probs most accurate way to measure waves; give us capability to monitor hazards and study change in coastal settings |
| Pros of hardwired wave measurements | data can be monitored continuously and problems addressed immediately on-site; sampling only limited by storage capacity of linked computers; instrument can be re-programmed on-site; electrical power supplied from 'base-camp' |
| Cons of hardwire wave measurements | cables are heavy, expensive and vulnerable; need backup power supply; installation of cable and equipment can be difficult and involves major field effort; spatial coverage limited; field costs high, often needs onsite technician |
| Pros of standalone wave measurements | simple deployment, easy to attach to fixed objects; easily transported to remote areas; can usually survive extreme conditions; usually easy to obtain permits to deploy |
| Cons of standalone wave measurements | data collection time limited (internal memory, power); compromise between sampling density and length of time between maintenance visits; battery capacity limiting; can't monitor instrument performance; difficulty finding; instrument failure catastrophic |
| What are the two ways of measuring currents? | eulerian (measurement from a point, e.g. current meter); langrangian (moving with the current, e.g. floater) |
| Eulerian measurement method | monitor flow past a fixed point; e.g. ducted flow meters, electromagnetic flowmeters, tilt current meters |
| Langrangian measurement method | follow the motion of flow; e.g. dye, drifters, floaters; tend to provide less information in terms of the resolution that you can sample at |
| What are tidal currents? | currents generated by tides; most pronounced in locations that have large tides or small volumes for the tide to flow through (e.g. tidal inlets); the dominant force in most eastuaries due to lack of wave processes |
| What are wave generated currents? | currents generate by waves; creates a longshore drift; when waves arrive obliquely to the coast it creates currents on the shoreline; the greater the angle between the wave crest and the coast, and the larger the waves, the greater the current |
| How to define ocean waves? | primarily done through wave height, length and period; the wave below is an idealised sine wave where halfway between the crests and the troughs is the still water level |
| What is sea? | the area where wind waves are generated, mixed period and wavelengths; typically a chaotic jumble of waves of many different sizes |
| What is swell? | refers to an increase in wave height due to a distant storm; typically more organised into a narrower range of wave heights and periods |
| What makes waves? | fetch (area where winds interact with water surface); duration (length of time winds blow); wind velocity |
| What is the definition of a wave in statistics? | the profile of the surface elevation between two successive downward zero-crossings of the elevation; this is the standard method to determine individual waves in a time-series |
| What pattern do wave heights follow? | follow a Rayliegh distribution, not a normal distribution; most of the waves are smaller with an extended tail towards higher wave heights |
| What is the wave spectrum? | allows for the partitioning of the wave climate into distinct frequency bands; wave spectrum shows the wave energy contained within each frequency; will describe the sea surface for all frequencies for a given period |
| Sea surface record | can be broken down to a number of different harmonic (sine) waves that add together to create the time-series sea surface; each harmonic wave has its distinctive period, wave height, and frequency |
| What changes wave height in shallow water? | refraction, diffraction, shoaling, breaking, friction |
| What is shoaling? | the deformation of incident waves on the lower shoreface that starts when the water depth becomes less than about half of the wavelength; causes waves to become steeper (increase in amplitude, decrease in wavelength) |
| What is wave diffraction? | refers to 'spreading' of waves in shadow zones in leee of an obstruction (e.g. island); wave energy transferred lateral along wave crests; independent of water depth and is not related to interaction with sea bed |
| What is wave refraction? | the bending of wave crests due to shallowing of water; wave energy is redistributed; wave crests will bend until they are parallel with the depth contours; tends to focus enery on rocky headlands |
| What is longshore drift? | the angle between the shoreline and the wave crest governs the speed of the longshore current and eventual sediment transport; current strongest at an angle of 45 degrees |
| What happens in wave breaking? | governed by local water depth; occurs after refraction and diffraction and leads to the removal of most of the wave energy in the surf zone; when waves start breaking, on average over time the height of those waves is controlled by depth |
| What happens to wave energy in shallow water? | is either reorganised by refraction, diffraction and shoaling; or is dissipated by breaking and bed frictional processes |
| What is bed friction dissipation? | dissipation of wave energy due to certain friction sea beds; usually most pronounced over 'rough' sea beds (e.g. coral reefs); mangroves can also dissipate waves and often provide coastal protection services to tropical coastlines |
Created by:
tkeen40
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