Lecture 8 - Coastal Processes Flashcards
coastal issues:
- future sea-level = tidal levels (MHWS, HAT), storm surges.
- coastal erosion and protection = hard defences, down-drift problems, soft and flexible defences, managed realignment.
- utilisation by humans = conventional energy, nuclear power, renewable energy.
the dynamic coast:
form and process = shape is a function of coastal dynamics, energy and material inputs.
the process - response system of coastal zone:
waves, tides and wind > sediment sources (beach material) > coast profile and plan > coastal zone management.
coastal processes - tides:
- position and relative motions of Earth, Moon and Sun.
- gravitational attraction.
- centrifuge force.
- spin and rotation of Earth.
- water/coast interaction.
- resonance of water bodies.
- water depth.
- coastal morphology.
law of gravitation - Newton, 1666:
- ‘the attractive force between Earth and Moon (Sun and Planet) is inversely proportional to square of distance between them’.
- force of attraction is proportional to 1/distance^2.
- F = G x ME x Mm/R^2.
equilibrium theory of tides:
- resultant equilibrium tide = F - Cf.
- where F = gravitational attraction + Cf = centrifugal effect.
equilibrium theory of tides - temporal variation:
- eastward rotation of Earth through ‘symmetrical’ tidal bulges (‘propagation of tidal wave’).
- semi-diurnal tide = time delay due to Earth-Moon mutual rotation giving complete diurnal cycle (24 hours, 50.47 minutes).
the Earth-Moon-Sun system:
- Tsolar = 0.46 x Tlunar.
- Ms = 27 x 10^6 x Mm.
- Dists = 400 x Distm.
combination of lunar and solar tides:
- spring tide.
- neap tide.
Earth-Sun-Moon alignment - syzygy:
- spring tides = high tides higher, low tides lower.
Earth-Sun-Moon at 90 - quadrature:
- neap tides = high tides lower, low tides higher.
dynamic theory of tides:
- equilibrium tides = 2 symmetrical bulges, bulges track from East-West, max semi-diurnal range ≈ 0.5m.
- observed tides = open ocean (0-1m range), shelf seas (>10m range), e.g. Bay of Fundy (Bristol Channel).
1. North-South continental boundaries impede East-West travel of tidal wave - tidal wave reflection at sudden changes in depth (sandbanks) and at coastal barriers.
2. attenuation due to speed of tidal wave - C = √gD, g = gravitational acceleration, D = water depth, in deep ocean (4km) tidal wave travels at 198 m/s, sub-lunar point travels at 450 m/s at equator.
3. natural mode/period of oscillation amplifies tide-generating forces (resonance) - T = 2L/√gD, T = period, L = length of basin, D = depth of basin, g = gravitational acceleration.
4. solid Earth also responds to tide.
5. coriolis force affects water movements = rotational tides, e.g. path curves to right in N. Hemisphere.
coastal processes - waves:
“driving force behind almost every coastal process is due to waves” - Pethick, 1984.
wave parameters:
- amplitude, a = height difference between crest/trough and equilibrium water level (m).
- wave height, λ = distance between 2 wave crests (m).
- water depth, D or d = depth of water from mean water level to bed (m).
- wave period, T = time taken for completion of wave cycle (s).
- frequency, f = number of completed wave cycles per second (Hz), also = 1/T.
- wave base = water depth where orbital motion of water particles becomes negligible, i.e. where water no longer causes movement (m) = λ/2.
deep to shallow water transition:
generation (sea) > propagation (swell) > shoaling + breaking (surf).
shallow water wave modification:
- importance of wave base.
- bottom friction causes H/λ increase = attenuation.
- slowing down of wave = C = √gD.
- kinetic energy = potential energy = H increase, λ decrease.
- particle orbits become elliptical = to and fro movement.
- orbits ultimately become incomplete and asymmetrical.
- energy lost against bed via friction = H decrease (proportional to roughness).
- wave profile changes with advance.
shallow water processes:
- refraction.
- diffraction.
- reflection.
- breaking.
diffraction:
- energy transfer along wave crest.
- occurs where: waves cross, shadow zones, piers and headlands.
reflection:
occurs at:
- cliffs, seawalls, jetties.
- beaches.
- shoals, bars, ridges.
breaking:
- energy dissipation as heat and sound in foaming water and sediment.
- stages: spilling, plunging, collapsing, surging.
energy dissipation post-breaking:
- surf zone.
- swash zone = final energy dissipation.
- jet-like excursions that run-up and run-down beach = swash and backwash.
coastal processes - storms and storm surges:
- “tide modified by weather…” - Pugh, 1987.
