MSG beaches / Hapua & Waituna Flashcards
(25 cards)
types of beaches
purse coarse grained, composite, MSG, pure sand
MSG beach characterisitcs
low and narrow
sand and gravel particles mixed
very reflective (lots of wave energy reflected back to sea)
single plunging breaker line
nearshore - fine, foreshore - coarse (dual transport system)
composite beach
distinct break btw fine sand (nearshore - dissipative) and coarse gravel (backshore - reflective)
mix of sediments but particles not MIXED
pure sand
large surf zone
dissipative
waves break far out
less steep beach profile
bar and trough
berm at top of beach profile
shoreline retreat sand beach vs gravel beach ridge
sand - retreat of sand due to SLR and response of erosion (sand erodes off beach face transported to immediate offshore deposits on sea floor),
gravel ridge - gravel carried inward through overwash (moves backwards)
MSG occurence
- common in NZ, russia, canada, uk.
- found where there is high wave energy
- where there is a corase sediment supply: on margins of fluvio-glacial outwash fnas and paralagical coasts.
- microtidal setting
small river concept
a river is small if it does not supply sufficient quantity or coarse enough sediment to maintain the coast against erosion. (not complete concept)
why is swash the most important control on MSG foreshores?
timing and interaction of swash/backwash very important - if backwash cannot escape before next breaker swashes onshore erosion is enhanced (high phase difference)
- waves in low phase difference - crests align so backwash can escape before swash arrives, promotes accretion.
estuary classifications
geomorphology, hydrodynamics, tidal, sediment, ecosystem
why are estuaries always in a state of infill
receive particles from rivers and sea, low energy environments, trap particles which leads to gradual sediment accumulation
froude number (x axis) represents what in estuarine parameter space ?
strength of river flow compared to wave speed - higher values means stronger river influence
mixing number (y-axis) estuarine parameter space?
strength of tidal mixing btw fresh and salt water - higher values mean more vertical mixing (estuary is well mixed) low number = strongly stratified
rivermouth waterbodies in NZ
- 90% drained
- susceptible to flooding
- ecologically productive
- historically used as waste dump
- amenities - recreational use
- sensitive flow, sediment changes, quakes (uplift)
navigation - entrances often maintained by dredging
estuaries are dominated by?
tides
deltas are dominated by ?
rivers
Hāpua and waituna are dominated by?
waves (lagoon)
where do river deltas form?
at the river mouth, sed builds up as the river enters sea
what is a salt wedge?
layer of saltwater beneath freshwater in stratified estuary typically inland of river mouth (lower river reaches)
inlet channel
ebb and flood tides pass through here
what is backwater effect and where does it occur?
when tidal forces slow or reverse river flow upstream causing water to back up inland from estuary
hydrodynamics depending on tidal velocity and river discharge
tide influence - seawater more dense than fresh so wants to sink
freshwater influence - smaller freshwater flow = more well mixed, large freshwater flow = stratified
Hāpua
wave dominated predominantly freshwater, backwater effect, no tidal prism, micro-tidal, MSG, breach of barrier opposite river then salt water enters. vulnerable to storms/ river floods.
estuary def
tide dom with tidal flows in and out of inlet on daily basis, brackish water bodies.
Waituna
small river flows, long water resisdence times, large spread out water bodies in coastal plain depressions or valleys, wave dom.
