Lecture 9

what’s the morphological difference between composite beaches and msg?
composite beaches have gravely dunes and fine sand further out
how does the fluvio-glacial nature of NZ relate to MSG beaches?
the fluvio-glacial nature of NZ means the river deliver coarser sediment, which contributes to the creation of MSG beaches
MSG beaches and glaciers
MSG beaches are close to glaciers in space or time. for NZ, the relation to glaciers is river based. NZ is fluvio-glacial, meaning the rivers carry coarse sediment
why are MSG beaches under-investigated gloabally?
the reason MSG beaches are under-investigated globally is that they are far from population centers, and people don’t vacation on them
explain the small rivers concept
the small rivers concept is based on relative, not absolute river size. A small river is one that cannot deliver enough sediment to stop erosion. This takes into account that silt and clay can’t maintain a beach against erosion
describe the tides of MSG beaches
MSG beaches are generally microtidal
what does it mean that MSG beaches are wave-dominated
when we say MSG beaches are wave-dominated we mean waves define the beaches more than tides
are MSG beaches wave dominated or tide dominated?
MSG beaches are wave dominated. they are generally microtidal
draw a MSG beach profile
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describe the breaker system of MSG
MSG have a single line of plunging breakers
where is the surf zone of MSG?
MSG beaches have no surf zone, as all waves break at the nearshore face

4 MSG beach characteristics relating to 

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 1. width

2. sediment sorting 

3. what type of breakers

4 MSG beach characteristics

1. fairly narrow compared to sand beaches

2. poorly sorted, with “armor” layer of gravel on top

3. single line of plunging breakers 

why is swash the “engine room” of MSG beaches?
swash controls the sediment balance of MSG beaches
what happens when backwash collides with swash from the next wave on MSG beaches?
when backwash and swash meet on MSG, it is an erosional environment. This is indicative of a high phase difference
when waves are in phase, or at least close, how does this affect MSG beaches?
if waves have a small phase difference, than backwash can clear the beach before the next wave sends it’s swash up there. this is an accretionary MSG environment
what determines if a MSG beach is accretionary or erosional?
if incoming waves are in phase or at least have a small phase difference, then swash and backwash don’t meet and you have an accretionary MSG environment. if incoming waves are out of phase swash meets backwash and you have an erosional environment
I still don’t think I understand the dual transport system
the Canterbury Bight is a MSG beach. what are some coastal process that contribute to this?

the Canterbury Bight’s characteristics that make it a MSG beach:

1. chronic erosion

2. small rivers

3. rivers bring poorly consolodated fluvio-glacial sediment 

4. high wave energies b/c exposed to open southern ocean

 

3 hot research topics for MSG beaches

3 hot research topics for MSG beaches

1. how changes in sediment input from rivers affects beaches 

2. how climate change will affect the sediment buget

3. beach nourishment

how does percolation relate to MSG beaches?
because MSG is easy to percolate too, often lagoons behind MSG beaches don’t have channels to let water run back to the sea through. instead the water just percolates. but because percolation is slower than water running through a channel, there is more variations in water levels in lagoons behind MSG beaches
how can clay be an important factor in MSG beaches?
sometimes, MSG beaches are on top of a clay “plug” that is far less porous
major affect of storm overtopping of beaches
one major affect of storm overtopping of beaches is that it ruins agriculture for many years
what human intervention is common for lagoons behind MSG beaches?
lagoons behind MSG rely on percolation to allow water to flow to the ocean. because percolation is so slow, water can build up behind these lagoons and threaten to flood infrastructure. so people create an artifical opening
3 river-meets coasts landforms. what is each dominated by?

3 river-meets coast landforms 

1. delta: river dominated

2. estuaries: tide dominated

3. waituna+hapua type lagoons: wave dominated

The Avon-Heathcote Ihutai Estuary (AHIE) is protected from the sea by what?
the AHIE is protected from the sea by the new brighton spit
what is the fresh backwater effect?
the fresh backwater affect is when water flowing downstream to an estuary “piles up” during high tide and the water level rises
are water velocities equal for tides going in and out at the AHIE Inlet?
at the AHIE inlet, tides flood in and ebb out
why is there net input of sediment via the AHIE inlet?
the reason the AHIE gets more sediment than it loses through the inlet is that water is moving faster coming in than going out. Tides flood in, and ebb out
why can’t we deepen the river channels of the Avon and Heathecote rivers?
the Avon and Heathcote rivers see tidal inputs. deepening the channels would just increase tidal inflow
what defines estuary mixing?
the balance of sediment/water inputs of the rivers and the sea defines estuary mixing.
from a physics perspective, why are estuaries sometimes stratified?
saltwater is denser than freshwater, which is why estuaries are sometimes stratified
how do tidal velocities effect estuary mixing?
higher tidal velocities means more estuary mixing
how does the ratio of river inputs to tidal inputs affect estuary mixing?
if river inputs are high compared to tidal inputs, then estuary is stratified. if river inputs are low compared to tidal inputs, estuary is well mixed
how could an extended dry period with no rain affect an estuary?
an extended dry period with no rain would reduce river flow into estuary, meaning the estuary would get more well mixed
what is the tidal prism?
the tidal prism is the total amount of water that comes in and out of an estuary every tidal cycle
how did subsidience affect the AHIE?
subsidience lead to realtive sea level rise, which led to saltmarsh retreat. this increased the storm and tsunami hazard
name three factors that change a hapua that relate to the Sea’s input

3 sea factors that effect hapua 

1. hydraulic head between lagoon and sea

2. wave overtopping

3. percolation rate 

what process creates a hapua?
the process that creates a hapua is when longshore transport protects a river mouth from an ocean
what are the two outlet system a hapua can have?
the two outlet systems a hapua can have are a single long-lived outlet or several shorter-lived outlets
are hapua wave, tide, or river dominated?
hapua are wave dominated, except during river floods
what kind of beach is between hapua and the sea?
a MSG beach is between hapua and the sea
why are hapua adaptable and long-lived
hapua can migrate with the coast when erosion causes relative sea level rise.
what will shrink/flood hapua?
river abstractions and dams can shrink/flood hapua
hapua are associated with what rivers?
hapua are associated with “small” rivers
difference between estuary and hapua rivers
hapua rivers are “small”, meaning they can’t bring enough sediment to keep the beach from eroding. By this measure, estuary rivers are “big”
sketch a hapua from birds eye view
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