Weeks 9 & 10 - Geomorphological features of wave-dominated coasts: Beaches, Barriers, Lagoons Flashcards
What is a Macrotidal coastline?
coastal areas where the tidal range is in excess of 4 m.
What is Mesotidal estuary?
characterized by large sand bodies deposited and molded by tidal currents and, to a lesser extent, by waves.
The principal sand deposits are tidal deltas that form adjacent to the tidal inlets.
an intermediate tidal range (between 2 and 4 metres)
define microtidal
Having a small tidal range (less than 2 metres)
Wave action dominates the processes active in microtidal areas
What is meant by the term surf zone?
As ocean surface waves approach shore, they get taller and break, forming the foamy, bubbly surface called surf. The region of breaking waves defines the surf zone, or breaker zone.
what is Swash/Backwash
When a wave breaks, water is washed up the beach. This is called the swash . Then the water runs back down the beach, which is called the backwash
what is wave run-up
the maximum onshore elevation reached by waves, relative to the shoreline position in the absence of waves
what is wave over-topping
the amount of water flowing over the crest of a coastal structure such as a seawall, a dike, a breakwater, due to wave action
what is wave overwash
Wave overwash is the flow of water across the surface of a thin floating body due to passing waves
What is the breaker coefficient?
The nature of energy transfer at the shoreline is dependent on what types of waves are forming as they are breaking
I.e., a wave’s ‘breaker type’ is given by the breaker coefficient
what are the components of the breaker coefficient?
wave height (h) - the difference in height between the crest and trough of a wave
wave period (T) - the time taken for two successive crests to pass a specific point i.e., completion of a wave cycle
wavelength (lambda) - the distance between two crests
beach slope (s) - determines how quickly a wave becomes unstable
what are the two biggest influences of breaker type?
beach slope and wave period
the greater these values, the smaller the breaker coefficient
describe the energy of waves in terms of erosion
arching waves have a lot of downwards energy which can lead to erosion. waves with not a lot of arching and just spill over at the crest don’t have as much downward energy
list wave breaker types in terms of decreasing breaker coefficient
spilling, plunging, collapsing, surging
describe sediment transportation via wave action
when a wave is breaking at the shoreline it has a lot of forward momentum which means it has the ability to transport sediment
when it loses energy, water drains off the beach due to gravity therefore sediment transport can occur in that direction too
this means that beaches tend to accumulate
why are beaches a good natural sea defence?
a healthy beach has lots of sediment that can protect the shoreline from the energy thrown at it
describe how tides influence sediment transportation via wave action
high tide - in favour of landward movement
mid tide - for a period of time, in favour of seaward movement
low tide - lots of opportunity for seaward movement
top part of water column moves sediment landward but deeper waters have just as much energy to move sediment seaward
Masselink and Turner, 1999
describe how seasons influence sediment transport via wave action
in the summer there is less energy on the sea surface than in winter
more energy = greater opportunity for influx of sediment seaward
in the summer, waves break, run up and deliver sediment to the shoreline
in the winter, better opportunity for sediment to return to the subtidal region due to longer wave periods, especially during storms
i.e., more sediment in supratidal region during summer and more in subtidal during the winter
describe how El Nino influences sediment transport via wave action
El Nino in the Pacific: beaches in California coast are much more eroded during an El Nino phase, removing sediment from the upper shore to the lower part where it will accumulate (Storlazzi and Griggs, 2000)
describe how beach profiles influence sediment transport via wave action
shallow, sloping, finer beaches tend to be more dissipative whilst steeper and coarser beaches tend to be more reflective
describe dissipative beaches
wide, shallow, sloping beach
water is below wave base for a lot of the time
therefore energy is lost from the sea surface to the seabed through dissipation
describe reflective beaches
steeper, narrow beach
water is less than the wave base for much shorter distance
therefore not much opportunity for energy dissipation
however, steep part of the beach profile enables energy to be reflected
describe intermediate beaches
can come in different forms (four or five)
have morphological variability
some parts are dissipative and some can be more reflective
what paper describes how beach profiles influence wave activity
Wright and Short, 1984
what is dimensionless fall velocity?
represented by omega
the parameter that determines whether a beach is reflective, intermediate or dissipative
omega = Hb / ws . T
Hb = breaker height
ws = mean sediment fall velocity
T = wave period
describe what parts of the dimensionless fall velocity equation has hydrodynamic and morpho-dynamic influence
breaker height = hydrodynamics but also morpho-dynamic (beach slope)
mean sediment fall velocity = morpho-dynamic (sediment particle size)
wave period = hydrodynamic
what paper describes how beach profiles influence wave activity in the UK?
Scott et al (2011)
examples of reflective, intermediate and dissipative beaches in the UK
talks about how grain size influences profile (coarser grain = reflective, finer grain = dissipative)
what influences how a beach will change?
Tidal level
Wave climate
Beach morphology
Sediment supply
Human intervention (defences → sediment)
how does tidal level influence beach response?
it influences where waves are breaking at any given time
modulates where waves are operating in the intertidal zone
has the greatest influence from sea level rise because sea level rise adds to every tidal level
how does wave climate influence beach response?
influences energy transfer, where sediments can be moved, how they’re accumulating, whether they’re moving onshore/offshore
sea level does have the potential to change wave climate because higher water level = higher tidal level = waves can come further inshore
how does beach morphology influence beach response?
the roughness of the beach, change in height according to distance, how variable e.g dissipative/reflective profiles
how does sediment supply and human intervention influence beach response?
adding a sea defence to a location cuts off sediment supply to the coastal zone
e.g England south west coast is sediment supply limited
what is the most widely used model for projecting how much erosion is going to take place on a beach as a result of sea level rise?
Bruun Rule (R):
shoreline height (Z)
shoreline width (X)
sea level rise (S’)
R = (X.S’)/Z
the bigger the Z value, the smaller the retreat rate
in terms of the bruun rule, how does beach profile effect retreat rate?
the bigger the Z value, the smaller the value of R
the steeper (reflective) the profile, the smaller the retreat rate
the shallower (dissipative) the profile, the greater the retreat rate
e.g., Titus, 1987 a wider (dissipative) Florida beach experiences faster retreat than a steeper (reflective) New Jersey beach
what determines the value of X and Z of the Bruun Rule?
if we removed sediment from the upper part of the profile, it is the extent to which no change will be seen on the seabed (depth of closure)
entirely theoretical point
how do storms influence the Bruun Rule?
if the storm frequency happens to match the rate at which the beach accretes during the inter-storm period, then there is no retreat
if we were to increase the time between storms, there would be more building up than is lost (accretion)
if storms were more frequent it has the opposite effect (erosion)
what is a barrier beach?
in many microtidal systems, beaches tend to be located in the mid tidal zone as a barrier beach
there is a lagoon that sits behind the barrier system
what is a problem with the bruun rule?
it is based on sandy beaches that are attached to the shoreline where as many microtidal system have barrier beaches
how a barrier responds is going to be determined by the energy available to cause it to move as opposed to its inertia that is enabling it to stay where it is
how does sea level rise influence barrier beaches?
Carter, 1988
SLR can cause the barrier beach to migrate: sediment is being taken from the seaward part of the barrier and the energy is being taken over the crest and deposited on the landward side (rollover- mass relocation onshore)
Overstepping (drowning of barrier): the energy that is available as sea level is rising cant overcome the barrier inertia (e.g barrier is made of coarse material or anchored on a geological feature)
what are the main points of Vousdoukas et al’s (2020) paper?
globally, sandy coastlines are under threat of erosion driven by: sea level rise, geological and physical factors
coastlines are also experiencing an increase in human activity and populations near coastlines will continue to increase
there are coastal management strategies that could be put in place to mitigate these threats
what are the main points of Cooper et al’s (2020) paper?
the use of Bruun Rule is 2-D
shorelines are retreating but not extinguished
sandy beaches react differently to SLR, they can:
mitigate landwards w/o loss of beach width
experience recession due to offshore sediment transport
be stranded on seabed as intact sand bodies
Justify the use of the Bruun Rule
Vousdoukas et al (2020):
it is the most widely used concept and any shortcomings are amended using correction factor E
it is true that global models rely on assumptions and cannot be applied to individual regions accurately but this gives way for more site specific research that can take local morphology into account
why is the Bruun Rule and other models useful for coastal stakeholders?
coastal erosion due to SLR will be a problem for sandy beaches in the future and whilst global scenarios may be inaccurate they can be seen as worse case scenarios
therefore it is important to implement SLR mitigation strategies
it would be appropriate to commission studies using local morphology to better understand local SLR and coastal erosion and what mitigation strategies can be put in place
