Weeks 9 & 10 - Geomorphological features of wave-dominated coasts: Beaches, Barriers, Lagoons

Question 1

What is a Macrotidal coastline?

Answer 1

coastal areas where the tidal range is in excess of 4 m.

Question 2

What is Mesotidal estuary?

Answer 2

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)

Question 3

define microtidal

Answer 3

Having a small tidal range (less than 2 metres)

Wave action dominates the processes active in microtidal areas

Question 4

What is meant by the term surf zone?

Answer 4

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.

Question 5

what is Swash/Backwash

Answer 5

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

Question 6

what is wave run-up

Answer 6

the maximum onshore elevation reached by waves, relative to the shoreline position in the absence of waves

Question 7

what is wave over-topping

Answer 7

the amount of water flowing over the crest of a coastal structure such as a seawall, a dike, a breakwater, due to wave action

Question 8

what is wave overwash

Answer 8

Wave overwash is the flow of water across the surface of a thin floating body due to passing waves

Question 9

What is the breaker coefficient?

Answer 9

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

Question 10

what are the components of the breaker coefficient?

Answer 10

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

Question 11

what are the two biggest influences of breaker type?

Answer 11

beach slope and wave period
the greater these values, the smaller the breaker coefficient

Question 12

describe the energy of waves in terms of erosion

Answer 12

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

Question 13

list wave breaker types in terms of decreasing breaker coefficient

Answer 13

spilling, plunging, collapsing, surging

Question 14

describe sediment transportation via wave action

Answer 14

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

Question 15

why are beaches a good natural sea defence?

Answer 15

a healthy beach has lots of sediment that can protect the shoreline from the energy thrown at it

Question 16

describe how tides influence sediment transportation via wave action

Answer 16

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

Question 17

describe how seasons influence sediment transport via wave action

Answer 17

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

Question 18

describe how El Nino influences sediment transport via wave action

Answer 18

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)

Question 19

describe how beach profiles influence sediment transport via wave action

Answer 19

shallow, sloping, finer beaches tend to be more dissipative whilst steeper and coarser beaches tend to be more reflective

Question 20

describe dissipative beaches

Answer 20

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

Question 21

describe reflective beaches

Answer 21

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

Question 22

describe intermediate beaches

Answer 22

can come in different forms (four or five)
have morphological variability
some parts are dissipative and some can be more reflective

Question 23

what paper describes how beach profiles influence wave activity

Answer 23

Wright and Short, 1984

Question 24

what is dimensionless fall velocity?

Answer 24

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

Question 25

describe what parts of the dimensionless fall velocity equation has hydrodynamic and morpho-dynamic influence

Answer 25

breaker height = hydrodynamics but also morpho-dynamic (beach slope)
mean sediment fall velocity = morpho-dynamic (sediment particle size)
wave period = hydrodynamic

Question 26

what paper describes how beach profiles influence wave activity in the UK?

Answer 26

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)

Question 27

what influences how a beach will change?

Answer 27

Tidal level
Wave climate
Beach morphology
Sediment supply
Human intervention (defences → sediment)

Question 28

how does tidal level influence beach response?

Answer 28

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

Question 29

how does wave climate influence beach response?

Answer 29

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

Question 30

how does beach morphology influence beach response?

Answer 30

the roughness of the beach, change in height according to distance, how variable e.g dissipative/reflective profiles

Question 31

how does sediment supply and human intervention influence beach response?

Answer 31

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

Question 32

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?

Answer 32

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

Question 33

in terms of the bruun rule, how does beach profile effect retreat rate?

Answer 33

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

Question 34

what determines the value of X and Z of the Bruun Rule?

Answer 34

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

Question 35

how do storms influence the Bruun Rule?

Answer 35

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)

Question 36

what is a barrier beach?

Answer 36

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

Question 37

what is a problem with the bruun rule?

Answer 37

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

Question 38

how does sea level rise influence barrier beaches?

Answer 38

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)

Question 39

what are the main points of Vousdoukas et al’s (2020) paper?

Answer 39

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

Question 40

what are the main points of Cooper et al’s (2020) paper?

Answer 40

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

Question 41

Justify the use of the Bruun Rule

Answer 41

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

Question 42

why is the Bruun Rule and other models useful for coastal stakeholders?

Answer 42

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