WA L3: Waves on Shorelines

Question 1

What does the location of the coast depend on?

Answer 1

global tectonic activity (uplift and subsidence) and the volume of water in the ocean (sea level rise or fall)

Changes to these occur over very long-term periods, from 100s to 1000s of years.

Question 2

What is the shape of the coast determined by?

Answer 2

Many processes of varying timescales:

Sediment transport and deposition

Erosion

Uplift and subsidence

Question 3

Waves usually approach a shore at an angle. T/F?

Answer 3

True

Question 4

Which segment of the wave feels the bottom and slows down before the rest? Why?

Answer 4

Segment of wave closest to shore. Because waves slow down as the depth becomes shallower.

Question 5

How is wave refraction formed?

Answer 5

When waves slow down and the segment of the wave that is closest to the shore will be feeling the bottom and slowing down before the rest.

Question 6

What process is wave refraction closely associated with?

Answer 6

Shore straightening

Question 7

What kind of coasts tend to be irregular?

Answer 7

Geologically young coasts

Question 8

When waves hit an irregular coast, they hit which part first?

Answer 8

The headlands. These jut farther out into the sea relative to the rest of the coast

Question 9

Which part of an irregular coast erodes faster? Why?

What happens to the rest of the wave?

Answer 9

Headlands absorb most of the energy transported by waves and tend to erode faster.

The rest of the wave travel farther before they hit shore to the left and right of the headland, almost like wrapping around the headland. These hit the shore with much less energy, hence cause less erosion.

Question 10

What are wave normals or orthogonal lines?

Answer 10

Lines perpendicular to the wave crests, outlines how energy is transported and focused on headlands.

Question 11

Waves are in fact refracting as they hit an irregular coast, causing wave energy to ____ on headlands, and ____ (spreads out) across bays.

Answer 11

focus; disperse

Question 12

Over time, what happens to the shoreline as waves continuously hit an irregular coast?

Answer 12

shoreline straightens out as waves erodes sediments from headlands and deposits them in bays

Question 13

What is the ultimate effect of wave convergence on headlands and divergence in embayments?

Answer 13

Headlands shrink (erode), and embayments fill in

Question 14

Breaking Waves. We learned earlier that as a swell reaches the coast, its steepness increases to a point where H / L (the ratio of wave height to wavelength) nears the value of ___. At the same time, the ratio of the wave height to the depth of the water, H / d also approaches a value of ___. Beyond these critical points, the wave crests become unstable and the wave breaks. _____ (froth) begin to form as the wave has turned to ___, the turbulent mass of agitated water rushing onshore.

Answer 14

1/7; 3/4; Whitecaps; surf

Question 15

When a wave breaks, its energy is _____ in the surf zone and transformed into ____ energy, affecting both the water and land, including the ocean ____ and the shore.

Answer 15

dissipated; kinetic; bottom

Question 16

What factors of the breaking wave does the slope of the ocean bottoms control?

Answer 16

The shape, size, and behaviour

Question 17

Which areas produce spilling breakers? What are these “waves” good for?

Answer 17

Areas where ocean bottom is flat or gentle. Good for safe surfing.

Question 18

Which areas produce plunging breakers?

Answer 18

Areas with moderately steep ocean bottoms. Only for expert surfers.

Question 19

Which areas produce surging breakers?

Answer 19

Areas where ocean bottom is very steep. These breakers release their energy in sudden bursts, making them potentially dangerous. Much of the wave energy is reflected back into water.

Question 20

Although the ___ of breaking waves expected at a specific beach does not change, ___ varies on a daily basis.

Answer 20

type; surf

Question 21

What variables affect surf on a daily basis?

Answer 21

swell from distant storms

local wind that may interfere with the waves generated much farther off shore

constructive or destructive interference from other swells

Question 22

What is a beach?

Answer 22

A beach is the zone of loose particles (sediment) covering part or all of the seashore.

Question 23

How are beaches produced

Answer 23

produced when sediment, mostly sand, is transported to the shore by rivers and streams (from land) and by waves (from the coastal ocean) and deposited there

Question 24

Why are beaches in a constant state of change?

Answer 24

Because of the continuous action of riverine input and waves

Question 25

What is swash?

Answer 25

Incoming turbulent layer of water that washes up on the beach after an incoming wave has broke.

Question 26

What is backwash or backflow?

Answer 26

Water that recedes and returns to the sea. Mass of water usually returns at 90 degree angle, perpendicular to the shore.

Question 27

What is longshore drift or longshore transport?

Answer 27

The process by which sediments move parallel to the shore.

Question 28

What kind of coastal features does longshore drift produce?

Answer 28

delicate coastal features like barrier islands and tombolos

Question 29

What are barrier islands?

Answer 29

sandbars that are usually very long and narrow, are built parallel to shore but separated from it by a narrow body of water

Question 30

What are tombolos?

Answer 30

Sandbars that connect one island to another.

Question 31

What do humans value in sandbars?

Answer 31

Although these sandbar features are quite fragile and are hardly permanent, they can acquire high value as seaside real estate.

Question 32

Why do people make efforts to attempt to control beach erosion?

Answer 32

Because the cyclical nature of sediment erosion and deposition is not widely understood, people regard beach erosion as permanent loss.

Question 33

What are groins?

Answer 33

elongate structures that protrude perpendicular to the shoreline

Question 34

What is one of the most popular ways for people to control beach erosion?

Answer 34

building groins

Question 35

What is successful about groins? What is not realized about groins?

Answer 35

Successful in trapping and retaining sediments on the up-drift side (facing the wave front)

What is not realized is that groins interfere with longshore transport of sediments such that deposition and erosion occurs in unexpected places: deposition in the up-drift (upstream) side and erosion in the down-drift (downstream) side.

Question 36

Side effect of groins?

Answer 36

Groins interfere with the movement of sediments along the beach, causing deposition of sediments upstream of the groin and erosion immediately downstream

Question 37

What is a jetty built for? What’s the build configuration?

Answer 37

to protect an inlet or harbour. Jetties lock the inlet in place (longshore drift naturally moves inlets over time- one fills in, another opens. jetties limit this.) Often built in pairs, one on each side of an inlet.

Question 38

Side effect of jetty?

Answer 38

interfere with longshore drift in a manner similar to groins. accretion on updrift side, erosion on downdrift side

Question 39

What is a breakwater?

Answer 39

type of protective structure built to defend against wave action and erosion. These structures are built at a distance away from the coast being protected

Question 40

As waves hit a breakwater, their energy is ____ onto the structure. The area behind the breakwater becomes a _______ for boats and ships as they are protected from the ____ and rolling effects from waves.

Answer 40

dissipated; safe harbour; pitching

Question 41

In addition, the coastline behind a breakwater is protected from the _____ action of waves. Because the waters are calm, _____ occurs and beaches build up behind the breakwater (see figure below). However, because the breakwater cuts off the energy required to drive the longshore transport, adjacent unprotected sections of the coastline do not receive fresh supplies of sediments and gradually shrink due to _____.

Answer 41

erosive; deposition; erosion

Question 42

What is a tethered-float breakwater?

Answer 42

Innovative variation of the standard breakwater. Has the advantage over traditional structures of removing energy from waves without interfering with sediment transport along the shore. The system consists of 1.5-metre spheres of steel placed 1.5 m apart. The spheres are anchored to the ocean floor and held just beneath the ocean surface.

Question 43

What is a seawall? What is its purpose?

Answer 43

form of coastal defense to reduce the effects of strong waves and to defend the coast around a town or harbour from erosion. The walls are constructed to reflect wave power and can be sloping, vertical or curved.

Question 44

How does a seawall differ from groins and breakwaters?

Answer 44

seawalls are constructed on the inland part of a coast (not built on the seaward side of the coastline)

Question 45

Why does the seawall only get bigger?

Answer 45

Waves rebounding off the wall erode the beach and deepen coastal waters. Beach sand is removed by waves, which then work to undermine the wall. So bigger wall must be built. Bigger wall reflects even bigger waves, and the beach disappears again. And it goes on…

Question 46

How is the Stanley Park Seawall a good example of the good and bad effects of seawalls.

Answer 46

The Stanley Park Seawall is a very good example of the good and bad effects of seawalls. This seawall is an effective defense in the short-term (years to a few decades) as it reflects the wave energy off the protected coastline (photo in left panel below). However, it has caused increased erosion in the long-term (many decades). The energy of the backwash is reflected off the beach material beneath and in front of the seawall, so the beach material is gradually eroded (photo in right panel below). The seawall will eventually collapse when the sediment foundation it is built on becomes fully eroded.