In 1990, 50% of the U

ClassNotes, Geology 1330: Dr Dupre'

The Coastal Zone

In 1990, 50% of the U.S. population lived within 75 km of a coast.

In 2010, 75% of the U.S. population will live within 75 km of a coast.

"The opinion held by some who are unacquainted with the actual condition of things, that Galveston will some time be seriously damaged by some such disturbance [hurricane] is simply an absurd delusion"

Issac Cline (meteorologist), Galveston News, 1891

"The beaches at Galveston slope so gently to the ocean depth that destructive waves will be broken and their force dissipated before reaching the shore…An inundation might be wasteful and damaging, but there is no possibility of serious loss of life"

Editor of the Galveston Tribune, Sept8, 1900

What happened in Galveston on September 9^th, 1900?

"Gulf Coast areas ... are experiencing erosion rates of 1 ft/year to as much as 6 ft/year." Source: Heinz Center Report to FEMA, 2000

"... Galveston and Brazoria areas are hardest hit with erosion rates 5-6 feet annually" Source: Heinz Center Report to FEMA, 2000

Average loss due to shoreline erosion in the Gulf of Mexico are estimated to be$50 million/year.$10 million/year will come from federally subsidized insurance. Source: Heinz Center Report to FEMA, 2000

30-50% of all the structures within 500 feet of the present Gulf shoreline will be lost due to erosion in the next 60 years Source: Heinz Center Report to FEMA, 2000

Bermuda Beach, Galveston: Shoreline in 1956,1996, and 2056 years later! Source: Texas Shoreline Change Project

Coastal Erosion Rates in the U.S.

Q. What determines whether a beach is eroding or stable?

: The rate at which sediment (mainly sand) is removed (wave energy) verses the rate at which it is being re-supplied to the beach (sediment supply). The ratio of wave energy to sediment supply.

Variables controlling Wave Energy

Waves of Oscillation: "Going in circles and getting nowhere!" (fig. 20.4)

Wave-generated orbital waves (fig. 20.3)

Important Wave Terms include:

- Wave Length (= )

- Wave Height (= )

- Wave Period (= )

- Wave Base (= )

- Water Depth (= )

- Deep Water Wave ( )

- Shallow Water Wave ( )

Changes in waves as they approach the beach (fig. 20.5)

Q. How fast do wind-generated waves travel?

A. It depends on whether or not the waves are deep-water or shallow-water waves!

What is a "deep-water wave"?

What is a "shallow-water wave"?

How fast do deep-water waves(D>L/2) travel?

- Their speed depends on their ____________.

- Long period waves travel _______________ than short period waves.

- The first storm-generated waves to arrive on the beach will be the longest period waves (the forerunners), announcing the pending storm offshore.

How fast do shallow-water waves(D<L/2) travel?

- Their speed depends on _____________

- The more shallow the water, the __________ the waves.

How fast do wind-generated waves travel?

- Deep-water waves: Speed = f(__________)

- Shallow-water waves: Speed = f(__________)

What happens when the water depth becomes less than one-half the wave length?

- the deep-water wave becomes a ___________________________.

- the wave ________________.

- the wave enters the zone of ______________________.

Where does the Zone of Wave Buildup occur?

What happens when the wave slows down (i.e. in the zone of wave buildup)?

- Wave period _____________

-Wave length ______________

- Wave height ______________

:The bending of waves as they change velocity.

Wave bending as they approach the beach

Wave refraction off east Galveston

Refraction at headlands and bays (fig. 20.9)

Wave refraction usually accelerates at headlands.

Wave refraction usually accelerates at bays.

Breaker zone?

Surf Zone?

Swash Zone?

:The transport of sediment (e.g. sand) parallel to the shoreline, mainly in the surf and swash zone.

Figure 20.10

It is driven by longshore currents set up by waves coming into the shoreline at an angle less than 90^o.

Longshore drift off east Galveston

What about Sources of Sediment?

1) Rivers

2) Erosion

3) Longshore Drift

4) Others?

Surfside beach-Freeport Region: an example of multiple sediment sources.

Quintana Beach in 2056! (click here to view erosion rates from Bolivar to Brazoria Counties)

How can we prevent coastal erosion?

I. Structural approaches: e.g.

- Jetties

- Groins

- Seawalls

- Breakwaters

II. Non-structural approaches: e.g.

- Beach Nourishment

- Abandonment/Relocation/Zoning

: Impermeable structures perpendicular to the shoreline.

Purpose: to trap sediment, thereby preventing or reducing beach erosion

Problems?

How can you tell the direction of longshore drift where groins are present? (figure 20.18)

Long Island case study

: Impermeable structures perpendicular to the shoreline, on either side of an inlet.

Purpose: to reduce sedimentation in the inlet, thereby improving navigation.

Problems? (figure 20.17)

A __________________ is a naturally occurring connection of the ocean and a bay or lagoon across a barrier island or peninsula.

Ocean City Case Study:

: Impermeable structures parallel to the shoreline, usually on the landward side of the beach.

Purpose: to prevent erosion of the land adjacent to the beach.

What event caused the Galveston Seawall to be built?

How the seawall was built?

What is happening at the end of Galveston Seawall? – Why?

What happened to the beach at Seabright, New Jersey after a seawall was built to protect the town? (fig. 20.20)

: Impermeable structures parallel to and offshore from the shoreline.

Purpose: to protect a beach or boat anchorage from incoming waves

Santa Monica Breakwater Case Study: (fig. 20.19)

Q. What do jetties, groins, seawalls, and breakwaters all have in common?

Texas coastal erosion problems in the news:

Are our beaches eroding????? If not, what is going on?

: The artificial addition of sand to the beach to reduce the rate of beach erosion.

Fig. 20.21

Galveston Beach Nourishment Case Study:

Alternatives?

What is Texas doing about all of this?