SHORELINES AND WAVES Chapter 16

NOTE: Omit pp. 367-376 that deal with the Seafloor

A. Changes in Sea Level: Sea level is always changing, so although at any instant, shorelines constitute only a small amount of land area, over time large areas have been a shoreline environment at one time or another Not discussed in the textbook

1. Causes for changes in local sea level changes: local tectonic uplift/subsidence, isostatic rebound

2. Causes for changes in worldwide sea level changes: extent of glaciation, change in volume of ocean basins (long term)

B. Waves

1. Terminology pp. 377-378; see Fig 16-10, p. 378

Crest, trough, wavelength, wave height, wave front, wave form

2. Formation and Size of Waves pp. 378--379

a. All waves (except Tsunamis) are caused by wind blowing across the water

b. The height (= size) of a wave depends on : wind speed, duration of wind, fetch

3. Wave and Water Motion see Fig 16.10, p. 378

a. Although the wave form may travel 1000’s of miles, the water does not go far (makes vertical circles)

b. Wave base is the depth to which the water is affected by the wind and is about one-half of the wavelength

4. Breaking of waves near the shoreline p 379; see Fig 16.10 p. 378

a. Waves begin to drag bottom when the wave base intersects the bottom

b. The wave begins to pitch forward and at some point breaks and washes up on the beach

c. The force of the wave drives it onto the beach but then the water flows back due to gravity

5. Tsunamis or Seismic Sea Waves (“tidal waves”) see p. 199 in Chap. 9 (Earthquakes)

a. Caused by sudden disruption of ocean floor: earthquake, volcanic explosion, submarine slides

b. Most common in Pacific: Japanese & Hawaiian islands are particular susceptible

c. Recent Sumatra earthquake cause Tsunami in Indian Ocean (12-26-04) worst in history

d. Generally have 2-4 waves that may travel up to 500 mph

e. Behavior is unspectacular until approaches shore (several miles off shore)

f. Because water is disturbed to depth, get tremendous build up of wall of water

g. Steep slope to shore magnifies height of all waves including Tsunamis

C. Two types of Shorelines: Don’t be concerned about “emergent coastlines” (falling sea level) or “submergent coastlines" (rising sea level) p. 386-387

1. Rugged Coastlines such as those found in New England and northern California

2. Flat Coastlines such as those found along the Mid-Atlantic and Southeastern states and along the Gulf Coast.

D. Waves as a geologic agent

1. Erosion see page 375

a. Erosional processes

i. Hydraulic action = wave action: energy dissipated when wave crashes on shore

ii. Abrasion=sediment carried by the wave abrading against the shore

iii. Solution: factor only if costal rock is carbonate. Not as efficient as fresh water in dissolving rock

b. If a coastline is uneven (headlands and bays), the waves will be bent = refracted toward the headland areas and they will be eroded first. Over time erosion tends to straighten coastlines. pp. 379-380 See Fig 16.11, 16.12, p. 389

c. Erosional Features of “Rugged” Coastlines (those with rock exposed at the shoreline)

i. Wave cut benches

ii. Sea stacks See Fig 16.22 a & c p. 388; Fig 16.24, p. 389

iii. Sea arches See Fig 16.22 a & b p. 388; Fig 16.24, p. 389

iv. Sea cliffs See Fig 16.21 p.387

2. Transportation see p. 380, see Fig 16.14, p. 381

a. Most sediment is transported by “long shore currents”

b. Mostly sand size sediment transported

c. Most sand on beaches in the eastern and Gulf US coasts very dumped by streams into the ocean

d. The clay and silt fraction is mostly carried further offshore (in suspension) leaving sand on the beach

3. Deposition See pp. 380-385 (Note: more detail that I cover)

a. Bars (including Baymouth Bars and Barrier Islands)

b. Beaches

c. Spits, Hooks, etc.