I. The Coast is an Especially Dynamic Environment

• A. Subject to Many Processes
  
  • 1. Waves, tides, and currents
    
  • 2. Storms, wind, etc
    
  • 3. Subsidence
    
  • 4. Rise of sea level due to global climate change
    
• B. Natural Disasters May be Especially Severe
  
  • 1. Hurricanes - killed 13,000 in the US since 1900
    
  • 2. Tsunami
    
  • 3. Earthquakes
    
• C. A Huge Population Lives in Coastal Areas
  
  • 1. Half of the US population lives within 50 miles of the ocean or the Great Lakes
    
    

II. Beaches and Beach Processes

• A. The beach is a buffer zone protecting the onshore area
  
  • 1. Typically wide in the summer and narrow in the winter
    
  • 2. A beach may be in any of the following conditions
    
    • a. Growing
      
    • b. Stable
      
    • c. Eroding - greatest concern
      
• B. Sand Sources and Beach Supply
  
  • 1. Rivers and Streams
    
    • a. In Southern California beaches are wider near river mouths
      
  • 2. Eroding beach cliffs - 5 - 10% of beach sand
    
    • a. Generally sandstone and granitic cliffs are good sand sources
      
    • b. Off-shore reefs may supply sand
      
  • 3. Sand from the seafloor immediately offshore
    
    • a. Barrier islands of the Atlantic coast are examples
      
• C. Threats to the Sand Supply
  
  • 1. Flood control dams
    
    • a. They have virtually eliminated the sand supply to S. California beaches
      
    • b. Ventura River sand supply is reduced to 66% of normal
      
  • 2. Southern California has 77 sand and gravel quarries in stream channels
    
    • a. An annual sand and gravel production of 20 million tons
      
  • 3. Paved river channels - reduces channel widening
    
  • 4. Seawalls and riprap reduce cliff erosion and focus wave energy onto the beach
    
• D. Longshore Sand Drift
  
  • 1. Rate of drift
    
    • a. 150,000 to 300,000 m³/yr on the Atlantic seaboard
      
    • b. 750,000 m³/yr in Oxnard
      
  • 2. Drift is interrupted by jetties, groins and breakwaters
    
• E. Loss of Beach Sand
  
  • 1. Rip currents or other strong currents carry sand offshore into deep water
    
  • 2. Submarine canyons
    
    • a. Fans at canyon mouths are proof of sand loss down canyons
      
    • b. La Jolla canyon swallows 150,000 m³/yr
      
  • 3. Wind transport to dunes
    
    • a. Sand migrates inland burying everything and is lost to the coast
      
    • b. Oregon dunes have migrated 3 miles inland
      
      • 1) The most extensive dune field is 50 miles long
        
    • c. Thousands of east coast houses are built directly on active dunes
      
      • 1) Dunes are ephemeral features
        

I. The Coast is an Especially Dynamic Environment | II. Beaches and Beach Processes | III. Coastal Erosion | IV. Seismic Sea Waves (Tsunami) | V. Coastal Zone Management | Links | top | Geol 357 Lecture Home

III. Coastal Erosion

• A. Erosion may be a Seasonal Event
  
  • 1. Small, long wave length waves push sand onto the beach in the summer
    
  • 2. High, short wave length waves in the winter erode sand
    
  • 3. The worst erosion may occur in the rare severe storm
    
    • a. A wave 10 feet high and 90 feet in wave length can exert 10 tons of pressure/m2 on a seacliff
      
• B. Mechanism of Erosion
  
  • 1. Hydraulic impact - water is driven into the cracks and rock breaks loose
    
  • 2. Abrasion - sand and boulders abrade the cliff
    
  • 3. Solution
    
  • 4. Biological activity - animal borings, people
    
• C. Factors Affecting How Much Erosion Occurs
  
  • 1. Natural factors are very important in most areas
    
    • a. Width of the beach
      
    • b. Amount of wave energy
      
    • c. Nature of the rock present
      
  • 2. Humans have become very important at changing erosion rates
    
    • a. Cliff tops are loaded with buildings
      
    • b. Surface drainage is altered
      
    • c. Roads, seawalls, etc. are constructed
      
• D. Pacific Coast Erosion
  
  • 1. Facts:
    
    • a. The coast is actively eroding.
      
    • b. Natural sand supply to beaches has decreased.
      
    • c. Storm activity alters the coast.
      
    • d. Coastal population continues to increase
      
    • e. Natural processes and human activities do not respect political boundaries
      
  • 2. 1940's - 1970's
    
    • a. Mild weather
      
    • b. Intense coastal development
      
  • 3. Mid 1970's - 1990's
    
    • a. Shift to increased storm activity
      
    • b. El Niño winters of 1977-78, 1982-83 and 1997-98
      
    • c. 1982-83 El Niño episode:
      
      • 1) Strongest recorded in California
        
      • 2) $116+ million in coastal losses
        
      • 3) destroyed 33 homes,
        
      • 4) Damaged 3000 homes and 900 businesses
        
      • 5) $35 million in damages to public recreational facilities
        
• E. California Coastal Erosion Planning And Response
  
  • 1. Three primary management strategies:
    
    • a. Hazard avoidance
      
    • b. Relocation
      
    • c. Coastal protection.
      
  • 2. Hazard Avoidance
    
    • a. Designing public infrastructure to discourage development in high geologic hazard areas along the coast.
      
    • b. Creating construction setbacks to reduce the risk to structures in the vicinity of high geologic hazard areas that may be vulnerable to damage.
      
    • c. Requiring full disclosure rules on real estate transactions in high geologic hazard areas.
      
    • d. Acquiring and conserving undeveloped coastal property in high geologic hazard areas.
      
  • 3. Relocation
    
    • a. Construction setbacks to avoid risks posed by structures located close to, or within, high geologic hazard areas.
      
    • b. Rolling easements that allow structures to be developed but condition their removal to allow for natural coastal processes.
      
    • c. Creating tax and other incentives when viable, to encourage property owners in high-risk areas to relocate out of harm's way.
      
    • d. Full hazard disclosure rules on real estate transactions in high geologic hazard areas.
      
    • e. Prohibitions against rebuilding damaged structures in high geologic hazard areas.
      
    • f. Acquiring and conserving endangered or undeveloped property for conversion to public parkland.
      
  • 4. Coastal Protection Strategies
    
    • a. "Soft" solutions include beach nourishment
      
    • b. "Hard" solutions include seawalls, joins, etc.
      
• F. Control of Coastal Erosion
  
  • 1. Riprap - large resistant boulders are placed at the base of a seacliff
    
    • a. Riprap should not be discontinuous
      
  • 2. Seawalls
    
    • a. They will accelerate the erosion of adjacent property
      
  • 3. Groins - are built to block the longshore drift
    
  • 4. Breakwaters
    
    • a. They reduce wave action on the coast
      
    • b. Santa Monica breakwater requires continuous dredging
      
    • c. The Santa Barbara breakwater is attached to the coast and requires 350,000 m³/yr of

I. The Coast is an Especially Dynamic Environment | II. Beaches and Beach Processes | III. Coastal Erosion | IV. Seismic Sea Waves (Tsunami) | V. Coastal Zone Management | Links | top | Geol 357 Lecture Home

IV. Seismic Sea Waves (Tsunami)

• A. Cause - major displacement of the ocean by:
  
  • 1. Earthquakes
    
  • 2. Volcanic Eruptions
    
  • 3. Submarine landslides
    
• B. Characteristics
  
  • 1. Travel 435 mph across the Pacific ocean basin
    
  • 2. Rise to maximum height as they rush into shallow water
    • a. Flat land more susceptible than steep terrain
      
    • b. Tsunami run up, the height above sea level to which the tsunami wave rises above land, must be greater than the local topography
      
  • 3. Arrive as a series of waves
    
  • 4. Most dangerous when they occur locally because there is no warning time
    
• C. Examples
  
  • 1. Nicaragua 9/18/92
    
    • a. occurred 35 miles off the coast
      
    • b. killed ~ 200
      
    • c. El Tranisto (population 1,000)
      
      • 1) Sixteen people were killed (14 children and two elderly men)
        
      • 2) 151 were injured.
        
      • 3) >200 houses were destroyed by waves that reached more than 9 meters at this site
        
  • 2. Hilo, Hawaii in 1946 as a result of an Alaskan earthquake
    
    • a. Result of an Alaskan earthquake (7.4 M)
      
    • b. 5 hours later the tsunami reached Hawaii
      
    • c. Estimated maximum wave heights:
      
      • 1) 55 feet – Hawai’i
        
      • 2) 36 feet – O’ahu
        
      • 3) 33 feet – Maui
        
    • d. 159 people died in Hawai’i
      
  • 3. Crescent City, CA - 1964
    
    • a. 9.2 Mw Alaskan Earthquake
      
    • b. 4.8 m wave
      
    • c. Destroyed 30 blocks
      
    • d. 12 deaths
      
    • e. $27 million in damage
      
  • 4. The French Riviera (10/16/79)
    
    • a. Cause: Submarine Landslide (part of the Nice international airport)
      
    • b. Sea retreated 975 feet before rushing back in the form of two 32-foot-high waves
      
    • c. 6 deaths
      
    • d. 3 missing persons
      
• D. Two Serious Concerns
  
  • 1. Collapse of the Las Palmas volcano in the Canary Islands could create an Atlantic-wide tsunami
    
  • 2. Collapse of the Kilauea volcano could cause a Pacific-wide tsunami
    
• E. Solutions
  
  • 1. Restrictions on building in flat, low-lying coastal areas
    
  • 2. A warning system as in Hawaii
    

I. The Coast is an Especially Dynamic Environment | II. Beaches and Beach Processes | III. Coastal Erosion | IV. Seismic Sea Waves (Tsunami) | V. Coastal Zone Management | Links | top | Geol 357 Lecture Home

V. Coastal Zone Management

• A. Federal Efforts
  
  • 1. The Coastal Zone management Act of 1973 was passed to support state efforts to reasonably manage coastal development
    
  • 2. Flood Insurance Act of 1973
    
    • a. It requires mapping and avoidance of coastal hazards
      
    • b. The program has not worked well and owners are rebuilding in sites of homes destroyed by storms
      
• B. State Efforts
  
  • 1. The Coastal Initiative was passed in 1972
    
  • 2. The state passed the California Coastal Act and established the Coastal Commission in 1976
    

I. The Coast is an Especially Dynamic Environment | II. Beaches and Beach Processes | III. Coastal Erosion | IV. Seismic Sea Waves (Tsunami) | V. Coastal Zone Management | Links | top | Geol 357 Lecture Home