Waves, Beaches & Coastal Erosion Coastal ErosionCoastal Erosion

Would You Live Here? Florida Keys

Waves Transport energy over a body of waterTransport energy over a body of water Can cause coastal erosionCan cause coastal erosionCan cause coastal erosionCan cause coastal erosion

Waves Generation Generated by ForceGenerated by Force – Sea Wrinkles  ripples  wind waves  swells Generated at storm centersGenerated at storm centers Separation of waves by differing rates of travel - dispersionSeparation of waves by differing rates of travel - dispersion – wind or air

Waves Size Factors determining size:Factors determining size: º Fetch º Wind velocity º Duration – Distance wind blows over open ocean – Speed of wind – Length of time wind blows

Wave Behavior Refraction Wave crests approach shore at an angleWave crests approach shore at an angle Therefore, Wave bendsTherefore, Wave bends Crest over shallow water slows; crest over deeper water does notCrest over shallow water slows; crest over deeper water does not

Wave Behavior Beach Erosion Wave rays travel perpendicular to wave crestWave rays travel perpendicular to wave crest Energy focused on headlands  erosionEnergy focused on headlands  erosion

Wave Behavior Longshore Current and Transport Current that moves sediment parallel to shoreCurrent that moves sediment parallel to shore – Swash moves sediment onto beach at an angle – Backwash moves sediment down perpendicular to shore Zig-zag motion has an overall net transport; longshore transportZig-zag motion has an overall net transport; longshore transport

Beach compartments in southern California Beach compartments include: Beach compartments include: –Rivers –Beaches –Submarine canyons Figure 10-12

What is a beach? Area along coast where sediment accumulatesArea along coast where sediment accumulates Coastal zones begin when waves interact with seafloorCoastal zones begin when waves interact with seafloor

Beaches Shoreline Divisions Littoral zone Littoral zone oShoreface oForeshore oBackshore – Part of coast that stretches from land plants to where waves start to break – Permanently covered by water – Exposed at low tide and covered at high tide – above mean high tide – only affected by storms

Beaches Types Shape and structureShape and structure CompositionComposition SizeSize ColorColor wide narrow Quartz Cobble Pink sand Shell sand Black sand

Beaches Summer and Winter Summer Summer Winter Winter – Erosive storm waves – Gentle waves – Sand carried seaward to offshore sandbars – Narrow beach – Carry sand to shore but too weak to carry back to sea – Wider beach results winter summer

Beach Dynamics Deposition and Erosion DepositionDeposition ErosionErosion – Reshaping of coastline by wave action – Coasts build out by waves and currents Sandy shore Rocky shore – Little incoming sand, usually no major river

Beaches Depositional Features Sand spitsSand spits Barrier islandBarrier island – Long sand island enclosing a bay or lagoon Bay mouth barsBay mouth bars – When a sand spit closes off a bay – Longshore current deposits sand in a line downcurrent of a headland

Barrier Islands 15,000 years ago sea level was ~85 m below present 15,000 years ago sea level was ~85 m below present As sea level rose, a breach in ridge resulted in flooding As sea level rose, a breach in ridge resulted in flooding As sea level continues to rise, ridge migrates landward As sea level continues to rise, ridge migrates landward North Carolina Galveston, Texas

Baymouth Bars Bars are temporary structures Bars are temporary structures Beach through bar Beach through bar Alabama

Sandspit Depositional features are constantly migrating Depositional features are constantly migrating New Zealand

Beaches Erosional Features Wave cut benchWave cut bench CliffCliff – Cliff bottom attacked by waves – Waves undercut cliff to form wave cut notch – Overhanging rock collapse – As cliff retreats, area of flat rock exposed Cliff Wave cut bench

Wave Cut Terrace Santa Barbara, California

Beach Erosion California Beach Erosion California – 86% of California coasts are classified as actively eroding – 1 in 4 homes within 500 feet of coast will be lost to erosion in 60 years Beach erosion is a serious problem for coastal communitiesBeach erosion is a serious problem for coastal communities Some causes:Some causes: – Sea level rise – Building near coast – Efforts to reduce erosion

Palos Verdes Peninsula Malibu, California

Beach Erosion California Santa Barbara, California

Sand Sources and Beach Supply Rivers and Streams Rivers and Streams – In Southern California beaches are wider near river mouths Eroding beach cliffs – 5 – 10% of beach sand Eroding beach cliffs – 5 – 10% of beach sand – Generally sandstone and granitic cliffs are good sand sources – Off-shore reefs may supply sand Sand from the seafloor immediately offshore Sand from the seafloor immediately offshore – Barrier islands of the Atlantic coast are examples

Threats to the Sand Supply Flood control dams Flood control dams – They have virtually eliminated the sand supply to S. California beaches – Ventura River sand supply is reduced to 66% of normal Southern California has 77 sand and gravel quarries in stream channels Southern California has 77 sand and gravel quarries in stream channels – An annual sand and gravel production of 20 million tons Paved river channels – reduces channel widening Paved river channels – reduces channel widening Seawalls and riprap reduce cliff erosion and focus wave energy onto the beach Seawalls and riprap reduce cliff erosion and focus wave energy onto the beach

Coastal Erosion Erosion may be Seasonal Erosion may be Seasonal –Worst erosion may occur during severe storm –Low pressure raises water level –Waves stronger during storm A wave 10 feet high and 90 feet in wave length can exert 10 tons of pressure/m 2 on a seacliff A wave 10 feet high and 90 feet in wave length can exert 10 tons of pressure/m 2 on a seacliff

Mechanism of Erosion Hydraulic impact – water driven into cracks and rock breaks Hydraulic impact – water driven into cracks and rock breaks Abrasion – sand and boulders abrade cliff Abrasion – sand and boulders abrade cliff Solution-- chemically dissolving rock Solution-- chemically dissolving rock Biological activity – animal borings, people Biological activity – animal borings, people

Factors Affecting How Much Erosion Occurs Natural factors Natural factors – Width of the beach – Amount of wave energy – Nature of the rock present

Factors Affecting How Much Erosion Occurs Humans Impacts Humans Impacts – Cliff tops are loaded with buildings – Surface drainage is altered – Roads, seawalls, etc. are constructed

Pacific Coast Erosion Sea cliff retreat rates vary from nearly zero to 9 feet per year Sea cliff retreat rates vary from nearly zero to 9 feet per year – Marine terraces in many places are eroding 1 foot/yr Erosion has caused some developments to establish a setback calculated to last 50 years. Erosion has caused some developments to establish a setback calculated to last 50 years. – The setback serves as a common park area

Planning for Erosion Erosion must be incorporated into coastal land use plans Erosion must be incorporated into coastal land use plans – Setbacks must be established Real estate agents must by law reveal problems to coastal buyers Real estate agents must by law reveal problems to coastal buyers

Control of Coastal Erosion Riprap – large boulders at base of sea cliff Riprap – large boulders at base of sea cliff – Riprap should not be discontinuous Seawalls Seawalls – Accelerate erosion of adjacent property Groins – are built to block the longshore drift Groins – are built to block the longshore drift Breakwaters Breakwaters – Reduce wave action on the coast – Santa Monica breakwater requires continuous dredging – Santa Barbara breakwater is attached to the coast and requires 350,000 M 3 /yr of dredging

Beaches Human Impacts Sea wallsSea walls BreakwatersBreakwaters – Wall parallel to shore – Reduce wave energy – Interrupts beach drift JettiesJetties – Designed to maintain navigation routes GroinsGroins – Wall perpendicular to shore – Restore erosion – Erodes downcurrent side – Parallel concrete structures – Absorbs wave energy – Actually promotes erosion – Erosion and accretion

Tsunamis Cause erosion, destruction of property, loss of lives Cause erosion, destruction of property, loss of lives

Coastal Zone Management Federal Efforts Federal Efforts Coastal Zone management Act of 1973 passed to support state efforts to manage coastal development Coastal Zone management Act of 1973 passed to support state efforts to manage coastal development Flood Insurance Act of 1973 Flood Insurance Act of 1973 – Requires mapping and avoidance of coastal hazards – Program has not worked well, owners rebuilding in sites of homes destroyed by storms State Efforts State Efforts – The Coastal Initiative passed in 1972 – California Coastal Act established the Coastal Commission in 1976

Beaches Human intervention Beach nourishmentBeach nourishment DredgingDredging – Import sand into coastal areas – Underwater excavation to clear waterways before after Miami Beach

Let Nature Takes Its Course Remove Artificial Barriers Remove Artificial Barriers Move houses away or back Move houses away or back Learn to live with nature not change it Learn to live with nature not change it