ESYS 150LECTURE 11

COASTAL PROCESSES

Coastline

Waves in general

Waves on the coastline

Human effects on the Coast

COASTLINECOASTAL USRates of erosion on US beaches

Severely eroding – (red) Most people want to live on

Moderating eroding – (orange) beach but it is disappearing

Reasonably stable – (purple) from most places.

WAVESENERGY PULSEParticles rotate in place

Particles rotate in the direction of the wave front.At the surface, the diameter of particle motion = HOrbit decreases with depth ~ 0 when z = L/2

Waves formed by frictional drag of wind on water surface.Wave height a function of wind velocity, consistency, duration and fetch.Can travel thousands of miles from where formed.

WAVESENERGY PULSEParticles rotate in circles

Involve both longitudinal and transverse motion. As wave travels through water the particles rotate in a clockwise fashion.

Depth of the water is greater than the height of the waves.

Blue particles show that each particle travels in a circle.

WAVESSWELLS AND ROGUE WAVE

a) Usually many different storms, producing waves of different characteristics. They interfere with each other to produce small swell.

b) When they become synchronized they can produce huge waves that can overwhelm small boats. Waves can have amplitudes of between 5 and 15 m.

WAVES ON COASTLINEWHY WAVES BREAKWave moves into shallow water

At water depths less than 1/2 the wavelength waves change shape.

Friction on the ocean floor causes particle motion to go elliptical and the waves get larger.

Thus H increases and L decreases.

WAVES ON COASTLINEWHEN WAVES BREAKSimple theory (Miche 1944)

In deep water when d/L > 0.5 or d > L/2 then only the wavelength is important.

Breaking occurs when H/L ~= 0.14 or (H:L ~= 1:7)

In shallow water, when d/L < 0.05 or d < L/20 then only the depth of the water is important.

Breaking occurs when H ~= 0.8d or (d~= 1.3H)

In practice H/d at breaking depends also on both the wave steepness and the beach slope.

WAVES ON THE COASTLINEZONES ON THE BEACHFormation of actual breakers

Schematic cross section showing deep-water waves entering shallow water.

Wavelength decreases as wave height increases and the wave breaks.

Note the transition, breaker, surf and swash zones.

WAVES ON COASTLINEWHY WAVES CURL

In a breaking wave, circular rotating water slowed at base.

Rolls forward at the top. Creates a curl.

WAVES ON THE COASTLINESUMMER/ WINTER BEACHESSummer

Summer conditions at Boomer Beach, La Jolla.

Summer storms less active, waves short wavelength, small height.

Abundant small waves push off-shore sand up the beach.

WAVES ON COASTLINESUMMER/WINTER BEACHESWinter

Active winter storms, long wavelength waves with abundant energy.

Large breakers, large backwash, pulls the sand off the beach.

WAVES ON COASTLINEREAFRACTION OF WAVES AROUND A POINTFalse Point, La Jolla

a) View north of wave refraction around False Point. b) Wave refraction around a headland. Wave first entering shallow water slows down and converges onheadland. Wave portion still in deep water races ahead and stretches out creating a bending process known as wave refraction.

WAVES ON COAST LINEREFRACTION OF WAVESDirection of prevalent swell and Scripps Canyon control structure of the waves at Blacks Beach.

WAVES ON COAST LINEREFRACTION OF WAVESCreates high wave heights north of canyon head

WAVES ON COASTLINELONGSHORE DRIFTRiver of sand

Schematic map of longshore drift. Waves run up the beach at an angle but return to sea as a perpendicular backwash.

Creates a longshore transport of water and sand along the coast.

A river of sand.

WAVES ON COASTLINESUBMARINE CANYONSCoast of San Diego County.

Two different cells.

Silver Strand cell from Tijuana river sands driven north by waves from summer hurricanes off Mexico.

Oceanside cell is beach and sand moving south pushed by waves from north Pacific winter storms.

The sand flow ends when it pours into La Jolla Canyon - a submarine canyon.

HUMAN EFFECTS ON COASTDAMSMalibu Canyon Dam

Dam built in 1925 was filled by sediment 13 years later.Coastal zone short of fresh water. Build dams across rivers.Dams also catch sand, cuts off beach supply, reduces protection against

erosion. Torrey Pines beach losing sand. Eroding faster.

HUMAN EFFECTS ON COASTCLIFF PROTECTIONRetreating cliffs in Solana Beach

Condo’s have wonderful views but beach retreating at 10 ft per 100 years.

Want to protect the beach from erosion. How?

HUMAN EFFECTS ON COASTCLIFF PROTECTIONConcrete wall and rocks

Sunset Cliffs San Diego. Protected by a cement wall and “riprap”.

Waves erode underneath and around the structure.

Also wall reflects waves hitting them. Rebounding water surges power fully erode the beach.

“Riprap” reduces reflection but eventually waves work way under the “riprap”.

Eventually have very serious problem

HUMAN EFFECTS ON COASTSEA WALLBuilt on beach to provide protection

A before the wall,

B build he first wall - waves bounce off wall erode the beach

C 2 - 40 years - all sand removed by waves which work to undermine wall

D Later 10 - 60 years - build bigger seawall, increases reflection, beach disappears again and ocean much deeper.

Longshore drift can offset this

HUMAN EFFECTS ON THE COASTGROINSShip Bottom New Jersey

Groins - short elongate mass perpendicular to coast line to trap sand.

Interfere with longshore drift with deposition on upside and erosion on downside.

What is the direction of longshore drift?

HUMAN EFFECTS ON COASTBREAKWATER Built parallel to coast to provide protection

Stops waves from hitting the beach. Sand is deposited behind and on the upside of the breakwater.

Needs permanent dredging to keep the harbor or sheltered area open.

HUMAN EFFECTS ON COASTJETTIESCreate harbor and channels for boat passage.

Need to be built out just the right length to create a large enough tidal prism (in-and-out volume) of seawater to keep them clear.

Extend well beyond the breaker zone and interfere with longshore drift.

Have deposition on the upside and erosion on the down-drift side.