1ESYS 150 LECTURE 11 COASTAL PROCESSES CoastlineWaves in generalWaves on the coastlineHuman effects on the Coast
2COASTLINE COASTAL US Rates of erosion on US beaches Severely eroding – (red) Most people want to live onModerating eroding – (orange) beach but it is disappearingReasonably stable – (purple) from most places.
3WAVES ENERGY PULSE Particles 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/2Waves 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.
4WAVES ENERGY PULSE Particles 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.
5WAVES SWELLS AND ROGUE WAVE 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.
6WAVES ON COASTLINE WHY WAVES BREAK Wave 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.
7WAVES ON COASTLINE WHEN WAVES BREAK Simple 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 ~= 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.
8WAVES ON THE COASTLINE ZONES ON THE BEACH Formation 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.
9WAVES ON COASTLINE WHY WAVES CURL In a breaking wave, circular rotating water slowed at base.Rolls forward at the top. Creates a curl.
10WAVES ON THE COASTLINE SUMMER/ WINTER BEACHES Summer 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.
11WAVES ON COASTLINE SUMMER/WINTER BEACHES Winter Active winter storms, long wavelength waves with abundant energy.Large breakers, large backwash, pulls the sand off the beach.
12WAVES ON COASTLINE REAFRACTION OF WAVES AROUND A POINT False Point, La Jolla View north of wave refraction around False Point.Wave refraction around a headland.Wave first entering shallow water slows down and converges onheadland. Wave portion still in deep water races ahead and stretchesout creating a bending process known as wave refraction.
13WAVES ON COAST LINE REFRACTION OF WAVES Direction of prevalent swell and Scripps Canyon control structure of the waves at Blacks Beach.
14WAVES ON COAST LINE REFRACTION OF WAVES Creates high wave heights north of canyon head
15WAVES ON COASTLINE LONGSHORE DRIFT River 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.
16WAVES ON COASTLINE SUBMARINE CANYONS Coast 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.
17HUMAN EFFECTS ON COAST DAMS Malibu 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.
18HUMAN EFFECTS ON COAST CLIFF PROTECTION Retreating 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?
19HUMAN EFFECTS ON COAST CLIFF PROTECTION Concrete 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
20HUMAN EFFECTS ON COAST SEA WALL Built on beach to provide protection A before the wall,B build he first wall - waves bounce off wall erode the beachC years - all sand removed by waves which work to undermine wallD Later years - build bigger seawall, increases reflection, beach disappears again and ocean much deeper.Longshore drift can offset this
21HUMAN EFFECTS ON THE COAST GROINS Ship 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?
22HUMAN EFFECTS ON COAST BREAKWATER 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.
23HUMAN EFFECTS ON COAST JETTIES Create 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.