The Coast
I. BEACH ANATOMY
Anatomy of Coastal Regions Terms for different parts of beaches and coastal regions Are all about ENERGY- ie, where the ocean s energy Mostly through tides and waves, and shape the land.
Some Definitions COAST: coastline to inland as far as ocean related features found (< 1 km - 10s km) Coastline - where influence of storm waves ends SHORE: coastline to low-tide shorelineand forth shoreline migrates back OFFSHORE - beyond breakers, where waves do not influence bottom WAVE CUT BENCH: Common overall feature of coastal zone is wave cut bench - flat rock over which sand and sediments constantly change. BERM: The sand piled up on the wave-but bench beneathespecially, the dry sand above the typical high water mark that we typically think of as the beach.
Coasts are ephemeral: always changing! These changes happen on both short and long time scales. Daily: Swash and backwash reshape the berm, and change the beach texture. Seasonally: Winter vs. Summer beaches- Decadal and longer: Erosion pulls down cliffs, moves shorelines, reshapes coves..
Anatomy of Coastal Regions Offshore Nearshore Shore Coast Backshore Foreshore low-tide breakerline low-tide shoreline high-tide shoreline
What we typically think of as the beach is technically Called a berm - ie, where dry sand (most of the time) accumulates Point ReyesCapistrano Beach, Ca Let s go to the BERM!
Beach Anatomy Offshore Shore Coast low-tide breakerline low-tide shoreline high-tide shoreline longshore bar wave-cut bench longshore trough berm BERM = dry sand at foot of cliffs/dunes Wet only during storms or large waves
Example of a Wave cut bench exposed during low tides Wave cut bench is rocky underlayer, cut flat by waves Over which the sand migrates with storms and seasons
Point ReyesCapistrano Beach, Ca Let s go to the BERM!
Wave cut bench exposed during low tide
II. MAIN SAND TRANSPORT MECHANISM
Movement of Sand QUESTION: How is it that WAVES can both erode a beach - and also deposit a beach? IE, how can what might seem like the exact same process ( a wave striking a beach) sometimes take sand away yet other times deposit sand?
Swash / soak / backwash Backwash SWASH (+soak)
Swash - Soak dominates Beaches with light waves - sand added to berm Reduced longshore bars Sandy wide berm Steep beachface Classic SUMMERTIME
Backwash
BackWash Dominates Energetic beaches - sand eroded from berm Longshore bars built up Rocky, narrow berm Flattened beachface Classic WINTER TIME
Summer Beach: Swash/Soak dominates Mitchell's cove series
Winter Large waves, Backwash dominates, beaches Mitchell's eroded away! cove series
Mitchell s cover, summer
BAXTER RETURNS
Mid- Winter
Spring
Quantitative? Spring vs. Winter 2*B 3.5*B
rrrrrrrr r-woof! But when did this happen to my beach..?..
Mitchell's cove series
Mitchell's cove series
Mitchell's cove series
Mitchell's cove series
Mitchell's cove series Mitchel s cove: Summer vs. Winter
A dramatic storm-cut cliff Mitchell's cove series
Summer vs winter beach Result: classic seasonal Beach differences! Summertime beach Wintertime beach
Winter Pocket cove: Extreme Armor
Another form of Armored beach: due to high wave energy all year
III. Sand Migration The river of sand
Longshore drift = movement of sand down down coast-lines, in general direction of most swell
Longshore Drift Path of sand particles - zig zag downstream Waves approach beach at an angle
Aside: Rip currents Backwash is concentrated in narrow area Can be 4-5 mi/hour
Huge Rip (hopefully those are surfers..) Backwash is concentrated in narrow area Can be 4-5 mi/hour
IV. Beach texture and Composition
Beach Texture tells you about: 1) Sand Sources and 2) ENERGY of Local Environment
Aside: Maturity of a sediment is based on hydrodynamic sorting
LARGE Sizes = HIGH Energy environment ( all small stuff is moved away..)
kelp beach - why does this kelp accumulate here Backwash is concentrated in narrow area Can be 4-5 mi/hour
So what are Sand Sources?
Do Grains MATCH local rock types?
Do Grains MATCH local rock types?
What are Sand Sources? SOURCES: On our coast, mostly rivers bringing sediments and sands from inland, also some erosion of coastal cliffs.
Sand Sinks = Submarine Canyons!
Sand sources + sinks : Beach compartments.
Features of Beach Compartments River (or erosian) to supply sand Beach with longshore transport Submarine canyons
Constant sand migration ( river of sand ) via long-shore drift sets up: Beach Compartments = defined by sand sources and sand sinks within a given region of coast.
Finally: HUMANS AND BEACH EROSIAN
ATTEMPTS TO STOP COASTAL EROSION-
Groin Barrier to longshore sediment transport Built perpendicular to coastline
Series of Groins, Santa Monica, California
Jetty Barrier to wave action at harbor entrance Built perpendicular to coastline
Breakwater Barrier to wave action Built parallel to coastline
Mission Bay, California
Santa Cruz Harbor
What its supposed to look like
Every few winters..what it really looks like
But makes a great..(if illegal) wave..
Beach Armoring/Hardening: attempts to save human-built on coast
Waves hit seawall- Bounce Energy Back- and carry sediment back offshore
Beach loss due to a SEAWALL at Waikiki
END