181.%20Coasts

181.%20Coasts - Coasts Lecture 18 The Coast is Many Things...

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Unformatted text preview: Coasts Lecture 18 The Coast is Many Things To Many People Lecture 18: Coasts Page 1 of 34 Lecture 18: Coasts Lecture 18: Coasts Page 2 of 34 Lecture 18: Coasts Lecture 18: Coasts Page 3 of 34 Lecture 18: Coasts Lecture 18: Coasts Page 4 of 34 Lecture 18: Coasts Geomorphology of the Coast • • • • Four Basic Coastal Forms Depositional – clastic sediment rich Carbonate – coral reefs and carbonate beaches Tectonic – fault dominated, complex coast Glacial – complex coastal system related to downed glaciated terrain In Short: Coastal geomorphology is simple the drowned geomorphology that existed before sea level rise Depositional Coast Lecture 18: Coasts Page 5 of 34 Lecture 18: Coasts Characteristics • • • • sediments Sediment source local/regional rivers and coastal erosion “Flat” topography Characterized by beaches and barrier islands Broad Continental shelf • Dominated by clastic • • • • Sedimentation > Erosion Large delta complexes and wetlands Low energy environment Commonly contains barrier island systems Page 6 of 34 Lecture 18: Coasts Lecture 18: Coasts Highly complex series of depositional environments Lecture 18: Coasts Page 7 of 34 Lecture 18: Coasts Barrier Islands Characteristics • Series of narrow • sandy islands that parallel mainland Formed as sea level rose following the Pleistocene glacial “low stand of sea level” Waves and river sediment transport built modern coastal system Page 8 of 34 • Lecture 18: Coasts Lecture 18: Coasts Complex Environment Is this a Winter or Summer beach? Active Sediment Transport Lecture 18: Coasts Page 9 of 34 Lecture 18: Coasts Tidal Inlet • Provides water exchange between bays and open sea • Dynamic features • Carry coastal sediment inland and deposit it in the bay/lagoon Geomorphology of an Inlet Lecture 18: Coasts Page 10 of 34 Lecture 18: Coasts Inlet Hydrodynamic Stability • Flow velocity through inlet is controlled by the size of the inlet (hydraulic radius) Erosion is controlled by flow velocity Hydrodynamic stability occurs when flow velocity cannot cause erosion but transports sediment through inlet • • Shoreline Erosion Lecture 18: Coasts Page 11 of 34 Lecture 18: Coasts Characteristics • Erosion > Sedimentation – – – Subsidence Starvation Trapping • Commonly results in “cliffed” coast Texas Coastal Erosion Lecture 18: Coasts Page 12 of 34 Lecture 18: Coasts Land Subsidence Sargent Beach, Texas Why the sudden increase in shoreline erosion? Lecture 18: Coasts Page 13 of 34 Lecture 18: Coasts Recession Rate Recession rate at Sargent Beach shows a significant increase starting in the early 1940”s. 1. Is this Natural? 2. Is this Human caused? Sediment vs. Drainage Area vs. Drainage Lecture 18: Coasts Page 14 of 34 Lecture 18: Coasts Sediment Budget • 7104 mcf required to maintain No erosion conditions • Actual sediment supply is 3736 mcf, resulting in 13 ft/year loss • Construction of upstream dams reduces sediment supply to 1931 mcf resulting in increase of 7 ft/year loss Storm Wave Attack Lecture 18: Coasts Page 15 of 34 Lecture 18: Coasts Rock Weathering-Rock Fall Weather Induced Erosion Landslides Lecture 18: Coasts Page 16 of 34 Lecture 18: Coasts Wave Scour Carbonate Coast Lecture 18: Coasts Page 17 of 34 Lecture 18: Coasts Characteristics • Dominated by coral • • reefs and calcareous sand beaches Often exists in barrier reef or atoll forms Tropical region environment (modern) Lecture 18: Coasts Page 18 of 34 Lecture 18: Coasts Atoll Lecture 18: Coasts Page 19 of 34 Lecture 18: Coasts Tectonic Coast Characteristics • Very steep coastal • • • slope Deep water close to the shore Limited to non-existent continental shelf Minimal beaches and barrier islands Lecture 18: Coasts Page 20 of 34 Lecture 18: Coasts Lecture 18: Coasts Page 21 of 34 Lecture 18: Coasts Glacial Coast Lecture 18: Coasts Page 22 of 34 Lecture 18: Coasts Characteristics • Highly irregular coast • line Deep drowned glacial valleys flanked by glacial sediment shoals Pocket beaches, minimal barrier islands or regional beaches • Lecture 18: Coasts Page 23 of 34 Lecture 18: Coasts Coastal Dynamics Lecture 18: Coasts Page 24 of 34 Lecture 18: Coasts Wave Properties Wave Mechanics • Deep Water Waves • Transitional Water Waves • Shallow Water Waves – Depth/Wave Length ≤ ½ – Wave base does not “feel” bottom – 1/25 ≤ Depth/Wave Length ≤ 1/2 – Depth/Wave Length ≥ ½ – Waves “feel” bottom – Waves become unstable and break Lecture 18: Coasts Page 25 of 34 Lecture 18: Coasts Beach Slope and Waves • Rapid change in topography generates plunging wave • Higher energy generates complex beach profile Beach Profiles • Winter Profile (High Energy Waves) – Steep, narrow beach front, offshore bars • Summer Profile (Low Energy Waves) – Flat, wide beach front, minimal offshore bars Lecture 18: Coasts Page 26 of 34 Lecture 18: Coasts Waves and the Coast • Refraction: controlled by changing water depth and wave slowing in shallow water Reflection: controlled by vertical wall in deep water Diffraction: controlled by size of the opening that allows wave to enter bay/estuary • • Wave Refraction and Sediment Transport Lecture 18: Coasts Page 27 of 34 Lecture 18: Coasts Coastal Sediment Transport • • • • Waves approach at an angle Wave run-up controlled by approach direction Wave run-back controlled by gravity Net result is a zig-zag path of sediment movement Impact on Shoreline Protection Lecture 18: Coasts Page 28 of 34 Lecture 18: Coasts Groins for Beach Stabilization Erosion Deposition Lecture 18: Coasts Page 29 of 34 Lecture 18: Coasts River—Ocean Interaction • Fresh water is less dense than salt water • Fresh water in river moves bed-load and carries • • • suspended-load Fresh water “floats” on sea water and drops bed-load forming “river mouth” bar Suspended-load and dissolved chemicals are carried seaward in a floating fresh water lens Can create “dead zone” in marine environment Tides • Earth dynamics • Lunar gravitational forces • Solar gravity • Tidal Ranges Interaction between: – Spring Tide = Sun and Moon are aligned – Neap Tide = Sun and Moon are opposed Lecture 18: Coasts Page 30 of 34 Lecture 18: Coasts Tides Tides • Diurnal: 1-high tide • • and 1-low tide in a 24-hour period Semidiurnal: 2-high tides and 2-low tides in a 24-hour period Mixed: 1-higher high, 1-lower high, 1-lower low and 1-higher low tide in a 24-hour period Page 31 of 34 Lecture 18: Coasts Lecture 18: Coasts Coastal Protection Groins Designed to trip sand and allow the beach to reach equilibrium with the wave field Lecture 18: Coasts Page 32 of 34 Lecture 18: Coasts Seawall Designed to: • Provide rigid barrier against coastal erosion • Fix shoreline position • Stabilize coast Lecture 18: Coasts Page 33 of 34 Lecture 18: Coasts Bulkhead (Harbor Structure) Break Lecture 18: Coasts Page 34 of 34 Lecture 18: Coasts ...
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This note was uploaded on 04/16/2011 for the course GEOL 320 taught by Professor Mathewson during the Spring '11 term at Texas A&M.

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