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Unformatted text preview: Beaches Be Beach Composition Beaches are composed of whatever is available. They’re a product of wave reworking. Types include: Gravel (pebble to boulder-sized) Sand and Gravel Sand (terrigenous, from rocks) Shells (gastropods & bivalves) Carbonate (reef material, oolites) Volcanic (black and green sands) Jasper Beach, Maine G ra vel B ea ch es High Latitudes Gravel Can be Sand & Gravel or entirely boulders Approximately ½ of the world’s beaches consist of gravel Gravel derived from glacial sediments or cliff erosion Queensland, Australia Bowling Ball Beach
• T h e b o u l d e r s a r e a p r o d u c t o f th e p r e c i p i ta ti o n o f C a l c i u m C a r b o n a te (CaCO3) within the underlying s a n d s to n e s . • T h e s e a r e c o n c r e ti o n s c o n s i s ti n g o f c e m e n te d s a n d g r a i n s . • A s th e s a n d s to n e i s b e v e l e d b y th e w a v e s , th e s e c o n c r e ti o n s a r e e x p o s e d a t th e s u r f a c e a n d r e m a i n b e c a u s e th e s e a r e m o r e r e s i s ta n t to e r o s i o n th a n th e s a n d s to n e . Mid -Latitude Beaches Sand Derived from Rivers Cape Cod- glacial outwash Continental Shelf California- cliff erosion Sand composed of quartz, feldspar, and FeMg, iron-oxides (Fe2O3) Derived from most resistant minerals in Granite Back Beach-Dune Area Composed of low dunes vegetated by Sea Oats Holden Beach, NC Volcanic Sand Volcanic
Black and green sand Hawaiian Islands Iceland Olivine Sand on the Island of Hawaii Low Latitude
Biogenic Sand Composed of Shell, Coral and terrigenous Sand Includes oolites precipitated from seawater oolites Cobble Beaches Cobble Some English beaches composed of flat, roller, spherical, brick-shaped gravel Rounded Why? Beach Morphology
Function of wave conditions which is controlled by Storm versus Non-Storm Conditions Need to know something about Waves D e e p W a te r W a v e M o d e l Wave Conditions Erosional-Depositional Beach Profile Governed by Wave Steepness = H/L S lo p e = r ise/ r u n Steep wave causes beach erosion Low steepness wave caues beach depositional
L Storm waves
STEEP H Large Wave Steepness Swell waves
FLAT L Low Wave Steepness H Accretionary Beach Profile Fore Dune Ridge- sand dune that backs the beach Berm- soft sand, where you spread your towel Beach Face- steep portion of beach where most wave energy is expended Low Tide Terrace- flat hard sand, where you play Frisbee, football, soccer, baseball, run Beach Morphology Angular Wave Approach Approach Lowtide Terrace Beachface Berm Beach profile Storm Erosional Profile
Waves get steeper primarily due to higher wave heights Dune is eroded and scarped Berm is eroded Some sand is deposited on the Low-tide terrace Sand is transported seaward to an offshore bar Storm Erosional Profile
Waves get steeper primarily due to higher wave heights Dune is eroded and scarped Berm is eroded Some sand is deposited on the Low-tide terrace Sand is transported seaward to an offshore bar West Coast East Coast
Pre-storm Erosion StormProfile Important Points:
• S to r m e r o d e s d u n e / c l i f f Post-Strom Profile • S a n d i s tr a n s p o r te d o f f s h o r e a n d a lo n g s h o r e • S o m e o f th e s a n d m o v e d o f f s h o r e i s d e p o s i te d o n th e l o w ti d e t e r r a c e • E r o s io n a l p r o file ex p en d s W a v e E n er g y • R e b r o k e n w a v e e x p e n d e d a l o n g a n i n c r e a s e d a n d w i d e n e d i n te r ti d a l z o n e Process of Beach Rebuilding Process
Occurs when wave steepness decreases H/L, smaller waves Sand moves onshore from offshore bar 1. Onshore migration of Ridge and Runnels 2. Vertical Accretion 3. Construction of Aeolian Ramp Onshore Migration of Ridge and Runnel Ridge Runnel
Ridge slipface Runnel Ridge Sand transport • Waves breaking over the ridge carry sand and deposit it on the slipface of the ridge • Ridge migrates onshore by the incremental build-up of sand on the ridge • Runnel forms in front of the ridge as the ridge migrats up the beachface Multiple Ridge and Runnel System • Note that ridges are fronted by shallow runnels • Along individual ridges runnel outlets allow water (from the breaking wave) to escape back to the sea. • Eventually the ridges migrate and weld to the upper beach forming a berm Vertical Accretion Vertical 3. A eo lia n R a m p
Wind Blown Sand Post Storm Dune Scarp Aeolian Ramp rebuilds FDR R h izo m es Commonly, Rhizomes from existing dune plants allow recolonization of aeolian ramps by vegetation In New England, the grass is called American B ea ch G r a ss G ra vel B ea ch es
High Latitudes Gravel Can be Sand and Gravel or entirely boulders Ca Approximately ½ of the world’s beaches consist of gravel Gravel derived from Glacial sediments or Cliff erosion Mid -Latitude Beaches Sand Derived from Rivers Continental Shelf Sand composed of quartz, feldspar, and FeMg, Fe2O3 Derived from most resistant minerals in Granite ...
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This note was uploaded on 04/14/2011 for the course GEO 142 taught by Professor Fitzgereld during the Spring '11 term at BU.
- Spring '11