Beaches

What is a beach? Area along coast where sediment accumulatesArea along coast where sediment accumulates Coastal zones begin when waves interact with seafloorCoastal zones begin when waves interact with seafloor

Beaches Shoreline Divisions Littoral zone Littoral zone oShoreface oForeshore oBackshore – Part of coast that stretches from land plants to where waves start to break – Permanently covered by water – Exposed at low tide and covered at high tide – above mean high tide – only affected by storms

Beaches Shoreline Divisions Divided by wave action: Breaker zoneBreaker zone – Where waves become unstable and break Surf zoneSurf zone – Shallower waves move up beach face Swash zoneSwash zone – alternately covered by “swash” and uncovered by “backwash”

Beaches Sediment Profile Longshore barsLongshore bars BermBerm – Flat-top ridge of backshore Berm crestBerm crest – High point on a beach Beach faceBeach face – Sloping portion of the beach – In winter, sediment from berm is transported below breaker zone

Beaches Types Shape and structureShape and structure CompositionComposition SizeSize ColorColor wide narrow Quartz Cobble Pink sand Shell sand Black sand

Beaches Summer and Winter Summer Summer Winter Winter – Erosive storm waves – Gentle waves – Sand carried seaward to offshore sandbars – Narrow, rocky beach – Carry sand to shore but too weak to carry back to sea – Wide, sandy beach summer winter

Beach Processes Refraction Wave crests approach shore at an angleWave crests approach shore at an angle Energy focused on headlands  erosionEnergy focused on headlands  erosion Wave rays travel perpendicular to wave crestWave rays travel perpendicular to wave crest Wave bendsWave bends Slows over shallow waterSlows over shallow water

Beach Processes Rip Currents Incoming waves create underwater sandbar close to shoreIncoming waves create underwater sandbar close to shore Waves cause sandbar to collapseWaves cause sandbar to collapse Excess water is pushed through gapExcess water is pushed through gap Creates a strong, narrow current away from shoreCreates a strong, narrow current away from shore

Beach Processes Longshore Current and Transport Longshore current moves parallel to shoreLongshore current moves parallel to shore – Swash moves sediment onto beach at an angle – Backwash moves sediment down perpendicular to shore Longshore transport moves sediment along shore by LS currentLongshore transport moves sediment along shore by LS current Zig-zag motion has an overall net transportZig-zag motion has an overall net transport

Beach Dynamics Deposition and Erosion DepositionDeposition ErosionErosion – Reshaping of coastline by wave action – Coasts build out by waves and currents Sandy shore Rocky shore – Little incoming sand, usually no major river – Accumulation of sediment – Sediment is removed

Beaches Depositional Features Sand spitsSand spits Barrier islandBarrier island – Long sand island enclosing a bay or lagoon Bay mouth barsBay mouth bars – When a sand spit closes off a bay – Longshore current deposits sand in a line downcurrent

Beaches Erosional Features ArchesArches TomboloTombolo – Sand bar connecting to a rock or offshore outcrop StacksStacks – Offshore column formed from collapsed arch – Weaker rocks erode Arch Stack Tombolo

Beaches Erosional Features Wave cut benchWave cut bench CliffCliff – Cliff bottom attacked by waves – Waves undercut cliff to form wave cut notch – Overhanging rock collapse – As cliff retreats, area of flat rock exposed Cliff Wave cut bench

Beaches Erosional Features Marine terraceMarine terrace – If sea level lowers or land uplifted, wave cut bench becomes marine terrace

Beaches Human Impacts Sea wallsSea walls BreakwatersBreakwaters – Wall parallel to shore – Reduce wave energy – Interrupts beach drift JettiesJetties – Designed to maintain navigation routes GroinsGroins – Wall perpendicular to shore – Stops erosion – Erodes downcurrent side – Parallel concrete structures – Absorbs wave energy – Actually promotes erosion – Erosion and deposition

Beaches Human intervention Beach nourishmentBeach nourishment DredgingDredging – Import sand into coastal areas – Underwater excavation to clear waterways before after Miami Beach