Class 30 -- The OceansClass 30 -- The OceansBEACH PROCESSESBEACH PROCESSES

–Transport of sediments along Transport of sediments along shorelinesshorelines–Human intervention in beach Human intervention in beach processesprocesses

Wave fronts and onshore "swash" -- oblique to Wave fronts and onshore "swash" -- oblique to beach frontbeach front

"Backswash" perpendicular to beach front"Backswash" perpendicular to beach front

Net Result: Net Result: Longshore transport,Longshore transport, Water and sediment move parallel to the Water and sediment move parallel to the

beach frontbeach front

Longshore transportLongshore transport

1.1. It’s strength depends on wave energy…It’s strength depends on wave energy…2.2. Sediment moves rapidly in areas of greater Sediment moves rapidly in areas of greater

wave energywave energy3.3. Sediment dropped where wave energy is lesserSediment dropped where wave energy is lesser

• Erosion occurs in areas of strong wave action Erosion occurs in areas of strong wave action (headlands)(headlands)

• Longshore transport tends to be away from Longshore transport tends to be away from headlands and toward quieter areasheadlands and toward quieter areas

• Both sediment and water converge in recessed Both sediment and water converge in recessed areasareas

• Sediment collects there (beaches)Sediment collects there (beaches)• Water turns back seaward Water turns back seaward • Rip currents move water offshoreRip currents move water offshore

• Beach = temporary accumulation of sedimentsBeach = temporary accumulation of sediments• Seds come from rivers, headland erosionSeds come from rivers, headland erosion• Beach dynamics -- continuous sediment movement Beach dynamics -- continuous sediment movement

due to waves and longshore currents.due to waves and longshore currents.

BEACH PROCESSES -- NaturalBEACH PROCESSES -- Natural

• Most beaches constantly gain and lose sedimentMost beaches constantly gain and lose sediment• The budget can have a surplus or a deficitThe budget can have a surplus or a deficit

BEACH PROCESSES -- NaturalBEACH PROCESSES -- Natural

(1) (1) Dams on riversDams on rivers • Reduce sediment input to coasts; beaches Reduce sediment input to coasts; beaches

become “sediment starved”become “sediment starved”• Longshore transport continues carrying sedsLongshore transport continues carrying seds• Beaches are erodedBeaches are eroded

Human Impact On Beach/coastal ProcessesHuman Impact On Beach/coastal Processes

(2) (2) "Groins" to "Groins" to minimize loss of minimize loss of beach sandbeach sand

• Upwind side: Upwind side: depositiondeposition

• Downwind side: Downwind side: erosion; beaches erosion; beaches starved of their starved of their sediment supplysediment supply

Human Impact On Beach/coastal ProcessesHuman Impact On Beach/coastal Processes

(3) Breakwaters to protect harbors(3) Breakwaters to protect harbors•Reduce wave energy --> deposition in harborReduce wave energy --> deposition in harbor

–Need frequent dredgingNeed frequent dredging–Erosion of downwind beaches- sediment Erosion of downwind beaches- sediment supply cut offsupply cut off

Beach dynamicsBeach dynamics2 Common themes:2 Common themes:

1)1) Sediment transport links all beaches, Sediment transport links all beaches, sediment sources, and sediment traps on a sediment sources, and sediment traps on a coastlinecoastline

2)2) Intervention to address a single demand of Intervention to address a single demand of society often leads to a chain of unforeseen society often leads to a chain of unforeseen consequences due to erosion or deposition.consequences due to erosion or deposition.