The Coast: Shoreline Processes

Sand movement along beach Perpendicular to shoreline (toward and away) Swash and backwash Parallel to shoreline (up-coast or down-coast) Longshore transport

Swash and backwash After wave breaks, up rush of water (swash) on beach Sediment moved toward land Backwash Water returns to ocean Sediment moved away from shore

Swash and backwash Light wave activity Fair weather Summertime beach Swash dominates Sediment moved toward shore Wider beach Fair weather Summertime beach

Swash and backwash Heavy wave activity Sand forms offshore sand bars Backwash dominates Sediment moved away from shore Narrower beach Sand forms offshore sand bars Stormy weather Wintertime beach

Seasonal Change Summer: waves have a short wave height and a long wave length. Low energy Swash dominates Summer berm (builds the beach) Erodes offshore sandbars Winter: wave height increases and wave length decreases. High energy Backwash dominates Winter berm (erodes the beach) Builds offshore sandbars

Longshore current The longshore current occurs when waves strike the beach at an angle. Wave refraction causes water and sand to move parallel to shore Zigzag motion in surf zone Longshore current – a current running parallel to shore caused by waves approaching the beach at an angle Longshore transport (drift) – movement of sediments parallel to shore, driven by wave energy

longshore current longshore drift swash backwash ______________(net movement of water) longshore current longshore drift swash backwash

longshore current longshore drift (longshore transport)

Longshore transport Millions of tons of sediment moved yearly Direction of transport changes due to wave approach In general, sediment transported southward along Atlantic and Pacific coasts of U.S.

Rip Currents A very strong, narrow surface current that flows seaward through the surf zone caused by the escape of excess water Not rip tides! High energy beaches Swim parallel to shore 15

Coastal Cells Sections of coast in which sand input and sand output are balanced are referred to as coastal cells. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Human Interference on Coastal Processes Structures built to decrease coastal erosion and interfere with sand movement Often results in unwanted outcomes Some structures may increase wave erosion

Groins and groin fields Human Interference on Coastal Processes Groins and groin fields

Human Interference on Coastal Processes Jetties

Human Interference on Coastal Processes Breakwaters

Human Interference on Coastal Processes Seawalls

Human Interference on Coastal Processes Importing Sand