Chapter 11: The Coast: Beaches and Shoreline Processes

Coastal region   Shore between low tide and highest elevation affected by storm waves Coast from shore to farthest inland ocean features Fig. 11-1

Beach    Actively changing Shore: backshore, foreshore, nearshore, offshore Berm, beach face, longshore bar, longshore trough Fig. 11-1

Beach composition and shape    Locally available material Coarser sediment  Beach steeper Finer sediment  Beach more gentle

Sand movement  Swash and backwash    Sediments moved up and down beach face Smaller, low energy waves sand moved up beach face  Summertime beach Larger, high energy waves sand moved off shore  Wintertime beach

Sand movement  Longshore current    Longshore drift or transport Parallel to shore Increasing strength     More wave energy Higher wave frequency Steeper beach Greater angle

 Longshore current main way sediments moved along coasts  Southward along both U.S. coasts most of the time

Erosional-type shore    Headlands eroded   Sea arches Sea stacks Wave-cut cliffs  Sea caves Tectonically active coasts  Example Pacific coast U.S.

Depositional-type shores  Sediments re-distributed by ocean processes  Typical in tectonically passive areas

Depositional features    Spit Tombolo Bay barrier or baymouth bar Fig. 11-7

Barrier islands        Ocean beach Dune Barrier flat High salt marsh Low salt marsh Lagoon Rising sea level pushes barrier island landward Fig. 11-10

Deltas    Rivers deposit sediment at coast If more fluvial influence than ocean influence  Bird’s foot delta If more ocean influence than river influence  Arcuate delta FIG. 11.11b

Shepard classification of coasts  Primary   Geologically youthful Controlled by non-marine processes  Examples  “Drowned” river or glacial valleys  Volcanic coasts  Coasts shaped by recent tectonic movements

Secondary coasts  Geologically mature   Controlled by marine processes Tectonically passive  Examples  Straightened coasts  Coral reefs  Marsh grass Fig. 11-13f

Emergent coasts  Uplift or sea level drops  Once marine is now land  Marine terraces  Stranded beach deposits

Submergent coasts   Subsidence or sea level rises Once land (or shallower) is now underwater (or deeper)    Drowned beach deposits Drowned river/glacial valleys Submerged dunes

Tectonic and isostatic changes    Tectonic uplift and subsidence   Larger scale: major parts of continents Smaller scale: local deformation Isostatic adjustments to loading   Sediments or ice Regional and local effects Affect sea level

Eustatic changes in sea level Fig. 11-15     Global Changes in spreading rates of ocean floor Changes in ice volumes Changes in ocean temperature  Global warming  Sea level rises  Ice melts, ocean warmer

Atlantic coast of U.S.

 Mainly submergent  Barrier islands  Glaciers shaped some shorelines  Eroding coasts  Subsidence in most areas

Gulf coast of U.S.

 Mainly submergent  Barrier islands  Mississippi delta  High rates of erosion  Tectonic subsidence

Pacific coast of U.S.

 Mainly emergent  Less erosion  Mainly rocky shoreline  Dams on rivers beach starvation  Tectonically active

U.S. coastal erosion, deposition Fig. 11-17

Hard stabilization   Groin and groin field  Perpendicular to shore   Traps sand between groins Interrupts longshore current Jetty similar   Built to protect harbor entrance Sand on upcurrent side

 Breakwater  Parallel to shore  Offshore  Deposition behind breakwater

    Seawall   Built on beach Parallel to beach Erosion enhanced seaward of wall Seawall destroyed Beach narrowed Fig. 11-25

Alternatives to hard stabilization  Limit construction near shore  Relocate businesses/homes destroyed by wave erosion

End of Chapter 11: The Coast: Beaches and Shoreline Processes