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 Sand movement

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

Erosional-type shore Headlands eroded Wave-cut cliffs 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 FIG. 11.11b 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

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 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 Fig. 11-15

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 Jetty similar 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

Erosion enhanced seaward of wall Seawall destroyed Beach narrowed 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