Shorelines

Coastal Sediment BudgetThe sediment budget determines whether a beach will shrink or grow.

• If sediment gain is greater than loss, a beach becomes wider (and visa versa).

Fig. 19.58

Coastal Sediment Budget

Gains include sediment:

• transported onshore by waves and longshore currents.

• newly eroded along a shoreline.

• delivered by rivers.

Fig. 19.26

Coastal Sediment Budget

Losses include sediment:

• transported offshore by waves and longshore currents.

• sand blown further onshore by wind.

Fig. 19.26

Coastal Sediment BudgetHuman activities can increase or decrease sediment supply.

• damming rivers decreases supply

• ag. erosion increases supply

Fig. 19.28

Barrier IslandsBarrier islands – long, narrow islands parallel to coast.

Wind and waves pile up sediment to form islands.

Fig. 19.29

Barrier Islands

Importance of barrier islands:• major cities (Atlantic City,

Miami, Galveston) built on them• protect mainland from

storm surge• recreation• lagoons behind islands are

marine nurseries

Barrier Islands

Constantly changing (human time scale).

Highest point only a few meters above sea level.

Waves wash over low points during storms.

Storms cut inlets through islands.

Barrier Islands

Changes that occurred over an 8 year period.

1984 1992

Barrier Islands

Longshore currents seal inlets over time.

Fig. 19.23

Barrier IslandsInlets serve as passages from protected marinas to open ocean.

Inlets will close over time unless they are stabilized by jetties.

Disrupt longshore sediment transport.

Fig. 19.30

deposition

erosion

Barrier Islands

Barrier islands will migrate as sea level rises.

Storms erode sand on ocean side and deposit it on bay side.

Fig. 19.48

Shoreline Features

Spits form by longshore currents depositing sediment off the end of a headland.

Fig. 19.22

Shoreline Features

Cape Cod is a glacial moraine that has been modified by longshore currents.

Fig. 19.22

Deltas

Delta – sediments deposited when river enters ocean/lake.

• water velocity decreases, sediment load deposited

Fig. 16.15

DeltasMain river channel splits into distributaries.

Sediments deposited over broad area.

Waves rework sediments (islands and bays).

Delta MigrationPosition of main channel on a delta periodically shifts.

• sediment deposition raises elevation near main channel• compaction lowers elevation away from main channel, • eventually main channel shifts to lower elevation

Delta Migration

What will become of New Orleans when the Miss. River shifts course in the near (geologic) future?