Shorelines

Coastal Sediment Budget The 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

Coastal Sediment Budget Gains include sediment: transported onshore by waves and longshore currents. newly eroded along a shoreline. delivered by rivers. Fig

Coastal Sediment Budget Losses include sediment: transported offshore by waves and longshore currents. sand blown further onshore by wind. Fig

Coastal Sediment Budget Human activities can increase or decrease sediment supply. damming rivers decreases supply ag. erosion increases supply Fig

Barrier Islands Barrier islands – long, narrow islands parallel to coast. Wind and waves pile up sediment to form islands. Fig

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

Barrier Islands Longshore currents seal inlets over time. Fig

Barrier Islands Inlets 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 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

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

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

Deltas Delta – sediments deposited when river enters ocean/lake. water velocity decreases, sediment load deposited Fig

Deltas Main river channel splits into distributaries. Sediments deposited over broad area. Waves rework sediments (islands and bays).

Delta Migration Position 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?