Coastal Erosion
Factors:
Waves
Wind
Currents
Sea Level Rise
Human Modification of the Coast
Waves-energetic waves result from strong winds and storms. Larger waves produce faster longshore currents.
Low lying sandy coast will experience sediment washoever and sediment carried off shore into deeper water
Post-storm beach is usually narrower and steeper than before the storm.
Wind-causes dune fields to migrate landward (usually slower than washover events)
Sea level rise (.25-.5 meters/100 yrs) is causing beach erosion in many locations
Local subsidence (Hawaii due to moving away from hotspot, Louisiana due to compaction of the deltaic sediment)
Example from text: typical beach has a slope of about 1:50 (1 meter rise for every horizontal distance of 50 m). If sea level is rising at 10 mm/yr or about 1 m/100 years, then the coastline would retreat by 50 meters in 100 years. Think about some of the development we have seen along the world’s coastlines. How many buildings are within 50 meters of the shore?
Human modification of the coastline
Seawalls-reflect much of the energy of the waves back into the ocean, carrying sediment with them
Groins-trap sediment from continuing along the natural longshore drift
Breakwaters-trap sediment along the coast-may impede longshore drift of sediment.
Examples:
Mississippi River Delta (erosion and subsidence of the delta)
Ganges River Delta and Bangladesh (erosion and subsidence of the delta)
“Washaway Beach” in Washington (redirection of tidal channel)
Barrier Islands in Texas (damming of Brazos River, Jetties along river mouth, hardening of the island to protect it