Coasts and the Coastal Processes Coasts

can be active (i.e., plate boundaries) or passive Following are examples of coastal erosion by waves:

(i.e., non-tectonic), and erosional – stream erosion, wind abrasion and

glacial activity (e.g., fjords), or– erosion by waves and currents (e.g.,

effects of surging, plunging and spilling breakers) and The example below is that of a delta:

depositional (i.e., beaches, deltas etc).

Deltas are of three types: (a) river-dominated (e.g., Mississippi, Danube,

Po); (b) wave-dominated (e.g., Nile, Niger, São Francisco); and (c) tide-dominated (e.g.,

Colorado, Ganges-Brahmaputra, Mekong).

Longshore current and littoral drift Wave interference creates a “longshore current” parallel to the

shoreline (i.e., along the shore). This longshore current straightens the shoreline, and carries the

sand in littoral drift that it may dump in a submarine canyon or use for creating a barrier island and the like features.

Wave refraction along a straight coast

B e a c h

Wave touches bottom here and slows down

Wave refraction along a straight coast

B e a c h

Wave touches bottom here and slows down

1940 1963

XX

N

Shown on the left, for instance, are two pictures of Sandy Beach (shown by the cross here), NJ, one taken in 1940 and the other in 1963. They show that the longshore current here flows to the north — notice how littoral drift has built a barrier island here.

These two cartoons below show how longshore current straightens the shoreline by eroding the headlands and filling the bays by this eroded material

As shown on the right, berms form above the highest wave activity, beachface (or the backshore) always lies above water,

and foreshore or tidal terrace is only exposed at the low tides. Beaches

Beach bearing coasts have backshore, foreshore and offshore regions that are defined mainly by the tidal heights and shore geometry and thus have tidal flats or tidal terraces.

Sand stays on the beach in the tropics, where the thermocline is permanent.

Sand at the temperate latitudes is conti-nually recycled between the shore and the surf zone so that the longshore bar grows at the cost of the berm here during winter, when wave activity is strong, but berm grows at the cost of the longshore bar insummer, when the wave activity is weak. Boomer Beach, San Diego, CA, seen in summer (left) but gone in winter (right).

Human interference often exacerbates coastal erosion: Breakwater wall grows beach behind it,

intensifies coastal erosion on the flanks. Groins produce beach-growth upstream of

longshore current, shifting erosion down-stream.

Seawalls often end up eroding the beach that they were initially designed to protect.

Original shore-line

New shoreline

Longshorecurrent

Breakwater WallWave

motion

Bea

ch b

uild

-up

behi

nd

the

brea

kwat

er w

all

Beacherosion up-

stream of the breakwater

wall

Original shore-line

New shoreline

Longshorecurrent

Breakwater WallWave

motion

Bea

ch b

uild

-up

behi

nd

the

brea

kwat

er w

all

Beacherosion up-

stream of the breakwater

wall

Orig

inal

sho

relin

e

Net longshore

current

Sand deposited on groin’s upstream side

Erosion downstream by the sand-starved water

Groin

Direction of wave motion

Orig

inal

sho

relin

e

Net longshore

current

Sand deposited on groin’s upstream side

Erosion downstream by the sand-starved water

Groin

Direction of wave motion

Orig

inal

sho

relin

e

Net longshore

current

Sand deposited on groin’s upstream side

Erosion downstream by the sand-starved water

Groin

Direction of wave motion

Notice how waves reflecting off the seawall at Galveston, TX, have carried away much of the beach sand.