Chap 12 Coasts
The Coast
Is the area of a continent or island that is affected by the adjacent ocean or sea. These areas are affected by the tide, storms that come from the sea, and the moderating influence of the ocean. The coast can extend several 10s of kilometers inland.
The shore is the area where the sea meets the land, and is generally less than 1 km wide. The shore is either a rocky area or a sandy beach.
Classifying the Coast
One way of classifying the coast is according to tectonic position 1. Active coast – near the leading edge of moving continental plates 2. Passive coast – trailing the edges of the plates Sea level changes are important in classification of coastal areas
The most popular classification combines both elements This divides the coastal areas in: 1. Primary coast – young coast with terrestrial influence, stable since the last ice ages, retain evidence of the process that produce it 2. Secondary coast – older than primary and has been significantly changed since the last changes in sea level, little is any evidence of the process that produce them
Primary Coast
Characteristics: 1. Rough and irregular 2. Features of activities like scouring of glaciers, deposition of sediment, and volcanic activity are present
Rivers empty into the sea through a mouth which may have one of 2 shapes: 1. the river mouth is extended into the sea as a delta. (triangular)(f.12.4) 2. the river mouth is cut back into land as an estuary or fjord (pronounce the 'j' as a 'y'). ( f.12.3) The principle difference between the two main shapes is how much sediment the river is carrying.
Estuaries were once valleys carved out by rivers during the last glacial maximum. Since sea level has risen over the last 18,000 years, these river valleys became drowned. Fjords are a type of estuary that was carved by glaciers during the last glacial maximum. Since sea level rise, these glacial valleys became drowned.
For a delta to maintain its size or grow, the river must carry enough sediments to keep marine process in check The combined effects of waves, tides and river flow determines the shape of the delta Fig 12.6 shows the classification of deltas according to this factors
Volcanic Coast
Coast of volcanic island will consist of lobed lave flows extending seaward Concave may also occur when a small volcano explodes or collapses and fills with seawater (f. 12.8)
Earth Movement
Coast can coincide with plate boundaries If the crust moves downward, it creates a steep escarpment If the crust moves upward, it exposes a previously submerged area (f.12.9)
In general, these are areas of erosion, and many of the features we see at rocky coasts are formed by waves eroding the uplifted rocks. There are 4 features caused by erosion at rocky shores ( f. 12.12):
1. sea stacks - isolated pillars of rock left standing after all the rock around it has been eroded away 2. arches - rocks from which the bottom has been eroded away by waves
3. blowholes - cracks and crevices in rocky coasts through which wave water is forced upward 4. wave-cut platforms and -notches - the notches are cut-outs at the base of a rocky cliff; the platforms are flat-lying areas
The rocky part of land sticking out toward sea is called a headland. The indentation into land where sand can accumulate on a beach is called a cove beach.
Below is a sea stack and an arch. These features are caused by wave erosion.
Pounding waves can erode cracks and crevices into rocky cliffs. When waves break at the base of these cliffs, the water is forced up into the cracks, forming blowholes.
Here is a wave-cut notch (the indentation at the base of the cliff) and the flat-lying wavecut platform at the base of the cliff, above the waves. Both indicate that sea level in this area was higher, and have since been uplifted by tectonic or isostatic activity.
Beaches
The principle feature of a sandy shore is the beach (f. 12.15). Zone of unconsolidated particles that cover part or all of the shore A typical sandy beach has a number of features, some of which are formed by deposition, and some by erosion.
1. 2. 3. 4.
Agents of deposition include: the wind (on land) waves longshore currents tidal currents
If you slice a beach, like a giant slice of cake, the following profile would appear (f. 12.16) 1. The Backshore is usually above water except during big storms or unusually high tides.
• a high-standing area that is either a sea cliff (rocky shores) or a set of sand dunes • sand dunes are deposited by the wind and are usually in motion (shifting sands) • The dunes are important areas for sand storage
A flat area called the berm, a good place to lay your beach towel because it is usually above water except when there is a storm or very high tide • The Foreshore area, which is where all the wave action takes place
• Note the foreshore area is underwater at high tide, and above water at low tide. • This is a tough environment to adapt to. • The Offshore area, which is underwater all of the time, and also below wave base most of the time (except during big storms). • This is where longshore currents move sand around, forming longshore bars.
If you climb the sand dunes (don't! they are usually vegetated with a beautiful grass, which are protected by law) or the sea cliff, and keep heading away from the ocean, you are heading landward.
The beach profile varies from season to season. In general, there is more deposition, and a smoother profile during the summer when waves are gentle and big storms are scarce. Conversely, there tends to be more erosion in the winter when many large storms lash the coast with big waves.
Longshore currents (12.21-22) are local currents that are largely generated by waves that strike the shore at some angle (not parallel). This pushes the water into a current that parallels the shoreline, the longshore current. Longshore currents are important movers of sand at the shore. They are one of the causes of sediment drift, which is the constant motion of sand grains parallel to the shore.
Rip Currents
Another effect of waves striking the shore at an angle is that water can be piled up on the beach (12.19) This is particularly likely after a big storm and big waves have brought lots of water to the beach. The pile of water flows back to the ocean in a fairly narrow (10s of feet wide) and strong current, the rip current.
It usually flows between longshore bars. Sometimes rip currents are visible as a patch of muddy water (they're carrying sediment in that case), but often they are not visible to swimmers. If you feel yourself moving seaward faster than you would like, don't panic! Swim parallel to shore and you eventually get out of the current.
Estuaries
Estuaries are environments similar to that of a lagoon, particularly with regards to salinity This is a curious place, where freshwater meets the ocean, and a peculiar type of water circulation occurs here called 'estuarine circulation Freshwater from the river flows out into the estuary, and being less dense than seawater, floats on top of the seawater.
The outgoing river water drags the underlying seawater with it, and this causes seawater to flow into the estuary, up to a point ( f.12.30) This results in a stratified estuary, with little mixing of the overlying fresh- and the underlying salt waters. The point to which the salt water wedge flows (hand below) is the same point at which the river water loses contact with the river bottom.
At this point sediment from the river is deposited, and any sediment from the sea water is also deposited. In places where the tide is vigorous, the fresh- and salt waters are mixed by internal waves. This results in no stratification, and a mixed estuary. Estuaries are also good places to anchor a ship, since wave energy is minimized.
Along our east coast, most of the cities in existence by the time of the Revolution were built along estuaries: Charleston, Richmond, Philadelphia, Savannah, Baltimore, Newport... Millions of people live around estuaries, and this puts a lot of pressure on these ecosystems.
Human Activities
1. Construction of dams reduces river sediment inflows. 2. Extraction of sediments from rivers alters the sediment inflows. 3. Extraction of fossil fuels and water causes subsidence.
Consequences
1. Loss of natural habitats and biodiversity 2. Loss of beach areas and their recreational value. 3. Potential loss of infrastructure and buildings. 4. Reduction of natural protection against floods
Humans attempt to stabilize the entrance position by (f. 12.34): 1. building structures on the coast: a. groins, seawalls, long jetties, 2. increase navigable depths by dredging straight channels offshore.
In the coastal region of the southeastern U.S., beach restoration has become the method of choice for alleviating threats to property arising from erosion, this method is controversial due to economic and environmental concerns.
The Santa Barbara, California, breakwater is a classic case study of the effects of shoreline engineering. To create a harbor for small boats, an Lshaped breakwater was completed at Santa Barbara in 1929.
The original design left a gap in the west side, almost immediately sand began to pour through this opening to fill the harbor.
The breakwater that protected the boats from waves also protected the beach from the wave and current action that are essential to maintaining the coastal sediment budget. To halt the deposition within the harbor, the shoreward leg of the breakwater was completed in 1931.
This literally "stop-gap" measure resulted in deposition along the seaward side of the breakwater which filled the area west of the shore leg in less than two years. The University of California, Santa Barbara's athletic stadium is built on this "new" land.
At Miramar Beach, four miles east of the harbor, erosion completely removed the 200-foot wide sandy beach by 1938, leaving behind only the rubble that was too large to be moved.
Six miles farther east at Sandyland, the beach was cut back 245 feet during 1940, undermining and destroying the cottages built upon it. Less severe erosion occurred as far along the coast as Rincon Point, about 20 miles east of the harbor.
The lack of sufficient sand on the downdrift shoreline destroyed valuable beaches and waterfront property The only solution was to move the sand from the harbor to the beaches, to artificially recreate the longshore drift with dredges and pumps.
Dredging began in 1938 and has taken place at two- to three-year intervals ever since. The beaches have been partially replenished and the harbor is still in use The annual bill for dredging the harbor has varied from $350,000 to $1.7 million, depending on the amount of sand moved.
Santa Barbara is only one example of the type of problem that has occurred repeatedly with artificial harbors in California and elsewhere along coastal areas. Dredging to prevent the closure of these harbors was costing the Federal government $7 to $8 million annually in the early 1980's.