lec 13: Coastal environments

• Coastal Zone


  Portion of the ocean affected by processes that occur on or near land (e.g., runoff, tidal currents, waves)

  
• Open Ocean

  Beyond the coastal zone
• Coastal zone is usually defined as the area between coastline and shelf break

  
• Coastal zone is 10% of total area of the oceans and harbors 99% of world fishery catch
• Activities in the coastal zone

  • Harbors
  • Recreation
  • Mining of sand and other minerals
  • Oil exploration
  • Waste disposal
• Coastal Waters


  Relatively shallow-water areas that adjoin continents

  
• Because of their proximity to land, coastal waters are influenced by processes that occur on or near land (e.g. river runoff or tidal currents) more than the open ocean

  
• Characteristics of Coastal Waters

  • Temperature and salinity are more variable
  • Freshwater from river runoff + small vertical mixing → well-developed halocline (low to high salinity)
  • No river runoff + dry winds blowing from land + small vertical mixing → well-developed halocline (high to low salinity)
  • Strong vertical mixing: isohaline (more consistency)

  
• Characteristics of Coastal Waters - Temperature

  • Low- and high latitude coastal waters → isothermal ("same temperature")
  • Mid-latitudes → well-developed thermocline (surface water warmer during summer and cooler during winter)
  • Vertical mixing may make thermocline less pronounced

  
• Coastal Environments

  • Estuaries
  • Fjords
  • Lagoons
  • Beaches

  
• Estuary


  Partially enclosed coastal body of water in which freshwater from runoff dilutes salty ocean water
• Origins of Estuaries

  • Coastal-plain estuary (former river valley now flooded with seawater)
  • Bar-built estuary (lagoon separated from open ocean by sand bars or barrier islands)
  • Tectonic estuary (faulting or folding of rocks creates a restricted down-dropped area which was flooded)
  • Fjord (former glaciated valley now flooded with seawater)
• Estuarine Circulation


  1. Surface flow of less dense freshwater toward the ocean

  2. Opposite flow below the surface of salty seawater into the estuary

  3. Vary with tidal currents mixing, wind-driven wave mixing, shape and depth of the estuary, rate of freshwater discharge, friction, Coriolis effect, human-made structures

  
• Types of Estuarine Circulation

  • Vertically mixed (salinity uniform from surface to bottom)
  • Slightly stratified (two water layers can be identified, separated by zone of mixing)
  • Highly stratified (deep-water layer has uniform open-ocean salinity)
  • Salt wedge (wedge of salty water intrudes from the ocean beneath the river water)

  
• Estuaries are important breeding grounds and protective nurseries for many marine mammals
• Estuaries also support shipping, logging, manufacturing, waste disposal, and other activities damaging to the environment
• Estuaries can be threatened both where human populations are modest (e.g. Columbia River estuary) or are large (e.g. Chesapeake Bay)
• Columbia River Estuary

  • Salt-wedge estuary at its entrance to the Pacific ocean
  • Flow of river and tides drives a salt wedge as far as 42-km upstream, raising river level by 3.5-m
  • When tide falls, the huge flow of freshwater creates a freshwater wedge that can extend 100's of kms into the Pacific
  • Affected by interference with floodplains: dikes to prevent flooding of agricultural areas, logging has increased sediment load into the river, multiple hydroelectric dams have altered ecosystem, no salmon ladders to help fish "climb" around dams to reach their spawning grounds at headwaters
• Chesapeake Bay Estuary


  • Fed by 19 major rivers and 400 creeks and tributaries
  • Slightly stratified
  • Seasonal changes in salinity, temperature and dissolved oxygen
  • Leads to development of a strong halocline and pycnocline preventing fresh surface water and saltier deep water from mixing
  • Anoxic conditions below pycnocline in summer as dead organic matter decays
  • Anoxia leads to major kills of commercially important marine animals (e.g. blue crab, oysters)
  • Degree of stratification and extent of mortality of bottom-dwelling animals has increased since 1950s
  • Tied to increased nutrients from sewage and agriculture fertilizers, increasing productivity of algae
  • In drier years with less river runoff, less widespread anoxia because less nutrients are supplied and less stratification
• Fjord

  Flooded glacial valley (u-shaped), tend to be deeper than adjacent seas, long narrow inlet with steep cliffs, sill (or shoal) often present near mouth, located on rugged mountainous coastlines
• Fjords


  • Sills substantially modify estuarine circulation patterns
  • Can create isolated deep water landward of sill
  • Deep water is dense (saline) and has limited mixing with layers above
  • Deep water can become anoxic (oxygen deprived)
  • Renewal of deep water determined by exchange flow over sill
• Lagoon


  Protected, shallow body of water, landward of barrier islands, form in bar-built estuaries, restricted circulation with the ocean, results in 3 zones: freshwater zone, transitional zone, saltwater zone

  
• Beach


  Deposit of the shore area, entire active areas of a coast that experiences changes due to breaking waves, wave deposited accumulation of sediment located at the shoreline, can be thought of as material in transit along the shoreline, area of beach above the shoreline is the "recreational beach"
• Beach Glossary


  • Shore
  • Coast
  • Coastline
  • Backshore
  • Foreshore
  • Shoreline
  • Nearshore
  • Offshore
  • Beach
  • Berm
  • Beach face
  • Longshore bars
• Shoreline


  The water's edge (migrates with tides)

  
• Shoreline


  • Can be thought of as material in transit along the shoreline
  • Area of beach above the shoreline is the "recreational beach"
• Beach Glossary

  • Shore = zone between lowest tide level and highest elevation affected by storm waves
  • Coast = from shore to farthest point where ocean-related features can be found
  • Coastline = boundary between shore and coast
  • Backshore = above high tide, covered with water only during storms
  • Foreshore = zone between low and high tide
  • Shoreline = water's edge (migrates with tides)
  • Nearshore = from low tide shoreline to breaker line
  • Offshore = waves rarely affect the bottom
  • Beach = entire active area of a coast that experiences changes due to breaking waves
  • Berm = dry, gently sloping, slightly elevated margin of the beach (dry sand)
  • Beach face = wet sand, exposed during low tide
  • Longshore bars = sand bars parallel to the coast

  
• Dissipative beach


  • Almost flat
  • Spilling breakers
  • Broad surf zone
  • Fine sediment
• Reflective beach

  • Steep
  • Surging breakers
  • Narrow surf zone
  • Coarse sediment
• Beach composition dependent on locally available material
• Movement of sand on the beach occurs both perpendicular to the shore and parallel to the shore
• Movement of sand perpendicular to the shore

  1. Toward shore

  2. Away from shore
• Movement of sand parallel to the shore

  1. Upcoast

  2. Downcoast
• Swash


  Water rushes up the beach
• Backwash


  Water drains back to the ocean
• Swash and backwash


  Transport sediment up and down the beach face perpendicular to the shoreline
• Whether swash or backwash dominate

  Determines whether sand is deposited or eroded
• Low-energy waves

  Backwash is reduced, net transport of sand up to the beach face, well-developed berm
• High-energy waves


  Beach saturated with water from previous waves, backwash dominates, net transport of sand down the beach face, erodes the berm, sand accumulates beyond where the waves break, forms sand bars
• Light/heavy wave activity


  May alternate seasonally