Beach Zones Lecture 8 PDF

Beach  Zones   Backshore:  berm  crest  to  cliff  or   dunes   Foreshore:  berm  crest  to  level  of   low  tide   Inshore:  low  tide  level  to  far  side   of  longshore  bars   Offshor...

Beach  Zones   Backshore:  berm  crest  to  cliff  or   dunes   Foreshore:  berm  crest  to  level  of   low  tide   Inshore:  low  tide  level  to  far  side   of  longshore  bars   Offshore:  longshore  bar  to  mud/ sand  contact.   Chapter 10 Page 1  Beach  Zones  Shaped  by   Wave  Ac3on   Features  are  formed  in  response    to   dynamics  of  water  motion.   – Offshore  zone  of  wave  build-­‐up  and   seaAloor  shallowing.   – Breaker  zone—above  longshore  bar:   Convergence  on  the  bottom  builds  the  bar.   – Surf  zone.   – Swash  zone  –  on  foreshore.   Chapter 10 Page 2  Beach  Zones   Berm Off- shore Inshore crest Foreshore Backshore Dunes behind the backshore Chapter 10 Page 3  Longshore  bars  of  the   Inshore   The  beach  is  dynamic  and  does  not   stay  in  one  place.     – Sand  is  transported  on  and  off  the   beach  perpendicular  to  the  shore.   – Sand  is  also  transported  along  the   beach  in  longshore  currents.   Chapter 10 Page 4  The  Geologists’  Beach   Why  is  the  beach  sandy?     – Winnowing  due  to  high  wave  energy.   Why  does  the  sand  stay  in  the  beach   zone,  and  why  does  the  mud  move   offshore?   Hjulstrom’s Diagram Transportation Chapter 10 Page 5  Fair  and  storm  beach  at   NarraganseB  Pier,  RI   Fair weather Winter beach Chapter 10 Page 6  Storm  and  fair  weather   waves  in  the  surf  zone   Waves move sand toward or away from the beach depending on wave size. Fair-weather waves Storm waves Vectors Chapter 10 show water motion and strength ┴ toPage the7 beac  Fair  Weather  Waves   Transport  Sand  toward   the  Beach   http://gore.ocean.washington.edu/fluids/fluids97/1997/carmine/ Chapter 10 Page 8  Beach  –  foreshore  steepens    with   increasing  grain  size,  both  of  which  are  a   response  to  increasing  wave  energy  (and   par9cle  sizes  provided  by  the  source).   Fine  sand   Niger  Delta  beach   Medium  sand   Assateague  Island,  MD   Cobbles   PatreksMördur,   Wellington, NZ Iceland   Chapter 10 Page 9  Types  of  Beaches:   composi3on     Also depends on environ- ment and source of sediment Bonnet Shores, RI glacial outwash, mineral sand Antarctica, glacier. OK, maybe not a Beach! Jewfish Cay: Sand made of carbonate shells. Chapter 10 Page 10  Review   DeAined  the  features   of  the  beach  and  the   surf  zone  that  shapes   the  beach.   The  beach  is  a   dynamic  balance   between  input  and   removal  of  sand.   The  sand  moves   inshore  and  offshore   with  fair-­‐  and  storm-­‐   wave  conditions.   Now,  consider   movement  of  sand   alongshore.   Chapter 10 Page 11  Longshore  Current     The  longshore  current  moves  parallel  to   the  beach  in  the  foreshore  and  inshore.   What  drives  it?  Waves.   Waves  often  approach  at  an  angle  to  the   shore,  but  always  a  small  angle.       Chapter 10 Page 12  Wave  Refrac3on   Shallow  water  wave  velocity:   V  =  √(gd)  ,  where  d  is  depth  and  g  is   gravitational  acceleration.   ! Waves  slow  with   decreasing  depth—   they  “bend”  to  conform   with  bottom   bathymetry.   Chapter 10 Page 13  Longshore  Currents  –  two   mechanisms   Swash  and  backwash  carry  sand  in   a  “river  of  sand”  that  parallels  the   beach.   Chapter 10 Page 14  Longshore  currents   ! Wave  setup  also  can  drive  longshore   currents   ! When  waves  are  parallel  to  the  shore,  there  is   no  well-­‐organized  longshore  current.   ! Water  piles  up  in  the  surf  zone  from  the   repeated  breaking  waves,  especially  where   the  breakers  are  tallest  and  carry  the  most   water.     ! Water  Alows  down  hill,  so  the  water  setup  in   the  surf  zone  will  diverge,  moving  in  opposite   directions  parallel  to  the  shore.   ! Longshore  currents  (L.  C.)  converge  where   the  least  water  is  piled  up  by  the  shortest   waves.  Rip  currents  return  Alow  offshore.   Shore Convergent Divergent L. C. Converg. Surf zone Shortest waves Tallest Shortest ripcurrent Wave Crests ripcurrent Chapter 10 Page 15  Coastal  Water  Movement   Refraction  does  more  than  drive   longshore  currents.   It  focuses  energy  on  headlands  and   disperses  energy  in  coves.   Vectors of wave energy Chapter 10 Page 16  Coastal  Water  Movement   Erodes  the  Coast   ! Here’s  how  this  works;  watch  the  coast   become  straight  due  to  erosion:   Chapter 10 Page 17  Coastal  Water  Movement   This  coast  is  being  smoothed:   – Promontories  are  being  eroded.   – Coves  are  being  Ailled.   Chapter 10 Page 18  Re-­‐cap.  Coastal  Environs     Model  of  surface  sediment   (lithofacies)  on  a  “typical”  shelf  .   Descriptions  of  coastal  environments   of  the  temperate  climate  zone.   – Estuaries   – Deltas   – Beaches   – Barrier  islands   Return  to  the  open  continental  shelf:   models  connected  to  process.   Chapter 10 Page 19  Beaches  are  dynamic   Beaches  Move   – They  move  on  shore  and  offshore         with  waves  –  seasonal  storm  and  fair-­‐ weather  waves.   – Beaches  also  move  downcurrent  due   to  longshore  transport.   The  beach  position  is  a  dynamic   balance  among   – Sediment  supply  (nourish  or  starve),   – Longshore  movement,   – On-­‐  and  offshore  movement,   – Sea-­‐level  and  “land-­‐level”  changes.     What  happens  when  a  beach   moves  shoreward?   – Naturally,  no  structures   Sea-­‐level  rise  or  sediment  starvation   – When  structures  are  involved?   Chapter 10 Page 20  Cape Lookout, NC Longshore     transport   An  example  in  a   natural  setting   An  example  in  an   unnatural  setting.   starvation Chapter 10 Page 21  Miami Beach Threatened beach No more beach at all. Chapter 10 Page 22  Barrier  Island  Zona3on   Islands  made  of  transported  sand.   – Beach  environmental  zone  (facies)   – Dune  zone  (and  associated  facies)   – Upland,  vegetated  zone  (facies)   – Marsh  (facies)   Lagoon or Ocean estuary Chapter 10 Page 23  Longshore  Barrier  Island   Several  theories  about  formation   – Spits  supplied  from  river-­‐mouth  sands  or   other  sources  of  plentiful  sand;     – Longshore  bar  that  gets  big;   – Coastal  sand  ridge  at  low  stand,  moves  inland   with  rising  sea  level;     – All  require  nourishment.   Chapter 10 Page 24  Barrier  Island  Processes   Migrate!   – Along  shore  with  the  longshore  current.   – Landward  with  storms  that  wash  over   sediment  from  the  beach  side  to  the  lee   side.   – Landward  with  rising  sea  level   (seaward  with  regression  of  the  sea).   – Seaward  (landward)  with  sediment   nourishment  (starvation).   Migrating Ocean City, MD landward due to sediment starvation (and maybe sea Retreat level rise). Assateague Island, MD Chapter 10 Page 25  Barrier  Islands   Evidence  for  retreat  landward.   – Presence  of  salt-­‐marsh  peats  seaward  side  of   many  barrier  islands.     – The  peats  are  the  remains  of  salt  marshes   that  once  lined  the  backshore  region,  and   were  buried  as  the  island  moved  shoreward.   Outcropping Marsh Marsh peat – Dramatic   example:  A   guard  house   that  once  sat  in   the  barrier   island  forest  is   now  buried   beneath  the   dunes.     Chapter 10 Page 26  Erosion  by  storms   Washover  events  -­‐  storms   – Move  sand  from  the  beach  to  the  backshore   or  lagoon.  Erosion!  Migration!   – Washover  opens  inlets  or  closes  them.   – Denudes  vegetation,  levels  dunes.     – Particular  problem  if  the  barrier  has  no   modern  source  of  sand  to  rebuild  itself.   Storm-breached barrier island, Laguna Madre Chapter 10 Page 27  Mississippi  Barrier  Islands   ! Mississippi  islands:  Part  of  Gulf  Island   National  Seashore,  owned  and  managed   by  the  National  Park  Service.   ! Cat  (private,  state  and  fed.  owned:  preserve)   ! Ship   ! Horn   ! Petit  Bois   ! Barrier  islands     ! Protect  the  coast;   ! Nursery  for  seabirds,  Aish;   ! Habitat  for  numerous  species,  including  rare   and  endangered    species,  migratory  birds  …  .   Chapter 10 Page 28  Erosion  of  Ship  Island,  MS   1952, before Camille 1996, after Camille 1998, after George Chapter 10 Page 29  Major  shelf    types   Lithogenous  shelves   – Relict  sediment  cover   Storm-­‐dominated  shelves   Tide  dominated  shelves   – Recent  sediment  cover   Carbonate  shelves   – Tropical/subtropical  (coral,   coralline  algae,  and   heterozoans)   – Temperate  (bryozoans  and   other  heterozoans  )   Chapter 10 Page 30  Storm-­‐dominated   Shelves   ! Storm  dominated  and  sediment  starved.   ! Lithogenous,  relict  sediment  cover   ! Storms  are  the  primary  movers  of   sediment.     ! 6-­‐12  storms/y,  typically  in  winter   ! Sand  waves  and  ridges  dominate.   ! Examples:  U.S.  east  coast,  FL  Panhandle   Chapter 10 South Carolina Page 31  Storm  dominated  shelf  with   high  sediment  supply   The  relict  sands  are  covered  to  varying  degrees   (liIle  to  lots)  by  recent  muds  on  most  storm   dominated  shelves. Sediment  supply   near  Mazatlán,     Mexico  is  large,   Clay larger    than  at   South  Carolina.   Sand Silt Storms   Mud transport  much   Mud sediment  off  the   shelf,  but  it  is   replenished  by   the  rivers  in  a   Sand dynamic   equilibrium.   Mexico Chapter 10 Page 32  Tide  dominate  shelf   Middle  latitude  tides  are   semidiurnal,  so  strong  current   reverse  twice  daily   – Bedforms  form  along  current     gradients  with  every  tidal  cycle!   – North  Sea  and  English  Channel  are   prime  examples.   Chapter 10 Page 33  Tide-­‐dominated   Saint John, Bay of Fundy Asymmetric current Symmetric current Chapter 10 Link Page 34  Carbonate  shelves   Carbonate  shelves  covered  with  debris   from  coralgal  reefs  and  associated   fauna  in  the  tropics.    They  are  starved   of  lithogenous  material.   Shelves  starved  of  mineral  sands/mud   in  temperate  regions  are  also   carbonate,  covered  by  mixtures  of   bryozoans,  coraline  algae,  mollusks,   foraminifers,  etc.     – Temperate-­‐water  bryozoan  mounds  in   some  locales,  like  the  south  coast  of   Australia.   Chapter 10 Page 35  Bryozoans   ! A  bryozoan  colony  consists  of  microscopic,   connected  individuals  called  zooids.     ! Each  bryozoan  zooid  has  a  boxlike  or  tube-­‐ shaped  body  that  contains  Aluid  and  a  U-­‐ shaped  gut.     ! A  cluster  of  tentacles  called  a  lophophore   extends  into  the  water  to  trap  small  particles   of  food.     ! A  thin  crust  formed  around  each  zooid   consisting  of  a  protein  and   mucopolysaccharide  material  than  may  calcify.     ! Bryozoan  fossils  date  from  500  million  years   ago.   Zooid Chapter 10 Page 36  Bryozoans  –   temperate    carb.   ramp  builders   Chapter 10 Page 37  Great  Australian  Bight   ! Bryozoan  carbonate  ramp  complex  commenced   ~  40  million  years  ago.       ! Constituents  include  bioclasts  and  shells  of   gastropods,  pelecypods,  pteropods,  scaphpods,   echinoid  spines,  sponge  and  tunicate  spicules,   ostracodes,  calcareous  nannofossils,  planktonic   and  benthic  foraminifers,  bryozoans  and  rare   quartz  grains.   ! Little  terrigenous  material  shed  from  the     continent,  which  has  been  sinking  throughout   the  Cenozoic  Era.   Chapter 10 Page 38  Summary   ! Models  of  shelf  sedimentation   ! Features  of  the  coast   ! Estuaries   ! Beaches   ! Deltas   ! Barrier  islands   ! Types  of  continental  shelf   deposition   ! Lithogenous  shelves   ! Relict  sediment  cover   ! Storm-­‐dominated  shelves   ! Tide  dominated  shelves   ! Recent  sediment  cover   ! Carbonate  shelves   ! Tropical/subtropical  (coral,  coralline   algae,  and  heterozoans)   ! Temperate  (bryozoans  and  other   heterozoans  )   Chapter 10 Page 39  Laguna  Madre,  MX   Its  northern  limit  is   the  Rio  Bravo  delta,   and  its  southern  limit   the  mouth  of  the  Soto   La  Marina  river.   – River  freshwater   input  which  might   Alow  into  the  system   is  diverted  for   agricultural  and   urban  purposes.   – The  climate  is  arid.   Evaporation  is  1,900   mm/y,  and  average   precipitation  is  600   mm/y.  Salinities  are   as  high  as  40  psu.   Chapter 10 Page 40  Laguna  Madre,  MX   – Average  depth  of  0.7   m,  and  an  area  of   about  2,000  km2     – It  is  separated  from   the  sea  by  a  barrier   island.  The  lagoon   has    13  inlets  that   communicate  with   the  sea    only   intermittently,   because    they  are   often  closed  by   sediment   accumulation  events   due  to  cyclones  and   hurricanes   (Contreras,  1993).   Chapter 10 Page 41  Lagoons     ! Like  estuaries,  they  are  semi-­‐enclosed.  They   differ  from  estuaries  in  that  there  is  liIle  or   no  fresh-­‐water  input  and  oQen  less  contact   with  ocean  water.   ! Poten9al  for  large  salinity  varia9ons  due   to  evapora'on  or  heavy  rainfall!   ! Typically,  lagoons  lie  behind  barrier   beaches  with  9dally  scoured  inlets   between  barriers.   ! Poor  circula9on   in  the  narrow,   shallow     lagoons–liIle     9dal  flushing,   limited  fetch  for   wind-­‐driven   currents,  no   salinity   gradients.   Chapter 10 Page 42  Lagoons   High  salini9es   Bordered  by   may  drive   9dal  flats,  a   precipita9on  of   sink  of  fine-­‐   various  salts.     grain  sediment.   Some  sand  is   – Evaporites   Calcium  sulfates   washed  over   Mg  chlorides   the  barrier  into   Mg  sulfates   the  lagoon   Na  chloride   during  storms.   Na  sulfates   Generally,   – May  even  get   there  is  not   concentra9ons  of   much  sediment   carbonate:   moving  into  or   precipita9on  of   out  of  a  lagoon.   ooids.   Ooids Chapter 10 Page 43  Summary  -­‐  up  to  this  point   Features  of  the  temperate  shore   – Estuaries   – Deltas   – Beaches   – Barrier  islands   – Lagoons   Shelf  processes     – Description  of  facies  on  the  modern  shelf   – The  beach  zone  as  a  grinding  mill   – Effects  of  sea  level  change:  Transgression  and   Regression  of  the  sea   Shoreface  and  river/estuary  viewed  as  a  source  or   sink  of  sediment  during  change  of  sea  level.     Now  for  the  closing  –  MS/AL  barriers   Chapter 10 Page 44  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 45  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 46  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 47  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 48  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 49  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 50  Migra3on  of  Mississippi-­‐ Alabama  Barrier  Islands   (Morton,  Journal  of  Coastal   Research,  2008) Chapter 10 Page 51