Chapter 11 Coastal Processes and Related Hazards

Environmental  Geology   Chapter  11  COASTAL  PROCESSES  and  RELATED  HAZARDS     • Introduction   • >50%  of  world  population  concentrated  in  the  coastal  zones   • ~75%  of  U.S.  population  living  in  coastal  states   • Coastal  hazard  problems  compounded  by  increased  human  activities     • Tides  -­‐  Tides  are  the  oscillations  in  local  sea  level  produced  by  the   gravitational  pull  of  the  moon  and  the  Sun  (to  a  lesser  extent)  on  the  oceans.     As  the  Earth  spins,  the  “overhead”  position  of  the  moon  sweeps  across  all   latitudes.    When  the  moon  is  overhead,  gravity  from  it  is  a  maximum,  and  the   water  is  pulled  upward  to  form  high  tide.    This  also  happens  on  the  opposite   side  of  the  earth  at  the  same  time  as  water  is  pulled  away  from  the  “sides”  of   the  planet  (where  low  tide  is  occurring).       • There  are  two  high  tides  and  two  low  tides  in  every  24-­‐hour  period.     When  the  moon  is  closest  to  Earth,  and  the  moon  and  sun  are  lined  up   together  (either  new  or  full  moon),  then  tides  are  maximum  (spring   tides).    When  the  sun  and  moon  are  at  90  degrees  to  one  another  with   respect  to  the  earth  (first  and  last  quarter  moon),  then  tides  are  at  a   minimum  (neap  tides).     • Coastal  Hazards   • Tropical  cyclones  (hurricanes  in  the  Atlantic  and  typhoons  in  the  Pacific)   and  other  severe  storms   • Marine  floods  and  inland  floods  due  to  tropical  storms   • Coastal  erosion  -­‐  MOST  dynamic  coastal  environment:  Convergent  zone  of   continental  and  oceanic  processes   • Tsunamis  and  tidal  currents   • Rip  currents     • Coastal  Processes:  Waves  are  formed  by  winds.    The  magnitude  of  waves   is  controlled  by:   Ø Speed  of  the  wind   Ø The  duration  of  the  wind   Ø The  fetch  of  the  wind—The  area  or  distance  blown  by  winds     • Tsunamis:  Caused  by  earthquake  or  other  seafloor  vibration       • Waves  -­‐  The  height  of  a  wave  is  the  distance  from  the  top  (crest),  or  highest   point  of  the  wave  to  the  bottom  (trough)  or  lowest  point.    The  distance   between  successive  wave  crests  (or  troughs)  is  called  the  wavelength.    The   wave  height  and  wavelength  are  controlled  by  wind  speed  (wind  blowing   over  water  produces  waves),  length  of  time  the  wind  has  been  blowing,  and   the  area  over  which  the  wind  is  blowing  over  the  water.     • • •   • • Wave  and  Water  Movement   Motion  of  water  particles  in  waves  is  up  and  down  in  a  circular  (orbital)   fashion.    The  wave/disturbance  moves  forward,  but  the  water  itself  does  not.     Waves  expend  their  energy  along  the  coastline     • Wave  translation:  Decrease  in  wave  length  and  velocity,  but  increase  in   wave  height;  deep  water  wave  energy  translating  into  wave  breakers,   pounding  the  shore.    The  long-­‐term  effect  of  wave  translation:  Coastal   erosion  and  beach  modification   • Wave  refraction:  The  bending  of  waves  and  concentration  of  its  energy  on   the  protruding  areas.    The  long-­‐term  effect  of  wave  refraction:   Straightening  the  irregular  shoreline,  erosion  of  headlands     Plunging  and  Spilling  Breakers  -­‐  Beach  erosion/wave  erosion  is  produced   by  abrasion  and  by  the  impact  pressure  of  waves  breaking  against  the  beach   or  headlands.    Plunging  breakers  are  the  curled-­‐over  type  of  waves  favored   by  surfers  who  want  to  ride  the  pipeline  in  Hawaii,  and  spilling  breakers  are   the  more  low-­‐key  type  that  just  break  apart  into  white  foam  without  the   overturned  lip.   Coastal  Processes:  Currents  -­‐  Horizontal  movement  of  a  large  volume  of   seawater:   Ø Due  to  oblique  waves   Ø Due  to  differences  in  water  temperature   Ø Due  to  differences  in  water  salinity   Ø Global  currents  or  more  local  currents       Longshore/Beach  Drift  =  Littoral  Transport  -­‐  Longshore  currents  move   sand  down  along  the  beaches.    Eroding  in  one  area  and  then  redepositing  in   another.    Beach  drift  and  longshore  drift  combine  to  move  sand  along  the   coastlines  (collectively  known  as  littoral  transport).     • Swash  pushing  sediment  onshore  in  an  oblique  angle   • Backwash:  Back  flow  of  water  and  sediments  perpendicular  to  shore   by  gravity   • Net  effect:  Zigzag  longshore  beach  drift   • Changes  of  seasonal  and  weather  conditions  resulting  in  changes  of   beach  faces  and  textures     • • Beach  Processes  -­‐  The  place  where  waves  break  is  known  as  the  “surf  zone”,   and  is  the  only  place  where  significant  forward  motion  of  water  is  actually   occurring.     Sand  tends  to  be  pushed  onto  the  beaches  by  mild  summer  waves  and  be   eroded  into  offshore  bars  during  stronger  winter  storms.    Offshore  sand  bars   • • • • • • • • can  form  and  block  bays  (baymouth  bars)  or  continue  as  spits  beyond  the   end  of  headlands.     When  offshore  sea  stacks  or  breakwaters  decrease  wave  action,  sand  bars   can  build  up  and  connect  these  structures  to  land  via  a  sand  peninsula  known   as  a  tombolo.     Rip  Currents  (aka  rip  tides  or  undertow)  -­‐  Formation:  A  series  of  large   waves  surging  to  shore,  then  the  rapid  backflow  of  the  piled  up  water  in   narrow  zones.    Up  to  200  people  killed  and  20,000  rescued  from  rip  currents   per  year  in  the  U.S.    It  is  important  for  people  to  recognize  rip  currents  and   to  understand  how  to  avoid  or  escape  the  hazard  –  Swim  parallel  to  the   shore  until  outside  the  current….then  head  in  to  the  beach.     Coastal  Erosion  -­‐  Factors   Ø Tropical  cyclones,  Nor’easters   Ø Tsunamis   Ø Tidal  actions   Ø Long-­‐term  rise  of  sea  level   Ø Human  activities   Sea  Levels  and  Climate  Change  -­‐  Sea  level  rises  during  warm  climates  when   glaciers  are  melting  away,  and  drops  during  ice  ages  as  water  is  taken  from   the  oceans  and  supplies  the  growing  glaciers.    Increased  coastal  erosion   occurs  during  ice  ages  as  water  can  flow  down  farther  to  reach  the  lowered   sea  level.     Seacliff  Erosion  (e.g.  landslide,  mudflow,  runoff)  -­‐  Human  activities   promoting  seacliff  erosion  through:   Ø Urbanization   Ø Added  structures  along  the  edge—pools,  and  patios   Ø Irrigation  and  other  activities   Hazard  reduction:  Reducing  runoff  (drainpipes),  planed  development  and   activities     Rivers  supply  the  sediment  for  beaches  -­‐  So  long  as  there  is  a  constant   supply  of  sediment  from  rivers,  no  net  erosion  has  to  occur.    When  sand   resupply  is  blocked  by  groins,  jetties,  etc.  (or  is  removed  from  a  channel  by   dredging),  enhanced  beach  erosion  can  occur  downstream  from  these   structures,  while  increased  deposition  occurs  upstream  of  these  features.     Coastal  Engineering  Structures  -­‐  Common  structures:  Seawalls,  groins,   breakwaters,  and  jetties   • Benefits   Ø Improve  navigation   Ø Retard  erosion,  recreational  beach  expansion   • Problems   Ø Interference  with  longshore  currents,  causing  unintended  adjacent   local  erosion  and  deposition     Building  Beaches  with  Groins  –  On  the  updrift  side  there  is  deposition  &   erosion  on  the  downdrift  side     • Beach  Nourishment  –  This  is  an  Alternative  to  coastal  engineering   structures.     • Constructing  a  positive  beach  sand  budget  involves  simply  putting  more   (and  hopefully  slightly  coarser)  sand  onto  a  beach  to  maintain  a  positive   “sand  budget”  from  year  to  year.     • A  successful  case:   Ø Miami  Beach,  Florida,  1970s–1980s   Ø 200  m  wide  beach,  survived  major  hurricanes  in  1979  and  1992   • Remaining  debate:  Value  of  beach  nourishment     • Barrier  islands  are  long  narrow  islands  (typically  equivalent  to  big  sand   bars)  separated  from  the  main  coastline  by  a  body  of  water  (e.g.,  a  bay).    The   Outer  Banks  of  North  Carolina  (of  Wright  Brothers  at  Kitty  Hawk  fame)  are  a   prime  example.       • Barrier  islands  are  dynamic  places,  shifting  back  and  forth,  with  inlets   forming  and  reforming,  under  the  influence  of  large  storms.    People   occupy  these  naturally  changing  landscapes  at  their  peril,  as   structures  are  not  as  naturally  mobile  as  the  sand  bars  on  which  they’ve   been  built     • Human  Activity  and  Coastal  Erosion  -­‐  Atlantic  Coast:  Barrier  islands  from   FL  to  NY   • Example:  Ocean  City,  Maryland   Ø Summer  resort  city,  high-­‐rise  developments   Ø Inlet  opened  in  1933  hurricane,  vulnerable  for  future  hurricane  or   other  severe  storms,  only  matter  of  time   Ø Local  erosions    due  to  development  and  removal  of  natural  dunes   Ø Rapid  shoreline  erosion  in  Assateague  Island  due  to  the  starvation  of   sand  supply  blocked  off  by  the  Ocean  City  inlet  jetties   •   • Human  Activity  and  Coastal  Erosion  -­‐  Gulf  Coast   • Last  100  years,  coastal  erosion  along  Texas  coast  accelerated  by  30–40%   • Coastal  erosions  along  the  Gulf  of  Mexico   Ø Due  to  land  subsidence  from  the  groundwater  withdrawal  and   petroleum  exploration   Ø Reduction  of  sand  supply  from  the  damming  of  rivers   • Galveston  Seawall  -­‐  Constructed  after  1900  Hurricane  Disaster   (~12k  killed).  All  sand  eroded  away  due  to  wave  energy  reflected  off   seawall   • • • •   • • Human  Activity  and  Coastal  Erosion  -­‐  The  Great  Lakes   • Periodic  problems  along  the  coasts  of  the  Great   Lakes   • Coastal  erosion     Ø Fluctuations  of  lake  water  level   Ø Lack  of  natural  frontal  dunes   Ø Erosion  more  severe  along  the  Lake  Michigan  shoreline     Tropical  Cyclones  -­‐  Warm  ocean  water  transformed  into  rotating  bands   of  thunderstorms-­‐warm  air  around  clear  “eye”  of  subsiding  air     Ø Called  Typhoons  in  Pacific   Ø Called  Hurricanes  in  Atlantic   Ø Form  where  thunderstorms  from  over  land  move  out  over  warm   ocean  waters  (in  tropical  latitudes)  and  gather  energy  from  the   warmth   Ø Counterclockwise  rotation;  low  pressure  system     Tropical  Cyclone  -­‐  hazards   • Strong  winds:  Hundreds  mph  winds  damaging  structures,  power  lines,   and  trees   • Intense  precipitation:  Marine  floods  and  inland   floods     • Storm  surge,  if  compounded  with  high  tide:  Great  amount  of  flood  and   coastal  erosion   • Annual  average:  Possible  impact  from  5  hurricanes  along  the  Atlantic   Coast     The  Saffir-­‐Simpson  Scale  (from  1  to  5)  –  used  to  describe  the  “level”  or   intensity  of  a  cyclone   Responses  to  Tropical  Cyclones  and  Beach  Erosion   Ø Do  nothing  and  bear  the  occasional  losses   Ø Evacuation,  relocation,  and  change  in  coastal  land  use   Ø Beach  nourishment   Ø Shoreline  stabilization  (groins,  jetties,  breakwaters)  and  raising   buildings  on  stilts     Perception  and  Adjustment  -­‐  Perception:  Largely  affected  by  past   experience,  proximity  to  the  coast,  and  probability  of  suffering  damages     • Adjustment     Ø Better  protective  structures   Ø Better  land  use  zoning   Ø Better  coastal  mitigation  planning  and   emergency  management  (preparation,   evacuation  and  warning,  post-­‐storm  management  procedures)   • •   Once  humans  become  involved  in  altering  the  coastal  zone,  they  must  stay   involved,  or  reap  the  consequences  (down-­‐current  erosion,  channel  filling,   structure  damage  and  destruction)  of  the  changes  such  involvement   produces     Managing  Coastal  Erosion  -­‐  Five  general  principles   • Coastal  erosion:  A  natural  process,  posing  natural  hazards  as   development  approaching  shore  fronts   • Shoreline  construction  causes  changes:  Often  better  for  some,  worse  for   others   • Stabilization  of  the  coastal  zone:    Protecting  the  interests  of  few  or   general  public?   • Engineering  structures:  Design  and  consequences   • Structural  vs.  nonstructural  alternatives  to  coastal  erosion  problems