10: The coast

10: The coast

1. shore erosion causes

   • riverdams
     • we've dammed nearly every river
     • our beaches are shortening and eroding because of dams
     • ex: columbia river dammed multiple times
     • act as sediment traps
       • reduce amount of material that reaches the shore environment
     • beaches that used to receive a constant supply of new sediment are now "starved"
   • storms
     • beceaches can get eroded away completely
       • sediment eroded is brought back
     • rough seas can mobilize large amounts of near shore sediments and increase wave impact on seacliffs
     • storm effect: tempestites
       • storm layers formed on the inner shelf by the movement of sediment offshore
         • taking sediment out of system
   • sea level rise
     • global sea level has been rising since the last ice age
       • rate exacerbated by global warming
   • urbanization
     • increases runoff which speeds up seacliff erosion
     • lose local source of sediment
   • coastal development
     • construction along coast can alter currents and sediment in and output of a system
     • examples
       • creation of beach for holiday resort
         • hotel wants wider beach
           • put in a groin
             • traps sediment
               • longshore current west to east
             • sediment fills side of groin
               • otherside: wind erosion
                 • groin interrupts longshore current
                   • turns beach into "starved beach"
                 • continual groin making
       • shopping channel
         • longshore current continuously dumps sediment into shipping channel
           • build a jetty
             • interrupts longshore current
               • starves beach on other side of channel
         • longshore current switches direction
           • shipping channel more likely to fill up quickly
             • build jettys on both sides
               • both beaches are starved
                 • sediment from the river is dumped into deeper water
                   • removed from beach system
       • protection of harbors and moorings
         • resort wishes to construct breakwater to protect marina against storms
           • beach builds up but blocks dock
             • erosion down stream
       • protection of coast from wave impact
         • lighthouse requires protection from storm wave impact
           • breakwater in
             • wider breach in front of lighthouse
               • erosion down stream

2. stabilizing shores

   • groins
     • sediment traps
     • break waves that would otherwise erode shore
     • sediment is moved parallel to shoreline
       • groin stops this from travelling, intercepts currents
     • problem: traps on one side, erodes on the other
       • you very rarely ever see one groin
         • you mess up the flow of the entire shore
   • jetty
     • to protect rivermouth, keep it open
       • artificially lengthen channel of river
         • keeps current of river high until you dump sediment offshore
           • safe shipping
   • parallel breakwater
     • break down the waves,, stop shoreline erosion
       • traps sediment behind breakwater
         • can't see the horizon

3. longshore drift and current

   • waves will always affect the current: creating these two effects
   • natural system finds equillibrium
   • movement of sediment along the beach by longshore drift and current
   • path of grain
     • zigzag motion at an angle
       • net movement of longshore drift
   • wave vector
     • coming to beach at angle then wash down
       • longshore current: parallel

4. north carolina barrier islands

   • a ton of shore line
     • sediment supply comes in from 4 major rivers
       • sediment ultimately comes into pamlico sound
   • come from ~20,000 years ago
     • sealevel was almost 400 ft lower than today
       • coastal plain of NC extended all the way to today's shelf edge
     • why was sealevel so low during the last ice age?
       • significant amounts of water were stored as ice on land
     • barrier island formation: spit model
       • ice age ends, sea level rises
         • retreat the shoreline, erode into shoreline and up river valleys
           • longshore current east to west
             • picks up sediemnt grains, moving them in direction of longshore
               • forms "spits" - arms of sand
                 • shoreline gets higher
                   • developed, elongageds pits
                     • eventually land behind it disappears
                       • spits get separated from land behind it by laggoon/sound
                         • = barrier island
       • requires
         • steady sediment supply
           • to keep barrier islands above water
     • formation: ridge submergence
       • during ice age- stable sealevel
         • winds caused high dune ridge on shore
           • sea level rises
             • dune ridge completey cut off from land from erosion
               • becomes island
     • formation: offshore bar
       • offshore: large piles of sand
         • current system bringing back offshore sand
           • motion of sand toward the beach
             • sand bar migrates out of water
               • becoming shallow sand bar on shore
                 • eventually becomes island
                   • accretionary view-- building
   • zonations
     • laggoon side
       • salt marsh: on barrier island side
         • protected by island
         • maritime forest
           • been there long enough to grow trees
           • dune field
             • dune sand blown onto island
             • oceanside beach
               • other side of island
     • all these have very unique characteristics
       • different eco systems