Shorelines and related environmental concerns

Environmental Geology lecture outline

Contents:


An introductory case history

Galveston, Texas, on Sept. 8th 1900 and now

View of Galveston, with breakwaters, seawall and buildings built on raised platform.

Factoids on 1900 storm:

  • 20 foot high storm surge covered island.
  • 4' increase in less than a minute in some locales.
  • 6000 fatalities.
  • 3000 homes demolished.
  • subsequently built up sand/soil platform to reconstruct on (see above).
  • 'protected' by a seawall.
  • Wikipedia article with more information.
  •  

    View of Hotel Galvez from breakwater made of granite blocks. The thin beach and the seawall are also evident in the background. Why was this shoreline control structure built? What determined the size of the blocks of rock they built it out of? How much did it cost? These are all reasonable questions to ask.


    Two important questions

    What basic forces are involved in shaping shorelines?

    What are general environmental concerns associated with shorelines?

     


    Shoreline types

    There is a great diversity of shoreline types - we will look at three general types listed below:

    Image of a delta building out into a fjord in Svalbard. Since this fjord was occupied by ice some 15,000 years ago or less, we know this delta has built out in that time period. Note the active portion to the left where transport and deposition is occurring via a braided stream. The oldest portion of the delta surface is the darker surface, where tundra vegetation has had a chance to establish itself. The braided stream has wandered back and forth over the delta surface with time. Note also the thin narrow beach ridge that has trapped small ponds behind it.

    Air photo mosaic images of barrier island complexes SE of Savannah Georgia taken from USGS (terraserver site). This morphology can also be nicely explored in Google Earth.

    From Bear Island, north of Norway in the Barents Sea. The surf zone outlines a shallow and relatively flat area that is being cut by constant wave action - i.e. a wave cut terrace in the process of being formed. Note the sea cliffs in the back, with some talus at their base. Here the process of undercutting and mass wasting are important. Typically not a lot of talus accumulates because the waves grind it up and carry it into deeper water.

    Note the very flat surface. This is a wave cut terrace above present sea cliffs. Image source USGS - http://3dparks.wr.usgs.gov/3Dbayarea/html/WilderRanch.htm


    Deltas

    Why focus on deltas? A lot of people live on deltas! Why do a lot of people live on deltas??

    How are deltas classified and how do they behave? Three end member types of deltas are defined on basis of a dominating process that shapes the delta: river dominated deltas, wave dominated deltas, and tide dominated deltas.

    The Mississippi delta is a river dominated delta, characterized by lobe progradation -> lobe switching.

    Image of the Atchafalaya control structures with the Mississippi to the upper left. It may not be obvious at first what is going on in this image. The river more in the distance with the two barges on it is the Mississippi, and it is flowing away from the viewer. The three or four dam structures (count them) are the control structures that keep water in the present day Mississippi channel, and prevent much of it from going into the Atchafalaya river branches to the right. What would happen if one of these were to fail (e.g. during floods)? More information and image source at https://en.wikipedia.org/wiki/Old_River_Control_Structure.

    Taken from USGS site: http://gulfsci.usgs.gov/missriv/aerials.html.

    Satellite image of very end of Mississippi delta - a classic bird's foot geometry of distributory channels.

    Process of environmental consequence associated with deltas include compaction and subsidence, flooding, off shore mass wasting and oil infrastructure, shifting channels and ecosystems.

    Flooding on the Bangla Desh delta (a tide dominated delta): in last 30 years typhoons have caused over 300,000 fatalities.

    Satellite image of Bangla Desh delta. Note how different the geometry is from a bird's foot delta. Also note the turbid brown water in front of the delta. The deltaic sedimentation extends far offshore, and these types of deltas are typically muddy. Image source: NASA Visible Earth http://visibleearth.nasa.gov/view_rec.php?id=4576

    Satellite image of Nile delta. Note the smooth, curved nature of the shoreline, and the fan shaped area where branches of the river bring water to the desert. Upper portion of image from NASA's Visible Earth at http://visibleearth.nasa.gov/view.php?id=59656 .


    Barrier Island complexes and shifting shorelines

    Examples occur along both the lower U.S. east coast (e.g. Hilton Head, Tybee Island) and along the Gulf Coast (e.g. Padre Island), but also occur in many other places in the world.

    Photograph of Pea Island, North Carolina showing an oblique view of a where a tidal channel cuts through the barrier island. Notice how the road is truncated indicating the tidal channel has formed or enlargened relatively recently. The image was taken in 2011 three days after Hurricane Irene came through the area. Note also the relatively sparse vegetation that might otherwise serve to stabilize the island. Image source: USGS - http://coastal.er.usgs.gov/hurricanes/irene/photo-comparisons/ .


    Rocky shorelines

    California is one example where mass wasting triggered by wave undercutting has caused sea cliffs to retreat destroying and threatening various structures. The fundamental factor here is a tectonically active landscape, where uplift can occur, which the waves can then cut into.

    California sea cliff. Note the breakwater built to protect the houses right next to the beach. Image source: USGS site: http://soundwaves.usgs.gov/2007/07/pubs2.html

    Close up image of sea cliff retreat in California threatening structures. What were they thinking when they build these structures and what are the owners thinking now? Source: USGS http://soundwaves.usgs.gov/2008/07/meetings.html .

    Famous chalk cliffs along southern coast of England near Newhaven. The sloping wave cut terrace at the foot of the retreating cliff face is quite evident. At low tide and on a nice day such as this the beach is a pleasant place to stroll. During a storm and at high tide the waves are crashing into the base of the cliffs and moving around the good sized cobbles (the greyish material on the beach face. The small restaurant to the right should make its money while it can. Another building nearby has already been taken by the retreating cliff and the sea.

    UNO undergraduate in a sea cave along the chalk cliffs depicted in the above photo. Fractures in the chalk associated with a small fault make it easier for the waves to erode and undercut here.


    Engineering of shorelines

    Images from the USGS showing seawalls, one standard way of attempting to "draw a line in the sand". Image source: http://pubs.usgs.gov/of/2008/1206/html/figs/fig1_7.html .

    What are engineering responses to shifting shorelines?

    These efforts are highly subsidized by the government.

    What is an alternate approach to shifting and flooded shorelines?


    Some links to explore more:
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