Environmental Geology lecture outline
- Shorelines and related environmental concerns. Some answers
What are general
environmental concerns associated with shorelines?
- floods associated with large cyclonic
storms (e.g. hurricanes), and especially the damage caused by the storm surge (one of the biggest risk factors).
- long term shoreline erosion/retreat (especially during large storms).
- water quality and saltwater incursion
(water, water all around and not a drop to drink).
- maintenance of important biotic systems
(e.g. spawning grounds for fisheries).
- sites of damage from oil spills.
- stability of navigation channels.
Stunning image of result of washover of Dauphin Island during Hurricane Katrina in 2005. Photo source: USGS - http://coastal.er.usgs.gov/hurricanes/coastal-change/overwash.html
The influence of three hurricanes on a barrier island complex. Image source USGS: http://marine.usgs.gov/coastalchangehazards/research/storm-impacts.html .
Estimated recurrence intervals for hurricanes for different parts of the coast. Image from NOAA - http://www.nhc.noaa.gov/climo/ .
What basic forces
are involved in shaping shorelines?
- waves: wave size and wave base, wave refraction,
longshore drift and movement of sediment.
- tides: tidal range as indicator of power
to move sediment.
- rivers: supply of sediment, deltas, fresh
- mass wasting: sea cliff retreat.
- sea level change: local and due to tectonism
vs. eustatic (global).
- storms: hurricanes to storms.
- biologic activity: reefs and mangroves
- the effect of day to day events vs. that
of large magnitude, infrequent events?
Amount of subsidence
for a few major cities:
- Venice, .22 m subsidence over 150 km2, high tides can flood the plazas, large
flood gates being built.
- Shanghai, 2.63 m over 121 km2.
- Tokyo, 4.5 m over 3000 km2.
- Houston, 2.7 m over 12,100 km2. Due in some
large part to oil withdrawal.
- New Orleans, 2 m over 175 km2.
- source: Dolan, R. and Goodell, H., 1986,
Sinking Cities, American Scientist, vol. 74, p. 38-47
How do you determine
and what are rates of shoreline retreat?
- Consider that retreat during small storms versus big
storms is of course different.
- Thus, your rate is time scale dependent meaning that you get different answers depending on the time frame you are measuring it over. There can be a problem
of short sampling spans under representing the long term rate.
- Air photos over time are enormously useful for gauging the changes.
- Rates on east coast can be on the order
of feet per year over the last century.
- Rates of Great Lakes shoreline retreat are locally as much as 10 m/yr (Rahn, 1989). This is another important consideration - rates will very spatially, and can be much higher in one place than another.
Shoreline before Hurricane Ivan along part of the Chandeleur Islands off the Louisiana coast.
Islands after Hurricane Ivan. Note lighthouse for position. Photo sources (and more information): USGS sitehttp://www.nwrc.usgs.gov/hurricane/postivanphotos.htm
How does sand
move in shoreline setting?
- onshore during small wave action (visible
beach often grows in width in the summer).
- offshore during large storms (storm set-up
and bottom return flow is a primary mechanism).
- from barrier island into lagoon during
storm break through and washover fan development.
- along shore due to longshore drift.
- back and forth through tidal channel.
- from lagoonal and terrestrial environment
to barrier island where trapped by vegetation.
- by wind transport, which can be quite significant. Vegetation traps such wind-blow sediment.
Schematic cartoon of sources that produce sediment and of processes that move sediment around in a low-slope coastal environment.
How can barrier
island morphology change with time?
- whole complex can shift landward or seaward (see images of Dauphine Island above).
- the complex can disappear or accrete to
- tidal channels can migrate in the direction
of longshore drift.
- new tidal channels can get formed during
- tidal channels can be clogged and closed
by long shore drift.
- storms can break through and form spillover
- basic conclusion: these are dynamic places.
What are possible
causes of shoreline retreat?
- glacial melting related eustatic (global sea level) rise.
- change (decrease) in sediment supply to shoreline due
to sediment capture by dam basins.
- compaction and subsidence of wet sediment
below at depth.
- change (increase) in storm size frequency and shore
- local tectonic subsidence (sinking crust).
are engineering responses to shifting shorelines.
- groins and long shore drift.
- breakwaters: designed to provide relief
from the waves
- disturbs sediment flow, sediment collects
in back of it.
- Santa Barbara built one in 1930, Miramar
Beach Hotel 5 km down drift direction sued city.
- response is often a distinct narrowing
of beach (increase offshore transport of sand).
- must be deeply anchored
- jetties: designed to stabilize navigation
- beach nourishment: trucking or pumping sand to the beach.
- how long does it last.
- cost as a subsidy to those living on coast.
Groins, rip-rap and seawall employed to stop erosion. Image source USGS site: http://coastal.er.usgs.gov/coastal-classification/figures/fig11.html
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