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).
- shoreline erosion/retreat.
- water quality and saltwater incursion
(water, water all around and not a drop to drink).
- maintenance of important biotic systems
(e.g. spawning grounds).
- sites of damage from oil spills.
- stability of navigation channels.
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.
- the effect of day to day events vs. that
of large magnitude, infrequent events.
Rates of subsidence
for a few major cities:
- Venice, .22 m subsidence of 150 km2, large
flood gates being built.
- Shanghai, 2.63 m of 121 km2.
- Tokyo, 4.5 m of 3000 km2.
- Houston, 2.7 m of 12,100 km2. Due in some
large part to oil withdrawal.
- New Orleans, 2 m of 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?
- retreat during small storms versus big
- time scale dependent rates and the problem
of short sampling spans.
- air photos over time.
- 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 a much as 10 m/yr (Rahn, 1989).
How does sand
move in shoreline setting?
- onshore during small wave action (visible
beach often grows in width).
- offshore during large storms (storm set-up
and bottom return flow).
- from barrier island into lagoon during
storm break through.
- along shore due to longshore drift.
- back and forth through tidal channel.
- from lagoonal and terrestrial environment
to barrier island where trapped by vegetation.
- from near shore to offshore environments
by wind transport.
How can barrier
island change with time?
- whole complex can shift landward or seaward.
- 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
What are possible
causes of shoreline retreat?
- post-glacial eustatic rise.
- increase in temperature of seawater.
- change (decrease) in sediment supply due
to dam basins.
- compaction and subsidence of wet sediment
- change in storm size frequency and shore
- local tectonic subsidence (sinking crust).
are engineering responses to shifting shorelines.
- groins and long shore drift.
- widely used on E and Gulf coast.
- interrupt longshore 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 downdrift 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.
- how long does it last.
- cost as a subsidy to those living on coast.
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