Unformatted text preview: Envir Sc 1G03 Started November 22nd, 2011 Earth & the Environment On the Beach
1. Why do we need to understand
coastal processes? Reading: 2. Sediment transport on coastlines Chapter 18
Coasts & Oceans - waves, longshore drift, rip currents,
beaches 3. Tides 1. Why do we need to understand
- Canada has the longest coastline in the world -- (over 200,000 km)
- coastlines are dynamic; global sea level is rising ---- (23cm/100yrs)
- need to carefully plan coastal land uses Hurricane Hugo
1989 South Carolina
5m storm surge
$10 billion damage
500,000 people evacuated
29 killed Hurricane Katrina
1322 lives lost
$70 to $130 billion
storm in US history
>1 million people
damage Katrina coastal
- elevation gains
- losses (red) http://coastal.er.usgs.gov Katrina coastal
Dauphin Island, AL.
3-D views looking
along the island
http://coastal.er.usgs.gov 2. Sediment transport on coastlines
Waves and currents move sediment
How do waves form?
- induced by ﬂow of wind across water surface Fig. 18.2 .
- water molecules have circular orbits below waves
- orbits become smaller with depth Wave base - depth at which water movement is negligible
* Depth of
wavelength Why is it important to know the depth of wave base? the impact on sediment movement Waves...
- As waves approach shore...
orbits become elliptical, develop
into breakers Fig. 18.4 Longshore drift
What can you tell
about the movement
of sediment here?
- Sediment is moving
parallel to coastline
- LONGSHORE DRIFT What conditions do
you need for
longshore drift? Longshore
When waves approach coast at an oblique
- sediment moves obliquely up the beach
- moves directly downslope in backwash
- transports sediment along coast Land
Beach face Sea Wind direction Net sediment movement Longshore drift...
- Maintains supply of sediment to
beach Fig. 18.11
Forms 'spits' (ridges of sediment
that extend into open water) Forms 'bars' (ridges of sediment
that cut o bays from open water) Fig. 18.12 Local example of a bar???
Burlington Bar, one in Cootes Paradise What happens if something is built out into
longshore drift system?
'Groynes' - trap sediment
- used to widen beaches Problem! May cause
downcurrent Jetties - May cause deposition & erosion of sand Fig. 18.14 Breakwaters - Santa Monica Protect areas of the shore from waves that can be
deposited in protected area Rip currents
- Narrow currents that ﬂow out to sea
- returning water seaward
- Travel at water surface
- transport sediment
- and swimmers! Fig. 18.6 ** Shoreline environments
Shoreface - below low tide mark - sediment gets sandier towards shore
(coarser grain b/c of higher energy) Beach face - Steepest part of the beach
- lots of wave action Backshore/dune - landward of beach
- sand blown inland from dunes Backshore dune systems
Along coast of Lake Erie
over 75m high How do beaches develop?
Beaches usually composed of Sand - shell fragments in tropical areas
- gravel in colder areas Seasonal cycle
- storm waves in winter erode beach
face, build oshore sand bar
- low energy waves in summer move
sand onshore Canadian beaches
Commonly affected by ice
- lake or sea ice blown onshore in winter
- boulders can be removed onshore to form
'boulder barricades' or 'ramparts' lake erie in january 3. What are tides?
- Sea level on the earth's surface rises and falls once or twice daily as a 'tide'
- Caused by tide-generating force created by gravitational attraction of
'moon' and 'sun' - Creates two bulges in the oceans –– one closest to the moon and the other on
the opposite side of the Earth
- As Earth rotates -- water levels rise ('ﬂood tide')
- water levels fall ('ebb tide') Tidal Range
dierence in sea
high tide and low
tide Fig. 18.7 Where would you find the highest
tidal range in the world?
- Bay of Fundy
- Open ocean
toward coast Fig. 18.8 Bay of Fundy
- Tidal range >16m
- 'Tidal Bore' can reach speeds of 35 km/hr highest
world Fig. 18.8 Bay of Fundy tidal power Annapolis Power Tidal Facility
- Generates 20 megawatts/day
- Water moves through barrage
- need >5m tidal range
- Environmental concerns ...
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- Fall '11
- Coast, Tide, Longshore drift, Bay of Fundy