Transcript Longmuir Circulation and Seiches

Lake Morphology Affects on
Circulation & Productivity
Lake Morphometry (shape and size)
Fetch, Wind Waves & Mixing
Langmuir Circulation & Seiches
Lake Origins
What’s a Lake?
Bathymetry Map of Lake Erie:
Isobaths are lines of constant depth;
here they’re given at 1 m increments.
Morphometric Parameters
• Surface Area: important in determining total incoming solar radiation; many
parameters are reported in areal units (m-2). How can this be measured?
• Depth (maximum depth; mean depth): lakes with low mean depth (shallow) are
often more productive (why?); the relative importance of littoral habitat can be
determined by knowing depth at 1% surface irradiance (~3 * ZSD)
• Volume: can be calculated from surface area and mean depth. If the amount
of discharge into the lake from streams and other inputs (groundwater, direct
runoff) is known then a retention time (residence time) may be calculated.
Retention Time = (volume/discharge into lake)
• Retention time may be from hours to thousands of years; useful in determining
the impact of pollutants and relative influence of particular inputs (like a
tributary).
• Watershed Area: important influence on discharge into the lake; thereby
retention time; larger watersheds also may contribute more nutrients, leading to
greater productivity.
•Shoreline Development (DL): irregularity or degree of convolutions of the shore.
DL 
L
2 A0
DL = Length of lake shore
divided by the circumference of
a circle with equal surface area;
DL = 1.0 is a perfect circle.
Reservoirs vs Natural Lakes:
Reservoirs are dendritic in
shape due to filling of stream
channels.
Very high DL value; ~ 4-5.
Overall, low mean depth, only
deep area by dam.
Watershed surface area very
high relative to reservoir area.
Trap nutrients and sediments.
Productivity high (eutrophic).
Benton Res., TX
Flathead L., MT
Fetch, Wind Waves and Mixing
Lake shape affects mixing:
• Wave size (height) depends on:
- Wind speed
- Wind duration
- Fetch
• Fetch is the length of lake across
which wind interacts at the water
surface. Greater fetch; greater height.
• Fetch will change with wind direction
for most lakes; unless DL = 1.0.
• Watershed topography and
vegetation cover can influence the
“effective” fetch of a given lake.
• What happens to epilimnion depth
after deforestation around a lake?
Wind Waves, Lake Depth and
Sediment Suspension
• Wave mixing with depth can
reach the lake bottom and
suspend sediments.
• Waves over a particular
location can be classified
based on wavelength (L)
relative to water depth (z).
- Deep-water wave
- Shallow-water wave
• Shallow-water wave behavior
will suspend sediments and
potentially release large stores
of nutrients.
Langmuir Circulation (“Windrows”)
Wind causes water to move forward; underlying water rises to replace it; the net
effect is a spiral motion in the direction of the wind. For reasons, not completely
understood, these “spiral circulation cells” will oppose each other in direction.
Convergence and divergence zones are created; particles collect at convergences.
Seiche
(sāsh)
Lake water can resonate in its basin under certain conditions of
sustained unidirectional winds followed by calm. The seiche
may be externally visible or unseen, only in hypolimnion
(internal seiche). Seiche cause entrainment (mixing) of layers.
Lake Origins
1) Tectonic Movements
• Movement of tectonic plates has created some of the oldest, deepest
lakes on earth.
• Graben, or rift, lakes form where fault allows a block to slip down,
causing a massive depression that fills with water. Horst lakes form
similarly however the block tilts, or slips more severely on one side.
• Uplift Lakes are due to epeirogeny, the rising of the large crustal blocks.
2) Volcanism: “Grew, Blew, Fell, Fill”
* Caldera: A notable example is Crater Lake in Oregon. This was
formed after Mt. Mazama erupted ~7700 yrs ago. The resulting
collapse of the mountain walls left a deep crater that became filled
with water. Residence time is very long!
3) Damming:
* Natural processes can back-up the flow of water:
- landslides
- lava flows (coulee lakes)
- animals constructed dams
- thick vegetative growth (Sphagnum bogs)
- glaciers (ice dams)
* Manmade dams create reservoirs.
Castor canadensis
4) Glacial activity:
Glaciers may form a variety of lakes in the landscape following their
retreat (melting) up mountain valleys.
Glaciers aggressively scour bedrock, creating depressions throughout
mountain valleys, particularly at the “head” of a valley.
- Cirque (valley head or “amphitheater”)
- Paternoster (down valley; smaller)
- Fjords (deeply scoured valleys)
Glaciers move large amount of soils and rocky debris forward and to
its sides as it flows (till; moraine). Much of this is entrained within the
ice on the underside of the glacier. As the glacier retreats large
deposits of moraine can accumulate; or chunks of ice can break off
and becoming embedded into the till.
- Terminal moraine lakes (wall of moraine builds at end)
- Kettle (“prairie pothole”) lakes (melted ice chunks in till)
Glacial Lake Types
5) Dissolution:
Aggressive weathering of limestone bedrock can
create sinkhole or cinote (typical in karst topography)
6) Fluvial:
River / stream flow dynamics (meanders, etc.) can
create fluvial lakes:
- Oxbow lakes
- Levee lakes
7) Aeolian:
Scouring by abrasive forces of wind and sands
- buffalo wallows
- perched dune lakes
8) Cosmogenic:
meteor impact craters.