Lecture 19 Flashcards
Quantifying reoccurrence/risk interval of a flood
- prediction essential
- plot frequency of last floods
- calculate reoccurrence interval (average time interval between two floods of the same magnitude)
- real time monitoring
- mapping and assessing flood hazard risks
Lakes
Standing bodies of water filling depressions on land
In general, an obstacle to flow is necessary to produce a lake
Over time the obstacle tends to be eroded; most lakes are geologically short lived
Causes of lakes
- Glacial erosion and deposition
- volcanic activity
- tectonism
- deposition and erosion of sediment by water (e.g. oxbow lake in meandering rivers)
Open lakes
Inlet and outlet streams
- water level tends to stay constant in short term
- sediments are typically muds, with sand near shore
Closed lakes
- no stream outlet
- water levels fluctuate due to evaporation
- sediments typically include salt deposits formed by evaporation
Oceans
Cover 71% of earth’s surface
Uneven distribution in northern (53%) and southern (88%) hemispheres
Distribution of ocean water:depth
-average depth 4.5 km
-max depth 11 km
-volume of ocean water 1.35 x 10^18 m^3
(Varies over time-amount of ice)
Salinity
Totally salinity (proportion of dissolved ions) is ~3.5% or 35%o (parts per thousand)
Chlorine and sodium make up 80%, rest is trace ions
Dissolved ions
- sodium
- chloride
- sulphate
- magnesium
- calcium
- bicarbonate
- potassium
Distribution of salinity
- salinity is high where evaporation is high (equator)
- salinity is low where river water enters ocean
Temperature variation
Temperatures highest in Tropical pacific and Indian oceans, decreases poleward
Temperature and salinity variation with depth
-cold water is denser so it sinks
Thermocline: zone of rapid temp change
-saline water is denser so it sinks
Halocline: region of rapid salinity change
Forces driving ocean currents
- air currents (wind on surface)
- sinking of denser (colder or saline) water
- effects resulting from rotation of the earth
Ocean surface circulation
- surface circulation is dominated by coriolis driven gyres
- major westward drift at equator
- eastward circulation in southern ocean, N.Atlantic, N.Pacific
Thermohaline currents
-high salinity descending currents
AABW Antarctic bottom water
NADW North Atlantic deep water
-north flowing surface currents
Coriolis effect
- surface of solid earth is rotating about vertical axis (except at equator)
- CCW in North hemisphere
- CW in southern
- objects moving in a straight line curve
- right in northern hemisphere
- left in Southern Hemisphere
Ekman transport
- because of Coriolis effect, wind driven current in northern hemisphere is slightly CW of actual wind direction
- deeper currents are progressively rotated CW
- in Northern hemisphere, overall average water movement is 90 degrees CW of wind direction
- no deflection at equator
Ekman transport, upwelling and downwelling
Upwelling where ekman transport is offshore (brings nutrients to shore)
Downwelling where ekman transport is onshore
Tides
- moons gravity on near side is stronger than necessary to maintain orbit, pulling water towards moon
- moons gravity on far side is weaker, allowing water to spin away from earth
- hydrosphere is pulled into an elliptical shape
Spring tides
Sun also causes tides but not as effective as moon
When sun and moon are in line:
- effects are combined
- high tidal range
- high high tides
- low low tides
Neap tides
- when sun and moon are at 90 degrees
- low tidal range
- low high tides
- high low tides
Changes in relative sea level
Submergence or emergence
These changes results from a combination of
- worldwide sea level change
- local effects
Eustatic (worldwide change)
Changes in ice volume (glaciation / deglaciation)
Changes in volume of ocean basins (volume of mid ocean ridge system)
Local changes
- tectonic movements in active areas
- isostatic response to ice caps and ice sheets
- compaction of sediment
