Environmental Geology
Tu. Oct. 14, 1996
BREAKING NEWS
- Volcano in Iceland and flood threat
- Trash Landslide in Portugal
LECTURE: Mass Wasting/Rivers
- A) Mass Wasting Summary
- B) The Hydrologic Cycle
- C) Human River Impacts
- D) The 500 year flood
MASS WASTING HIGHLIGHTS
- Weakened geologic material
- Geologic structure
- Triggering mechanisms
- Slow and fast both cause much damage
Association with other geologic hazards
- Earthquakes
- Volcanic eruption
- Flood
HYDROLOGIC CYCLE
OUR PLANET IS JUST RIGHT -We are the right distance from our sun so
that water can exist in three states
Differential heating of our planet by the sun drives the movement of
the atmosphere
- Pole to equator differences
- Day/Night by rotation of planet
- Continent/Ocean heating cooling contrasts
The main features can be summarized by the HYDROLOGIC CYCLE
The HYDROLOGIC CYCLE
- Water evaporated from ocean
- Precipitation on land and ocean
- Liquid and solid water are agents
- Water runs back to the oceans
- Carries solid and dissolved material
- Reevaporation of seawater leaves both dissolved and solid matter behind
in the ocean
The cycle is driven by SOLAR RADIATION
- Solar radiation drives evaporation
- Differential heating and evaporation/precipitation cause winds
- Day night, seasonal, and long term climate changes change distribution
of glaciers and rivers
- Solar radiation is used by plants for photosynthesis and biologic weathering
Water Reservoirs
| RESERVOIR |
VOLUME
106 km3
|
% OF TOTAL |
| Ocean |
1,400 |
95.6 |
| Glaciers |
43.4 |
2.97 |
| Groundwater |
15.3 |
1.05 |
| Lakes |
0.125 |
0.009 |
| Atmosphere |
0.0155 |
0.001 |
| Rivers and Streams |
0.0017 |
0.0001 |
Water Transfers FLUX
| PROCESS |
UNITS
103 km3/YR
|
| Evaporation from the Ocean |
434 |
| Precipitation on the Ocean |
398 |
| Atmospheric transer from ocean to land |
36 |
| Precipitation on Land |
107 |
| Runoff (rivers to the ocean) |
36 |
| Evaporation from the land
(Evapotranspiration)
|
71 |
EROSION RATES AND GLOBAL RIVER COMPARISONS
Ex: Mississippi River
QUESTIONS:
- What is the annual rate of weathering and erosion from the whole of
the Mississippi River drainage basin?
- How does one go about obtaining such information?
- Given the estimated rates what will be the rate of lowering of the
continent over millions of years?
- What effects have humans had on the river?
MEASUREMENTS
Measure the annual DISCHARGE
Measure the average suspended matter
Measure the average dissolved material
Sample over the year and average values
- Determine cross-section of river by series of soundings.
- Measure flow at different depths at a number of distances from the
river bank across the whole of the river cross section.
- Sample water and sediment with all measurements for later estimates
of suspended solids and dissolved matter.
- Calculate the flow rate across the cross section by summing up the
flows from all the point measurements.
- Compare the flow measurements with the river stage (level) and plot
flow versus stage
- From the plot determine a rating curve so that most measurements of
flow can be calculated from stage measurements.
Staff Gauge Calibration
- Note rarity of low and high flows
CALCULATIONS
- Calculate the FLUX of dissolved and suspended material per year.
- FLUX = Volume discharge times the concentration
- dissolved load= 125 x 1014 gms/year suspended load= *210
x 1014 gms/year
- Divide flux by the drainage area of the Mississippi River (3,270,000
km2)
- Equals grams of solid and dissolved matter eroded per square meter
per year (0.106 kg/m2/yr combined)
* Current estimate (with dams, levees, and locks)
FINALLY!
- Divide by density of rock (2.67 gms/cm3) = lowering rate
of land per year
[0.059 mm/yr = 5.9 cm/1,000 yr]
- Multiply by 10,000 (time since last glaciation) = average weathering
rate of the whole drainage basin
59 cm in 10,000 years!! 23 inches!
BIG DEAL??
- Is the Mississippi R. unusual?
- What does this mean in terms of long term changes on the earth?
- How long would it take to reduce continents to low elevation?
- What are the major controls on weathering and erosion?
- What are human effects?
Rates depend on position in the watershed
- Highest in upper reaches (mts.)
- Lowest in lower reaches (river mouth)
RATES OF EROSION CONTROLLED BY:
- Topographic RELIEF in drainage area
- TOTAL PRECIPITATION
- GLACIERS IN THE HEADWATERS
- LENGTH OF STREAM REACH
- HUMAN ACTIVITIES IN BASIN
- Cropland development
- Desertification
- Levee construction
COMPLICATONS
Bed load vs Suspended Load
- (20-80 % of a riverÌs sediment may be carried by BED LOAD
Riverbank storage
Much of glacial sediment from 10,000 years ago has not left the river
basin
Dams and locks along river
Accelerated by conversion of natural vegetation
- If Forest = x
- Then Grassland = 3x
- And Cropland = 8x (U.S.) or 10x (World)
Current estimates are that modern erosion rates are approximately 2.8
times that of pre-European settlement for the United States.
FLOOD FREQUENCY
The 500 year flood concept
- Need to deal with some simple statistics.
- Statistics (probability, relative risk, average behavior, ...) play
important role in environmental geology.
- Need to predict infrequent events from observations of the distribution
of more frequent events.
- Flood frequency defined by two measures:
- Average (mean discharge per year/event)
- Deviation about the average (± discharge)
AN ILLUSTRATION
- What percentage should be heads or tails?
- 50% right? - It depends on how many times you flip the coins.
- Experiment: Flip 10 coins and count number of heads and tails. Do experiment
100 times and plot the results in a HISTOGRAM
- Possibility of getting 0 heads and 10 tails in one flip much less likely
than 5 heads and tails at a time.
A Second Experiment
- Do same experiment, but now flip 100 coins, add up the number of heads
and number of tails.
- Do experiment 100 times and plot a histogram of the results.
- The possiblity of getting 0 heads and 100 tails in one experiment is
EXTREMELY RARE. 40 heads and 60 tails less rare, and 50 heads and 50 tails
most probable.
1,000 coin flips, 100 at a time?
The distribution depends on the number of observations
Thursday
Important concepts from Rivers
The Colorado River and the Arid West
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