Environmental Geology lecture outline
- Fluvial systems: floods and more.
How are rivers
used by people?
- commerce avenue.
- domestic source of water.
- hydro-electric power production.
- industrial cooling/processing.
- habitat preservation.
- clean up of wastes (natural bioremediation).
Note that some of the multiple 'users' will
be in natural conflict. You might explore the tensions between
various users. Kansas and Nebraska are now engaged in a legal
contest over the Republican River.
influence fluvial behavior (including flood behavior)?
- drainage basin characteristics: size,
geometry, ground seepage traits, gradients
- channel configuration.
- type of substrate: e.g. hard rock versus
loose sediment, sand versus mud.
- biologic processes: riparian corridors,
bank stability, evapotranspiration.
- human activity: dams, irrigation, siltation,
change in drainage basin characteristics (urbanization).
- climatic factors: annual rain fall, precipitation
flux, snow pack, ice.
- crustal setting: uplift vs. stability
vs. subsidence, crustal warping or faults.
A long list! A complex system.
Can floods be
predicted/forecast and how? This involves the development of flood
recurrence interval estimates - e.g. the proverbial hundred year
Basis of estimating recurrence interval?
Analyze for pattern in historical data. If one exists extrapolate
it into the future.
Methodology of stimation of flood recurrence
intervals (can be easily done in Excel, or a variety of other
- Obtain historical discharge data - source
(United States Geological Survey)
and nature of.
- Rank the discharges from 1 the maximum
on down to n the minimum discharge per year.
- Calculate recurrence interval: RI = (n+1)
/ rank. This is nothing but asking how often did a discharge
of this size or bigger occur in the historic time span.
- Plot the log of the RI versus the discharge.
Usually a line will fit the line fairly well. Compute best fit
mathematical relationship for the data.
- Extrapolate mathematically from this history
to longer time period of interest - 100 year flood or 1000 year
- example for Elkhorn River near Omaha, from Geodata
Uses recurrence intervals put to?
- how big to build the bridge (flood waters
need to fit under).
- how big to build a dam and its spillway.
- size of required flood control structures.
- what size rock or what type of bank stabilization
- flood hazard map and associated zoning
and planning ordinances.
- to decide what should flood insurance
premiums should be.
- where to site critical facilities.
What is the basic assumption in this type
How may this assumption be violated?
- drainage basin characteristics change
(e.g. logging, urbanization, farming practices).
- climate change.
Is there a better way?
structures and their effects:
- levees: an extensive system of along Missouri
and Mississippi. 1993 floods extensive failures.
- channelization: Keystone trail as an example.
- dams: very effective and proven history.
- alternates: back to the river Army Corp
Project and restoration of wetlands. Hidden Lake and Boyer Chute
as local examples.
How do dams pertubate
the fluvial system?
What will determine how fast a dam silts
up and how do you predict how long that will take for a specific
- West Pakistan Mangla Reservoir cost $600
million and filled up in 50 years before it was paid off. Bad
- Lake Powell formed by the Glen Canyon
dam may loose up to 33% of its storage capacity per year to seepage
into the underlying Navajo sandstone. 10% of the flow is lost
Dam safety considerations: Johnstown, Pennsylvania
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