Environmental Geology lecture outline - Fluvial systems: floods and more.

How are rivers used by people?

• commerce avenue.
• domestic source of water.
• irrigation.
• hydro-electric power production.
• industrial cooling/processing.
• recreation.
• 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.

What factors 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 flood.

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 spread sheets):

• 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 flood.
• example for Elkhorn River near Omaha, from Geodata analysis course.

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 to use.
• 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 of analysis?

How may this assumption be violated?

• drainage basin characteristics change (e.g. logging, urbanization, farming practices).
• climate change.

Is there a better way?

Flood control 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 proposed dam?

Relevant factoids:

• West Pakistan Mangla Reservoir cost \$600 million and filled up in 50 years before it was paid off. Bad deal!
• 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 to evaporation.

Dam safety considerations: Johnstown, Pennsylvania story.

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