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Rivers and Flooding - Environmental Geology - Lecture Slides, Slides of Earth Sciences

In these Lecture Slides, the Lecturer covered the following basic concepts of Environmental Geology : Rivers and Flooding, Effects of Urbanization, Drainage Basins, Adjustments, Environmentally Preferable, Benefits, Channelization, Channel Restoration, Implications, Pakistan

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2012/2013

Uploaded on 07/23/2013

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Introduction to
Environmental Geology, 5e
Chapter 9
Rivers and Flooding
Chapter 9: Overview
Understand basic river processes.
Understand the nature of the flood hazard.
Understand the effects of urbanization on
flooding in small drainage basins.
Know the major adjustments to flooding and
which are environmentally preferable.
Know the potential adverse environmental
effects of channelization and the benefits of
channel restoration.
Case History: Pakistan Floods of 2010
The number of people killed or affected, along with
economic loss, is the greatest in Asia
A monsoon refers to a seasonal shift in air pressure
and precipitation patterns (dry winter to wet summer)
August of 2010, the greatest monsoon rains in
decades, caused catastrophic flooding in Pakistan
July 29th, 12 inches of rain fell in the Upper Indus
Killed about 1,600 people, 20 mill ion people were
affected, 1.4 million acres were flooded
Case History: Implications
The population of Pakistan has grown from about 34 million
in 1951 to 170 million in 2010
Most people in Pakistan live close to the river
About 20 percent of Pakistan was flooded in 2010
How to compare this catastrophe with the US:
Rethink our philosophy of how we adjust to the flood
hazard in the United States as population grows
Plans for future flood hazard reduction that do not require
massive evacuation from flood prone areas
Avoid the hazard through land use instead of evacuation
Case History
Figure 9.2a/b
Rivers: Historical Use
For more than 200 years, Americans have lived
and worked on floodplains, depending on soil,
water supply, ease of waste disposal, and the
commerce
If the floodplain and its relation to the river are
not recognized, flood control and drainage of
wetlands become prime concerns
The pioneers moved west modifying the land:
cutting and burning the trees, and then modifying
the natural drainage
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Introduction to

Environmental Geology, 5e

Chapter 9 Rivers and Flooding

Chapter 9: Overview

  • Understand basic river processes.
  • Understand the nature of the flood hazard.
  • Understand the effects of urbanization on flooding in small drainage basins.
  • Know the major adjustments to flooding and which are environmentally preferable.
  • Know the potential adverse environmental effects of channelization and the benefits of channel restoration.

Case History: Pakistan Floods of 2010

  • The number of people killed or affected, along with economic loss, is the greatest in Asia
  • A monsoon refers to a seasonal shift in air pressure and precipitation patterns (dry winter to wet summer)
  • August of 2010, the greatest monsoon rains in decades, caused catastrophic flooding in Pakistan
  • July 29th, 12 inches of rain fell in the Upper Indus
  • Killed about 1,600 people, 20 million people were affected, 1.4 million acres were flooded

Case History: Implications

  • The population of Pakistan has grown from about 34 million in 1951 to 170 million in 2010
  • Most people in Pakistan live close to the river
  • About 20 percent of Pakistan was flooded in 2010 How to compare this catastrophe with the US:
  • Rethink our philosophy of how we adjust to the flood hazard in the United States as population grows
  • Plans for future flood hazard reduction that do not require massive evacuation from flood prone areas - Avoid the hazard through land use instead of evacuation

Case History

Figure 9.2a/b

Rivers: Historical Use

  • For more than 200 years, Americans have lived and worked on floodplains, depending on soil, water supply, ease of waste disposal, and the commerce
  • If the floodplain and its relation to the river are not recognized, flood control and drainage of wetlands become prime concerns
  • The pioneers moved west modifying the land: cutting and burning the trees, and then modifying the natural drainage

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Figure 9.3 a/b Floodplain: the flat surface adjacent to the river channel that is periodically inundated by floodwater.

Streams and Rivers

Part of the hydrologic cycle:

  • Streams = Small rivers
  • River components-
    • Network of streams
    • Watershed or drainage basin
    • Base level and slope/gradient
    • Latitudinal profile
    • Longitudinal profile
    • Grading processes

Streams and Rivers

Figure 9.5a Figure 9.4b

Sediment in Rivers

Stream total load = total amount of sediments

  • Bed load: coarse particles moving along the bottom of river channel, <10% of total load
  • Suspended load: accounts for about 90% of a river’s total load, river can look muddy
  • Dissolved load: carried in chemical sollutions such as HCO 3 - , SO 42 - , Ca2+, Na+, Mg+

River - Fluid Dynamics

  • Continuity equation
    • Discharge: the volume of water passing through a given location of a river per unit of time ( cfs ) Q = W x D x V
  • Gradient: vertical drop over horizontal flowing distance, expressed in percentage, ft/mi, or degree of the slope
  • Stream velocity: largely depending on the stream gradient, discharge, channel shape, and channel roughness

Sediment in Rivers

Stream competence and capacity

  • Competence: measuring the maximum size of the sediments transported by a river - The largest size particle transported
  • Capacity: total amount of sediments a river is capable of transporting - Volume-how many truckloads of material transported

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Effects of Land-Use Changes

  • Changes in infiltration rate: Change of the amount of water flowing into a river
  • Soil erosion: Change in the amount of sediments in a river
  • Amount of water and sediments in river: Changes in the velocity of water flow
  • Changes in river’s velocity: Leading the change in river dynamics

Land use

changes

Figure 9.9 & 9.

Effects of Land-Use Changes

Forest to farmland

  • Increases soil erosion, stream deposition
  • Increases gradient and velocity
  • Increases river-channel erosion Urban build-up
  • Increases impervious cover
  • Increases lower-magnitude flood frequency
  • Reduces the lag time of flood

Channel Patterns and Floodplain Formation

  • Braided : river’s longitudinal profile is steep and there is an abundance of coarse bed load sediment - Braided channels tend to be wide and shallow compared with meandering rivers - Associated with steep rivers being rapidly uplifted by tectonic processes or rivers receiving water from melting glaciers
  • Meandering : channels often contain a series of regularly spaced pools and riffles - Meanders migrate laterally by erosion on the cut banks and by deposition on point bars Figure 9.11a (^) Figure 9.12b

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River Flooding

  • Flooding: Overbank flow condition, discharge greater than channel’s holding capacity
  • Stage: The height of the water level in a river at a given location at a given time
  • Hydrograph: Graphic representation of a river’s discharge over time
  • Lag time: The amount of time between the occurrence of peak rainfall and the onset of flooding

Flooding Data Measurement

Figure 9.D

Frequency and Magnitude of Flood

Recurrence interval:

  • R = (N + 1)/M … where N is the number of years of record, M is the rank of individual flow within the recorded years The probability of a given magnitude flood:
  • P = 1/R Statistical probability versus reality:
  • Probability; one 25-year flood on average
  • Reality; two 25-year floods consecutively Figure 9.E

Types of Flooding

  • By stream location
    • Upstream flood: Shorter duration, smaller area
    • Downstream flood : Longer duration, greater magnitude, larger area
  • By duration
    • Flash flood : High volume of flooding water in very short duration, characteristic short lag time, usually in upstream
    • Non-flash flood
  • By magnitude/recurrence interval
    • 100 - year, 50-year, 25-year, 10-year floods Figure 9.

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Adjustments to Flood Hazards

  • The structural approach:
    • Physical/Engineering barriers: Levee augmentation
    • Channelization
    • River-channel restoration
  • Flood insurance: Shared responsibility and accountability
  • Flood-proofing: Raised foundation, floodwalls, waterproof doors and windows, pumps

Artificial Levee

Figure 9. Channelization Figure 9.

Retention Pond

Figure 9.

Stream Restoration

Figure 9.

Closer Look: Mississippi Flood

  • Two major recent floods, 1973 and 1993
  • 1973 spring flood
    • Evacuation of tens of thousands
    • Inundation of thousands km^2 of farmland
    • $1.2 billion in property damage
  • 1993 summer flood
    • Century flood in magnitude
    • From climatic anomaly, unusual precipitation & snowmelt
    • Lasted from late June to early August
    • 50 deaths, $10 billion in damage
    • Levees can provide a false sense of security

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Figure 9.G Figure 9.H & 9.I Levee failure in Illinois

Adjustments to Flood Hazards

  • Floodplain regulation: Obtaining the most beneficial use of floodplains - Flood-hazard mapping - Floodplain zoning
  • Government buyout and relocating people from floodplain
  • Personal adjustments

Floodplain Zoning

Figure 9.30 & 9.

Perception of Flooding

  • Individual level: Variable in their knowledge of flooding, anticipation of future flooding, and willingness to accept adjustments
  • Local and state level: Mitigation plans
  • Federal government level
    • Mapping of flood-prone areas
    • Floodplain management plans
    • Public outreach

Critical Thinking Topics

  • As a planner, outline a plan of action working for a community that is expanding into the headwater portions of drainage basins.
  • What is the largest floods ever occurred in your area?
  • With the global warming, what do you think the frequency and magnitude of flooding would change?
  • Differentiate between competency and capacity. Does a stream’s competency and capacity change over time?

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