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© 2012 Pearson Education, Inc.
Lecture Presentation
Chapter 6
Flooding

Learning Objectives
 Understand basic river processes
 Understand the process of flooding and know the
difference between upstream and downstream
floods
 Know what geographic regions are at risk from
flooding
 Know the effects of flooding and the linkages with
other natural hazards

Learning Objectives, cont.
 Recognize the benefits of periodic flooding
 Understand how people interact with and affect
the flood hazard
 Be familiar with adjustments we can make to
minimize flood deaths and damage

An Introduction to Rivers
 Streams and rivers are part of the hydrologic cycle
 Evaporation of water from Earth’s surface
 Water returns to ocean underground or across the
land
 Runoff
 Surface drainage
 Streams merge into tributaries and then into rivers
 Drainage basin, watershed, river basin, or
catchment
 Area drained by a single stream

An Introduction to Rivers, cont.
 Gradient is slope of river
 is shown on longitudinal profile
 Steep at high elevations
 Headwaters
 Decreases as river reaches base level
 Lowest elevation of river, ultimately the ocean
 Floodplain
 Flat surface adjacent to channel

Materials Transported by Rivers
 Rivers transport materials along with water
 Total load consists of:
 Bed load
 Materials that roll, slide, bounce along bottom
 Suspended load
 Silt and clay particles that are carried in the water
 Dissolved load
 Materials carried as chemical solution

Velocity, Discharge, Erosion and
Deposition
 Rivers are the primary transportation and erosion agent
in the rock cycle
 Amount of erosion and deposition depends on stream
velocity and discharge
 Volume of water flowing through a cross section per unit time
(cubic meters per second)
 Discharge is constant along river
 Changes in area lead to changes in velocity
 Narrow channels have higher velocity than wide ones
 Stream flow widens and slows when moving from high
to low gradient
 Forms an alluvial fan or delta

Channel Patterns and Floodplain
Formation
 Braided channels
 Contain sand and gravel bars that divide and unite a single
channel
 Tend to be wide and shallow
 Meandering channels
 Migrate back and forth within a floodplain
 Velocity is greater on the outside of curves causing erosion
(cut banks)
 Rivers slow on the inside of curves causing deposition (point
bars)
 Floodplains are created during overbank flows
 During avulsion streams shift position
 Contain pools and riffles

Flooding
 Natural process of overbank flow
 Related to:
 Amount and distribution of precipitation in
drainage basin
 Rate at which the precipitation soaks into earth
 How quickly surface runoff reaches river
 Amount of moisture in the soil

Flood Description
 Flood discharge – discharge of the stream at the point
where water overflows the channel banks
 Flood stage – height of water in the river
 Shown on hydrograph
 Graph of stream discharge or water depth over time
 Flood stage
 Elevation of water surface that is likely to cause damage
to property
 Recurrence interval
 Average time between flood events of a certain size

Flash Floods
 Typical in upper portion of drainage basin and in small
basin of tributaries of larger rivers
 Caused by intense rainfall of short duration over a
relatively small area
 Common in arid environments with steep slopes or little
vegetation and following breaks of dams, levees, and
ice jams
 Most people who die during flash floods are in cars

Downstream Floods
 Cover a wide area
 Usually produced by storms of long duration
that saturate the soil and produce increased
runoff
 Can be caused by combined runoff from
thousands of tributary basins
 Characterized by large rise and fall of discharge
at a particular location

Geographic Regions at Risk
 Any place that receives precipitation has the
potential to flood
 Floods are number-one disaster in the United
States in twentieth century
 All areas of the United States and Canada are
vulnerable to floods
 A single flood can cause billions of dollars of
property damage and more than 200 deaths

Effects of Floods
 Primary
 Injury and loss of life
 Damage caused by currents, debris, and sediment
to farms, homes, buildings, railroads, bridges, roads
 Erosion and deposition of sediment related to loss of
soil and vegetation
 Secondary
 Short-term river pollution of rivers
 Hunger and disease
 Homelessness

Factors Affecting Flood Damage
 Land use on floodplain
 Depth and velocity of floodwaters
 Rate of rise and duration of flooding
 Season
 Quantity and type of sediment deposited
 Effectiveness of forecasting, warning, and
evacuation

Linkages with Other Natural Hazards
 Primary effect of hurricanes
 Secondary effect of earthquakes and landslides
 Fires
 Produce shorts in electrical circuits and erode
and break natural gas mains
 Coastal erosion

Natural Service Functions
 Fertile lands
 Periodic deposits of minerals enriches the soil for
agriculture
 Aquatic ecosystems
 Floods clear rivers of debris and sweep in
nutrients
 Sediment supply
 Periodic flooding builds up elevation
 Example: New Orleans

Human Interaction—Land Use Changes
 Rivers generally maintain a dynamic equilibrium
 Balance between gradient, cross sectional shape, and
flow velocity for sediment load
 That is, increase or decrease in the amount of water or
sediment received by a stream changes gradient or cross-
sectional shape, changing the velocity
 Land use changes can affect that equilibrium
 Forest to farming creates more erosion and sediment
 Sediment will build up the gradient of the stream
 Stream will flow faster until it can carry greater amount of
sediment
 Farming to forest sets the opposite into effect

Human Interaction—Dam Construction
 Upstream water slows down, deposits sediment, forming a
delta
 Downstream water devoid of sediment, will erode sediment
to transport
 Slope of the stream will decrease until equilibrium is reached
Figure 6.25

Human Interaction—Urbanization
 Increases magnitude and frequency of floods
 Urban areas have impervious cover and greater storm
sewers
 Carries water to stream channels more quickly
 Decreases lag time
 Causes flashy discharge – rapid rise and fall of floodwater
 Reduces stream flow during dry season
 Less groundwater is available
 Bridges block debris creating dams and flash flooding

Minimizing the Hazard—Physical Barriers
 Include earthen levees, concrete flood walls,
reservoirs, and storm water retention basins
 Levee breaks cause higher energy flows and
bottlenecks in upstream areas
 All physical barriers need to be maintained

Minimizing the Hazard—Channelization
 Straightening, deepening, widening, clearing, or lining
existing stream channels
 Can improve navigation and decrease flooding
 Some drawbacks:
 Drainage adversely affects plants and animals
 Cutting trees eliminates shading and cover for fish and
wildlife
 Cutting trees eliminates many habitats
 Changing the streambed destroys both the diversity of
flow patterns and feeding and breeding areas for aquatic
life
 Degrades the aesthetic

Minimizing the Hazard—Channel
Restoration
 Create a natural channel by allowing the stream
to meander and reconstruct variable water flow
conditions by:
 Cleaning urban waste to allow channel to flow freely
 Protecting existing channel banks by not removing
trees
 Planting additional trees or vegetation where
necessary
 Example: Kissimmee River Restoration in
Florida

Perception of Flood Hazard
 Most individuals have inadequate perception of
flood problem
 Local governments have prepared maps of flood
prone areas
 Federal government encourages local governments
to adopt floodplain management plans
 Public safety campaigns have been created to
educate public about flash flooding

Adjustments to the Hazard—Flood
Insurance
 FEMA manages U.S. National Flood Insurance
Program
 Maps of 100 year floodplain created to determine risk
 Areas where there is a 1 percent chance of floods in any
given year
 New property owners required to purchase flood
insurance
 Building codes limit new construction on floodplain
 Codes prohibit building on 20 year floodplain

Adjustment to the Hazard—Flood Proofing
 Raising foundation of buildings above the flood
hazard
 Constructing flood walls or mounds
 Using waterproofing construction
 Installing improved drains and pumps

Adjustment to the Hazard—Flood Plain
Regulation
 Obtain the most beneficial use of floodplains while
minimizing flood damage and cost of flood protection
 Structural controls may be necessary on heavily used
floodplains
 Less physical modification of river is ideal
 Flood hazard mapping
 Shows location of previous flooding
 Helpful in land use planning
 Relocation
 Government purchasing and removing homes damaged
by floodwaters

End
Flooding
Chapter 6

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