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

© 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

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

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