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Chapter 6 Flooding
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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
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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
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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
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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
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Figure 6.9
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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
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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
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Figure 6.10
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Figure 6.11 Figure 6.12
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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
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Figure 6.14
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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
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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
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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
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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
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Figure 6.20
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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
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Table 6.1
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Figure 6.21
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Effects of Floods Primary Secondary 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
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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
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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
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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
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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
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Figure 6.23 Figure 6.24
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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
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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
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Figure 6.28
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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
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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
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Figure 6.33
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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
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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
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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
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Figure 6.40
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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
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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
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Table 6.2
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End Flooding Chapter 6
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