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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.9

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.10

© 2012 Pearson Education, Inc. Figure 6.11Figure 6.12

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.14

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.20

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Table 6.1

© 2012 Pearson Education, Inc. Figure 6.21

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.23Figure 6.24

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.28

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.33

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Figure 6.40

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. 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

© 2012 Pearson Education, Inc. Table 6.2

© 2012 Pearson Education, Inc. End Flooding Chapter 6