Natural Hazards, 2e

Natural Hazards, 2e

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 Earths 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 6.11 Figure 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 InteractionLand 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 crosssectional 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 6.23 Figure 6.24 2012 Pearson Education, Inc. Human InteractionDam 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 InteractionUrbanization 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 HazardPhysical 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 HazardChannelization 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 HazardChannel 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 HazardFlood 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 HazardFlood 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 HazardFlood 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 2012 Pearson Education, Inc.

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