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Earth’s six water reservoirs

Earth’s six water reservoirs. Rivers: Historical Use. Americans have lived and worked on floodplains for 200 years Attracted to this area by rich soil, water supply, ease of waste disposal, proximity to river commerce (trade) Building on rivers invites disasters Floodplain

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Earth’s six water reservoirs

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  1. Earth’s six water reservoirs

  2. Rivers: Historical Use • Americans have lived and worked on floodplains for 200 years • Attracted to this area by rich soil, water supply, ease of waste disposal, proximity to river commerce (trade) • Building on rivers invites disasters • Floodplain • Defined as a flat surface adjacent to the river that is periodically inundated by flood water

  3. Streams and rivers • Part of the hydrologic cycle • Hydrology—study of water transport in a natural system • Runoff • Drainage basins • River slope or gradient • Drop of channel over horizontal distance • Steepest in the headwaters • Higher velocity, greater erosion

  4. Base level • Theoretical lowest level to which a river may erode • Usually sea level • Base level may be temporary, as with a lake

  5. Flooding • Natural process of overbank flow • Function of the amount and distribution of precipitation, rate of infiltration, and topography • Snow melt, failure of a dam • Stage—height of water in a river • Floodstage—high water level, damage to personal property

  6. Upstream and downstream floods • Upstream • Headwaters • Intense rainfalls of short duration • Hazard: autos • Downstream • Cover wide area • Long duration storms

  7. Flood alerts • Flood watch • High flow or overflow is possible • Flood warning • Flood is actually occurring or is imminent

  8. Dangers of floods • 80% of flood deaths occur in vehicles • The mistake: driving on flooded roads • Just 2 feet of water can float a car • 1/3 of all flooded roads are so damaged that a car has only a 50% chance of making it • Just 6” of rapid flood waters can knock you down • Flood waters are usually very cold

  9. Staying safe during floods • Know if you live in a flood-hazard area • Pay attention to signs of possible flooding • Evacuate if flood waters threaten • Don’t walk in flood waters • Don’t drive on flooded roads • If you get stuck in your car, wade out the way you drove in

  10. Closer look: Magnitude and Frequency of Floods • Related to amount and intensity of precipitation and runoff • Large floods from infrequent, intense storms • Described by magnitude (intensity) of discharge, units of cms, cfs • 10 yr flood, 1/10 or 10% chance of happening in any one year • 100 yr flood, 1/100 or 1% chance of happening in any one year

  11. Nature and extent of flood hazards • #1 disaster in 20th century • Factors that cause damage: • Land use • Depth and velocity of water • Rate of rise and how long the flooding lasts • Season • Effectiveness of forecasting

  12. Effects of flooding • Primary: directly by flood • Life, homes, bridges • Secondary: by disruptions • Pollution, hunger, disease

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