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Floods PowerPoint Presentation

Floods

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Floods

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  1. Floods • Definition: Natural process of overbank flow • Amount & distribution of precipitation in the drainage basin • Rate at which the precipitation soaks into the soil • Vegetated vs barren vs rocky • Rate at which surface runoff reaches the stream • Differs for agricultural vs forested vs urban • Amount of saturation of the soil at time of precipitation • > saturation > runoff > flooding

  2. Flood Measurements • Discharge (Q) • Amount of water per unit time that overflows channel banks • Q = H x W x V / 2 • Stage • Height of water in river • Recurrence Interval (R) • Average time between events of equal or greater magnitude • R = (N + 1) / M • R = Recurrence Interval • N is number of yrs of record • M is rank of flow within the recorded years • Hydrograph • Chart that shows change in Q, water depth, or stage over time

  3. Susquehanna River, NYBase-flow stage and flood stage

  4. Floods • Upstream • Upper portions of drainage basin or tributaries • May be intense • Steep topography • Thin soil horizon • Short duration over relatively small area • Flash floods • Downstream • Wide area • Longer duration • Downstream movement of large Q variation • Gentle topography (large floodplain) • Thick soil horizons • Long-term floods

  5. Effects of Flooding • Primary • Injury • Fatalities • Water damage to structures, transporation and communications systems • Erosion and deposition of soils, agriculture, vegetation • Secondary • Short-term pollution of river/stream systems • Natural sediments, leaching of septic or sewer systems • Contamination of drinking water sources • Displacement of population • Hunger and disease • Factors • Land use of floodplain • Depth & velocity or water • Rate of rise and duration of Q • Season in which flooding occurs • Quantity and type of sediments either eroded or deposited • Effectiveness of forecasting, warning and evacuation

  6. Change in Lag Time & Peak Flood • Land-use change from rural to urban • Lag time between precipitation and flood peak shortens • Flood peak increases

  7. Flash flooding Sabino Canyon, Catalina Mtns., Tucson

  8. End of the road….

  9. Slope-scars Sabino Canyon, Catalina Mtns., Tucson

  10. Bed-load debrisSabino Canyon, Catalina Mtns., Tucson

  11. Long-term floodingMissouri/Mississippi Confluence1993 • Upper image is during drought season in 1988 • Flow restricted to channel • Note difference in vegetation signature • Lower image show flood stage in July, 1993 • Wettest spring and early summer in 20th century • 14 million acres flooded • 50,000 people displaced

  12. Natural Service Functions • Fertile lands • As water overflows banks, velocity <, causing fine-grained particles to be deposited • Creates and replenishes minerals vital to agricultural processes • Aquatic systems • Provide nutrients to wildlife • Sweep or clean out accumulated debris • Sediment supply • Keep deltaic areas above sea-level • Build beaches and bars for vegetation to take root • Replenish fertile soils

  13. Human Interaction • Land-use changes • Alter amount of sediments impacting stream systems • Introduce new or different solutes into system • Change gradient of stream to compensate for sediment loads • Alter runoff capabilities • Dam construction • Deposition dominates new upstream environment • Erosion and downcutting dominates downstream \ environment • Urbanization • Increases both magnitude and frequency of flood events • Can increase runoff 5x • Reduces lag time between rainfall and flooding • Reduce groundwater by preventing recharge

  14. Mississippi Delta • Over 5-6,000yrs built 7 coalescing sub-deltas • Present “bird-foot” represents the last 500yrs • Channelization, dredging, levee construction have restricted flow to present river configuration while decreasing sediment recharge • Wants to follow Atchafalaya River course