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Chapter 06


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Chapter 06

  1. 1. New vocab: “polar vortex” Feature of [usually] polar climate where strong winds circulate and make a very cold air mass Held together by jet sream The idea is that this feature is actually weaker, now shifting and moving south [that is over us]
  2. 2. The flood threat comes as Congress is to consider whether to help fund a nearly $2 billion diversion channel around the Fargo-Moorhead area, a project that has come up against roadblocks. Fargo-Moorhead residents battled three straight major floods beginning in 2009, when the record crest forced thousands to evacuate and caused an estimated $100 million in damage. Fargo has spent $100 million on flood protection since the 2009 flood, buying out hundreds of homes in low-lying areas and building about 20 levees. Moorhead has invested more than $88 million on similar projects in the last four years.
  3. 3. Water cycle Stream characteristics Sediments Floods Floodplains
  4. 4. Lots of freshwater in hydrosphere, but not much in circulation Huge amounts of water in oceans, glaciers Loads of water moves slowly through groundwater and stays in storage for up to millions of years Notice all the water goes down  except of course for the vapor
  5. 5. Basin = entire drainage bowl Edges of basins are marked by divides where water flows in different directions Network has many levels of feeder streams = tributaries divide Very consistent relationship between stream size and abundance – many more small streams than big ones – pattern is the same everywhere tributaries
  6. 6. Land surface shape reflects moving water = erosion + underlying soil/geology In this map, streams are shown in V shapes and linear valley features
  7. 7. Why do streams look and act the way they do?
  8. 8. Stream profiles start steep and end shallow – this drop or slope of stream is called its gradient More feet drop per mile = more energy Coast – 2‟/mile mountain – 200‟/mile Up  downstream Overall course is also different Up  downstream
  9. 9. Rocky, multi ple channels Two classic types – braided and meandering Downstream parts often have more meanders floodplains deltas
  10. 10. Transport energy is related to the discharge and area across the stream, and so gradient matters also Higer discharge, higher competency
  11. 11. Coarse  fine Size, amount and transport distance is directly linked to energy Bedload >> suspended load
  12. 12. Dynamic interplay of erosion and deposition Rivers erode and migrate over time, developing stream sinuosity
  13. 13. Simply, discharge > channel Above bankfull stage, water spills out past levees into floodplain Floods carry sediment, organic debris, and energy outward Energy  sediment load, so finer sediments travel farther and longer
  14. 14. Northern & Central, Spring 2010 Southern & Central, 9/24-26 2010 Minnesota & Mississippi Rivers + tributaries [Zumbro, Cannon, etc] Southern & Central, 2011? 63% chance „major‟ St Paul flooding
  15. 15. Frequency and especially recurrence – episodes or trends? The “100-year flood” Our forecasts depend on pas measurements
  16. 16. Floods affect large areas of inhabited land Direct effects -drowning -sedimentation Indirect effects – long term devastation is much more important in economic and human terms …such as? What are the human / biotic impacts?
  17. 17. So… what do we do? De-urbanize / preserve channels and floodplains Increase wetlands / recharge Sandbags? Levees? Zoning? Forecasting?