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Hydrology

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  1. Hydrology The flow or movement of water

  2. Hydrologic cycle

  3. Surface Runoff • Surface hydrology focuses primarily on streams and their channels

  4. Measuring Flow • Discharge or gage height commonly used for surface water flows • Discharge or flow are measured in ft3/sec or m3/sec • Width x depth x velocity = flow • Width to depth ratio

  5. Hydrographs • Graph of a stream’s discharge over time • “Real-time” and historical data may be available through Water Resources Dept, your local watermaster or USGS • http://waterdata.usgs.gov/nwis • Water Year = Oct - Sept

  6. Annual Hydrograph 10/7/01 is when flow begins to increase above 10 cfs

  7. Peak Flows • Watermaster has determined 1,873 cfs to be “flood stage” for this site • Gales Creek exceeded this level 4 times in water year 2001 • Highest discharge for year = 4,622 csf on 1/8/02

  8. Jan-02 Multi-year Hydrograph Flood Stage

  9. Flood Frequency • Gales Creek appears to reach “flood stage” at least once per year (except 2001) • 1999 appears to have highest discharge for the 8 years we have data for – even higher than 1996 • What makes a 100-year flood?

  10. Flood Frequency • Find maximum annual discharges • Can query “Peak Streamflow” on USGS site • Rank peak discharges from largest to smallest • Calculate recurrence interval (RI) RI = # years + 1 flood rank

  11. Gales Creek Peak Streamflow

  12. Recurrence Interval • Recurrence Interval = # years + 1 flood rank = 7 + 1 = 4 2 For Gales Creek, 1996 was a “4-year flood”

  13. 100-Year Flood • Requires 99 years of data • Only the largest flood during that 100 year period would be a “100-year flood” • Very few sites with 100 years of data • The Flood of 1996 was far from a 100-year event

  14. 1996 1862 is the Historic Peak for this site

  15. Flood Recurrence at Albany = 117 + 1 = 3.5 34 For the Willamette in Albany, 1996 was a “3-4 year flood”

  16. Flood Recurrence at Portland = 29 + 1 = 30 1 For the Willamette in Portland, 1996 was a “30-year flood”

  17. Stream Channel • The energy from gradient can alter the • Sinuosity • Entrenchment • Width to Depth Ratio of a stream channel below bankfull

  18. Determining “Bankfull” channel • Top of point bar • Change in vegetation • Topographic break in slope • Staining or change in substrate material or size • Change in nature or amount of debris deposits

  19. Entrenchment • Vertical containment • Estimated as: Width of stream at 2x bankfull depth Width at bankfull Width @ 2x Bankfull Depth (Floodprone Width) Width @ Bankfull

  20. Entrenchment Entrenchment of 1.0 means the floodprone width = bankfull width

  21. Width to Depth Ratio • Width / Depth at Bankfull

  22. Width to Depth As the width to depth ratio increases, the stream gets wider and shallower.

  23. Sinuosity • Is stream straight or does it meander? • How much longer would channel be if it were stretched into a straight line? • Estimated as: Channel length Straight length

  24. Sinuosity Sinuosity of 1.0 means the stream channel has the same gradient as the valley

  25. Gradient • Channel slope (Rise over Run) • Can be difficult to measure, estimate by characteristics:

  26. Rosgen Stream Classification