Hydrology
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Hydrology. The flow or movement of water. Hydrologic cycle. Surface Runoff. Surface hydrology focuses primarily on streams and their channels. Measuring Flow. Discharge or gage height commonly used for surface water flows Discharge or flow are measured in ft 3 /sec or m 3 /sec

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Hydrology l.jpg

Hydrology

The flow or movement of water



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Surface Runoff

  • Surface hydrology focuses primarily on streams and their channels


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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


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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


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Annual Hydrograph

10/7/01 is when flow begins to increase above 10 cfs


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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


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Jan-02

Multi-year Hydrograph

Flood Stage


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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?


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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



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Recurrence Interval

  • Recurrence Interval

    = # years + 1

    flood rank

    = 7 + 1 = 4

    2

    For Gales Creek, 1996 was a “4-year flood”


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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


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1996

1862 is the Historic Peak for this site


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Flood Recurrence at Albany

= 117 + 1 = 3.5

34

For the Willamette in Albany, 1996 was a “3-4 year flood”


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Flood Recurrence at Portland

= 29 + 1 = 30

1

For the Willamette in Portland, 1996 was a “30-year flood”


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Stream Channel

  • The energy from gradient can alter the

    • Sinuosity

    • Entrenchment

    • Width to Depth Ratio

      of a stream channel below bankfull


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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


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Entrenchment

  • Vertical containment

  • Estimated as:

    Width of stream at 2x bankfull depth

    Width at bankfull

Width @ 2x Bankfull Depth

(Floodprone Width)

Width @ Bankfull


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Entrenchment

Entrenchment of 1.0 means the floodprone width = bankfull width


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Width to Depth Ratio

  • Width / Depth at Bankfull


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Width to Depth

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


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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


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Sinuosity

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


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Gradient

  • Channel slope (Rise over Run)

  • Can be difficult to measure, estimate by characteristics:



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