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

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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 ft3/sec or m3/sec

    • Width x depth x velocity = flow

  • Width to depth ratio


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


Annual Hydrograph

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


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


Jan-02

Multi-year Hydrograph

Flood Stage


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?


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


Gales Creek Peak Streamflow


Recurrence Interval

  • Recurrence Interval

    = # years + 1

    flood rank

    = 7 + 1 = 4

    2

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


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


1996

1862 is the Historic Peak for this site


Flood Recurrence at Albany

= 117 + 1 = 3.5

34

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


Flood Recurrence at Portland

= 29 + 1 = 30

1

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


Stream Channel

  • The energy from gradient can alter the

    • Sinuosity

    • Entrenchment

    • Width to Depth Ratio

      of a stream channel below bankfull


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


Entrenchment

  • Vertical containment

  • Estimated as:

    Width of stream at 2x bankfull depth

    Width at bankfull

Width @ 2x Bankfull Depth

(Floodprone Width)

Width @ Bankfull


Entrenchment

Entrenchment of 1.0 means the floodprone width = bankfull width


Width to Depth Ratio

  • Width / Depth at Bankfull


Width to Depth

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


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


Sinuosity

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


Gradient

  • Channel slope (Rise over Run)

  • Can be difficult to measure, estimate by characteristics:


Rosgen Stream Classification


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