Hydrology

							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
Multi-year Hydrograph
                                  Gales Creek at Hwy 43

                  7000
                  6000
Discharge (cfs)




                  5000
                  4000
                  3000
                  2000                                                                            Flood
                  1000
                                                                                                  Stage
                     0




                                                                                         Jan-02
                         Jan-95

                                   Jan-96

                                            Jan-97

                                                     Jan-98

                                                              Jan-99

                                                                       Jan-00

                                                                                Jan-01
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
Year   Flow    Rank Year   Flow
       (cfs)               (cfs)
1995   2490     1   1999   6170
1996   4780     2   1996   4780
1997   2660     3   2001   4622
1998   2770     4   1998   2770
1999   6170     5   1997   2660
2000   2500     6   2000   2500
2001   4622     7   1995   2490
    Recurrence Interval
 Recurrence Interval    Year   Discharge
                                  (cfs)
  = # years + 1          1999      6170
     flood rank          1996      4780
  = 7+1         =4       2001      4622
       2                 1998      2770
For Gales Creek, 1996    1997      2660
  was a “4-year flood”   2000      2500
                         1995      2490
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 Frequency for Willamette River
                                at Albany

                  400,000
                  350,000
Discharge (cfs)




                  300,000
                  250,000
                  200,000
                  150,000
                  100,000
                   50,000
                       0
                            0   20   40    60    80   100
                                          Rank
 Flood Recurrence at Albany
= 117 + 1 = 3.5 Rank    Year   Discharge
                                 (cfs)
      34           1    1862       340,000
For the            2    1890       291,000
  Willamette in    3    1881       266,000
  Albany, 1996    34    1996       125,000
  was a “3-4
                  35    1922       122,000
  year flood”
                  36    1909       119,000
                  117   1977        18,000
                             Willamette River at
                            Albany and Portland

                  500,000
                                                  Albany
Discharge (cfs)


                  400,000                         Portland

                  300,000
                  200,000
                  100,000
                       0
                             1973
                                    1977
                                           1981
                                                  1985
                                                         1989
                                                                1993
                                                                       1997
                                                                              2001
                                             Water Year
 Flood Recurrence at Portland
= 29 + 1 = 30      Rank   Year   Discharge
                                   (cfs)
      1              1    1996       420,000
For the              2    1997       293,000
  Willamette in      3    1974       283,000
  Portland, 1996    26    1992       105,000
  was a “30-year
                    27    1991       102,000
  flood”
                    28    1977        58,100
                    29    2001        53,000
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
                               Width @ Bankfull
(Floodprone Width)
Entrenchment
                                    Entrenchment

Entrenched                              <1.4

Moderately Entrenched                 1.4 – 2.2

Slightly Entrenched                     >2.2

Entrenchment of 1.0 means the
floodprone width = bankfull width
Width to Depth Ratio
   Width / Depth at Bankfull
Width to Depth
                 Width/Depth Ratio

Low              <12
Moderate         12 – 40
High             >40

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
Low                   <1.2
Moderate            1.2 – 1.5
High                   >1.5


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:
Gradient   Characteristics
 0-2%      Calm water surface; almost no sound

 2-4%      Rough surface; must raise voice
           slightly to be heard
    >4%    Turbulent flow; must shout to be heard
Rosgen Stream Classification