# Hydrology

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10/4/2011
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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
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
   Channel slope (Rise over Run)
   Can be difficult to measure, estimate by
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

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