USGS Procedures for High-Flow Measurement using the Price AA Meter

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USGS Procedures for High-Flow Measurement using
the Price AA Meter
Annual Tri-Agency Coordination Meeting Memphis, TN
Mark E. Smith mesmith_at_usgs.gov USGS CRSO, Denver,
CO November 17, 2009
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Discharge and Velocity Measurement

• A current-meter measurement is the summation of
  the products of the subsection areas of the
  stream cross section and their respective average
  velocities
• USGS uses the mid-section computational method

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Stream discharge is sum of discharges in all
sub-sections (mid-section method)
Total Discharge ((Area1 x Velocity 1) (Area2
x Velocity2) .. (Arean x Velocityn))
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Point Measurement of Velocity
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Water velocity in each sub-section
estimated using a current meter to measure water
velocity at selected vertical locations in the
water column.
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Velocity Measurement

• Discharge measurement using a current meter
  requires determination of the mean velocity in
  each of the selected verticals.
• The mean velocity in a vertical is the average of
  many point velocities in that vertical, but it
  can be approximated by a few velocity
  observations and a known relation (for normal
  conditions) between those observations and the
  mean in the vertical.

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Velocity Measurement

• Flow velocity in natural channels generally
  pulsates. The USGS measures velocity for 40 - 70
  seconds to better represent average velocity at a
  point.

Comparison of pulsations for two different mean
velocities measured in a laboratory flume, 12 ft
wide. (Rantz, 1982 vol. 1)
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Vertical-Velocity Curve

• The vertical-velocity profile under normal
  conditions tends to have the shape shown below.
  We can use this relation to compute a mean
  vertical velocity based on measurements at
  certainpoints in the vertical.

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Six-Tenths Velocity Method

• In the 0.6-depth method, an observation of
  velocity made in the vertical at 0.6 of the depth
  below the surface is used as the mean velocity in
  the vertical
• The U.S. Geological Survey uses the 0.6-depth
  method under relatively shallow conditions

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Two-Point Velocity Method

• When velocity profiles are relatively normal, the
  average velocity can be adequately estimated by
  averaging velocities at .2 and .8 of the depth
  below the water surface
• The two-point method is used with the Price AA
  meter when depths are gt 2.50 ft

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Three-Point Velocity Method

• In the three-point method velocities are observed
  at 0.2, 0.6, and 0.8 depths.
• Usually, the mean velocity is computed by
  averaging the 0.2- and 0.8-depth observations and
  then averaging that result with the 0.6-depth
  observation.
• The three-point method is used when velocities in
  the vertical likely are distributed abnormally
  because of disturbing elements in the water.
• Note With a Price AA current meter, the
  three-point method cannot be applied unless the
  depths are greater than 2.5 ft (0.76 m).

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Vertical-Velocity Curve
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Current-Meter Measurementof High Flows(Bridges,
Boats, or Cableways)
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Current-Meter Measurementof High Flows

• The Price AA current meter generally is used when
  making a conventional high-flow measurement.
  Depth is measured using asounding reelvelocity
  ismeasured afterpositioning themeter in
  thevertical.

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Current-Meter Measurementof High Flows

• Some reels are equipped with a computing depth
  indicator. To use the computing spiral, the
  indicator is set at zero when the center of the
  current-meter rotor is at the water surface. The
  sounding weight and meter are then lowered until
  the weight touches the streambed.

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Preparation for Measurement

• Spacing of verticals is determined to provide
  about 25 to 30 subsections
• Verticals should be so spaced that no subsection
  has more than 10 percent (ideally 5 percent) of
  the total discharge (approximated using the most
  current shift-adjusted rating)

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Current-Meter Measurementof High Flows

• The depth at each observation vertical (recorded
  to nearest 0.1 foot) determines the method of
  velocity measurement to be used, normally the
  two-point or the 0.6-depth method. If a
  non-standard velocity distribution is suspected,
  we use the three-point method.
• After the meter is placed at the proper depth and
  pointed into the current, the number of
  revolutions made by the rotor is counted for a
  period of 40 to 70 s. The stopwatch is started
  simultaneously with the first signal or click,
  which is counted as "zero," and not "one."

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Current-Meter Measurementof High Flows

• The size of the sounding weight used in
  current-meter measurements depends on depth and
  velocity in the cross section.
• The size of the weight (lbs) should be greater
  than the maximum product of velocity (ft/s) and
  depth (ft) in the cross section (more if there is
  debris or ice). If insufficient weight is used,
  the meter assembly will be dragged downstream,
  resulting in errors in the depth computation.

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Velocity Adjustment forDirection of Flow

• Required velocity is the component normal to
  measurement section
• At vertical locations where the meter points into
  an oblique current, multiply measured velocity in
  the vertical by thecosine of angle, a.

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Adverse Conditions Measurement of Swift, Deep
Streams
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Measurement of Swift,Deep Streams

• At sites where depth soundings are possible (i.e.
  weight will reach the bottom) but weight, meter
  drift downstream
• Depth measured by usual methods will be in error
  too large
• Error correction has 2 parts
• Air (dry-line) correction
• Wet-line correction

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Measuring Depth
a
Vertical angle of sounding line
Dry-line correction (cd) corrects for excess
amount of line above water
p
c
d
b
p
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Measuring Depth - cont.

• Wet-line correction corrects for excess amount of
  line below the water
• Not a linear function with depth
• Correction applied to nearest 0.1 foot
• Cannot be determined until the dry-line
  correction has been deducted from observed depth

p
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Measuring Depth -Vertical Angle of Sounding Line
Protractor
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Measuring Depth - cont.

• Best to eliminate dry-line correction by
• Using tags on the sounding line (bridges) or
• Using a stay-line to facilitate depth measurement
  from the water surface (boat deck)

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Measuring Depth - cont.
Both dry-line and wet-line corrections can be
computed using an Excel spreadsheet
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USGS Measurement using Price AAMississippi
River at Natchez, MS May 27, 2009
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Boat-Measurement Procedure

1. 300-lb weight used to suspend current meter
2. 300-ft stay line (attached to bridge) used to
   hold horizontal line and position at each
   vertical
3. Suspension of current meter initiated from boat
   deck, so no air-line correction needed
4. Vertical angle of sounding line measured from
   boat boom to determine and apply wet-line
   correction

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Boat-Measurement Procedure cont

1. Depths at each of 27 verticals measured using
   onboard depth finder (and compared to ADCP
   readings). Depths ranged from 24.0 to 118.0 ft
2. Velocities measured at .2, .6, and.8 depth points
   for40 seconds in each vertical. Average
   velocities in the 27 verticals ranged from 3.0 to
   10.0 ft/s(no direction-of-flow adjustments were
   needed)
3. Standard mid-section method used to compute
   discharge

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Discharge-Computation Results

• Cross-section Area 192,000 ft2 (by depth
  finder)
• Discharge by .2, .6, .8 method Q11,460,000
  cfs Avg. V7.60 ft/s
• Discharge by .2, .8 methodQ21,410,000 cfs
  (-3.4) Avg. V7.33 ft/s
• Discharge by .6 methodQ31,510,000 cfs
  (3.4) Avg. V7.88 ft/s
• Uncertainty (Q2 vs. Q3) 6.6 - 7.1

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Selected References

• Buchanan, T.J., and Somers, W.P., 1969, Discharge
  measurements at gaging stations U.S. Geological
  Survey Techniques of Water-Resources
  Investigations, book 3, chapter A8, 65 p.
• Rantz, S.E. and others, 1982, Measurement and
  Computation of Streamflow Volumes 1 and 2,
  Geological Survey Water-Supply Paper 2175,
  Washington D.C., 631 p.

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Questions?