http:water'usgs'govwaterwatch

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http//water.usgs.gov/waterwatch/
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Stream Flow Measurement Velocity-Area
method   Stream discharge Q U A   Q
volumetric flow rate in L3 T-1 U average
stream velocity L2 T-1 A cross sectional area
of stream flow (perpendicular to the direction
of flow) L2  
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In wide, strait streams with a moderate amount of
turbulence, the change of velocity with depth
generally follows a logarithmic velocity
profile.   u(yi) 1 ui ln (yi/yoi) k u(yi)
velocity at distance yi above the stream
bottom at vertical i k von Karmens constant
0.4 ui shear velocity (g Yi Sc)1/2 yoi
roughness height
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With a logarithmic velocity profile, velocity at
0.368 of the total water depth (distance above
the bottom, or 0.632 of the depth measured from
the surface) equals the average stream velocity.
For shallow stream segments, the USGS will
measure velocity at 0.6 of the depth measured
from the surface and consider this the average
water velocity. The average stream velocity
can also be determined from the average of the
velocities measured at 0.2 and 0.8 of the depth.
This approach is used in water deeper than 0.75
m. When stream flow gauging, USGS divides the
stream width into segments and then typically
measures velocity and calculates cross sectional
area for each width segment The flow within the
segment is the average velocity times the cross
sectional are of the segment, and the total flow
in the stream is the sum of the flows calculated
in each segment.  
Stream discharge Q U A Q ? Ui Ai U
A/Q
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Current meters for measuring stream velocity
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These stream flow measurements are conducted
approximately every 6 weeks, and special effort
is made to obtain such measurements during very
high flow events. If the stream channel shape is
relatively stable, a consistent relationship
between stream flow and water depth is often
observed. Stream depth is monitored continuously,
and the relationship between depth and discharge
(rating curve) is used to estimate stream flow.
  The vast majority of stream discharge reported
by USGS depends on a rating curve.    
Boneyard Creek Rating Curves
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Monitoring Water surface elevation (or stage) can
be measured by several techniques Float and
pulley Pressure methods Bubbler Transducer Ultr
asonic
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Problems with the Rating Curve Approach Discharge
at the highest flow depths is often based on
extrapolation because such flows are rare and it
is difficult to make many velocity measurements
during such events.   At very low flows in wide
shallow streams, it is difficult to make accurate
velocity measurements.   The relationship
between depth and flow is often different
depending upon whether flow is increasing or
decreasing. This is sometimes called hysteresis
in the stage- discharge curve because the
relationship depends on the recent history of
measurement. This is caused by there being
higher velocities in rising flow and lower
velocities in declining flow. When hysteresis
leads to large errors, it can be addressed by
measuring depth at two different locations and
the rating curve becomes a function of both depth
at one location and the difference in depth at
the two locations.   In rivers affected by
tides, or backflow from larger rivers it is also
necessary to measure depth at two different
points to correct for the influence of these
effects.
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Rating curves may change when significant change
occurs in the geometry of the stream channel or
if an obstruction to flow is introduced or
removed. Log jams, ice jams, beaver dams may
cause temporary deviations from a rating curve.
Erosion of streambed or banks, sediment
deposition, stream dredging or major construction
in a stream or flood plain can cause permanent
changes in a rating curve and a new rating curve
may need to be established.   In order to avoid
frequent changes in rating curves from erosion or
deposition, USGS attempts to locate flow gauging
stations in places where the streams are
relatively stable.  
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Dilution Gauging
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V-notch sharp crested weir
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Broad Crested Weir
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Parshall Flume
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H Flume