LEARNING OUTCOMES
• Understand the field techniques used for measuring velocity
• Understand various methods of computing discharge
STREAMFLOW METERING USING A CURRENT METER
• Set up a transect
• Measure width of channel
• Determine spacing of velocity readings based on width (verticals)
• Measure velocity based on depth of vertical
• Calculate total discharge (Q) at the transect location
SITE SELECTION• Straight channel with a uniform cross-section
• Laminar flows along transect
• Channels should be free of obstruction, debris and vegetation
• Avoid sites with dead water, turbulent flows, vortices and reverse
flow
• Depth of the water should allow for current measurements at all
river stages
• Ease of access to site
• Located away from pumps, outfalls, sluices and bridges (measure
upstream of the bridge)
MEASURE CHANNEL WIDTH• Choose a point on the bank to start the transect
• Stretch a measuring tape perpendicular to the direction of flow from
one bank to the next
• Secure the measuring tape on both ends (bank reference points)
• Record the channel width of the stream’s water surface
VERTICAL SPACING• Channel width >10 m – determine spacing to obtain 25 to 30 verticals
• Channel width <10 m – determine spacing to obtain 10 verticals
• Where water depth and velocity changes rapidly the verticals should
be spaced closer together
MEASURE VELOCITY• Measure the water depth at each vertical starting from the bank
reference points
• Position the wading rod vertically and ensure the current meter is kept perpendicular to the stream flow
• Stand downstream of the wading rod to avoid disturbance of the natural flow
• Depth >0.5 m:o measure velocity at 0.2 & 0.8 of depth below water surface
(average velocity of both depths will be used as the mean velocity for the vertical)
• Depth <0.5 m:o measure velocity at 0.6 of depth below water surface
• Ensure the meter adjusts to the current before starting the observation (low velocities <0.3 m/s require a longer adjustment time)
• Electronic current meters usually record a 16 second average of the velocity
VERTICAL-VELOCITY CURVE
• Velocity at depths between the water surface and streambed at each
vertical is taken (increments of 0.1 depth)
• Verticals are closely spaced where curvature of the streambed
increases
• Velocity is plotted against depth for each vertical
• Mean velocity is determined by calculating the area bounded by the
curve and dividing the area by the length of the y-axis
• Time-intensive
TWO-POINT METHOD
• Velocity measured at 0.2 and 0.8 of the vertical depth
• Mean velocity is calculated as the average of these two readings
• Used for depths >0.5 m
• Test for appropriateness of measuring mean velocity - 0.2 depth
velocity should be greater than 0.8 depth velocity but not more than
double the amount
0.6 DEPTH METHOD
• Velocity measured at 0.6 of the vertical depth is the mean velocity of
the vertical
• Used for channel depths between 0.1 and 0.5 m
• Suitable for changing stage when a quick velocity measurement is
required
0.2 DEPTH METHOD
• Velocity is measured at 0.2 of the vertical depth
• A coefficient is applied to this reading to determine mean velocity.
• Coefficient is derived from the point to mean velocity ratio. USGS
used 0.87.
• Vertical-velocity curves are used to establish a relationship between
mean velocity and velocity at 0.2 of depth.
• Velocity-relation curve is plotted by plotting the true mean versus
the 0.2-depth velocity. Coefficient is derived from this curve
• Used for rivers with high velocity flows
• Not as accurate as the two point or 0.6 depth methods
THREE POINT METHOD
• Velocity is recorded at 0.2, 0.6 and 0.8 of the vertical depth
• The average velocity of 0.2 and 0.8 depths are calculated and then
averaged with the 0.6 depth velocity
• Alternatively, the arithmetic mean of the three velocities can be
reviewed
• Vertical depth >0.5 m
• Used when velocities in a vertical are abnormally distributed due to
turbulence or an obstruction
SURFACE AND SUBSURFACE METHOD
• Surface velocity is measured using a current meter or timing surface
floats
• Subsurface velocity is measured at 0.6 depth or below
• Coefficients are used to convert the surface or subsurface velocities
to mean velocity
• Vertical-velocity curve is used to derive the coefficients
• Coefficients may range from 0.84 to 0.9 based on shape of vertical-
velocity curve:
o Higher coefficients – smooth stream beds and normal shaped curves
o Lower coefficients – irregular stream beds and irregular curves
INTEGRATION METHOD
• Current meter is lowered in the vertical to the streambed and slowly
raised to the surface at a uniform rate
• The total number of revolutions and elapsed time is recorded
• A horizontal axis current meter can only be used since the vertical
movement of the meter will affect the measurements taken using a
vertical axis current meter
• Velocity at the stream bed cannot be measured using this method
• Accuracy dependent on the ability to move the meter at a constant
rate
FIVE POINT METHOD
• Velocity measurements recorded at :
1. 0.22. 0.63. 0.84. Surface5. Streambed
• Velocity measurements plotted on a graph and mean velocity is
determined using a planimeter or equation:
V = 0.1 (Vsurface + 3V0.2 + 3V0.6 + 2V0.8 + Vbed)
SIX POINT METHOD
• Velocity measurements recorded at :
1. 0.22. 0.43. 0.64. 0.85. Surface6. Streambed
• Velocity measurements plotted on a graph and mean velocity is
determined using a planimeter or equation:
V = 0.1 (Vsurface + 2V0.2 + 2V0.4 + 2V0.6 + 2V0.8 + Vbed)
• Used when vertical velocity profile is distorted due to aquatic
growth or ice cover
MID-SECTION METHOD• Channel is divided into rectangular subsections and the velocity and
depth is measured at each vertical which is located at the centre of the rectangular subsections
• Assumption that mean velocity in each vertical represents the mean velocity of a rectangular subsection in the channel
• Partial Discharge (qi) of subsection
vi – mean velocity at location ib(i+1) – distance from initial point to next locationb(i-1) – distance from initial point to preceding locationdi – depth of water at location i
• Total Discharge is the sum of the partial discharge in each subsection
MEAN SECTION METHOD• Rectangular subsections are between successive verticals
• Mean depth of the subsection is the average of the depths in the two
verticals
• Width of the subsection is the distance between two verticals
• Velocity of the subsection is the average of the two mean velocities
taken in both verticals
• Total discharge is the sum of the discharges from each subsection
MANNING’S EQUATION
• Manning’s equation can also be to measure the free surface flow of
water by gravity or in open channel flow.
V = R2/3S1/2
n
R - hydraulic radius (ratio of cross-sectional area to the wetted perimeter); S – energy gradient or slope of the water surface; n – channel’s roughness co-efficient
MANNING’S COEFFICIENT
Surface Material Manning’s Roughness Coefficient (n)
Floodplains - pasture, farmland 0.035
Floodplains - light brush 0.050
Floodplains - heavy brush 0.075
Floodplains - trees 0.15
Natural streams - clean and straight
0.030
Natural streams - major rivers 0.035
Natural streams - sluggish with deep pools
0.040
www.engineeringtoolbox.comFurther Reading: Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains, USGS, 1989
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