Definition of Streamflow
� Streamflow, or discharge, is defined as the volumetric rate of flow of water (volume per unit time) in an open channel
� Streamflow in the USGS is usually expressed in English dimensions of cubic feet per second (ft3/s) or CFS.
Velocity-Area Method
Discharge = (Mean water velocity) x (Cross-section area of water)
orDischarge = (Water velocity) x ((Width) x (Depth))
Velocity-Area Method
Discharge =
Cross Section Area
X Mean Water Velocity
The Current Meter
Price AA PricePygmy
Measuring High Flow
Conventional Current Meter-Method
• The USGS measures water depth and velocity in at least 20 locations or verticals (generally >25 verticals) across the channel.
• Velocity is measured at each location at water depths that best represent average velocity at that location.
Conventional Current Meter-Method
� The places where depth and velocity are measured (verticals) define subsections of the channel cross section.
� Discharge is determined by summing discharge in these subsections using the “mid-section method”
Conventional Current Meter-Method
� The places where depth and velocity are measured (verticals) define subsections of the channel cross section.
� Discharge is determined by summing discharge in these subsections using the “mid-section method”
The Midsection method
Measurement of Velocity
� Current-meter measures velocity at a point� Discharge measurements require determination of
the mean velocity in each of the selected verticals� Mean velocity in a vertical can be approximated by
making velocity observations and using a known relation between those velocities and the mean in the vertical.
Velocity Curve
Two-Point Method
� Generally used for depths greater than or equal to 2.5 ft
� Velocity observations are made at the 0.2 and 0.8 depth below the surface.
� The average of the two observations is used as the mean velocity in the vertical
Six-Tenths-Depth Method
� Generally used for relatively shallow conditions at depths <2.5 ft.
� Observation of velocity at 0.6-depth below the surface is used as the mean velocity in the vertical.
Three-Point Method
� Used when the velocities in the vertical are abnormally distributed
� Velocities are observed at the 0.2, 0.6, and 0.8-depth
� Mean velocity is computed by averaging the 0.2 and 0.8-depth observations and thenaveraging that result with the 0.6-depth observation
Depth, in feet Current meter Velocity Method2.5 and greater Price Type AA 0.2 and 0.81.5 – 2.5 Price Type AA 0.60.3 –1.5 Price Pygmy 0.61.5 and greater Price Pygmy 0.2 and 0.80.3 –1.5 Acoustic Doppler
Velocimeter (ADV)0.6
1.5 and greater ADV 0.2 and 0.8
Surface-Velocity Method
• In a natural channel a surface-velocity coefficient of 0.85 or 0.86 is used to compute mean velocity
• In a smooth artificial channel a surface velocity coefficient of 0.90 is used
Selection of Measurement Cross Section� Cross section lies within a strait reach, and
streamlines are parallel to each other� Velocities are greater than 0.5 ft/s and depths are
greater than 0.5 ft� Streambed is relatively uniform and free of numerous
boulders and heavy aquatic growth� Measurement section is relatively close to gaging-
station control to avoid the effect of tributary inflow, and storage during periods of rapidly changing stage
Selection of Measurement Cross Section
Selection of Measurement Cross Section
Spacing of Observation Verticals
� Spacing of verticals is determined to provide about 25 to 30 subsections or more
� No subsection should have more than 10 percent of the total discharge
� Ideal measurement has no subsection more than 5 percent of the total discharge
� Spacing between verticals should be closer in areas of the cross section that have greater depths and velocities
� Note: Ideal measurements are seldom achieved with 25 subsections
Indirect Measurement of Discharge
1
10
100
1 10 100 1000 10000 100000Discharge (cfs)
Stag
e (ft
)
Rating Curve
Discharge Measurements
Discharge measurements are used to develop rating curves