U.S. Department of the InteriorU.S. Geological Survey

Measurement of Streamflow

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

Acoustic Current Meter

� SonTek/YSIFlowTracker

ADCPs

ADCPs

The Current Meter Method

Miscellaneous Measurement Methods

Miscellaneous Measurement Methods

Measuring High Flow

Measuring High Flow

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

The Midsection 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

Questions?