Name:___________________________

MATERIALS NEEDED: Paper and calculator. Meter tapes, staking pins, stream gage, hip waders, stop watches, floats, and current meters will be provided.

In principle, any stream can be gauged anywhere along its length by measuring cross-sectional area of flow and stream velocity. For very small streams, flow can be directed through a portable weir or flume, of known dimensions that allow accurate measurement of flow. For somewhat larger streams, measurement is usually by wading a measured cross-section, taking readings of depth and stream velocity simultaneously with a portable current meter mounted on a staff gauge. Even without a current meter, it is possible to estimate stream discharge with only a meter tape, meter stick, and float. The tape and meter stick are used to measure the area of the cross-section, and velocity is estimated from the time taken for the float to travel a known distance downstream through the measured section. The float can be almost anything that floats, but oranges and partly filled bottles are often used because they ride near the depth of maximum velocity and are not affected by wind as much as more buoyant objects. The U.S. Geological Survey estimates that, under ideal conditions, velocity estimates from float measurements are within 10% of the true value.

In this lab you will use both a simple current meter and the float method to measure velocity.

STEP 1: Select a smooth, non-turbulent cross-section along a relatively straight reach of stream. The site should be as free as possible of debris, vegetation, and large rocks.

STEP 2: Stretch a meter tape taut across the stream. The tape should begin near, but not at, the water margin and run at right angles across the channel above the water surface.

STEP 3: Wading the stream on the downstream side of the tape, measure the depth of the stream at selected points along the tape (every 10 cm is a good interval). Make sure you make your measurements upstream of the tape (while you are standing downstream of the tape) with a gauge held vertical next to the tape. At each location, record both water depth and position (distance from the beginning of the tape).

STEP 1: Stretch two meter tapes across the stream parallel to each other and at approximately equal distances upstream and downstream of the measured cross-section. The tapes should be far enough apart that a floating object will take at least 20 seconds to travel from one tape to another (try a distance of 5 m). Measure and record the distance between tapes.

STEP 2: Place the floats in the channel at approximately equal intervals across the stream and far enough above the upstream tape that the floats will reach constant velocity at the tape. Make a record of the positions of the floats. Floats should not be placed in water so shallow that they will touch bottom. You can launch floats at every meter along the tape, which usually gives good results. Record the positions of your floats from the bank.

STEP 3: Release the floats and record their travel time between the two tapes on the accompanying data sheet. The velocity is the distance (meters) divided by the time (seconds) the float takes to travel.

The mean velocity of flow through the cross-section using the floats measurements is estimated by:

NOTE: This correction factor is necessary to adjust for variation in stream velocity with depth. Velocity is usually greatest just below the water surface and decreases to zero at the bed; thus the float velocity is higher than mean velocity throughout the depth profile. The value of the correction factor can actually vary with stream conditions, but 0.85 is a commonly used value.

1. Compute the mean velocity for the cross-section using the floats measurements:

Average floats velocity: ________________

Corrected average floats velocity (see above):_________________

1. Make replicate readings using the current meter every meter along your cross-section. Your instructor will show you how to do this in the field. Record your results in the table provided below.

2. Compare the velocity value calculated from the floats with the average from the current meter.

Take three samples of sediment:

NOTE: These samples will be used later for analysis in Lab 11.

In Lab 8, you will use the data collected in the current lab to compute discharge. To answer the questions in Lab 8, you will need to remember accurately the settings of your cross-section. This is easily done by drawing a detailed sketch.

1. Draw a sketch of the stream section where the measurements have been made.

Mark the position of your cross-section and identify the other teams’ cross-sections as well.  Also mark the location of your three sediment samples, and any features along your cross-section (rocks, vegetation, wood debris, change in sediment type).

1. Draw a graph of the measured cross-section. Assume that the water surface is horizontal (flat), and reconstruct the configuration of the streambed by connecting the depth measurements below this horizontal surface. Convert all your measurements to meters before you start drawing the graph. Report the vertical exaggeration on your plot.

Back to Lab 7