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FLOOD03477
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Last modified
1/25/2010 6:27:24 PM
Creation date
10/4/2006 11:48:20 PM
Metadata
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Template:
Floodplain Documents
County
Statewide
Community
Statewide
Stream Name
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Basin
Statewide
Title
Velocity Profile, Water-Surface Slope, Bed-Material Size for Selected Streams in Colorado
Date
1/1/1984
Prepared For
CWCB
Prepared By
USGS
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />DATA COMPILATION <br /> <br />The data were compiled and basic calculations completed. Computations of <br />several important characteristics, velocity profiles, and bed-material size- <br />frequency distributions were made by standard methods. <br /> <br />Mean Velocity <br /> <br />The mean velocity in a vertical may be obtained from velocity observa- <br />tions at many points in that vertical (Rantz and others, 1982). This method, <br />commonly called the vertical-velocity curve method, was used to compute the <br />mean velocity in each vertical, for each current meter. Observations were <br />made at points distributed as previously described between the water surface <br />and the streambed. The mean velocity in each vertical was computed by weight- <br />ing each observed velocity in proportion to its corresponding increment of <br />depth (Hulsing and others, 1966). <br /> <br />There are numerous methods of estimating the mean velocity in a vertical <br />from only a few point-velocity observations (Rantz and others, 1982). Prob- <br />ably the two most common methods are the two-point and the six-tenths-depth <br />methods. <br /> <br />In the two-point method, velocity observations are made at 0.2 and 0.8 <br />depth in each vertical. The mean velocity in the vertical is assumed equal to <br />the arithmetic average of the two readings. This method normally is used in <br />depths greater than 2.5 feet when using a Price type AA current meter. In <br />shallower depths, the current meter is within 0.5 foot of the surface or <br />streambed. <br /> <br />I <br />I <br />I <br />--r <br /> <br />In the six-tenths-depth method, a single observation of velocity in the <br />vertical at 0.6 depth is assumed equal to the mean velocity. This method is <br />used when the measurement needs to be completed rapidly or when a reading can- <br />not be obtained at 0.8 depth. When a Price type AA current meter is being <br />used, the six-tenths-depth method normally is used if the depth of flow is <br />between 1.5 and 2.5 feet. In shallower depths, the current meter may be <br />affcct-ed-b)l PLoX";"IIl.i.ly to-tl.lt: /:) LLedUlLed-;---- -- <br /> <br />The presence of submerged objects such as large boulders will distort the <br />vertical-velocity curve (Rantz and others 1982). When this distortion occurs, <br />use of the three-point method of computing mean velocity in a vertical has <br />been recommended (Rantz and others, 1982). Usually, the average of the 0.2 <br />and 0.8 depth readings is averaged with the 0.6 depth reading to obtain the <br />mean velocity. If more emphasis on the 0.2 and 0.8 depth readings is desired, <br />then all three readings may be averaged directly. When using a Price type AA <br />meter, depths need to be greater than 2.5 feet to apply this method. <br /> <br />13 <br />
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