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    Floatmethod

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    siay mekonnen
    This document describes the float method for measuring stream discharge. Stream discharge (Q) is calculated as the product of cross-sectional area (A) and average flow velocity (V). To perform the float method, objects are released into the stream and the time taken to travel a known distance is measured, from which surface velocity is calculated. Since surface velocity is typically faster than average velocity, a correction factor between 0.8-0.9 is applied to estimate average velocity. Multiple trials are averaged to calculate velocity, and cross-sectional area is estimated by taking depth measurements across the stream's width. Discharge is then calculated as the product of average velocity and cross-sectional area.

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    Floatmethod

    Uploaded by

    siay mekonnen
    206 views2 pages
    This document describes the float method for measuring stream discharge. Stream discharge (Q) is calculated as the product of cross-sectional area (A) and average flow velocity (V). To perform the float method, objects are released into the stream and the time taken to travel a known distance is measured, from which surface velocity is calculated. Since surface velocity is typically faster than average velocity, a correction factor between 0.8-0.9 is applied to estimate average velocity. Multiple trials are averaged to calculate velocity, and cross-sectional area is estimated by taking depth measurements across the stream's width. Discharge is then calculated as the product of average velocity and cross-sectional area.

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    Teaching materials for hydrology

    Original Title

    floatmethod

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    This document describes the float method for measuring stream discharge. Stream discharge (Q) is calculated as the product of cross-sectional area (A) and average flow velocity (V). To perform the float method, objects are released into the stream and the time taken to travel a known distance is measured, from which surface velocity is calculated. Since surface velocity is typically faster than average velocity, a correction factor between 0.8-0.9 is applied to estimate average velocity. Multiple trials are averaged to calculate velocity, and cross-sectional area is estimated by taking depth measurements across the stream's width. Discharge is then calculated as the product of average velocity and cross-sectional area.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    206 views2 pages

    Floatmethod

    Uploaded by

    siay mekonnen
    This document describes the float method for measuring stream discharge. Stream discharge (Q) is calculated as the product of cross-sectional area (A) and average flow velocity (V). To perform the float method, objects are released into the stream and the time taken to travel a known distance is measured, from which surface velocity is calculated. Since surface velocity is typically faster than average velocity, a correction factor between 0.8-0.9 is applied to estimate average velocity. Multiple trials are averaged to calculate velocity, and cross-sectional area is estimated by taking depth measurements across the stream's width. Discharge is then calculated as the product of average velocity and cross-sectional area.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    of 2

    Measurement of Stream Discharge using the Float Method

    The amount of water passing a point on the stream channel during a given time is a function of
    velocity and cross-sectional area of the flowing water.

    Q = AV

    Where, Q is stream discharge (volume/time), A is cross-sectional area, and V is flow velocity

    Materials Required:
     Tape measure
     Stop -watch
     Rod , yard or meter stick to measure depth
     At least three highly visible buoyant objects such as a drifting branches or logs, pine
    cone, coffee stir sticks, half filled bottles, or oranges (objects buoyant enough not to be
    effected by the wind)
     Stakes for anchoring tape measure to stream banks
     Waders

    Approach

    Float method – inexpensive and simple. This method measures surface velocity. Mean velocity
    is obtained using a correction factor. The basic idea is to measure the time that it takes the
    object to float a specified distance downstream.
     
    Vsurface = travel distance/ travel time = L/t
     
    Because surface velocities are typically higher than mean or average velocities

    V mean = k Vsurface

    Where, k is a coefficient that generally ranges from 0.8 for rough beds to 0.9 for smooth
    beds (0.85 is a commonly used value)
    Procedure

    Step 1. Choose a suitable straight reach with minimum turbulence (ideally at least 3 channel
    widths long).
    Step 2. Mark the start and end point of your reach.
    Step 3. If possible, travel time should exceed 20 seconds.
    Step 4. Drop your object into the stream upstream of your upstream marker.
    Step 5. Start the watch when the object crosses the upstream marker and stop the watch when
    it crosses the downstream marker.
    Step 6. You should repeat the measurement at least 3 times and use the average in further
    calculations.
    Step 7. Measure stream’s width and depth across at least one cross section where it is safe to
    wade. If possible, measure depth across the stream's width at the start and stop

    1
    markers and average the two but if measuring one cross section choose the
    downstream side. Use a marked rod, a yard or meter stick to measure the
    depth at regular intervals across the stream. Ten depth measurements is
    the minimum required but more is better, especially in larger streams. Or
    another method, walks heal to toe, and measure stream depth every left big
    toe, along the downstream cross section. Average your cross-
    sectional areas (A): Using the average area and corrected velocity, you can
    now compute discharge, Q.

    Q = Cross section area (A) * mean velocity (V)

    Essentially the cross section technique estimates each of the terms on the right hand side of the
    equation(s) and multiplies them together. The cross section area of the channel is estimated at a
    transect, across which water depth and average water column velocities are measured at a series
    of points (verticals).

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