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Slope-area method of flow measurement
John FenwickNational Institute of Water & Atmospheric
Research Ltd
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Slope-area
A method when nothing else will work?
But also has application in hydraulic calculations – e.g. design & construction of canals
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Slope area
Uses the Manning formula:V= R2/3 Sf
1/2 /n
(Q = Area x V)
some uncertainty in estimating n
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Requirements
A suitable reachA means of defining/measuring surface slopeTwo (or more) cross-sections with their areas measured, at top and bottom of reach
(intermediate cross-sections can be helpful)
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A suitable reach..
Straight, long (> 5 x width)Contain flow without overflowMinimum of vegetationUniform cross-sections, preferably slightly converging
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Procedure
Select reachMeasure cross-sectionsMeasure slopeEstimate Manning nCalculate, using appropriate formula
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Measure cross-sections
Full flood waterway and beyondCalculate areasCalculate wetted perimeters and hence hydraulic Radii (R)
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WLs from flood-marks
Can be moderately accurateOften, all you haveNeeds some care in working out
Trees and scrub bend down in the flow and collect debrisSurge puts flood marks higher than actual
The slope parameter is very sensitive to the result, and very subject to error
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Accuracy of floodmarks
Flood marks are often elevated above actual water-levelForce of waterSurge
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Recorder can be one section
..If it survivesUse recorder as top or bottom section Should give good accurate level for that section
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Peg slopes if you are there
More accurate than using floodmarksAccess can be difficult – delays can give wrong levels