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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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Slope area

good for estimating flood peaks which cannot otherwise be measured

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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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Measure water-surface slope

between cross-sectionsUse pegged WLs, flood marks?

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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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Floodmarks

Not easy to interpret

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Not easy to find?

Sometimes

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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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Rough river beds give another set of problems

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Peg slopes if you are there

More accurate than using floodmarksAccess can be difficult – delays can give wrong levels

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Estimation of n

Use Hicks & Mason?Gauge at other flow and back-calculate n

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Compile stage/ncurves from gaugings

Stage

n

bankfull

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Slope-area exercise

Slope-area gauging cardData sheetCalculate, after assessing n