1. Basic Hydrology & Hydraulics: DES 601
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Rating Curve

2. Definition & terminology
Rating curve, also known as stage–discharge curve, is a graph showing the relation between the water height (stage), and the volumetric flow-rate (discharge).
Channels and hydraulic structures can have rating curves
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3. Definition & terminology
An accurate stage–discharge relationship is necessary for design to evaluate the interrelationships of flow characteristics (depth and discharge)
The stage-discharge relationship also enables evaluation of a range of conditions as opposed to a preselected design flow rate.
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5. How Rating Curves are Constructed - I
Record stage (depth) during an interesting runoff event
Measure velocity across the channel (or structure)
Compute discharge as produce of the flow area and velocity
Wading during flow
Cross Section – Measure V at different stations.
Q=S VA
ADV Probe
Boat mounted for
deeper/faster water
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6. How Rating Curves are Constructed - II
Stage and measured discharge recorded for several different flow events
A curve is fitted to these event pairs, and that curve is the “Rating Curve”
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7. Rating Curves for Hydraulic Structures
Certain hydraulic structures force flow through critical depth, where the specific energy is a minimum.
Knowledge that critical depth will occur along with geometry and an appropriate discharge coefficient produces an indirect measurement of discharge
Weir equations
Orifice equations
Sluice gate equations
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8. Weir flow Weir = obstruction in open channel over which water flows
Flow over a weir directly related to water depth (head) upstream of weir (A rating curve!)
Qweir
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9. Sharp crested weir Module 22 Width into the figure
Discharge Coefficient
(from tables, but derived from rated weirs)
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10. Sharp crested weir flow calculation
For US units, weir discharge coefficient of CD = 3.3 commonly used
Example: If upstream head is 1.2 ft above weir crest, and weir length (normal to the flow) is 8 ft, what is the flow over weir? Q = CDLh3/2 = 3.3(8)(1.2)3/2 = 34.7 cfs
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11. Broad crested weir Equation for discharge is the same
Head often measured to total energy line
Discharge coefficient CD = 2.34 to 3.32
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12. Broad crested weir discharge coefficients
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