STREAMFLOW and HYDROGRAPH ANALYSIS

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STREAMFLOW and HYDROGRAPH ANALYSIS

• Stream flow is one of the most important topics
  in engineering hydrology because it directly
  relate to water supply, flood control, reservoir
  design, navigation, irrigation, drainage, water
  quality, and others.

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Stream Flow Measurements

• Serves as the basis for many water resources
  engineering designs
• Two approaches
• Measurement of water stage
• Measurement of flow velocity
• Measurement of Water Stage
• Water stage the elevation above some arbitrary
  datum of water surface at a station
• Types of Gages Measuring River Stage
• Staff gage vertical or inclined
• Suspended weight gage
• Recording gage
• Crest stage gage ( used to indicate high water
  mark)
• Misc (Table 1).

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Figures of Stream Gauges
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List of StreamFlow Measurement Methods
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Stage-Discharge Relation

• When water stages are measured, we need
  additional information to estimate the flow rates
  (or discharges)

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Stage-Discharge Relation

• Typical relationship Q a(H b)c
• The function relationship between H Q has to be
  calibrated locally for different stations

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Storage Hysterisis

• In natural rivers, the H-Q relationship in
  general appears to be a loop, rather than
  single-valued.

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Devices for Flow Velocity Measurement

• Current Meters
• Cups
• Propellers
• V a b?N
• where V flow velocity a starting velocity
  to overcome mechanical friction b equipment
  calibration constant N revolutions/sec.
• Pitot Tubes Suitable only for clean water
• Floats Suitable for straight channel, V L/T

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Current Meters
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Mean Flow Velocity Estimation

• Velocity Profile

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Measurement of Stream Flow Discharge - 1

• Stage Discharge (Rating) Curve
• Mid-Section Method

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Measurement of Stream Flow Discharge - 2
(c) Mean-Section Method
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Measurement of Stream Flow Discharge - 3

• (d) Area-Slope Method

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Measurement of Stream Flow Discharge - 4
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Hydraulic Structures for Discharge Measurement
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Regime Theory

• W a?Qb D c?Qf V k?Qm
• Q V?W?D ? a?c?k 1.0
• subject to bfm 1.0
•  
•    Generally applicable up to mean annual
  discharge. For flows larger than the mean annual
  discharge, different relationships exist.

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Extension of Rating Curve

• - During the event of large flood, it is
  impossible or impractical to measure discharge
  directly. More often than not, the flood stage
  goes beyond the range of the data range used to
  define the rating curve. Therefore, extrapolation
  of the ration curve is needed when water level is
  recorded below the lowest or above the highest
  level.
• - Large errors can result if the functional form
  of rating curve, Q a?(Hb)c, is extrapolated
  beyond the recorded gauge discharges without
  consideration of the cross-section geometry and
  controls
• - Graphical extension or by the fitted Q-H
  relationship is adequate only for small extension
• - For large extrapolation beyond the active
  channel cross-section, hydraulic formula can be
  used to estimate the stage-discharge relation.

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Steven Extension
Based on Chezy formula, with A flow
cross-section area C Chezy Coefficient R
hydraulic radius, A/P and S channel
slope.   For a given section, constant
whereas for a wide channel (Wgt10D) R?D.
Therefore,
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Example (Extension of Rating Curve)