Hydrology

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Hydrology

• Measurement of
• discharge
• in a river
• Acad. year 2002-2003
• FLTBW

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Discharge in a river

• Aim to determine the runoff of a river system
  output of the hydrologic system
• Often continuous measurement of water level which
  is translated to a discharge
• Some principles of open channel flow are useful
• Hydraulic structures are most useful if possible
• Examples

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Usual practice

• Continuous measurement of water level (stage)
• Pressure transducer float ultrasonic etc
• Stage-discharge curve (debietdischarge)
• Occasional velocity-area measure velocity and
  cross-section area (float method quick idea
  current meter precise)
• Occasional dilution method (tracer injection)
• Hydraulic structures (e.g. Parshall Flume
  V-notch) well-known and precise relation
  discharge-stage
• New methods. Laser-doppler

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Bernouilli energy equation
Energy -loss
Total potential
V2/2g
Open water surface
H1
D
H2
Rivier bottom
z
Reference heigth
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Bernouilli energy equation

• Energy of the water on the bottom per unit of
  weight discharge
• Pressure D expressed in waterdepth (D P/?g)
• Height of bottom z
• Average velocity V in a cross-section
• Beware open canals (wet cross-section is not
  constant like in a filled pipe)
• Head losses hL friction and local
• In a vertical the potential remains constant

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Reynolds number laminair of turbulent

• Water ?10-6 m²/s normall turbulent
• 4 Rh characteristic length (diameter pipe)
• Rh A/P hydraulic radius (!!!!!!!)
• Sheet flow over smooth surface could be laminair
  (rather exceptional)

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Uniform open chanal-flow

• Uniform (eenparig) constant cross-section
  watersurface, bottom- and energy line are
  parallel
• gt friction loss in equilibrium
• Manning formula (turbulent)

Relation discharge- waterheight is uniform
Rahter rare in natural rivers
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(No Transcript)
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Rivier is not a simple channel
Uniform flow mostly in artificial canals (
irrigation) Backwater widening bends etc are
common in natural rivers
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Usually non-uniform (of niet eenparige) flow

• Gradually varying ( little local loss mainly by
  friction)
• Theoretical calculations are possible
• Backwater curves slow widening etc.
• Rapidly varying (local loss important)
• Over short distance (e.g. Hydraulic jump)
• Relation water level and discharge in natural
  rivers not necessarily uniform but more complex

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Specific energy ( typical concept for open
channel flow)

• Specific energy ( energy relative to bottom)
• Open channels for a given discharge several
  combinations of D and V are possible
• For a given discharge there is a minimal specific
  energy the critical depth or also the critical
  flow

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Critical flow

• Flow at a weir (overlaat)
• Waterfall
• Between mild and steep slope
• Critical flow
• Uniform relation between heigth and discharge at
  critical flow

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Froude number type of flow

• Open channel flow is
• Supercritical or shooting  Frgt1
• Critical Fr 1
• Subcritical Frlt1
• Froude number expresses the proportion of kinetic
  energy/ water depth in a dimensionless number.
• Measurement at Fr gt0.5 is difficult (lots of
  kinetic energy and a relatively low level)

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Good control section

• Critical depth stabilizes the relation between
  discharge and level
• However low levels high kinetic energygt very
  difficult for accurate and sensitive measurements
• Downstream of critical flow
• good control with good relation discharge and
  level and easy to measure large levels with low
  kinetic energy
• (Fr lt0.5 lower kinetic energy)

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Hydraulic structures

• Welldefined structures with well-known transition
  from critical to subcritical
• Uniform and constant discharge level relation
• Fr lt 0.5 (sufficient level and lower kinetic
  energy) and so high sensitivity to level
• Avoid sedimentation ( at lower Fr)
• If possible with a minimum of energy-loss

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Example of a structure Parshall flume

• Standardized design exact and different sizes
• Very good throughflow low sedimentation and low
  energy loss
• Expensive and rather complicated construction
• Not so flexible to fitt in a river or channel
• Design and construction errors common by not
  following the guidelines precisely!!!

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Parshall flume
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Parshall UCL
Common in Watertreatment plants
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Scharpcrested weir (scherpe overlaten V-notch
e.g.)

• Simple and cheap ()
• Easy to fit ()
• Large energy loss (- -)
• Fast sedimentation (with clean water no problem)
  (-)
• Can be very accurate if well constructed (/-)

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V-notch weir
H
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V-notch UCL
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V-notch Greece-drinking water
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V-notch Tanzania sedimented (-)
Level-Recorder
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RBC-flumes

• Good compromise flexible construction sediment
  passes well and little energy loss
• Can be fitted in most cross-sections (pipe,
  trapezoidal etc)

Critical flow
measurement
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RBC in irrigation channel
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Ad hoc structures

• Require calibration Cw by experiments
• Powerfunctions with sometime a known power
• Rectangular weir B 1.5 and W width
• Triangular weir B 2.5 (and W 1)

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V-notch rectangular for high flows
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Adhoc structures e.g. watermill
Voer Bertem (41 km²)
Voer Heverlee (51 km²)
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Volumetric Flow rate Measurements

• liters per minute
• cubic meters per second (m3/sec) or
• liters per second (l/s).

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Velocity-Area Flow Rate Measurements in rivers

• Q A V
• where
• Q - flow rate, volume per unit time
• A - cross-sectional area of flow
• V - mean velocity of flow

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Current meter velocity
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Velocity area