Part B1: Basics

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Part B1 Basics B1.3 Water conveyance
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B1.3 Water conveyanceTopics

• Inlet arrangements
• Diversion structures, settling, dealing with
  flood
• Water transport
• Limitations of canals
• Getting around obstacles
• Flumes, culverts, syphons, Inverted syphons,

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B1.3 Water conveyanceConveyance arrangements
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B1.3.1 Water conveyance Inlet arrangements
Considerations

• How much of the flow to divert
• Total flow needs weirs which are expensive and
  may cause problems
• Some fraction may be cheaper
• Dealing with abnormal flow
• Drought (low flow) lack of performance (may
  not work at all)
• Flood (high flow) things break!!!!
• Dealing with sediment
• Blocking of the inlet

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B1.3.1 Water conveyance Water transport
Intakes siting
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B1.3.1 Water conveyance Water transport
Intakes siting
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B1.3.1 Water conveyance Water transport
Intakes siting
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B1.3.1 Water conveyance Water transport Intakes
Direct Inlet
Side Inlet
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B1.3.1 Water conveyance Water transport
IntakesGabions
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B1.3.1 Water conveyance Water transport
Intakes Direct inlet
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B1.3.1 Water conveyance Water transport
Intakes Side inlet
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B1.3.1 Water conveyance Water transport
Intakes Pros and cons
Direct Side
Better transport of silt into the headrace More difficult to construct Needs special grill to self clean Easier to construct Self cleaning
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B1.3.1 Water conveyance Water transport
Intakes Grilles
Sloped grille for direct inlet
Plain grille for side inlet
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B1.3.1 Water conveyance Water transport
Intakes Stream bed
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B1.3.3 Water conveyance Water transport
Intakes Rate of inlet
over-top
weir crest
Normal water level (hr)
Headrace water level (hh)
Intake area (A)
From Bernoulli
Cd 0.6-0.8
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B1.3.3 Water conveyance Water transport
Intakes Rate of inlet
over-top
weir crest
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B1.3.3 Water conveyance Water transport
Intakes Rate of inlet Weir coefficients
Shape coefficient
Broad sharp edges 1.5
Broad round edges 1.6
rounded 2.1
Sharp 1.9
Roof shaped 2.3
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B1.3.1 Water conveyance Water transport
Intakes Spillway
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B1.3.1 Water conveyance Water transport
Intakes Spillway
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B1.3.1 Water conveyance Water transport
Intakes Settlement
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B1.3.1 Water conveyance Water transport
Intakes Settlement
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B1.3.2 Water conveyance Water transport Open
channels Manning's equation

V Stream velocity (m s-1) R Hydraulic
radius S Slope n Manning roughness
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B1.3.2 Water conveyance Water transport
Hydraulic radius producing the ideal cross
section
Shape Efficiency
Semi circular 1
Half hexagon 0.95
Vee 0.89
Half square 0.84
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B1.3.2 Water conveyance Water transport the
ideal cross section and variable flow
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B1.3.2 Water conveyance Water transport Shapes
for highly variable flow
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B1.3.2 Water conveyance Water transport Soil
and side slopes
Soil type Slope Slope
Sandy loam 2
Loam 1.5
Clay loam 1
Clay 0.58
Concrete 0.58
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B1.3.2 Water conveyance Water transport
Limitations to velocity

• To high channel erosion
• To low - silting

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B1.3.2 Water conveyance Water transport Maximum
and minimum speeds
Maximum speeds Clear Sedimented
Fine sand 0.45 0.45
Silt loam 0.60 0.60
Fine gravel 0.75 1.00
Stiff clay 1.2 0.90
Coarse gravel 1.2 1.8
Shale, hardpan 1.8 1.5
Steel - 2.4
Timber 6.0 3.0
concrete 12.0 3.6

Minimum speeds 0.1 0.31
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B1.3.2 Water conveyance Water transport getting
it wrong
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B1.3.2 Water conveyance Water transport grass
in channels
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B1.3.2 Water conveyance Water transport grass
in channels
Maximum speeds (m/s) Bare Medium grass cover Very good grass cover
Very light silty sand 0.3 0.75 1.5
Light loose sand 0.5 0.9 1.5
Coarse sand 0.75 1.25 1.7
Sandy loam 0.75 1.5 2.0
Sandy soil 1.0 1.7 2.3
Firm clay loam 1.5 1.8 2.3
Stiff clay or stiff gravelly soil 1.5 1.8 Unlikely to form
Course gravel 1.8 2.1 Unlikely to form
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B1.3.2 Water conveyance Water transport High
slopes Hydraulic jump
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B1.3.2 Water conveyance Water transport High
slopes Hydraulic jump Critical depth

A Cross sectional area (m) B breadth of
stream at the surface (m)
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B1.3.2 Water conveyance Water transport High
slopes Steps
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B1.3.2 Water conveyance Water transport High
slopes Steps
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B1.3.2 Water conveyance Water transport making
channels
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B1.3.2 Water conveyance Water transport making
channels
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B1.3.2 Water conveyance Water transport making
channels
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B1.3.3 Water conveyance Obstacles Flume
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B1.3.3 Water conveyance Obstacles Flume
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B1.3.3 Water conveyance Obstacles Pipe bridge
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B1.3.3 Water conveyance Obstacles Pipe bridge
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B1.3.3 Water conveyance Obstacles part full
pipes
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B1.3.3 Water conveyance Obstacles Culverts
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B1.3.3 Water conveyance Obstacles Culverts
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B1.3.3 Water conveyance Obstacles Inverted
syphons
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B1.3.3 Water conveyance Obstacles Inverted
syphons
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B1.3.4 Water conveyance Comparison between
closed pipes and open channels
Open channels Closed pipes
Susceptible to blocking Water protected from outside factors
Needs care with manipulating gradients to stay within limits Constant flow rate easy to maintain Variable gradient permissible
Cheap to build Expensive to build
Cheap to maintain Expensive to maintain blockages are hidden and difficult to remove Air locks
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B1.3 Water conveyance Summary

• Intakes should be carefully sited to avoid
  silting or damage. They should also be
  self-cleaning
• Water conveyance structures should be designed
  for both high and low flow conditions. A number
  of methods are available to do this such as
  weirs, spillways and sluice gates
• The height of the flow is predictable using
  Bernoulli and manning formulas
• Channel cross sections should take account
  limitations placed by the soil. Stepping the
  channels can be used to slow the flow and avoid
  hydraulic jump
• A number of methods can be used to overcome
  obstacles such as flumes, pipes bridges, culverts
  and inverted syphons

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B2.1 Next..Hydro power