1.3. Conveyance
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Transcript of 1.3. Conveyance
Part B1: Basics
B1.3 Water conveyance
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,
B1.3 Water conveyanceConveyance arrangements
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
B1.3.1 Water conveyance Water transport: Intakes: siting
B1.3.1 Water conveyance Water transport: Intakes: siting
B1.3.1 Water conveyance Water transport: Intakes: siting
B1.3.1 Water conveyance Water transport: Intakes
Direct Inlet Side Inlet
B1.3.1 Water conveyance Water transport: Intakes:Gabions
B1.3.1 Water conveyance Water transport: Intakes: Direct inlet
B1.3.1 Water conveyance Water transport: Intakes: Side inlet
Direct Side
• Better transport of silt into the headrace
• More difficult to construct
• Needs special grill to self clean
• Easier to construct
• Self cleaning
B1.3.1 Water conveyance Water transport: Intakes: Pros and cons
B1.3.1 Water conveyance Water transport: Intakes: Grilles
Sloped grille for direct inlet
Plain grille for side inlet
B1.3.1 Water conveyance Water transport: Intakes: Stream bed
B1.3.3 Water conveyance Water transport: Intakes: Rate of inlet
Normal water level (hr)
Headrace water level (hh)
weir crestover-top
1 22d r hQ AC g h h
Cd = 0.6-0.8
2
2net
vh
gFrom Bernoulli
Intake area (A)
B1.3.3 Water conveyance Water transport: Intakes: Rate of inlet
2 3
overtopw
Qh
C b
weir crest
over-top
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
B1.3.1 Water conveyance Water transport: Intakes: Spillway
B1.3.1 Water conveyance Water transport: Intakes: Spillway
B1.3.1 Water conveyance Water transport: Intakes: Settlement
B1.3.1 Water conveyance Water transport: Intakes: Settlement
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
2 3 1 2R SV
n
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
B1.3.2 Water conveyance Water transport: the ideal cross section and variable flow
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
01/41/23/41
Water level
Re
lati
ve
eff
icie
nc
y
half circle Half hexagon Half square Vee
B1.3.2 Water conveyance Water transport: Shapes for highly variable flow
Soil type Slope
Sandy loam 2
Loam 1.5
Clay loam 1
Clay 0.58
Concrete 0.58
B1.3.2 Water conveyance Water transport: Soil and side slopes
B1.3.2 Water conveyance Water transport: Limitations to velocity
• To high – channel erosion
• To low - silting
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
B1.3.2 Water conveyance Water transport: Maximum and minimum speeds
B1.3.2 Water conveyance Water transport: getting it wrong…
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
B1.3.2 Water conveyance Water transport: grass in channels
B1.3.2 Water conveyance Water transport: High slopes: Hydraulic jump
A = Cross sectional area (m)
B = breadth of stream at the surface (m)
B1.3.2 Water conveyance Water transport: High slopes: Hydraulic jump: Critical depth
2 3Q A
g B
B1.3.2 Water conveyance Water transport: High slopes: Steps
B1.3.2 Water conveyance Water transport: High slopes: Steps
B1.3.2 Water conveyance Water transport: making channels
B1.3.2 Water conveyance Water transport: making channels
B1.3.2 Water conveyance Water transport: making channels
B1.3.3 Water conveyance Obstacles: Flume
B1.3.3 Water conveyance Obstacles: Flume
B1.3.3 Water conveyance Obstacles: Pipe bridge
B1.3.3 Water conveyance Obstacles: Pipe bridge
B1.3.3 Water conveyance Obstacles: part full pipes
B1.3.3 Water conveyance Obstacles: Culverts
B1.3.3 Water conveyance Obstacles: Culverts
B1.3.3 Water conveyance Obstacles: Inverted syphons
B1.3.3 Water conveyance Obstacles: Inverted syphons
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
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
B2.1 Next…..Hydro power