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