Part B1: Basics B1.3 Water conveyance

B1.3 Water conveyance Topics Inlet arrangements –Diversion structures, settling, dealing with flood Water transport –Limitations of canals Getting around obstacles –Flumes, culverts, syphons, Inverted syphons,

B1.3 Water conveyance Conveyance arrangements

B1.3.1Water 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

B1.3.1Water conveyance Water transport: Intakes Direct InletSide Inlet

B1.3.1Water conveyance Water transport: Intakes:Gabions

B1.3.1Water conveyance Water transport: Intakes: Direct inlet

B1.3.1Water conveyance Water transport: Intakes: Side inlet

DirectSide Better transport of silt into the headrace More difficult to construct Needs special grill to self clean Easier to construct Self cleaning B1.3.1Water conveyance Water transport: Intakes: Pros and cons

B1.3.1Water conveyance Water transport: Intakes: Grilles Sloped grille for direct inlet Plain grille for side inlet

B1.3.1Water conveyance Water transport: Intakes: Stream bed

B1.3.3Water conveyance Water transport: Intakes: Rate of inlet Normal water level (h r ) Headrace water level (h h ) weir crest over-top C d = From Bernoulli Intake area (A)

B1.3.3Water conveyance Water transport: Intakes: Rate of inlet weir crest over-top

B1.3.3Water conveyance Water transport: Intakes: Rate of inlet: Weir coefficients Shapecoefficient Broad; sharp edges1.5 Broad; round edges1.6 rounded2.1 Sharp1.9 Roof shaped2.3

B1.3.1Water conveyance Water transport: Intakes: Spillway

B1.3.1Water conveyance Water transport: Intakes: Settlement

B1.3.2Water conveyance Water transport: Open channels: Manning's equation V = Stream velocity (m s -1 ) R = Hydraulic radius S = Slope n =Manning roughness

B1.3.2Water conveyance Water transport: Hydraulic radius: producing the ideal cross section ShapeEfficiency Semi circular1 Half hexagon0.95 Vee0.89 Half square0.84

B1.3.2Water conveyance Water transport: the ideal cross section and variable flow

B1.3.2Water conveyance Water transport: Shapes for highly variable flow

Soil typeSlope Sandy loam2 Loam1.5 Clay loam1 Clay0.58 Concrete0.58 B1.3.2Water conveyance Water transport: Soil and side slopes

B1.3.2Water conveyance Water transport: Limitations to velocity To high – channel erosion To low - silting

Maximum speedsClearSedimented Fine sand0.45 Silt loam0.60 Fine gravel Stiff clay Coarse gravel Shale, hardpan Steel-2.4 Timber concrete Minimum speeds B1.3.2Water conveyance Water transport: Maximum and minimum speeds

B1.3.2Water conveyance Water transport: getting it wrong…

B1.3.2Water conveyance Water transport: grass in channels

Maximum speeds (m/s)BareMedium grass cover Very good grass cover Very light silty sand Light loose sand Coarse sand Sandy loam Sandy soil Firm clay loam Stiff clay or stiff gravelly soil Unlikely to form Course gravel B1.3.2Water conveyance Water transport: grass in channels

B1.3.2Water conveyance Water transport: High slopes: Hydraulic jump

A = Cross sectional area (m) B = breadth of stream at the surface (m) B1.3.2Water conveyance Water transport: High slopes: Hydraulic jump: Critical depth

B1.3.2Water conveyance Water transport: High slopes: Steps

B1.3.2Water conveyance Water transport: making channels

B1.3.3Water conveyance Obstacles: Flume

B1.3.3Water conveyance Obstacles: Pipe bridge

B1.3.3Water conveyance Obstacles: part full pipes

B1.3.3Water conveyance Obstacles: Culverts

B1.3.3Water conveyance Obstacles: Inverted syphons

B1.3.4Water conveyance Comparison between closed pipes and open channels Open channelsClosed pipes Susceptible to blockingWater protected from outside factors Needs care with manipulating gradients to stay within limits Constant flow rate easy to maintain Variable gradient permissible Cheap to buildExpensive to build Cheap to maintainExpensive to maintain – blockages are hidden and difficult to remove Air locks

B1.3Water 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

B2.1Next…..Hydro powerNext…..Hydro power